PRINCIPLES OF
INTERCHANGEABLE MANUFACTURING
PRINCIPLES OF
INTERCHANGEABLE
MANUFACTURING
A TREATISE ON THE BASIC PRINCIPLES
INVOLVED IN SUCCESSFUL INTERCHANGE-
ABLE MANUFACTURING PRACTICE
COVERING DESIGN, TOLERANCES, DRAW-
INGS, MANUFACTURING EQUIPMENT,
GAGING AND INSPECTION
BY
EARLE BUCKINGHAM, A.S.M.E., S.A.E.
ENGINEER, PRATT & WHITNEY Co.
FIRST EDITION
FIRST PRINTING
NEW YORK
THE INDUSTRIAL PRESS
LONDON: THE MACHINERY PUBLISHING CO.. LTD
COPYRIGHT, 1921,
BY
THE INDUSTRIAL PRESS
NEW YORK
COMPOSITION AND ELECTROTYPING BY THE PLIMPTON PRESS, NORWOOD, MASS., U.S.A.
PREFACE
WHILE many articles dealing with various phases of inter-
changeable manufacturing have appeared from time to time
in the technical press, no complete and comprehensive treatise
dealing with this subject as a whole has heretofore been avail-
able to those interested in interchangeable manufacturing in
the machine building and metal working fields.
The development of interchangeable manufacturing is closely
interwoven with many distinctly American manufacturing
methods and processes. Every large American industry has
contributed its share to the progress made in interchangeable
manufacturing. Different plants working along independent
lines have often achieved the same results by widely different
methods. The author has attempted to define and emphasize
the underlying basic principles, using specific methods only
when necessary to illustrate the application of these principles
in actual manufacturing processes. He has gathered the in-
formation upon which this treatise is based from many manu-
facturing plants, both large and small, in this country and in
Canada. He has seen every method discussed in successful
operation, some in one plant, some in another but not all
in any one.
For more than ten years the author has been in constant
touch with many of the detailed manufacturing problems that
arise in the production of interchangeable mechanisms in large
quantities. During the World War his work took him for four
years into many manufacturing plants in connection with ord-
nance work, first for private corporations and later for the
Ordnance Department. When engaged in this work it became
apparent to him that the absence of common methods of inter-
pretation of drawings, tolerances, and specifications, the lack
of uniform gaging methods and misunderstanding of many of
v
822419
VI PREFACE
the factors of interchangeable manufacturing, presented an
urgent need for a complete treatise on this subject.
In arranging the material available on the subject of inter-
changeable manufacturing, the author has first taken up the
general principles involved in the industrial application of this
method of production, and has then devoted a separate chapter
to the definition of the terms used, so that there will be no mis-
understanding as to the meaning of the terms used later in the
book. The influence of interchangeable manufacturing processes
on machine design and the purposes of models are then dealt
with, followed by a complete and minute discussion on the
dimensioning of drawings intended for use in interchangeable
manufacturing. This is followed by a discussion of the principal
elements that govern mechanical production, the equipment
required for interchangeable manufacturing (including machines,
jigs and fixtures); the gaging equipment necessary; and the
principles of inspection and testing. Special chapters are also
devoted to the manufacture for selective assembly, and methods
used in small quantity production on an interchangeable basis.
An entire chapter deals with the service factor in interchangeable
manufacturing, because in the final analysis no manufactured
machine or device is ever purchased for itself alone, but is
acquired for the service which it is supposed to render.
The Pratt & Whitney Co., Hartford, Conn., with whose
cooperation this treatise is written, submits it to the public
as a part of the company's contribution to the art of inter-
changeable manufacturing with the hope that it will assist
manufacturers and mechanics to employ effectively the prin-
ciples of interchangeable manufacturing and to reap the benefits
that a rational application of these principles make possible.
The author also wishes to acknowledge at this time the assist-
ance that has been given him by many other manufacturing
plants that he has visited. To name them all would mean a long
list of prominent plants manufacturing machine tools, auto-
mobiles, tractors, ordnance, typewriters, watches, phonographs,
instruments, etc.
EARLE BUCKINGHAM
CONTENTS
CHAPTER I
PRINCIPLES OF INTERCHANGEABLE
MANUFACTURING
PAGES
Economy Extent of Interchangeability Clearances
Tolerances Component Drawings Specifications Gages
for Checking Results Manufacturing Equipment Pro-
duction Problems Inspection of Product 1-17
CHAPTER II
TERMS USED IN INTERCHANGEABLE
MANUFACTURING
Interchangeability - - Selective Assembly - - Function
Limit Tolerance Basic and Model Size Maximum and
Minimum Metal Size Maximum and Minimum Clearance
Interference Operating, Functional, and Clearance Sur-
faces Elementary and Composite Surfaces Compound
Tolerances Register Points Unit Assembly Component
and Operation Drawings 18-28
CHAPTER III
MACHINE DESIGN IN INTERCHANGEABLE
MANUFACTURING
Classes of Design Simplifying Design Choice of Ma-
terials Clearances and Tolerances Application of Inter-
changeable Principle Advantages of Unit Assembly -
Designing for Assembling and Service 29-39
CHAPTER IV
PURPOSE OF MODELS
Manufacturing Model to Test Functioning Experimental
Model Testing Tolerances Model for Standard of Pre-
cision 40-45
vii
viii CONTENTS
CHAPTER V
PRINCIPLES IN MAKING COMPONENT DRAWINGS
PAGES
Functional Drawings Manufacturing Drawings Laws of
Dimensioning Inspection Gage Requirements Composite
Surfaces Compound Tolerances Force Fits Profile Sur-
faces Dimensioning Holes Location of Holes Con-
centricity and Alignment Gears 46-76
CHAPTER VI
PRACTICE IN MAKING COMPONENT DRAWINGS
Maintaining Functional Requirements of a Mechanism
Basic Dimensioning Maintaining a Common Locating Point
Length Dimensions from Common Locating Point Draw-
ing of Separate Parts Compound Tolerances 77-104
CHAPTER VII
ECONOMICAL PRODUCTION
Specifications Functions and Requirements of Product
Manufacturing Data Factory Cost Direct Labor Cost
Machine Hour Rate Product Overhead Clerical and Ac-
counting Work Specific and General Information 105-120
CHAPTER VIII
EQUIPMENT FOR INTERCHANGEABLE
MANUFACTURING
Selection of Machine Tools Designing Jigs and Fixtures
Cutting Tools Locating Points Chip Clearances Check-
ing and Testing Jigs and Fixtures Maintaining Tolerances
Special Equipment for Machining Automobile Transmission
Cases Drilling Holes Simultaneously Pneumatic Clamp-
ing Devices Multiple-Tool Facing Bar Milling and Drill-
ing Fixtures 121-176
CHAPTER IX
GAGES IN INTERCHANGEABLE MANUFACTURING
Classification According to Use Accuracy Working and
Inspection Gages Interchangeability between Parts made in
CONTENTS IX
PAGES
Different Shops Snap, Ring, and Plug Gages Contour or
Profile Gages Receiving Gages Flush-pin Gages Sliding
Bar Gages Depth and Length Gages Hole Gages -
Gaging Threads Tolerances on Threaded Parts Wing and
Indicator Gages Functional Gages Gaging Gears -
Master and Reference Gages 177-216
CHAPTER X
INSPECTION AND TESTING
Discrepancy between Part and Drawing Incomplete
Drawings and Specifications Shop Inspection Final In-
spection Inspecting Gages and Material Testing As-
sembled Mechanisms 217-223
CHAPTER XI
MANUFACTURING FOR SELECTIVE ASSEMBLY
Clearances and Tolerances Dimensions and Tolerances on
Drawings Laws of Dimensioning Similarity of Specifica-
tions, Equipment, Gages, and Inspection Methods 224-225
CHAPTER XII
SMALL-QUANTITY PRODUCTION METHODS
Standardization of Nominal Sizes Clearances and Toler-
ances Economy of Standardization Standardizing Unit
Assemblies to Suit Several Machines Component Drawings
- Manufacturing Equipment Gages and Methods of In-
spection 230-240
CHAPTER XIII
SERVICE FACTOR IN INTERCHANGEABLE
MANUFACTURING
Functional and Manufacturing Designs Keeping Specifica-
tions up to Date Planning Production to Obtain Required
Service. . 241-245
PRINCIPLES
OF INTERCHANGEABLE
MANUFACTURING
CHAPTER I
PRINCIPLES OF INTERCHANGEABLE MANUFACTURING
INTERCHANGEABLE manufacturing consists of machining the
component parts of a given mechanism in the manufacturing
departments within such limits that they may be assembled in
the assembling department without fitting or further machining.
> Component parts may also be replaced or transferred from
one mechanism to another without detriment to the function-
ing and without machining. The advantages of such a method
of manufacture are self-evident, and need not be dwelt upon
further. It is obvious that with proper equipment and control,
the component parts of a mechanism can thus be manufactured
in large quantities at a low direct labor cost.
Economy of Interchangeable Manufacturing. In all private
industrial enterprises ultimate economy is the controlling factor
of any method of procedure. This does not necessarily mean
that the methods adopted always are actually the most economi-
cal. Methods which will promote this economy are, however,
the ideals toward which manufacturers are constantly striving.
Now, a careful analysis will show that interchangeability does
not always result in ultimate economy. In such cases the at-
tempt to maintain it is a fault, not a virtue.
To make this point clear, consider the matter first from the
standpoint of production alone. The equipment and prepa-
ration necessary to produce interchangeable parts are expensive.
In making only a small number of special mechanisms, it would
be gross extravagance to maintain any high degree of inter-
changeability. Viewed simply as a question of production, the
2 ; .; : \ .INTERCHANGEABLE MANUFACTURING
problem of interchangeable parts is solved by establishing a
balance between manufacturing and assembling costs, whether
the quantity of production be great or small, whether the mecha-
nism involved be a standard or a special product.
Ultimate economy, however, must include the factor of service.
Suppose automobiles, typewriters, sewing machines, or sporting
rifles are sold. Parts will wear out or be broken by accident.
The maintenance of service stations, where extra parts are
quickly available, tends to keep customers satisfied. Service
stations will be least expensive if the product is truly inter-
changeable and the agent can replace a part with the aid of a
screwdriver or wrench or, still better, if the customer can
replace it himself. Since the advent of the automobile, people
have been much more interested in things mechanical than
before, and have taken pride in making their own repairs. The
more nearly interchangeable mechanisms are made, the more
this desirable trait is fostered and the less will service stations
cost. Ultimate economy, then, requires that service costs be
balanced against total productive costs.
Degree of Interchangeability Desirable. It should not be as-
sumed from this that entire interchangeability or none at all
must be had. In almost every mechanism certain parts are
begun as separate units in order to simplify the manufacture,
but are later permanently assembled into a single unit and
machined to completion as such. In many such cases, the
expense of attaining interchangeability would be too great to
justify the attempt, because of the many mechanical difficulties
to be overcome. It would be more economical, in case of break-
age, to discard and replace the entire assembled unit. In other
cases, the functional requirements may be so severe that a
system of selective assembly will prove to be the proper course,
although this entails carrying a double or triple number of spare
parts in service stations, or involves some fitting when replacing
unserviceable parts.
In general, however, interchangeability is a desirable goal,
and is readily attained in the majority of cases if the proper
attention is given to the basic principles governing it, including
INTERCHANGEABLE MANUFACTURING 3
the design of the mechanism and the process of manufacture;
yet it is limited in several directions by the inadequacy of many
present manufacturing conditions. With improved facilities,
it may be that in future years a much greater degree of inter-
changeability will be possible than at present.
The following paragraphs, which are based on manufacturing
conditions as they now exist, trace the progress of a commodity
through all stages of its manufacture, from its inception as a
mechanical project to the final testing that determines its suc-
cessful completion. An attempt has been made to single out for
special comment those factors which make possible, or promote,
the interchangeable manufacture of its parts.
Design as it Affects Success. The development of any new
mechanism starts with a mental conception of some function
to be performed. This conception then takes detailed form,
first mentally, then on paper, and finally in metal. The experi-
mental model if such be constructed is usually made by the
cut-and-try method. Little attention is paid in the beginning
to future manufacturing requirements. The main object is to
construct a mechanism that will function properly regardless
of the exact design. When this end is reached, what may be
called the inventive or functional design has demonstrated its
success.
Before manufacturing is begun, however, a manufacturing
design must be perfected which will modify the inventive
design so as to allow its economical production on a large scale.
Several manufacturers recognize this twofold nature of design-
ing, and maintain a separate department for each type. In-
dispensable as is the original invention, it is the manufactur-
ing design which largely determines the success or failure of a
given project. This manufacturing designing necessarily con-
tinues throughout the whole course of production because of the
almost infinite number of petty detailed questions involved,
only a few of which can be foreseen and answered in advance.
One of the important functions of an engineering department is
to keep in close touch with the progress of the work in the
shops, deduce general principles therefrom, and apply these
4 INTERCHANGEABLE MANUFACTURING
principles not only to the work in hand, but also to all new work
that may be developed.
The Manufacturing Model. Assume that the functional re-
quirements of the mechanism are established and that the
manufacturing design has been adopted. The first concern is
to test this design as far as possible. The most certain method
of accomplishing this is to develop a physical model. Such a
model must not be confused with the experimental model,
as its purpose is quite different. The experimental model
shows that the mechanism will perform certain functions.
The manufacturing or physical model, if properly developed,
proves that the mechanism, as modified and developed to facili-
tate manufacture, still retains the functional advantages of the
experimental model.,, The manufacturing model is naturally
an expensive piecei of equipment, but if a large output of a new
commodity is under consideration, it is money well invested.
In the case of a small total output, a "pilot" mechanism is often
built for this purpose, which is not set aside for future reference
but incorporated in the product itself.
There are many other services which a manufacturing model
is capable of rendering. It may serve as a physical standard of
dimensions for the future product. In this case, it must be made
with much greater care than if it were to be used merely to test
the functioning of the manufacturing design. Such a model
will be of great value as a reference at all times during pro-
duction: In itself, it comprises an effective functional gage to
test any completed part. It should be used but rarely, however,
for that purpose. In addition, the component parts of the model
are of great assistance in checking the manufacturing equipment
in the early stages of the work.
Clearances. Clearances are vital factors in interchangeable
manufacturing. Fits can be secured without interchangeability,
but the latter cannot be maintained without proper clearances.
It is self-evident that a certain space must be left between
operating parts. The minimum clearances should be as small
as the assembling of the parts and their proper operation under
service conditions will allow. The maximum clearances should
INTERCHANGEABLE MANUFACTURING 5
be as great as the functioning of the mechanism permits. The
variation between a maximum and a minimum clearance de-
termines the manufacturing tolerance. It is clear, then, that
determining at the outset the permissible clearances establishes
also the extent of the tolerances which control the final inspection.
Clearances should be one of the principal considerations in
developing the manufacturing design. This design should aim
to allow the greatest possible amount of clearance between
companion parts. The more the design lends itself to this end,
the greater the economy of manufacture and the greater the
degree of interchangeability obtainable. In determining which
parts of a mechanism can be made interchangeable, this matter
of permissible clearances plays the largest part. A mechanism
which is so designed that it cannot permit fairly liberal clear-
ances is not a suitable one to be manufactured on a strictly
interchangeable basis with the standard equipment now avail-
able. Every operating part of a mechanism must be located with-
in reasonably close clearances in each plane. After such require-
ments of location are met, all other surfaces should have liberal
clearances, unless the factor of strength is the controlling one.
Manufacturing Tolerances. The general tendency in the past
has been to establish manufacturing tolerances by trying to
hold the product as closely as possible to a fixed size. The natural
result of this policy is that the tolerances established on paper
are often exceeded; yet the actual working variations remain
unrecorded, because it is argued that under certain conditions
the original requirements might be met and, therefore, the
tolerances noted are the proper ones, even though they are
not maintained. Every effort to make the recorded tolerances
represent the actual working tolerances is opposed on the ground
that such a procedure would lower the shop standards. As a
matter of fact, it is hard to understand how anything could
lower the standards of the shop more than the absolute disregard
of the rules it is supposed to be obeying.
There is a further argument for the acceptance of liberal
tolerances. Too often in manufacturing concerns, and especially
in the case of interchangeable manufacturing, one finds details
6 INTERCHANGEABLE MANUFACTURING
being made ends in themselves rather than means to a larger
end. In producing a component part, the main object should
not be to demonstrate how closely a fixed size can be approached;
the aim should be to construct, as economically as possible, a
mechanism that will satisfactorily perform certain functions.
The knowledge of how accurately a machining operation can
be performed is indeed invaluable in making the manufacturing
design; but when that design has once been completed, interest
should shift to the proper functioning of the completed mecha-
nism. Finally, it may be said that in most cases the tolerances
originally fixed are increased during the process of manufactur-
ing without detriment to the mechanism. It is rarely that a
tolerance has to be reduced.
The proper minimum clearances can be determined quite
readily and definitely for most cases in the early stages of the
work the manufacturing model is of great value in this
respect but the maximum clearances become established only
after extended experience with the particular mechanism. In
many cases the extreme maximum is never found, because
long before that point is reached, the tolerances have become so
liberal that there is no need, from the standpoint of economical
production, to increase them further.
Component Drawings. Component drawings have two main
functions to perform. The first is to give such information about
the design and the tolerances that the manufacture of the product
can begin. This does not seem like a very difficult task, but the
notation of the tolerances on component drawings has created
new problems of interpretation that have not, as yet, been
fully solved. At the present time, the language of drawings is
not altogether clear and exact.
The first tendency in introducing tolerances on drawings
seems to have been to attempt to express a permissible variation
on every dimension given. The results obtained in the shop
depend, then, upon the particular combination of dimensions
used. Different organizations using different combinations
could obtain radically different results; and of the possible
number of different combinations there is no end.
INTERCHANGEABLE MANUFACTURING 7
The existence of a tolerance on a drawing is an acknowledg-
ment that variations are inevitable in the physical dimensions
of the product. Any dimension given on such a drawing without
a tolerance should not be construed to denote an absolute size
without error, but rather to indicate either that the permissible
variation for that point or surface is controlled by tolerances
given on other co-related dimensions, or that the dimension is
so relatively unimportant that no attempt had been made to
determine its permissible variation.
In making component drawings, the effort should be made
to so give the dimensions and necessary tolerances that it would
be possible to lay out one, and only one, representation of the
" maximum metal" condition and one, and only one, of the
" minimum metal" condition. If such lay-outs were super-
imposed, the difference between them would represent the
permissible variation on every surface. Any condition of the
product which fell within the zone thus established could be
considered as meeting the requirements of the drawing. If
one will make a few such lay-outs, it will soon be clear to him
that there are always a number of dimensions that should be
given without tolerances if drawings are to be kept consistent
and intelligible.
Information on Component Drawings. It must be realized at
the start that it is impossible in every case to give on one com-
ponent drawing all the dimensions that are needed to construct
the patterns, tools, gages, and other manufacturing equipment,
without introducing many inconsistencies. Certain dimensions
could be correct if one set of holding points and one series of
operations were to be used, but would be incorrect under differ-
ent conditions. If the component drawings are made so that
they represent the proper completed conditions call them
inspection gage requirements if you will the end in view is
attained. Any figures that the shop desires to use are correct
if they insure this result.
It is impossible to amplify this point without entering into
a prolonged discussion of the effect of using different holding
or registering points in the manufacturing processes. Yet it
8 INTERCHANGEABLE MANUFACTURING
may be of interest to know that several manufacturing plants
solve this problem by adding operation drawings, which give
only the specific dimensions required at a particular operation.
Some of the dimensions are duplicates of those on the component
drawing, while others are computed to serve their restricted
purpose. This proves an effective means of recording additional
information required in the manufacturing departments, which
cannot be put on component drawings without danger of misuse.
After production is well under way, the component drawings
have served their first purpose. In the meantime, the actual
manufacturing operations have made available a store of new
information regarding the proper conditions to be maintained.
It should be the second function of the component drawings to
record as much of this information as possible. Conflicting
information or misinformation should be eliminated at the same
time; in short, the drawings should be revised to agree with
actual conditions and requirements. It has been a great fault
in the past to neglect this second function almost entirely. It is
a difficult task to make the component drawings represent from
the first conditions that must be maintained. In time, the
shop will discover many of them, often after bitter experience,
even though they have been omitted from the component draw-
ings. Frequently, however, it happens that this information
does not make its way back to the office, but is retained by the
shop men among themselves. Often this is the fault of the
office, which is prone to consider such information as criticism,
so that the shop, after a few rebuffs, makes no further attempt
to pass it along. It is most essential, however, that such in-
formation be recorded in permanent form, not only because of
its value to the work in hand, but also because of its helpful
application to new work in the future.
Dimensioning of Component Drawings. The problem of the
proper dimensioning of component drawings is strictly a mathe-
matical one. There are a few basic principles in regard to it
as fixed and simple as Newton's three laws of motion, but even
more difficult at times to apply correctly. When either of the
two following principles is violated, trouble will inevitably follow:
INTERCHANGEABLE MANUFACTURING 9
1. In interchangeable manufacturing, there is but one dimen-
sion (or group of dimensions) in the same straight line that can
be controlled within fixed tolerances. That is the distance
between the cutting surface of the tool and the locating or
registering surface of the part being machined. Hence, it is
incorrect to locate any point or surface with tolerances from more
than one point in the same straight line.
2. Dimensions should be given between those points which
it is essential to hold in a specific relation to each other. The
majority of dimensions, however, are relatively unimportant
in this respect. It is good practice to establish locating points
in each plane, and to give, as far as possible, all such dimensions
from these common locating points.
There are also a few other general principles which it is good
practice to follow. Although violations of them are not errors
in themselves, they lead to many unnecessary errors. In all of
this work it must be realized that it is impossible to create
anything that is altogether fool-proof; the best that can be
expected is to make conditions such that little or no excuse
remains for making a mistake. The three following principles
are of this order:
1. The basic dimensions given on component drawings for /
interchangeable parts should be the maximum metal sizes,
except for force fits and other unusual conditions. The direct 1
comparison of the basic sizes should check the " danger zone" I
or the minimum clearance conditions in most cases. It is evident !
that these sizes are the most important ones, as they control
the interchangeability. They should be the first determined
and, once established, they should remain fixed if the mechanism
functions properly and the design is unchanged. The direction
of the tolerances, then, would be such as would increase this
clearance. For force fits, such as taper keys, etc., the basic
dimensions should be those which determine the minimum inter-
ference (which is the " danger zone" in this case) and the direc-
tion of the tolerances for this class of work should be such as
would increase this interference.
2. Dimensions should not be duplicated between the same
10 INTERCHANGEABLE MANUFACTURING
points. The duplication of dimensions causes much needless
trouble, due to changes being made in one place and not in the
others. It causes less trouble to search a drawing to find dimen-
sions than it does to have them duplicated and, though more
readily found, inconsistent.
3. As far as possible, the dimensions on companion parts
should be given from the same relative locations. This pro-
cedure assists in detecting interferences and other improper
conditions.
If careful thought is given to these component drawings,
much time and effort will be saved later in the shop. If they
are neglected, all the future work will suffer. A large percentage
of. the mistakes made in the manufacturing departments may
be traced back to improper component drawings.
Specifications for Interchangeable Manufacturing. The in-
formation that can be included on component drawings, except
in the case of a very simple or familiar mechanism, is seldom
sufficient in itself to enable the manufacturer to proceed in-
telligently with a new product. It is desirable that he know the
particular purpose for which the mechanism is to be made.
The better he is informed on this subject, the greater service
he can render in promoting its economical manufacture and
future development. Specifications are supposed to supplement
the drawings by giving all the needed additional information
which has no place on the drawings. I say "supposed" because
it is only in rare cases that the specifications commonly en-
countered give all the desirable information. They usually
deal with only the most exacting requirements and make no
mention of the others, thus establishing a severe precedent for
the solution of all questions in regard to the requirements,
important and unimportant. They seldom indicate the essential
object in view, namely, the economical production of mechanisms
which will function satisfactorily.
Specifications should state the end to be accomplished, and
should give all possible information to assist in the attaining of
that end. Any unusual conditions should be explained in detail.
All exacting requirements should be specified with the reasons
INTERCHANGEABLE MANUFACTURING II
for the same, including requirements of functioning and of
materials to be used. But they should not stop here. The less
exacting conditions should be noted also. If a certain material
is specified, and the chief consideration is economy, it should
be so stated, with the substitution allowed. The material that
might be most economical under one set of conditions might be
otherwise under different circumstances. Parts which are
detailed on the drawings but for which commercial articles can
be substituted should be so designated. The specifications
should list those parts which must be made interchangeable and
those which need not be. A description of the tests for materials,
for physical dimensions, and for functioning should be included.
In fact, any information that will assist in the manufacture of
the product should be given. Some of it will specify the results
to be obtained; more of it should be information to assist the
manufacturer, not hard and fast rules which he must follow
regardless of consequences.
Such specifications would undoubtedly be far from com-
plete at first. Provision should be made to keep them abreast
of the actual progress of the work. The shop should use them
as a place to record as much of the experience gained as possible.
If certain methods have been found unsatisfactory, here is an
ideal place to record the fact, and perhaps save a duplication
of the mistake in the future. If other methods have proved
satisfactory, they, too, should by all means be recorded. In
fact, specifications of this kind, although they would in time
become voluminous, would be a history of a mechanism and
furnish valuable data to assist in developing new mechanisms.
Written specifications are held in low esteem by the majority
of manufacturers. They do without written specifications for
their own products, and when obliged to meet them for contract
work, find them an additional annoyance instead of a help.
This is due, in large measure, to the fact that this subject, as
also the matter of tolerances, has been regarded as an end in
itself instead of as a means to a larger end. Manufacturers do
have specifications, although they are seldom called by that
name and are seldom written or grouped together for ready
12 INTERCHANGEABLE MANUFACTURING
reference. Some of them may be found in the cost and pro-
duction records, some in the shop correspondence, but most
of them are carried in the memories of the foremen and older
employes who maintain the traditions of the shop.
Gages for Checking Results. Thus far those elements which
form the groundwork for actual manufacturing operations
have been discussed. The manufacturing design has been
developed; it has been tested with the manufacturing model;
the first guess as to the proper manufacturing tolerances has
been made; all suitable and available information has been
recorded on the component drawings; and the specifications
to supplement these drawings have been partially developed
by recording there all further information available that will
assist in accomplishing the main purpose, namely, to produce
satisfactory mechanisms as economically as possible. The
means of carrying on the work of actual production and the
facilities that should be provided for checking the results, must
now be considered.
There are two important reasons for inspecting the product
during manufacture: First, spoiled parts must be eliminated
as soon as possible to save the expenditure of useless effort on
unserviceable pieces. Second, the completed components must
be checked before assembly to eliminate the unserviceable parts
and thus insure the proper functioning of the mechanism. For
these purposes, gages are extensively employed.
A gage should be provided whenever its use is more economical
than the use of standard measuring instruments. For example,
if the total production of a certain mechanism amounts to about
a dozen units, it is extravagance to provide special gages. On
the other hand, if this production amounts to several thousand
units, a complete set of gages is both desirable and necessary.
The extent to which gages are necessary, therefore, depends in
great measure upon the amount of the total production. Further-
more, gages should be provided to check only those conditions
which it is essential to maintain. The nature and extent of the
gages required depend upon the manufacturing conditions.
In many cases, a check on one or two points is sufficient to detect
INTERCHANGEABLE MANUFACTURING 13
any unsatisfactory results. Under varying manufacturing con-
ditions different faults must be guarded against. Gages are
a preventive and not a cure. The point to be emphasized is that
they should be provided whenever their addition will result in
the production of more or better components with a total ex-
penditure of the same or less effort.
Main Classes of Gages. There are two kinds of gages to con-
sider, which, for want of better terms, will be called limit gages
and functional gages. A limit gage is one that checks a specified
dimension to specified tolerances. A functional gage is one that
checks the relationship of several dimensions to insure the
proper functioning of the assembled mechanism. As with other
manufacturing equipment, the exact design of a gage is unim-
portant, if it fulfils its purpose simply and efficiently.
The degree of accuracy or precision required depends upon
the extent of the tolerances. In all cases, on limit gages, the
variation must be inside the established limits of the component.
The dimensions given on component drawings are limit gage
sizes. For example, the limits given for the diameter of a stud
should be interpreted to mean that such diameter must be made
to satisfy ring or snap gages of the sizes specified.
As yet master gages, or reference gages, as they are variously
called, have not been touched upon. A master is a physical
standard of size or form used for reference purposes. It is needed
only where the degree of precision required is so exacting that
the errors inherent in direct measurements with standard measur-
ing instruments will be great enough to prevent the proper
functioning of the product. If a manufacturing model is care-
fully developed, few, if any, masters will be required. For
simple dimensions of length, it is usually sufficient to establish
reference pieces of, say, tenth-inch units. For important
functional contours, masters are essential.
Test pieces for individual gages are necessary only when the
amount of gage checking is so great that too much time is con-
sumed by using standard measuring instruments, or when no
skilled labor is available for this checking. Test pieces are
therefore desirable for checking complicated profile and fixture
14 INTERCHANGEABLE MANUFACTURING
gages that receive hard usage, but they are seldom necessary
for plain plug, ring, and snap gages.
Manufacturing Equipment. Suitable tools and equipment with
which to manufacture a product must also be provided. The
first logical step to this end is to make operation lists, planning
in detail the successive operations, and specifying the type of
machine, fixture, tool, and gage required. These operation lists
are an integral part of the specifications, subject, of course, to
such modifications as are found necessary. Of the machines
themselves but little mention need be made at this time. Stand-
ard machine tools are now on the market for making almost
every variety of machining cut. Special machines are required
only for very unusual operations or for extremely large pro-
ductions where many automatic operations are performed.
The design of the fixture and the tool depends to a great
extent upon the design of the piece to be machined. Great
care should be taken to maintain the same locating or register-
ing points in the fixtures as are used for the gages. The ideal
condition is to have the registering points for both fixtures and
gages identical with the points on the component drawings
from which the surfaces in question are dimensioned. After
the equipment is complete, the component drawings should be
checked and revised T ^here necessary to obtain this result.
Another factor which must be considered in the design of the
equipment is the required rate of production. In the case of a
small output, the cost of the equipment amounts to a large
percentage of the total cost of production. As the output in-
creases, the proportionate cost of the equipment decreases,
thus making it desirable to refine this equipment, if by so doing
the production can be increased with the expenditure of less
productive effort. Here, as elsewhere, it is a question of balanc-
ing the cost of one item against that of another and of selecting
the most economical combination.
In most cases, except with some automatic machines or on
very large work, the operator spends more time in handling the
work than the machine takes to perform the machining operation.
Therefore, whenever the rate of production is high enough to make
INTERCHANGEABLE MANUFACTURING 15
it economical, the fixtures should be made for rapid operation,
even though this greatly increases the initial cost of equiqment.
Production Problems. The actual production consists of taking
the raw material and passing it through the equipment until it
emerges as a finished component. The production problems
are many and varied. Any part of the preceding work which
has been slighted or left undone must be completed here in
addition to the many tasks which are involved in the production
itself. The greatest problem involved in production is that
most uncertain factor human nature. The present tendency
is to provide equipment that can be operated by semi-skilled
labor. Equipment, however, cannot be made altogether fool-
proof. As noted before, the best that can be done is to arrange
matters so that little or no excuse remains for making mistakes.
People thoughtlessly speak of unskilled labor. The more this
problem is studied, the more it is realized that there is no place
in interchangeable manufacturing for such assistance. That
is, there is no task so elementary but that better and more
economical results can be obtained by a certain degree of train-
ing or skill in the operator. An attempt is made to subdivide
productive operations into the most elementary tasks so that
labor can be readily trained to perform them satisfactorily.
Each manufacturer is forced to train the majority of his own
operators. Naturally, then, the shorter the time required for
this training, the sooner the results will show in the production.
On the other hand, the less skill required of the operator, the
more elaborate and complete the equipment must be. The
amount of supervision required for both operators and equip-
ment is also greatly increased, in both quantity and quality.
In any case, the better the training that these operators
receive, the higher is the quality of the work produced. And
the matter of honest, serviceable quality as distinguished from
mere appearance is more appreciated than formerly. The
operator should be taught to maintain the established tolerances.
If the specified tolerances prove too severe in practice for eco-
nomical production, they should be corrected, provided the
functional requirements of the mechanism will permit. If they
1 6 INTERCHANGEABLE MANUFACTURING
are not too severe, there is no excuse for violating them. The
practice of adhering to the specified tolerances will do much to
promote a high quality of product.
Shop Inspection of Product. The inspection and acceptance
or rejection of the components falls logically into two divisions.
The first is the shop inspection which is made while the material
is in process of manufacture. The object is to cull out defective
work as soon as possible and also to detect any defects in the
equipment that would result in faulty work. If the percentage
of rejections is normal, it is evident that the requirements speci-
fied and the manufacturing facilities provided are satisfactdry.
If the percentage is high, it is evidence of improper conditions
somewhere which should be investigated, and the trouble should
be corrected at its source. Sometimes an error occurs, with the
result that the requirements are exceeded on a large number
of parts. Such matters should be investigated and settled
according to their merits. If the pieces will be serviceable and
can be completed without undue cost, the factor of economy
will play a large part in the decision. In such cases, the require-
ments specified should not be changed unless it is evident that
such a change will result in an economic benefit in the future.
As in all other cases, ultimate economy is the goal.
Final Inspection. The second division of the inspection is the
final examination of the completed parts. The object of this
inspection is to see that all components which will function
properly are accepted and that all unserviceable parts are re-
jected. This inspection is largely governed by the requirements
of the component drawings often represented by gages and
by the specifications. It is therefore most important that the
drawings and specifications give as nearly as possible the limits
of parts which will function properly. Yet, as has been already
noted, these drawings and specifications are incomplete at the
beginning, and probably will always be so, to a certain extent.
Therefore, a rigid adherence to the letter but not to the spirit
of the drawings and specifications is unwise, as it will not aid in
the acceptance of all serviceable material, nor in the ultimate
economy of manufacture. In addition to the written require-
INTERCHANGEABLE MANUFACTURING iy
ments, inspectors must have a certain amount of education and
experience with the mechanisms involved, or with similar
mechanisms; otherwise the inspection will always prove a
hindrance to the main purpose.
The characteristic needed for a successful inspector is a
judicial mind. Since the requirements are laws, the inspection
should equitably enforce them. The spirit of the requirements
should be enforced in those cases where their exact expression
is incomplete. If the essentials are always specified definitely
and completely, it will be a fair assumption that incompletely
specified conditions are relatively unimportant. Wherever
possible, the requirements should be revised to make the letter
and the spirit agree, but the attempt to cover every minute and
unimportant detail will prove impossible in practice.
The functional requirements should be maintained in the
final inspection strictly according to the specified conditions.
The non-functional requirements should be handled in a more
judicial manner, each case being decided on its merits. As a
matter of fact, this final inspection should be in the nature of a
functional inspection only. Little attention should be given
here to the non-essentials other than, perhaps, a visual inspection
for general quality, and some supervision of the shop inspection
to see that proper precautions are taken during production to
insure a good product. In all cases, the main effort throughout
the work should be to establish, define, and maintain the essential
conditions first, letting the non-essentials develop in practice.
No secret, however, should be made of the fact that these non-
essentials are left to work out their own salvation.
Test of Success. The final and complete evidence as to whether
the aim has been accomplished is furnished after the mechanisms
have been assembled and tested. If the total costs have been
reasonable and the completed mechanisms assemble properly
and perform satisfactorily all the required functions, it is con-
clusive evidence that all essentials have been mastered. On
the other hand, if the costs are excessive or if the mechanism
fails to assemble or to operate properly after being assembled,
it is equally conclusive evidence of failure.
CHAPTER II
TERMS USED IN INTERCHANGEABLE MANUFACTURING
IN order to describe concisely characteristics peculiar to
interchangeable manufacturing, it is necessary to use many
words and phrases in an arbitrary sense. Therefore, to avoid
misunderstanding, space is taken here to define several of the
important terms. The interpretation of these terms is limited to
the ideas they express in this treatise.
Interchangeability. The term interchangeability, as used
here, refers to absolute interchangeability. In this sense, inter-
changeable parts are parts that are so made that they can be
assembled or interchanged after final inspection without machin-
ing or fitting, and any possible combination of these parts will
assemble, interchange, and function properly. To insure this
end, the most extreme limits permitted must be constantly
checked against each other.
Selective Assembly. Selective assembly refers to a method
of manufacturing similar in many of its details to interchange-
able manufacturing, in which component parts are sorted and
mated according to size and assembled or interchanged with
little or no machining. Companion parts made to the extreme
limits are not supposed to interchange. For instance, a maximum
male component will not assemble with a minimum female
part. However, the maximum male and female, or the minimum
male and female must interchange. A good example of this
method of assembling is found in the production of ball bearings.
The balls are sorted into groups, according to their size, to facili-
tate the assembly of any bearing with balls of uniform size.
As a matter of fact, nearly every so-called interchangeable
article represents a combination of the two methods of quantity
production interchangeable and selective.
18
DEFINITIONS OF TERMS 1 9
Function. The term function is used extensively and with
various shades of meaning. The word itself has many meanings.
The dictionary gives one as "fulfillment or discharge of a set
duty or requirement"; and another as "that mode of action
or operation which is proper to any structure," etc. As applied
to component parts, the word has been used to express both
these meanings. This includes all requirements of interchange-
ability and service which the part must render throughout the
normal life of the mechanism of which it forms a part. The
same meaning is intended when it is applied to the assembled
mechanism. The functional design refers specifically to the
combination of mechanical movements required to make the
completed mechanism perform its specified duties. Functional
gages are those which test the functional operation of components
without strict adherence to their exact physical dimensions.
Limit. In every interchangeable mechanism there are certain
maximum and minimum sizes for each part, between which the
parts will function properly in conjunction with each other
and outside of which they will not. These sizes are the absolute
limits of the parts. The established limits are the maximum
and minimum dimensions specified on the component drawings.
The established limits should approach as closely to the absolute
limits as normal manufacturing conditions require. Limits
established without regard to the absolute limits result either
in excessive cost of manufacture or faulty mechanisms or both.
If the established limits are much more severe than the absolute
limits, needless expense is incurred in manufacturing. On the
other hand, if the established limits are more liberal than the
absolute limits, unsatisfactory mechanisms will be produced.
Tolerance. Tolerance is the amount of variation permitted
on dimensions or surfaces. The tolerance is equal to the differ-
ence between the maximum and minimum limits of any specified
dimension. For example, if the maximum limit for the diameter
of a shaft was 2.000 inches and its minimum limit was 1.990
inches, the tolerance for this diameter would be o.oio inch.
By determining the maximum and minimum clearances required
on operating surfaces, the extent of these tolerances is established.
2O
INTERCHANGEABLE MANUFACTURING
The application of the tolerances to the basic dimensions fixes
the limits.
Basic and Model Size. Obviously, the absolute limits of the
various dimensions and surfaces indicate danger points, inas-
much as parts made beyond these limits are unserviceable. A
careful analysis of a mechanism shows that one of these danger
points is more sharply defined than the other. For example, a
certain stud must always assemble into a certain hole. If the
stud is made beyond its maximum limit, it will soon be tqo large
to assemble. If it is made beyond its minimum limit, it will be
too loose or too weak to function. The absolute maximum
HOLE 1.250 +. DIA.
STUD 1.248 Zows" DIA -
\
T
rm
i i! i II
VT'I'*
it 1
Machinery
Fig. 1. Graphic Illustration of the Meaning of the Terms Limit
and Tolerance
limit in this case can be defined within a range of o.ooi inch,
whereas the absolute minimum limit cannot be defined within
a range of at least 0.004 mcn - I n this case the maximum limit
is the more sharply defined.
The basic size expressed on the component drawing is that
limit which defines the more vital of the two danger points,
while the tolerance defines the other. In general, the basic
dimension of a male surface is the maximum limit which re-
quires a minus tolerance. Similarly, the basic dimension of a
female surface is the minimum limit requiring a plus tolerance,
DEFINITIONS OF TERMS 21
as shown in Fig. i. There are, however, dimensions which define
neither a male nor a female surface. Such are dimensions for
the location of holes. In a few cases of this kind, a variation in
one direction is less dangerous than a variation in the other.
Under these conditions, the basic dimension represents the dan-
ger point, and the tolerance permits a variation only in the less
dangerous direction. At other times, the conditions are such
that any variation from a fixed point in either direction is equally
dangerous. In such a case, the basic size represents this fixed
point. Tolerances, when given on the component drawing,
extend equally in both directions.
If a model is developed as a standard of precision, the model
parts become the physical representations of the basic sizes.
In other words, for all practical purposes, the model size and the
basic size are identical.
Maximum Metal Size. Maximum metal size is that limit at
which the part contains the maximum amount of metal. This
would be the maximum male limit and the minimum female
limit. In many cases, a careful analysis is necessary to determine
which limit represents the maximum metal conditions, as many
dimensions are neither male nor female. In other cases, such
as locations of holes, there are neither maximum nor minimum
metal conditions. With few exceptions, however, the maximum
metal sizes are also the basic sizes.
Minimum Metal Size. Similarly, the minimum metal size
is that limit at which the part contains the minimum amount
of metal. This is the minimum male limit and the maximum
female limit, when the dimensions can be so classified.
Minimum Clearance. It is evident that there must be a definite
amount of clearance between male and female components
which operate together. The minimum clearance should be as
small as will permit the ready assembly and operation of the
parts, while the maximum clearance should be as great as the
functioning of the mechanism will allow. The difference between
the maximum and minimum clearances defines the extent of the
tolerances. On companion elementary surfaces, the difference
between the maximum male limit and the minimum female
22
INTERCHANGEABLE MANUFACTURING
limit determines the minimum clearance, as shown in Fig. 2.
On composite surfaces, careful study is required to determine
which limit should be used. In fact, it is impossible in certain
cases to have the minimum clearance conditions at all points at
the same time. In general, however, the comparison of the
basic sizes of companion parts gives the minimum clearance
conditions. The minimum clearance is quite commonly known
as the " allowance."
Maximum Clearance. On elementary surfaces, the difference
between the minimum male limits and the maximum female
limits establishes the maximum clearances. In general, the
HOLE 1.250-^'' D IA,
6TUD 1.2483
T
5<
see
^2
1
-1
1
Machinery
Fig. 2.
Graphic Illustration of the Meaning of the Terms Maximum
and Minimum Clearance
terms maximum or minimum clearance refer only to the clear-
ance between surfaces which operate together or within close
proximity to each other. When surfaces stand well clear of each
other, and there is little or no danger of interference, as between
unfinished forged or cast surfaces, the matter of maximum and
minimum clearance plays little part in determining the toler-
ances.
Interference. If a male member is larger than a female mem-
ber, it is obvious that there will be interference when these parts
are assembled together. Such interference is required where
DEFINITIONS OF TERMS
force fits are specified. If interchangeable parts are to be forced
together, this interference performs a similar function to that
of clearance on operating surfaces. In this case, the minimum
interference establishes the danger point. This means that
for force fits the basic male dimension is the minimum limit
requiring a plus tolerance, while the basic female dimension is
the maximum limit requiring a minus tolerance. (See Fig. 3.)
When the component drawings permit an interference where
STUD 1.252 n'onn''DlA.
HOLE 1.250 +'DIA.
r
'L"TT *-
t KJ
fl
I m
Machinery
Fig. 3. Graphic Illustration of the Meaning of the Terms Maximum
and Minimum Interference
a clearance is required, they are wrong. The term interference
is often used to express such conditions of error.
Operating Surfaces. The term operating surface is used to
distinguish the working surfaces of the mechanism from the
others. It is clear that the working surfaces are the essential
ones; all others are present only because of the necessity of
holding the mechanism together. Generally speaking, the oper-
ating surfaces are the machined surfaces, while the others often
retain their original forged or cast finish. The operating surfaces
are divided into two classes, which are designated functional and
non-functional, or clearance, surfaces.
Functional Surfaces. The functional surfaces are those oper-
ating surfaces which control the functioning of the mechanism,
INTERCHANGEABLE MANUFACTURING
as shown in Fig. 4. These must naturally be held to the closest
limits. Every operating part of a mechanism must be controlled
in operation within reasonably close limits in each plane. After
these functional requirements of location are met, all other
surfaces should have as large clearances as possible, unless the
factor of strength is the controlling one. Those surfaces that
affect the relative location of the operating parts in operation
are the functional surfaces. For example, the surface of a pad
on which a bracket that carries operating parts is fastened is a
BREECH-RING
CLEARANCE SURFACES
CLEARANCE SURFACES
BREECH-BLOCK
DIRECTION OF
PRESSURE ON
BREECH-BLOCK
WHEN GUN
IS FIRED
Machinery
Fig. 4. Illustration showing the Meaning of the Terms Functional
and Clearance Surfaces
functional surface; whereas, the surface of a pad that supports
a bracket for holding wrenches or oil-cans is not.
Clearance Surfaces. Clearance surfaces are those operating
surfaces which are not functional surfaces. In this class are
surfaces which do not control the location of operating members
while functioning, but which either prevent them from being
disassembled or locate them approximately in their inactive
position, or both.
Atmospheric Fits. Atmospheric fits, as the name implies,
refers to those surfaces which, under all conditions, stand entirely
clear of any other operating or functional members of the mech-
anism. Such is the outside of a machine frame. Many surfaces
DEFINITIONS OF TERMS
on operating parts are themselves also atmospheric fits. With
few exceptions, the majority of the surfaces of all mechanisms
are atmospheric fits.
Elementary Surfaces. An elementary surface is one which is
defined with a single dimension, such as a cylinder, a plane, or a
sphere. For example, a reamed hole of a speicfied depth rep-
resents two elementary surfaces. The diameter defines one
and the depth the other. Obviously, most surfaces which are
not elementary in themselves are a combination of elementary
surfaces. In so far as such surfaces are machined and measured
according to their elements, they are considered elementary
surfaces. When the combination as a whole is measured or
y
Y
Machinery
Fig. 5.
Illustration showing the Meaning of the Terms Elementary
and Composite Surfaces
machined, and a variation on one surface affects the dimension
of another, they are not elementary but composite surfaces.
Composite Surfaces. Composite surfaces are those surfaces
which are required to maintain a co-relation which cannot be
expressed by a single dimension. For example, Fig. 5 shows a
yoke end. The over-all dimension (2.500 inches) controls
elementary surfaces. The dimension of the slot (0.750 inch) and
that of its location (0.875 inch) also define elementary surfaces
when used independently. If, however, surfaces marked A, B,
and C are required to be checked concurrently, these elementary
26 INTERCHANGEABLE MANUFACTURING
surfaces become composite. Irregular profiles, the co-relation
of several holes to each other, tapered surfaces, thread sizes
and forms, the contour and location of gear teeth, etc., are
examples of more complicated composite surfaces than the
example shown in Fig. 5.
Compound Tolerances. A compound tolerance refers to those
conditions where the established tolerances on more than one
dimension determine the required limits. These exist in con-
junction with the dimensioning of composite surfaces. For
example, a compound tolerance exists in establishing the location
of surface C in Fig. 5 from surface A. This condition of com-
pound tolerances will be covered in greater detail in a subsequent
chapter.
Working or Register Points. The working or register points
are those surfaces that are employed for locating the parts in the
jigs and fixtures during the process of manufacture. Sometimes
important functional surfaces are used for this purpose. In
other cases, for parts of irregular form, special lugs are provided
to serve this end. These are removed after the machining
operations are complete. Register points become functional
surfaces when they are employed to machine other functional
surfaces. As few locating points as possible should be estab-
lished; this practice simplifies the design of the gages and other
equipment.
Unit Assembly. Many mechanisms are a combination of
several semi-independent mechanisms which may be assembled
and tested individually before they are assembled together.
This is known as unit assembly; it is of particular value for a
plant which manufactures a varied product where such unit
assemblies are interchangeable. An example would be a feed-
box that could be used on several types of machines. This
practice enables a plant to obtain the benefits of quantity pro-
duction on these unit assemblies although the quantity of pro-
duction on any one type of machine is small.
Precision. There are two characteristics pertaining to the
physical dimensions of the parts manufactured. For purposes
of discussion, they will be called precision and accuracy. The
DEFINITIONS OF TERMS 27
two are often considered identical, and if ideal conditions could
be maintained, they would be identical. In ordinary manu-
facturing practice, however, precision alone is usually obtained,
and precision is all that is necessary in most cases. It is when
several factories attempt independently to produce a common
interchangeable product that accuracy is required. The degree
of precision is measured by the amount of variation that exists
between duplicate parts. For example, a reamed hole is dimen-
sioned as 0.500 inch in diameter. In manufacture, a product is
obtained in which the difference between the largest and smallest
holes produced does not exceed 0.005 inch. The degree of pre-
cision in this case would be 0.005 mcn > even though the absolute
size of the largest hole was 0.508 inch.
Accuracy. The accuracy of any determination is measured
by its limits of error from a fixed standard. For example, a
length of one inch is to be measured. We will assume, for the
sake of argument, that the measuring instruments are absolute.
This length measured with an ordinary steel scale gives a result
correct within a limit of error of about o.oi inch. If measured
with a micrometer, the result is correct within a limit of error
of about 0.005 i ncn - If measured on a sensitive measuring
machine, the result is correct within a limit of error of about
o.ooooi inch; while if measured by optical methods, advantage
being taken of the principle of interference of light waves, the
result is correct within a limit of error of o.oooooi inch or less.
It may be of interest to note here that standards of length have
been defined by the Bureau of Standards in terms of light waves.
By this means, an absolute standard is established, since the
lengths of light waves are absolute. The term accuracy implies
a comparison with a fixed standard. In the example given to
illustrate precision, the limit of precision is 0.005 inch, while
the limit of accuracy is 0.008 inch. It is obvious from this that
it is more difficult to maintain a limit of accuracy of o.ooi inch
than it is to maintain a limit of precision of the same amount.
Component Drawings. Component drawings are detailed
drawings of the component parts of a mechanism. For inter-
changeable manufacturing, these drawings show the completed
28 INTERCHANGEABLE MANUFACTURING
dimensions required (or inspection gage requirements) of the
parts. These include all tolerances and any sub-assemblies
that may be necessary to assist in their proper interpretation.
Operation Drawings. An operation drawing is a detailed
drawing or sketch which gives the dimensions required on an
individual machining operation. These drawings are used to
record much supplementary information that might be confusing
or misleading on the component drawings, such as allowances
for finishing cuts and grinding operations, etc.
CHAPTER III
MACHINE DESIGN IN INTERCHANGEABLE
MANUFACTURING
THE improvement in manufacturing methods and facilities
during the past forty or fifty years has been very rapid. Quantity
production is now the order of the day. New problems have
arisen and old ones must be constantly re-solved to meet the
situations thus created. Manufacturing on an interchangeable
basis has been a direct development of this advance. The pur-
pose of the present chapter is to discuss the effects of this develop-
ment on the design of a machine or device, and to emphasize
those practices which will promote economical manufacture on
an interchangeable basis.
In early days, the design of a new mechanism existed only in
the mind of the mechanic engaged in its construction. It was
made piece by piece, each detail taking definite shape as it was
constructed. The original mechanism was completed, tested,
and corrected or rebuilt before the design was finished. Dupli-
cate mechanisms that might be constructed were patterned
after the original, and modified or improved as suited the ideas
of the mechanics who performed the actual work. Needless
to say, interchangeability and quantity production were non-
existent factors.
Sketches and drawings were next employed to express the
ideas of the inventor, but little attempt was made to indicate
more than the general idea and construction. The details of
the design and the dimensions of the individual parts were
matters for the workmen to decide. A competent mechanic
was required to determine these for himself. It was part of his
training. Details and dimensions, more or less complete and
consistent, made their appearance on the drawings later. Errors
and omissions were of little moment, as the mechanic who worked
to the drawings expected to select and use the proper infor-
29
30 INTERCHANGEABLE MANUFACTURING
mation given and to ignore the incorrect, supplying all omissions
from his own store of mechanical knowledge and experience.
The quantity of production was small. Little or no importance
was placed on interchangeability. A few workmen thoroughly
acquainted with the requirements of the mechanism or with the
intentions of the designer performed all the actual work of con-
struction. Under these conditions, functional drawings, which
make no pretense of giving more than the general construction
or combination of mechanical movements and- the general out-
line of the detailed parts, are sufficient.
Function of Design. Under present manufacturing conditions,
with productive operations subdivided into elementary tasks,
with productive labor trained along specialized lines, with pro-
ductive equipment specialized and more nearly complete, with
the rate of production greatly increased, with larger organiza-
tions in which but few individuals are thoroughly conversant
with all the detailed requirements of the mechanism, the design
must cover a wider field and be much more comprehensive and
accurate. In addition to expressing the ideas of the inventor,
it must supply most of the knowledge and experience formerly
brought to this work by the mechanic.
We now have, therefore, two types of designing to consider,
which we will call the functional design and the manufacturing
design. The manufacturing design is a detailed development
of the functional design. It corrects and modifies the functional
design where necessary, to facilitate the economical production
of the mechanism, giving as much as possible of the information
previously supplied by the workman. It is evident that the
manufacturing design will always be incomplete to a certain
extent. Suitable provision for its modification must be made to
obtain the advantage of the new and improved methods of manu-
facture which are constantly developed. Changes, however,
in proved manufacturing designs should be avoided when pos-
sible. As much or greater care should be taken in adopting
changes as is exercised in establishing the original manufacturing
design. After equipment has been completed, changes are very
costly. A change which might be justified in the early stages of
MACHINE DESIGN 31
work often costs more than it is worth in the later stages. This
makes it of the utmost importance that great care be exercised
in the development of the original manufacturing design of a new
commodity which is to be manufactured in large quantities on
an interchangeable basis.
Classes of Design. For the construction of a small number of
special machines, or tools and fixtures, which are built in a
general machine shop or tool-room, the functional design is
all that is required. The number of men engaged in the produc-
tion is small, their training is general, and the requirements of
the mechanisms can be explained to them personally by the
designer as questions arise; therefore, the additional expense
of a manufacturing design is not justified. However, in the
manufacture of a large quantity of any article, particularly
if interchangeability is sought, a complete manufacturing design
is necessary. True, this design will work itself out in practice
in the course of time, but this is a very slow and expensive
method. It means that experimental work on a large scale is
carried on, whereas it can be done on a smaller scale with better
and speedier results. Furthermore, this method results in con-
tinual alterations in the equipment and a loss of interchange-
ability. However, this chapter is not concerned with designs of
special mechanisms, tools, fixtures, etc.; attention will here
be given to the requirements of designing as applied to the manu-
facture of a product in large quantities.
In both types of designing, the end in view is the same as far
as the functioning of the mechanism is concerned. This is to
develop a product capable of performing certain results which
will fill or create a public demand for itself. The means of at-
taining this are governed by various considerations. For the
functional design, any solution is satisfactory. As regards the
manufacturing design, the methods adopted must result in
ultimate economy. Also, the manufacturing design must re-
semble the functional to such an extent that all patents will be
retained, while those of competitors must not be infringed.
This is one of the commercial difficulties that, at times, prevents
the true economic development of a commodity.
32 INTERCHANGEABLE MANUFACTURING
It is plain that the manufacturing designer must take into
consideration every circumstance involved in the production
of the commodity. To be successful, he must work in close
cooperation with all who will be engaged in the development and
operation of the manufacturing equipment. This will include
the tool designers, and the superintendents and foremen of the
various manufacturing and assembling departments. In general,
there is too much detail involved for any one person to carry
it alone to a successful completion.
Simplifying Design. When considering the manufacture of a
new product, one of two conditions usually obtains. Either
it is to be produced in an established plant with an existing
variety of manufacturing equipment, or a new plant must be
created. In the first case, the designer must be familiar with the
available equipment and must modify the functional design
so as to utilize these facilities to the best advantage. In the
second case, he is not restricted to the use of any specified equip-
ment. In either case, unless the volume of production is to be
extremely large with many automatic operations, every effort
must be made to reduce the machined surfaces of the various com-
ponents to simple, elementary surfaces which can be readily
machined on standard machine tools with simple, rugged, and
inexpensive tools, jigs, and fixtures. If, in the manufacturing
design, the component parts are thus simplified, a further
advantage is gained. The productive operations on these parts
are resolved into simple, elementary tasks, and this simplifies
the problem of securing and training the necessary productive
labor. Simplicity is a primary source of economy. The number
of machining operations is reduced and the direct labor cost
thereby lowered. The amount of time that raw material is
tied up in process of manufacture is reduced and quicker re-
turns are secured on the money invested in direct labor and
materials. The many other economies resulting from simplicity
in design, such as lower equipment and maintenance costs are
obvious.
Factors Governing Choice of Materials. Those responsible
for the manufacturing design must pay close attention to the
MACHINE DESIGN 33
character of the materials they specify for the individual com-
ponents. Ultimate economy is the desired end. This is affected
by many different and sometimes opposing factors.
Cost. The first cost of the material is one of these. When
several thousand duplicate mechanisms are manufactured, the
slightest saving in the cost of direct materials is multiplied over
and over again in the course of time. As many parts as possible
should be made of the same size and kind of material. This
permits purchasing in larger quantities and reduces the gross
amount of raw material carried in the store-room. As far as
possible, this material should be of standard sizes and forms that
can be purchased in the open market at the lowest prices.
Source of Supply. Due consideration must be given to the
possible sources of supply for the materials specified. It is a
serious matter when production is held up because of lack of
material which has a limited or uncertain source of supply.
Every effort must be put forth, in making the manufacturing
design, to specify materials which are readily secured.
Machining Qualities. The actual economy of low-priced mate-
rial is governed by the ease with which it can be machined. If
a part requires many machining operations, a low initial cost
for material is often overbalanced by the greater cost of manu-
facture. Therefore, if a more expensive material can be machined
at a lower cost, ultimate economy dictates its purchase. For
this reason, the use of extruded or rolled bars of special form is
often adopted in the manufacture of small parts for adding
machines, typewriters, counters, and other similar mechanisms.
An illustration of this point occurred in a large plant which
makes small duplicate parts. Several of these parts were made
of brass castings because of the lower cost of machining, but the
price of copper began to rise and was soon about double its
normal price. It was decided to substitute cast iron for brass
because the difference in the cost of machining was less than the
difference in the market price of the materials. Luckily, an
investigation was made before the change went into effect.
This plant had its own brass foundry but no iron foundry.
It was discovered that the foundry had purchased no copper
34 INTERCHANGEABLE MANUFACTURING
for several years. In fact, a large stock of pig copper had been
stored in a shed and was never touched. Another department
of this plant was engaged in making copper matrices by a plat-
ing process, and the trimmings from these supplied all the pure
copper which the foundry required. This, with scrap brass
stock from other departments, made it unnecessary to purchase
any metal for the brass foundry in the open market. Needless
to say, no change was made in the material of the castings.
This incident indicates in some degree the many factors that
must be considered to secure genuine economy. It is not a
matter of mere addition and subtraction; every existing con-
dition must be taken into account.
Weight of Finished Product. Whenever the weight of the
finished product is an important consideration, as with auto-
mobiles, etc., the materials used in making it must be of a better
grade than when the weight is less important. In every case,
the materials specified must be sufficiently strong and rigid
to hold their form throughout the normal life of the mechanism.
Thus, the detailed design of the various components is governed
to a great extent by the nature of the materials which are used
in their manufacture. For example, if forged steel is substituted
for cast iron, the component will be of much lighter design.
Service Required. The composition of the materials used is
governed by the nature of the service which the part must render.
One that is subjected to excessive wear must be made of material
hard or tough enough to withstand it. Material for parts liable
to corrosion or other chemical action must be of the proper
composition to counteract it. Material for parts under constant
vibration must not crystallize readily. In every event, the
materials must be selected to withstand both the use and abuse
which they will eventually meet.
It is of interest to note, as an indication of the importance
of materials in relation to the total cost of production, that
census statistics show that the cost of materials direct and
indirect is from 30 to 60 per cent of the selling price of the
majority of mechanical products which are manufactured in
this country.
MACHINE DESIGN 35
Clearances and Tolerances. The establishment of suitable
clearances and tolerances is a vital, if not the most vital, factor
in the manufacturing design. Tolerances are, in many respects,
like laws. There are two classes of laws. One is so severe and
exacting in its nature that it cannot be enforced, and soon falls
into disrepute and is disregarded, even though it remains on the
statute books. The other is drawn up with a full understanding
of existing conditions, and its justice to all concerned is so
evident that it is readily and consistently enforced.
Similarly, tolerances fall into two classes. Those which
represent the extreme conditions of accuracy obtainable from
the equipment under ideal conditions can be specified without
regard to the functional requirements of the product. In such
cases they, too, soon fall into disrepute and are disregarded,
even though they still remain on the drawings. On the other
hand, tolerances are readily and consistently maintained when
they represent the widest variations that the functioning of the
mechanism will safely permit.
Liberal tolerances and clearances result in easier manu-
facturing conditions of every sort and thus promote economy;
they make quantity production possible. The serviceable
life of tools depends directly on the extent of the tolerances.
Every exacting tolerance is a direct check on the economical
and rapid production of the mechanism. On the other hand,
if the functional requirements do not permit wide tolerances,
the functional requirements must prevail.
It is evident, then, that the construction must be carefully
studied so that the manufacturing design will permit the widest
possible tolerances. It is only in exceptional cases that a mechan-
ism cannot be modified so as to retain all functional advantages
and yet allow liberal tolerances on the majority of its dimen-
sions. Very often, when there is a severe functional requirement
to maintain, the introduction of simple means of adjustment
promotes easier manufacturing conditions. In other cases,
a system of selective assembly is more desirable.
Applying Interchangeable Principle. The designer must de-
termine which parts will be interchangeable. Interchangeability
36 INTERCHANGEABLE MANUFACTURING
can be carried too far and thus allowed to defeat its own purpose
as noted in a previous chapter. Interchangeability and liberal
maximum clearances are closely connected. Whenever reason-
able clearances are out of the question on certain components,
these parts are not suitable ones to be manufactured on an inter-
changeable basis. In this matter, the relative accuracy of the
available equipment plays a large part. For example, if the
surfaces are elementary and can be finished by a simple grinding
operation, much closer tolerances can be economically main-
tained than if they are composite and require milling or turning
operations. The variations on work finished by grinding are
about one-third those resulting from milling and one-half those
from turning; and the effort expended is no greater. On the
other hand, grinding is not always suitable nor possible. There-
fore, in determining whether or not certain required conditions
permit reasonable tolerances, the designer must consider pos-
sible methods of manufacture and must be well informed
regarding the normal variations which result from them in
actual practice.
This knowledge is the outcome of experience in checking and
analyzing results previously secured. This is a matter to which
little attention has been paid in the past. For example, in a
large and long-established plant, where many milling operations
are performed, it had been assumed that these operations were
maintained within a tolerance of o.ooi inch. Actual measure-
ments brought out the fact that the normal variation was over
three times as great as that, and always had been. A similar
misconception of actual conditions was apparent in the majority
of shops engaged in government work during the recent war.
When their product was actually checked by limit gages and
held to the specified tolerances, a variation of 0.002 or 0.003 i ncQ
was found to be an extremely small manufacturing tolerance.
It is, therefore, one of the duties of the maker of the manu-
facturing design to specify the parts which are to be made inter-
changeable, those to be selectively assembled, and those to be
fitted to each other. Careful attention to this detail saves much
wasted effort in the shops subsequently.
MACHINE DESIGN 37
Advantages of Unit Assembly Construction. Almost every
mechanism can be subdivided into smaller units which are dis-
tinct in their purpose. For example, an automobile contains
an engine, transmission, axle drive, carburetor, magneto, etc.,
which are assembled and tested as units and later assembled
into the completed car. In like manner a typewriter is sub-
divided into the carriage, the escapement, the type-bar and the
segment assembly, etc. The assembly is greatly facilitated if
the design of the mechanism permits such unit assembly con-
struction; and efforts should be made to obtain this result
whenever practicable.
There are many other advantages of this unit assembly con-
struction. Not only the various manufacturing departments
of one factory but also entire plants are specializing more and
more. The automobile has hastened this trend more than any
other one thing. Where such unit assemblies are of equal value
on several articles, separate plants spring up to produce them as
a specialty. This gives the benefits of quantity production
where otherwise they would not exist. Therefore, as a direct
result of unit assembly construction, there are separate plants
specializing in engines, rear axles, carburetors, magnetos, etc.,
for automobiles; ball and roller bearings for all types of machin-
ery; and many other similar specialized products.
Standardization of Parts. Another practice which allows the
benefits of quantity production to be obtained in the production
of smaller numbers of complete mechanisms is the standard-
ization of many of the individual components. For example,
most manufacturing concerns have standardized their screws,
nuts, studs, rivets, and others mall parts. The majority of
machine tool builders also standardize their handwheels, mi-
crometer thimbles, gears, tool-holders, work-arbors, etc. A good
illustration of the economy of this practice is found in the ex-
perience of one plant which originally manufactured over one
hundred and fifty special screws and studs for its particular
product. Little effort was required to reduce this number to
less than half, thus increasing the rate of production of these
parts and also reducing the stock of spare parts. This practice
38 INTERCHANGEABLE MANUFACTURING
is extending to larger and more important components. Not
only are similar parts produced by individual plants being
standardized, but parts used in common by several manufactur-
ers are also standardized and often manufactured as specialties
by other concerns.
Designing for Assembling and Service. The design must per-
mit the ready assembly of the product. Parts which require
attention in service must be accessible. .Attention to these
details reduces assembling and service costs, and these must be
considered to insure ultimate economy.
The service requirements are the most difficult to determine.
Time alone brings the desired information. Experiments and
endurance tests in the factory are insufficient to give it. After a
mechanism is on the market, it receives use and abuse that the
makers never dreamed of. Yet if the product fails under these
unforeseen conditions, the manufacturing plant is blamed.
Naturally the nature of the commodity determines what sort
of service it must render. The service requirements of an
automobile are distinct from those of a typewriter; those of a
precision machine tool which is supposedly used by skilled
mechanics only differ from those of a lawn-mower; etc.
The service requirements include the protection of the working
parts from dirt and other foreign matter, the provision of proper
lubricating facilities, and the protection of the operator from
moving parts. The question of the best preservative finishes,
such as japanning, plating, painting, etc., must also be answered
to meet the service requirements, both of use and appearance.
For these and many other similar problems a solution is sought
that will result in the maximum amount of service at a minimum
expense.
It should be clearly understood that the manufacturing design
is not undertaken with the idea of wilfully altering the functional
design, but is*made to facilitate manufacture and to furnish as
much as possible of that vast amount of detailed information
previously brought to the productive work by the mechanic
who carried out the inventor's ideas. The alterations made in
the functional design by the manufacturing design should not
MACHINE DESIGN 39
be looked on as any criticism of the original lay-out. Each has
its distinct purpose to perform. Many large plants recognize
clearly the difference between the two types of designing and
maintain separate departments for each. The original research
and inventive work is carried on independently of the factory
operations. New or improved designs are turned over to the
factory organization where they are redesigned to meet the
manufacturing and service needs.
CHAPTER IV
PURPOSE OF MODELS
A MODEL mechanism, constructed personally by the inventor,
or by the workmen under his immediate direction, was the original
form of making and recording a new design. The introduction
and development of mechanical drawings superseded many of
the functions previously performed by the model. At the
present time, therefore, the practice of developing models has
been relegated to a comparatively insignificant place in most
lines of manufacturing. They are still employed to a limited
degree, however, by several manufacturers for a variety of
purposes.
The primary purpose of any model at the present time is
to prove not to originate a new or improved design that
has been developed only on paper. It may be either to prove
the possibility of the functional design or to check the manu-
facturing design. This may be done by a single mechanism
in some cases, or several duplicate mechanisms may be required
to prove its operation under various service conditions.
Manufacturing models may be used for one or more of the
following purposes: First, to check the operation of the manu-
facturing design against the experimental model; second, to
prove the manufacturing design in regard to the service require-
ments; third, to test the* manufacturing tolerances which may
be contemplated; and fourth, to create a physical standard of
precision for future manufacturing.
Manufacturing Model to Test Functioning. A manufacturing
model used to test the functioning of the manufacturing design
is merely a sample mechanism constructed in the tool-room or
machine shop to detect as many faults as possible in the design
or to discover possible errors in the component drawings. It is
essentially a precautionary measure. It is more economical to
40
PURPOSE OF MODELS 41
detect and correct a fault on one sample than it is to salvage a
large number of parts after production is started, with the addi-
tional expense of correcting the manufacturing equipment.
After such a model has demonstrated the success of the manu-
facturing design and the correctness of the component drawings,
its purpose has been achieved. Its future disposition is a matter
of little moment.
Such a model is seldom necessary on simple mechanisms that
are merely new combinations of old and proved mechanical
movements, or on minor variations of proved designs, such as
standard motors, dynamos, various types of engines, and many
machine tools. The actions of such mechanisms under many
conditions have been so well established that practically all of
the necessary experimental work can be accomplished on paper.
On many other mechanisms, however, such as typewriters, add-
ing machines, small arms, watches, etc., the mechanical move-
ments of which are delicate and intricate and not so positive,
manufacturing models are a vital necessity. In general, such a
model will be constructed when the insurance against the possi-
ble errors in the design is worth the expense entailed. For this
reason it is often customary to build a pilot machine before
putting through a lot of new or special machines.
If a new commodity is designed, particularly if it is to fill a
new demand, it is advisable to determine its action in actual
service before extensive productive operations are far advanced.
The only sure method of obtaining this information is to have
one or more mechanisms built and operated under the condi-
tions with which they are expected to contend. The manu-
facturing model, which is built to test the manufacturing design,
is often used for this purpose. As a matter of fact, the test for
functioning should also include the tests for service require-
ments, inasmuch as this factor of functioning should include the
measure of service which the mechanism must render throughout
its normal life.
For example, a large plant in the Middle West goes thor-
oughly into this preliminary work on all new models. Three
successive designs are developed and tested. First, the func-
42 INTERCHANGEABLE MANUFACTURING
tional or inventive design is made and tested. Second, the
manufacturing design is carefully developed and tested by
means of a manufacturing model. When this last design seems
satisfactory, it is turned over to the tool designing department
which goes over it a third time solely to simplify the tooling and
mechanical productive operations. The changes made at this
time, however, affect minor details only. From twenty-five to
fifty mechanisms are built to this design and sent into the field
for actual service. These last models must give satisfactory
service for from one to two years before further preparations
for manufacture are considered. It is of interest to note that
the manufacturing equipment provided by this factory is com-
plete; also that changes here in the process of manufacture or
in the product under production are very rare.
Many of the so-called improvements in new commodities
which result in frequent modifications of the product under
manufacture are only steps taken to correct mistakes, omis-
sions, and other faults due in large measure to neglect of the
manufacturing design (both neglect to make and neglect to test
it) because of haste to rush into actual production. This has
been forcibly brought out by the conditions which developed in
the manufacture of many devices during the war.
Models to Test Tolerances. It is desirable to know at the
earliest possible moment whether or not the specified tolerances
define the limits of parts that will function properly. The
sooner this information is obtained, the sooner can efforts be
concentrated on problems of production alone. Until this
matter is settled within a reasonable degree of certainty, each
problem in production is complicated by many considerations
relating to the design and tolerances. This causes innumer-
able revisions on the component drawings with the attendant
changes in tools, fixtures, and gages, resulting in delays in
production and additional expense.
Some concerns try to solve this problem by carefully build-
ing several models which represent as closely as practicable the
extreme conditions permitted by the component drawings.
These model parts are assembled and reassembled, and tested
PURPOSE OF MODELS 43
for operation after each assembly. Necessary alterations of
the drawings are made before manufacturing operations are
under way. This practice is, naturally, expensive, as each of
these models costs much more to construct than any of the
preceding ones. However, if insurance against future changes
is worth the expense, the practice is well worth while.
One typewriter manufacturer makes a practice of building
from six to ten sets of model parts before any new or revised
machine is manufactured. When only one unit assembly is
affected, model parts for that mechanism only are made. These
parts are then sent to the assembling department for trial.
Except on purely experimental models, the men who make the
parts are not allowed to assemble them. No effort is made to
do anything more than to duplicate the kind of work normally
produced in the manufacturing departments. The parts are
not cornered or burred unless that operation is required in
manufacture. In other words, the attempt is made to determine
how little effort will be required to manufacture the mechanism
satisfactorily. The sizes of these parts usually cover the entire
range between the specified limits, but no distinct effort is made
to have them meet either limit exactly. Any combination of
these parts must assemble and operate properly before changes
or new models are adopted. This practice has saved the com-
pany several times from making unnecessary and improper
changes.
Model for Standard of Precision. The component drawings
give many dimensions. Strictly speaking, the expressed di-
mensions represent absolute sizes. Dimensions of elementary
surfaces can be produced and reproduced within relatively small
limits of error. Often, however, these dimensions define de-
veloped and complicated profiles, locations, and other com-
posite surfaces which cannot be reproduced as readily. Yet
they must be reproduced many times over in the course of manu-
facture to within relatively small limits of error.
A choice must be made between accuracy and precision at
the outset. In the case of elementary surfaces, accuracy is
usually the better choice; but for many composite surfaces,
44 INTERCHANGEABLE MANUFACTURING
precision will often give the quicker, more economical, and
more practical results. On the other hand, it must be clearly
understood that when precision is chosen, it becomes prac-
tically impossible for another plant, working in entire indepen-
dence of the first, to produce a common interchangeable product.
If more than one plant is engaged on the production, they must
maintain close relations in almost every detail of the work. In
order to maintain this precision within reasonably close limits,
physical standards of some sort must be provided at the very
beginning. By developing a model for this purpose, two results
can be accomplished at the same time. Such a model will test
the manufacturing design for functioning, and will also provide
the desired physical standards.
This model must be made with the greatest care and should
represent the "danger zone" that is, in most cases, the
maximum metal or minimum clearance conditions. It is evi-
dent that these sizes are the most important. They control
the interchangeability. No cuts, other than slight cornering
and similar burring operations, should be made with a hand
tool, such as a file. Whenever the contour of the surface in
question is important, templets should be made to check the
special tools used. These templets are an integral part of the
model. All important locations of holes should be established
from master plates. Templets, master plates, etc., as well as
the model parts, are invaluable when the equipment is built;
properly utilized, they will insure a high degree of uniformity
at relatively small expense. Such a model must be used with
the greatest care and becomes the court of last appeal in many
of the perplexing questions which inevitably arise in the manu-
facturing departments of a plant engaged in producing an inter-
changeable product in large quantities.
This practice in regard to models is extensively followed in
the manufacture of small arms. As stated before, it has a
certain disadvantage when more than one factory is involved.
As it is impossible to duplicate the model exactly, one of two
courses is open. Either one model is standard for all plants
which entails much lost time in referring many detailed ques-
PURPOSE OF MODELS 45
tions back to the central plant or additional models may
be made, which results in different basic standards at the vari-
ous plants. If the second course is followed, and all parts of all
models are mutually interchangeable, the product of the various
factories will be interchangeable. However, this results in
reducing the amount of the absolute tolerances available for
manufacturing variations, as the variations in the different
models consume a certain amount. In cases where the func-
tional conditions are exacting, this method is often found im-
practicable. On the other hand, if the design of the mechanism
is such that the absolute tolerances are liberal, the second
method gives an economical solution.
All the foregoing model work, regardless of its purpose, is
essentially a preliminary measure in the manufacture of a new
or revised product. Properly conducted, it will stabilize the
manufacturing design at a minimum expense. It takes con-
siderable time, however, and that is one reason why models
are not more extensively employed. For any commodity that
is already under manufacture and the design of which is already
standardized, models are of doubtful value.
CHAPTER V
PRINCIPLES IN MAKING COMPONENT DRAWINGS
THE art of expressing mechanical information by means of
drawings is still in the process of evolution. Many details have
become conventionalized, yet these comprise little more than
the alphabet of the language of drawings and relate principally
to conventional meanings of the lines, figures, and relative
locations of the several projections which go to complete the
drawings. Such, for example, are the full lines which represent
the visible outlines of the part; the dotted lines which represent
the hidden outlines; the light dot-and-dash lines which indicate
center lines; the light dimension lines and all the other con-
ventional lines and characters which are employed. The third-
angle projection is also fairly well established in mechanical
drawing. This branch of drawing is fully covered in text-books,
so no further mention of it will be made here. The subject of
dimensioning, however, is so incompletely covered that this
chapter will be devoted to a detailed discussion of this subject.
The addition of tolerances on component drawings has created
new problems which have not, as yet, been fully solved, and
which, therefore, require considerable and thoughtful study.
The matter of dimensioning, as given in books and taught
in various schools, receives only minor attention. Little more
than the a b c of the subject is taught. In actual practice
particularly where tolerances are involved so many different
conditions are to be met, so many different shades of meaning
must be clearly expressed, and so many different types of work-
men must be informed by these drawings that this alphabet
must be fully understood and carefully used to enable it to serve
its purpose. It is necessary, in order to consider intelligently
this subject of dimensioning with tolerances, to discard all school
training in the application of dimension lines, etc.
4 6
COMPONENT DRAWINGS 47
The main purpose of a mechanical drawing is to express or
record information. This information is of many kinds and is
used for many purposes. The drawing, to be correct, must
clearly and consistently express the particular information re-
quired to serve its specific purpose. For example, a type of
drawing that may be correct for the use of a toolmaker in build-
ing a jig may be incorrect for the use of a machine operator in
the manufacturing department engaged in quantity production.
Inasmuch as the drawings are the written or pictured expres-
sion of the design, they may be roughly classified into func-
tional drawings and manufacturing or component drawings.
Functional Drawings. The functional drawing, like the func-
tional design, primarily expresses the functional conditions to
be maintained. The detailed information relating to many of
the manufacturing problems that are involved which does not
appear on these drawings is supplied by the mechanic who uses
them. Thus, in those cases where only a few special mechan-
isms, or jigs, fixtures, tools, etc., are to be made in a general
machine shop or tool-room, where the type of workman is such
that this detailed information is unnecessary, functional draw-
ings only are required. Such drawings need not express toler-
ances, clearances, and other minor details so essential on the
manufacturing drawings. For example, a notation such as
"drive fit" or " sliding fit" is sufficient to indicate and obtain
the desired results. Yet, even here, if the drawings are to serve
their purpose efficiently, the information given must be so
expressed that it may be used directly. In order to attain this
end, every line drawn and every dimension expressed must be
made with a full understanding of the final results required and
of the means to be employed to obtain them.
Manufacturing Drawings. The manufacturing drawings, to
be complete, must express all suitable information that is avail-
able. For the purposes of the present discussion, we will con-
fine ourselves to component drawings of an interchangeable
product. As stated in a preceding chapter, the proper dimen-
sioning of component draiwngs with tolerances is a mathe-
matical problem. Five laws are given, which, if carefully
48 INTERCHANGEABLE MANUFACTURING
observed, will simplify many of the equipment and production
problems.
Laws of Dimensioning, i. In interchangeable manufactur-
ing there is only one dimension (or group of dimensions) in the
same straight line which can be controlled within fixed toler-
ances. This is the distance between the cutting surface of the
tool and the locating or registering surface of the part being
machined. Therefore, it is incorrect to locate any point or
surface with tolerances from more than one point in the same
straight line.
2. Dimensions should be given between those points which
it is essential to hold in a specific relation to each other. The
majority of dimensions, however, are relatively unimportant
in this respect. It is good practice to establish common location
points in each plane and to give, as far as possible, all such
dimensions from these points.
3. The basic dimensions given on component drawings for
interchangeable parts should be, except for force fits and other
unusual conditions, the maximum metal sizes. The direct com-
parison of the basic sizes should check the danger zone, which
is the minimum clearance condition in the majority of cases.
It is evident that these sizes are the most important ones, as
they control the interchangeability, and they should be the first
determined. Once established, they should remain fixed if the
mechanism functions properly and the design is unchanged.
The direction of the tolerances, then, would be such as to recede
from the danger zone. In the majority of cases, this means that
the direction of the tolerances is such as will increase the clear-
ance. For force fits, such as taper keys, etc., the basic dimen-
sions determine the minimum interference, while the tolerances
limit the maximum interference.
4. Dimensions must not be duplicated between the same
points. The duplication of dimensions causes much needless
trouble, due to changes being made in one place and not in the
others. It causes less trouble to search a drawing to find a
dimension than it does to have them duplicated and more
readily found but inconsistent.
COMPONENT DRAWINGS
49
A
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INTERCHANGEABLE MANUFACTURING
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COMPONENT DRAWINGS
tions, it is evident from the foregoing that a different product
would be received from each plant. The example given is the
simplest one possible. As the parts become more complex, and
the number of dimensions increase, the number of different com-
binations possible and the extent of the variations in size that
will develop also increase.
Fig. 4 shows the correct way to dimension this part if the
length of the body and the length of the stem are the essential
dimensions. Fig. -5 is the correct way if the length of the body
and the length over all
are the most important.
Fig. 6 is correct if the
length of the stem and
the length over all are
the most important.
If the part is dimen-
sioned in accordance with
either Fig. 4, Fig. 5, or
Fig. 6, the product from
any number of factories
should be alike. There is
now no excuse for them to
misinterpret the meaning of the drawing. The point may be
raised that the manufacturer should study the drawing to deter-
mine what his sequence of operations should be in order to main-
tain all dimensions and tolerances given. On such a simple
part as was given for the first example, this would not be difficult.
On a more complicated piece, however, it would be almost im-
possible. Such conditions occur when the draftsman makes little
or no effort to reduce as many surfaces as possible to elementary
ones. Furthermore, when the manufacturer or workman sees
such dimensions on a component drawing, he is justified in as-
suming that the designer or draftsman who made them had little
or no idea as to the essential conditions to be maintained. In
such cases, the sequence of operations and the register points for
machining will be established to facilitate production, or to
suit the ideas of individuals as to the most essential conditions.
Machinery
Fig. 7. A Third Interpretation of Dimen-
sioning in Fig. 1
INTERCHANGEABLE MANUFACTURING
Often, this will result in some of the operations on a component
being arranged to suit one idea, while the remainder are com-
pleted in accordance with an almost diametrically opposed con-
ception. It cannot be too strongly impressed upon the drafts-
man that when a drawing leaves his hands it must not be open
to more than one interpretation. This, in turn, demands that
a uniform method of interpretation be adopted and published
by each plant for the guidance of all concerned. It is self-evident
that a universal method of interpretation of drawings with
tolerances would be of great benefit to all manufacturing plants.
ON EACH SIDE
WIN. CLEARANCE 0.005
MAX. CLEARANCE 0. 025*'
A \
FUNCTIONAL BEARING OR FUNCTIONAL
SURFACE
(MIN. CLEARANCE 0.002"
IMAX.CLEARANCE 0.012 1 '
Machinery
Fig. 8. Sketch showing Functional Requirements of Slide
This is a field where the various engineering societies, working
in close cooperation, could render valuable service.
Violation of the Second Law. Let us take as the second
example the slide shown in the sub-assembly, Fig. 8. This
sketch gives the functional conditions which must be main-
tained. It is well to note that it is a very desirable practice to
add to a set of component drawings a series of sub-assemblies
of this kind. These would show graphically the functional
requirements of the most important operating members of the
mechanism, when the detail drawings are insufficient, in them-
selves, to express them clearly. Such a practice will prove of
great assistance in limiting the interpretation of the component
drawings.
Fig. 9 illustrates a common method of dimensioning such
details. This is wrong, as it violates the second law previously
stated. These parts are dimensioned in Fig. 10 in accordance
COMPONENT DRAWINGS
53
with the foregoing laws. It will be noted that all dimensions
for height are given from the bearing surface A, which is the
most important in this case. If the slide should be designed
to bear at B instead of at A , surface B would become the most
important, and the various dimensions of height would be given
from there instead of from A. The same functional conditions
(see Fig. 8) are maintained in Figs. 9 and 10. Attention is
Marhincry
Fig. 9. Incorrect Dimensioning of Slide shown in Fig. 8
Fig. 10. Proper Dimensioning of Slide shown in Fig. 8
called to the fact that in Fig. 10 it is possible to allow a toler-
ance of o.o 10 inch on the dimension to the top surface B, whereas
in Fig. 9 only 0.005 mc h can be allowed as a manufacturing
tolerance when making this cut.
Increasing Possibility of Draftsman's Errors. Thus, the im-
proper and careless dimensioning of component drawings results
directly in reducing the manufacturing tolerances, in addition
to creating uncertainty by not indicating the essential surfaces.
Furthermore, the possibility of draftsman's errors is greatly
54
INTERCHANGEABLE MANUFACTURING
increased by dimensioning as shown in Fig. 9 because the drafts-
man, in this case, niust make several additions and subtractions
of basic figures and tolerances in order to check the maximum
and minimum clearances. In Fig. 10, on the other hand, the
direct comparison of the basic dimensions checks the minimum
clearance. The maximum clearance is readily checked by adding
the sum of the tolerances to this minimum clearance. In general,
the direct comparison of the basic dimensions should establish
the minimum clearances between elementary surfaces on com-
panion parts.
No mention has been made of the dimensions of width in
the previous example. Strictly speaking, dimensions so given
Machinery
Fig. 11. Graphical Illustration of Application of Tolerance
are central with the center line. Half of the tolerances for
width may be utilized on either side of the center line. This
does not mean that the surfaces must be absolutely central;
one side can be made to the maximum dimension and the other
side to the minimum. In general, the tolerances should be
understood to establish a parallel zone of acceptable work, all
parts falling within this zone being acceptable. Fig. n illus-
trates how the dimensions and tolerances in Fig. 10 establish
such a zone. The full lines show the basic or maximum metal
conditions, while the dotted lines show the minimum metal
conditions.
Inspection Gage Requirements. In the previous example,
each surface has been considered as an independent elementary
surface, and the meaning of the drawing interpreted accordingly.
But there is also a certain condition of alignment which these
COMPONENT DRAWINGS 55
various surfaces must maintain in relation to each other. When
considering this phase of the subject, the surfaces become com-
posite. Whenever composite surfaces are involved, the func-
tional requirements of these surfaces must be taken into con-
sideration. The only satisfactory method of solving such
problems is in terms of the inspection gage requirements. If
the succeeding solutions are accepted, the accompanying inter-
pretations, expressed in terms of functional gages, must also be
accepted.
To a certain extent, the amount of tolerance required to
machine a given surface depends on the methods employed to
check the results obtained. For example, the maximum thick-
ness of the tongue of the slide shown in Fig. 10 is 0.623 inch.
If this thickness is checked with an ordinary snap gage, prac-
tically the entire tolerance is available for variations in thick-
ness. If, however, the width of this snap gage were equal to
or greater than the length of the tongue, any deviations in the
surfaces checked from true parallel planes would tend to pre-
vent the part from entering the gage. In this case, part of the
tolerance would be consumed by the errors in alignment of the
two surfaces, leaving the remainder for variations in the dis-
tance between them.
One of the principal reasons for providing clearances in the
design is to discount this condition of misalignment. In develop-
ing functional gages to check these conditions, therefore, we are
justified in utilizing a fair percentage of the minimum clearance.
In order to insure strict interchangeability, the functional gage
for the male component should never be larger than the func-
tional gage for its companion female component. In general,
if the functional gages never invade this minimum clearance
more than fifty per cent, we shall remain on the safe side. Con-
ditions sometimes arise, of course, where it is desirable to utilize
a greater percentage on one component and a correspondingly
lesser percentage on the other. For the purposes of this dis-
cussion, however, we shall assume that the conditions are such
that a maximum of fifty per cent for each component represents
a fair distribution.
50 INTERCHANGEABLE MANUFACTURING
The dimensions for functional gages to check the parts shown
in Fig. 10 are given in Fig. 12. The various dimensions of the
parts should first be checked as elementary surfaces with limit
gages representing the specified limits. This functional gage
would then be employed to test the relative alignment of these
surfaces necessary to insure interchangeability.
|< 2.998" J
^
i
1
i h- 1 -
995"-~H
M~
P3
0.624 7
1
'\ \ i
5
l< 1.995"- >
\< 2 998"
-\
Machinery
Fig. 12. Dimensions for Functional Gages for Part shown in
Fig. 10
BEARING OR FUNCTIONAL
TO SHARP CORNERS
7
Jl ( MIN. |CLEARANCE^0.002"^ON EACH
P" t MAX.
/ALIGNING OR FUNCTIONAL
ff SURFACE
HNH
N. CLEARANCE O.OIo'J
AX. CLEARANCE O.OSo"
MAXJCLEARANCE 0.008" J SIDE
MIN. CLHARANCE 0.025||l E g H
MAX. CLEARANCE 0.045 J SIDE
!! (MIN. CLEARANCE 0.025"
~nP>AX. CLEARANCE 0. 045 f
ON
EACH
SIDE
Machinery
Fig. 13. Sketch showing Functional Requirements of Dovetail
Slide
Dimensioning Composite Surfaces. Thus far we have been
considering parts whose surfaces are susceptible of individual
checking as elementary surfaces. We must also consider parts
whose surfaces cannot be resolved into elementary ones and
checked as such. Take, for example, a dovetail slide, such as
shown in Fig. 13, which introduces an angular surface. Such
angular surfaces are almost always composite ones. Great care
must be exercised in such cases to avoid compound tolerances.
COMPONENT DRAWINGS
57
A compound tolerance exists when the application of a toler-
ance on one dimension develops a variation in another dimen-
sion which also has a tolerance specified. Such a condition
immediately raises the question as to whether the resultant
variation of both tolerances is permissible or whether the toler-
ances specified are final and complete for their respective di-
mensions. In either event, confusion and misunderstanding will
result. Here, as with the introduction of more than one dimen-
sion in the same straight line (see first law of dimensioning) to
locate a given surface, the final results will depend on the se-
quence of operations adopted, with all the attendant differences.
Machinery
Fig. 14. Correct Dimensioning of Dovetail Slide shown in Fig. 13
As stated in the chapter " Principles of Interchangeable Manu-
facturing," in making component drawings, the effort should
be made to so give the dimensions and necessary tolerances that
it would be possible to lay out one and only one repre-
sentation of the maximum metal condition and one and only
one minimum metal condition. If such lay-outs were super-
imposed, the difference between them would represent the per-
missible variation on every surface. If a few such lay-outs are
made, it will soon be evident that there are always a number of
dimensions that should be given without tolerances.
A compound tolerance is an error often a serious one. It
can and should always be eliminated. Fig. 14 illustrates a
method of dimensioning the dovetail slide shown in Fig. 13
which avoids compound tolerances. The dimensions that con-
trol the position of the angular flanks are given to the sharp
INTERCHANGEABLE MANUFACTURING
corners at the top of the dovetail. The tolerances on these
dimensions limit the permissible variation of these angular
flanks. The angle is given as a flat dimension. As is evident
from the functional drawing, Fig. 13, the bearing surface A
and these angular flanks are the essential functional surfaces
of this dovetail. All other surfaces are clearance surfaces, as
should be apparent from the extent of the tolerances, Fig. 14,
even though the functional drawing were not available. Fig.
15 shows graphically the applications of the tolerances given
in Fig. 14. The full lines represent the maximum metal con-
Machlncry
Fig. 15. Graphical Illustration of Application of Tolerances
ditions, while the dotted lines indicate the minimum metal
conditions.
Compound Tolerances. The dimensioning of tapered plugs
and holes introduces a somewhat similar problem which will
result in a condition of compound tolerances if great care is not
exercised. Fig. 16 shows such a tapered hole as it is usually
dimensioned. This method of dimensioning is wrong, as it
creates a condition of compound tolerances. With these di-
mensions, it is impossible to determine what final result is re-
quired, since there are so many possible combinations. It is
evident that as the diameter of either the large or the small hole
varies, the taper will change. This makes an uncertainty about
the reamers, as these tools have a fixed taper. If we assume
that the taper is constant, questions will be raised as to which
combination of limits to employ to establish the taper. If
we further assume that the basic dimensions are to be used
COMPONENT DRAWINGS
59
for this purpose, the next question will be whether this taper,
considered as a constant, is required to remain in the position
indicated by the dimension i.ooo inch -o'S^> under all con-
ditions, or whether it can also vary in addition by the amount
resulting from the variations in diameters. Also a tolerance is
given on the length of the taper. This is entirely meaningless.
It cannot be measured readily even with an elaborate laboratory
equipment and there is no use for this tolerance in the course of
manufacture. With a fixed taper, the variation in this length is
controlled absolutely by the relative size of the holes. All in
all, as the drawing stands, it is a puzzle without any solution.
r
rnA'-f- 0.010.,
500 0.000 "
Machinery
Fig. 16. Incorrect Method of dimensioning Tapered Hole
We will assume that the intent of Fig. 16 is to indicate a
constant taper with a tolerance of -fo.oio inch in regard to
its position. Fig. 17 shows the correct method of dimensioning
such a surface to maintain such a condition. An arbitrary point
is taken on the taper and a fixed dimension given for its diameter
at that point. The location of this fixed diameter is dimensioned
with the tolerance. Three methods of dimensioning this taper
are shown. Either of the first two, A or B, is preferable to the
third, C, because any reference figures desired can be readily
computed from them without recourse to trigonometry or any
tables or handbooks.
Fig. 17 gives the manufacturer definite information which he
can use and which he can use in only one way. The tolerances
6o
INTERCHANGEABLE MANUFACTURING
given on each dimension apply only to the specific surface
in question. No tolerance can be given on the diameter of
the taper nor on the angle without introducing compound
tolerances again, with resultant confusion. The permissible
variations on this tapered surface are fully established by
TAPER 0.400 PER INCH
B
THREE METHODS OF DIMENSIONING ANGLE
Machinery
Fig. 17. Correct Method of dimensioning Tapered Hole
Machinery
Fig. 18. Graphical Illustration of Application of Tolerance
the tolerance given on its location. Fig. 18 shows graphically
the maximum and minimum metal conditions established by the
dimensions and tolerances given in Fig. 17. It will be noted
that a parallel zone for the permissible variations has been
established on every surface. When this has been accomplished,
no further tolerances should be given.
COMPONENT DRAWINGS
6l
The method of dimensioning a taper shown in Fig. 17 usually
meets with more or less opposition from the shop men. The
objection is raised that more dimensions are necessary in order
to make up the proper reamers, etc. Although the needed di-
mensions can be readily computed, it is desirable to reduce
the amount of such computations in the shop as much as possi-
ble. This objection can be eliminated in several ways. First,
if a drawing is made for the reamers, all the additional checking
dimensions can appear on these drawings. Second, if opera-
tion drawings are provided, these dimensions would appear
BEARING OR FUNCTIONAL SURFACES
CLEARANCE
MIN. INTERFERENCE FOR KEY 0.002"
MAX. INTERFERENCE FOR KEY O.OOS"
Machinery
Fig. 19. Functional Requirements for a Taper Key
there. Third, if neither of the two foregoing practices is adopted,
the required dimensions may appear on the component drawing
in parentheses, and may be marked " Basic" or "Reference."
It should be clearly understood, however, that such dimensions
are supplementary and apply only in connection with the other
basic dimensions given. No tolerances should under any cir-
cumstances be given on such reference figures. As far as possi-
ble they should be eliminated from the drawing.
Dimensioning Force Fits. The dimensioning of a taper key
and its seat offers a very instructive example. In this case, we
have a drive fit so that instead of clearances we must concern
ourselves with the establishment of the proper interferences.
Fig. 19 illustrates such a key and its seat, The functional con-
62
INTERCHANGEABLE MANUFACTURING
ditions to be maintained demand that we have always an inter-
ference of at least 0.002 inch and never have a greater interference
than 0.008 inch. The illustration shows clearance at those points
at which no bearing is required. Often, however, we find draw-
ings for such functional conditions specifying fits on all surfaces.
Such conditions add nothing to the strength or effectiveness of
the construction but entail unnecessary refinement in the manu-
facture of the detailed parts with a correspondingly increased
II
=>=> 00
5
f-H
2
4-0.000.
-o.oio"
TAPER 0.625 INCH
PER FOOT 2>5001
TAPER 0.625 INCH
PER FOOT
_JL
Machinery
Fig. 20. Incorrect Dimensioning and Design of Details
shown in Fig. 19
cost. Fig. 20 illustrates the details of such a condition dimen-
sioned in a very common manner. This method of dimensioning
is wrong. The key in this sketch violates the first, second, third,
and fifth laws of dimensioning; the slide violates the second,
third and fifth laws; while the dimensioning on the seat vio-
lates the first, second, third, and fifth laws. With the dimen-
sions given as they are, it is impossible to specify tolerances that
will insure the required functional conditions unless we reduce
each tolerance to a fraction of a thousandth. The dimensions
and the tolerances as they stand permit, in some cases, the key
COMPONENT DRAWINGS
to be tight in the slide and loose in the seat of the slide. In
other cases, the reverse is true. Parts made to the basic di-
mensions will have a fit on all surfaces.
Fig. 21 shows these parts dimensioned in accordance with
the laws of dimensioning. It will be noted that with parts
made to the basic dimensions, the key will be driven home to
its head, with an interference on the bearing surfaces specified
of 0.002 inch. The direction of the tolerances on every dimen-
TAPER 0.625 INCH PER FOOT
(SIDES OF SLOT
MAY BE PARALLEL)
do
TAPER 0.625 INCHL<4o->
PER FOOT
Machinery
Fig. 21. Correct Dimensioning and Design of Details shown in
Fig. 19
sion affecting these bearing surfaces is such that this interference
is increased as the sizes of the parts vary from the basic dimen-
sions. Under maximum metal conditions, the bottom of the
key will be flush with the bottom of the slide with an inter-
ference of 0.008 inch on the functional surfaces. It should
be noted that by giving the dimensions in this manner, the
required conditions are always maintained, while the manu-
facturing tolerances are greatly increased. Both slots are made
with parallel sides to facilitate machining. Fig. 21 offers a
good example of the application of the fifth law of dimen-
INTERCHANGEABLE MANUFACTURING
sioning. This illustration should be carefully studied and com-
pared with Fig. 20. Note, in particular, the ease of checking
the functional conditions in Fig. 21 as contrasted with the diffi-
culty and confusion which arises if we attempt to determine the
possible combinations permitted in Fig. 20. Note also how the
relative extent of the tolerances specified in Fig. 21 calls atten
tion to the essential functional surfaces. These same relative
conditions exist between any drawings that are dimensioned
without careful study as compared with those which are ra-
tionally and logically dimensioned. No attempt has been made
0.560 RAD.
Machinery
Fig. 22.
Sketch showing Satisfactory Method of specifying
Tolerance on Contours
in this example to express any dimensions other than those which
affect the taper key and its seat. The example given in Fig. 14
shows the proper dimensioning of the dovetail slide.
Dimensioning of Profile Surfaces. The dimensioning of con-
tours with tolerances introduces still another problem. To give
tolerances on the various dimensions which establish the basic
contour inevitably introduces compound tolerances. On the
other hand, it is often impossible to resolve such composite sur-
faces into elementary ones for the purposes of dimensioning and
checking, because their dimensions and relative locations are
inseparable. Fig. 22 illustrates one satisfactory solution of
this problem. The basic dimensions of the profile are given
without tolerances. A dotted line is drawn parallel to the basic
COMPONENT DRAWINGS
contour which indicates the direction of the tolerance. A
dimension is given between the full (or basic) outline and this
dotted line which specifies the extent of the tolerances. This
method of dimensioning gives definite information which can
be used directly in the manufacturing departments.
Dimensioning of Holes. The dimensioning of the location of
holes with tolerances is a most difficult problem. These dimen-
sions are usually given to the centers of the holes and define
neither male nor female surfaces. They must be used in con-
junction with the diameters of the holes, thus establishing a
composite surface condition. The introduction of tolerances on
Machinery
Fig. 23. Diagram illustrating Conditions met with in measuring
Holes
these dimensions of location immediately will produce com-
pound tolerances.
We might dimension them as shown in Fig. 23 by giving one
dimension to the inside edges of the holes (which is a male
dimension), another to the outside edges of the holes (a female
dimension), and eliminate the dimension for diameter. This
would give us a better opportunity of applying the five laws of
dimensioning in a similar manner to that employed for elemen-
tary surfaces. However, this would prove unsatisfactory in
practice because it does not give directly the information which
is of most value in the shop namely, the diameters of the
holes and the center distances.
66
INTERCHANGEABLE MANUFACTURING
No rules can safely be given for dimensioning the location of
holes in which the permissible variations are distinctly expressed,
unless the required functional conditions are duly considered.
The following examples give possible solutions of a few of these
problems. If these solutions are accepted, the corresponding
interpretations, expressed in terms of inspection gage require-
ments, must also be accepted. For the first example, we will
take the base for a bracket and its pad on a frame illustrated
in Fig. 24. We will assume that the position of this bracket on
the frame is important and must be held as closely as manu-
facturing conditions will permit. We will assume also that the
DIAMETER OF STUD 0.746+'
Machinery
Fig. 24. Methods of dimensioning the Location of Holes
jigs from which these holes are drilled locate the parts on the
finished surfaces from which the dimensions are given.
Causes of Variation in Manufacture. Variations of locations
in manufacture develop from three main causes: First, from a
fixed error in the jig; second, from a difference in size between
the drill and its bushing in the jig; and third, from improper
location of the parts in the jigs. Variations occurring because
of the first cause will affect the locations of the holes both in
relation to each other and to their locating surfaces. Varia-
tions because of the second cause will have similar effects to
those developing from the first cause. Variations because of
the third cause will affect only the location of all holes as a unit
from the locating surfaces. Thus, with these problems, there
are always composite variations with which to contend. The
COMPONENT DRAWINGS
surfaces involved are always composite, and a condition of
compound tolerances is always present.
If precision, rather than absolute accuracy, is the main con-
sideration, and if these locations in the two jigs check with each
other, the variations due to the first cause may be disregarded,
provided that the gages which check these locations are made to
agree with the jigs.
The variations due to the second cause may be reduced to
comparatively small amounts by closely maintaining the rela-
tive diameters of the drills and their bushings. This naturally
involves a somewhat increased maintenance cost of the equip-
ment. The extent of the variations due to the third cause
I
_i_
r
^ r
^
i a ?
3.000
! -rr
4 T
^
f.
J ^
^ r
\)
*\
k?**II
\,
j \.
\)
2.000"
J_
__.
"^li.odd
h-
n
5.000 >
< 3.000--^
Machinery
Fig. 25. Functional Gage for Part shown in Fig. 24
depends upon the design of the jigs and the care exercised by
the operator. In general, the third cause is responsible for the
largest amount of variation.
The locations in Fig. 24 are given without tolerances, yet the
drawing should not be interpreted to mean that no variations
are permissible. The minimum clearance between the studs
and the holes is 0.004 mcn - This clearance is provided to allow
for the variations in their locations. Therefore, this clearance
should be considered in testing the locations of these holes.
Gages for Checking Location of Holes. The inspection gage
for testing these locations would be a functional gage which
invades this minimum clearance. There are two conditions to
be considered here which affect the amount of the minimum
68
INTERCHANGEABLE MANUFACTURING
clearance that may be used on the gage. If the studs used are
loose, individual pieces which pass through both parts and are
bolted or riveted at assembly, the functional gage may utilize
the entire minimum clearance. On the other hand, if the studs
are first driven or riveted into one member, these functional
gages could invade the minimum clearance not over fifty per
cent. In either case, it is possible to make a single gage which
will check both parts.
We will first consider the functional gage to check the first
of the above conditions. This gage would consist of a plate
with four pins, as shown in Fig. 25. It checks both the loca-
tions of the four holes relative to each other and the location of
\< 5.000"
Machinery
Fig. 26. Another Type of Functional Gage for Part shown in Fig. 24
the group from the edges of the part. The locations of the pins
are identical with the corresponding basic dimensions given on
the component drawings. The diameter of the pins is 0.746
inch (basic diameter of hole minus minimum clearance). The
gage must always enter all four holes on the part. When the
gage is held against the upper edge of the holes, the lower edge
of the gage must not project below the lower edge of the com-
ponent. When held against the lower edge of the holes, the
lower edge of the gage must not be above the lower edge of the
part. This checks the vertical position of the holes. The hori-
zontal locations are checked in a similar manner. The diameters
of the holes are checked as elementary surfaces with limit plug
gages made to the specified limits.
COMPONENT DRAWINGS
6 9
Thus, although both the drawings and the gages are made
to flat dimensions, a tolerance on all positions of the holes has
been established. If their relative locations were perfect, under
maximum metal conditions of the various holes, a variation of
0.002 inch either way would be permitted on their location from
the edges of the parts. If the various holes were made to the
maximum limits, this variation could be 0.005 mcn either way.
On the other hand, if the position of these holes as a unit were
perfect in regard to the edges of the components, a variation of
0.004 inch would be permitted on their relative locations under
maximum metal conditions, with a correspondingly increasing
T7
2.000^
7 f M)' + ' 006 i
iMpu_ 000
7 V +0.006"
IMOU_ O
>
} t
^ r
J
^
t
2.0JX)" s
>
r
J \.
^ /
JL
? ^
f
J
2.000"
I
\.
J t
>ti.odo
DIAMET
<- 4.j
Machinery
Fig. 27. Method of dimensioning Location of Holes without
Tolerances
tolerance as the parts approach minimum metal conditions.
This would amount to o.oio inch at the extreme minimum metal
condition. Inasmuch as variations will develop in both types
of locations, all that is not consumed by one is available for
the other.
We will now consider the functional gage to check the con-
dition where the studs are rigidly fastened to one of the parts.
A gage for this purpose would be similar to the one shown in
Fig. 25 except that it would contain four holes instead of four
pins. Such a gage is shown in Fig. 26. Four plugs would be
used with this gage for testing the locations of the holes in one
piece, while the holes in the gage would go over the studs fas-
tened to the companion part. The diameter of the holes in this
INTERCHANGEABLE MANUFACTURING
gage and also of the plugs is 0.748 inch (basic diameter of hole
minus one-half minimum clearance or basic diameter of stud
plus one-half minimum clearance). This gage would be used
in exactly the same manner as the first. The permissible varia-
tions under maximum metal conditions of the holes and studs
would be but one-half that permitted in the first case. As these
holes and studs approach minimum metal conditions, the per-
missible variations in location would increase in the same man-
ner and extent as in the first case.
In Fig. 24, the locating dimensions are given from common
locating surfaces in each direction. They could be given as
ONE METHOD OF DIMENSIONING
I C
b 1 (
h
>-
(
P ^
"^ C
0.002"
c
J C
1 f
J
3.000 0-002 j
< 7.000" 0.002- ;-
DIAMETER OF STUD= 0.746 Ij^w SECOND METHOD OF DIMENSIONING
Machinery
Fig. 28. Two Methods of Dimensioning with Tolerance which
maintain Identical Conditions
shown in Fig. 27. As long as no tolerances are expressed, the
method most convenient for the shop is best.
Expressed Tolerances on Location of Holes. If expressed
tolerances for locations of holes are insisted upon, it is impossi-
ble to avoid compound tolerances. Then an arbitrary method
of interpretations must be promulgated to prevent continual
argument and misunderstanding. Fig. 28 illustrates two
methods of indicating such tolerances. We assume that the
functional conditions are identical with those previously dis-
cussed in the first case of Fig. 24. This is one example where
the mean size is the proper basic dimension, and tolerances apply
equally plus and minus.
In order to establish the sizes of inspection gages we must
consider the tolerances, instead of minimum clearances. Re-
COMPONENT DRAWINGS 71
ferring to Fig. 23, an inspection gage to check the relative
locations of the holes must be made to the maximum dimension
A and the minimum dimension B. In the horizontal direction,
the maximum limit of A is equal to the maximum center distance
(4.004 inches) minus the minimum diameter of the hole (0.750
inch) which amounts to 3.254 inches. The minimum limit of
B in the same direction is equal to the minimum center distance
(3.996 inches) plus the minimum diameter of the hole (0.750
inch) which is equal to 4.746 inches. The difference between A
maximum and B minimum gives double the diameter of the
pins on the inspection gage, which amounts to 1.492 inches.
The diameter of these pins is therefore 0.746 inch, and the
inspection gage for the relative location of the holes is identical
with the one shown in Fig. 25.
In like manner, we must consider the location of these holes
from the edge of the components. For simplicity in notation,
call the dimension from the lower edge of the piece (in Fig. 28)
to the upper edge of the circumference of the lower left-hand
hole, C. Call the distance from the lower edge of the piece to
the bottom edge of the hole, D. On the gage, evidently, C must
be minimum, while D must be maximum. The minimum di-
mension of C is equal to 0.998 inch plus half the minimum
diameter of the hole (0.375 mcn ) which amounts to 1.373 inches.
The maximum dimension D is equal to 1.002 inches minus half
the minimum diameter of the hole, which equals 0.627 mcn -
The diameter of the pins in the gage is equal to the difference
between C and D, which equals 0.746 inch. Therefore, the gage
shown in Fig. 25 applies to both Fig. 24 and Fig. 28. Or, to
put it in other words, Fig. 24 and Fig. 28 express the same
information.
Therefore, in those cases where no tolerances are given for
center distance (and this applies equally to locations of holes or
grooves or slots, etc.), the minimum clearance must be analyzed,
and utilized accordingly to determine the inspection gage re-
quirements, and a suitable minimum clearance must be pro-
vided to allow for the inevitable variation in these dimensions.
On the other hand, where tolerances are specified on such di-
INTERCHANGEABLE MANUFACTURING
mensions, these must be analyzed and applied accordingly, to
establish the inspection gage dimensions, and the minimum
clearance between the holes and studs must be sufficient to
prevent interferences. In either case, the basic dimensions
should be identical, and the inspection gages would also be
identical. For conditions such as those described, experience
will teach that the safest plan is to eliminate the tolerances from
the component drawings.
The above interpretations of drawings are arbitrary to a cer-
tain extent. It would be possible to demonstrate that under
certain combinations of conditions, the exact letter of the com-
ponent drawings would be violated. This is inevitable in this
-J
H r 4<00 '' H
1 1.000"
0.040"
DIAMETER OF STUDS 0.746"+-OOjJ
. r
y
i
H r
>
N
">
V
^
J V.
>! r
.
C
J t
7
^.
< >
3.000 0.040"
< 4.000" *
M(
cftinc
Fig. 29. Another Method of Dimensioning with Tolerance Part
shown in Fig. 24
connection. As stated before, if these solutions are accepted,
the corresponding interpretations must also be accepted.
Tolerances for " Group " Locations. The same parts, but
with different functional conditions, will now be considered.
Naturally, the holes in the companion parts must line up suffi-
ciently to enable the studs to pass through. Therefore, the
importance of the location of these holes in relation to each other
is constant. We will assume, however, that in this case the
position of the bracket on the frame is unimportant. The only
method of indicating this condition that will be consistent with
the general practice of dimensioning discussed heretofore will
be to express a tolerance. This is done in Fig. 29. No toler-
ances are shown in this sketch on the dimensions controlling the
relative locations of the holes to each other.
COMPONENT DRAWINGS
73
The interpretation of this drawing is that a variation of 0.040
inch, plus and minus, over and above that allowed by the mini-
mum clearance between studs and holes is permitted on the
location of the holes as a group. The inspection gage for test-
ing this is shown in Fig. 30. This gage differs from that shown
in Fig. 25 only in the addition of the steps on the edges which
check the additional tolerance. It is used as follows: When
the gage is held against the upper edges of the holes, the mini-
mum lower step of the gage must not extend below the lower
edge of the part. When the gage is held against the lower edge
of the holes, the lower maximum step must not be above the
lower edge of the part. The horizontal locations are checked
Machinery
Fig. 30. Functional Gage for the Part shown in Fig. 29
in a similar manner. If the functional conditions permit a liberal
variation in one direction (say, horizontal) but not in the other
direction (vertical), a combination of the methods of dimen-
sioning and checking meets the situation.
Conditions arise where the locations of holes must be estab-
lished and checked from other points than a flat surface. This
often requires quite elaborate fixture gages. A full understand-
ing of the preceding principles and a careful study of the par-
ticular conditions will point the way to a consistent solution of
the problem. The present space is not sufficient to go into the
subject in greater detail. Simple examples have been purposely
selected to indicate and illustrate the general principles involved.
74
INTERCHANGEABLE MANUFACTURING
The preceding examples involve maintaining the relative
position of several holes with each other in addition to the loca-
tion of a group as a whole. In those cases where only a single
hole is involved which must maintain its position in relation to
elementary surfaces, the problem is simple. In most cases it
can be solved by the application of methods previously discussed
for elementary surfaces. In other cases, the functional require-
ments may be such as to demand a functional gage similar to
those shown in Figs. 25 and 30.
Concentricity and Alignment. The expression of permissible
variations in concentricity and alignment introduces another
difficult problem. The succeeding examples offer one solution.
U' " I
3.500 S:' *j h-2.0l6'i;^ ( ;->|
h-
Machinery
Fig. 31. Methods of dimensioning Holes and Studs to fit
As in the case of the locations of holes, if these solutions are
accepted, the corresponding interpretations, expressed in terms
of inspection gage requirements, must also be accepted.
We will assume that the stud shown in Fig. 31 must always
assemble into the hole shown in the same sketch. In the process
of manufacture, a certain amount of eccentricity will develop.
If we attempt to give on the component drawings the permis-
sible eccentricity in every case, the drawing will become more
and more complicated. The more complicated the drawings
become, the greater the possibility of undetected errors. With
the following interpretations of the drawings, the conditions of
eccentricity are almost automatically covered.
COMPONENT DRAWINGS
75
There is a minimum clearance on diameters of 0.004 inch
between the parts shown in Fig. 31. Inspection gages to test
the concentricity of these parts are functional gages and invade
this minimum clearance to a fair amount. In general, this
should not be over fifty per cent. The lengths of the studs and
depths of the holes do not enter into this discussion. They are
elementary surfaces which should be readily maintained and
checked. The gages for testing the concentricity of these parts
Machinery
Fig. 32. Functional Gages for Part shown in Fig. 31
are shown in Fig. 32. It will be noted that the diameters invade
the minimum clearance by an amount equal to fifty per cent.
A somewhat similar condition which involves the alignment of
slides, or profile grooves and tongues, has been previously dis-
cussed in connection with Fig. 10. Functional gages for such
conditions are shown in Fig. 12.
Occasionally, the situation arises where a sub-assembly as a
whole must meet such conditions as are described above. This
76 INTERCHANGEABLE MANUFACTURING
may entail individual tests for concentricity or alignment on
individual component parts of the sub-assembly. The mini-
mum clearances must, therefore, be subdivided proportionately.
In such cases, it is good practice to include on the component
drawing an outline with the dimensions of the functional gage
required to test the conditions of concentricity or alignment.
This practice will eliminate many arguments in the course of
future production.
Gears. Gear teeth offer another problem of composite sur-
faces. In general the tolerances on the tooth forms can best
be given by specifying the permissible amount of backlash
between the pair. No tolerances should ever be given on pitch
diameters of gears. Specifying a limit on the backlash makes
it possible to eliminate all compound tolerances. Furthermore,
the most effective inspection of the gears is obtained by measur-
ing this backlash with the gears at a fixed center distance. All
the foregoing examples are comparatively simple ones. They
should, however, be sufficient to indicate the manner in which a
component drawing with tolerances should be dimensioned.
As stated previously, it is seldom possible at the very start
to collect and record on the component drawings all of the de-
tailed information which belongs there. The development of
tools, gages, and other equipment and the final solution of many
of the manufacturing problems will make apparent omissions
and errors. Therefore, the component drawings should not be
considered as complete until the product is actually being pro-
duced in strict accordance with them. This requires that the
designer, responsible for the accuracy of the. drawings, keep in
close touch with both the designers of the manufacturing equip-
ment and the various manufacturing departments in order to
keep these component drawings up to date.
CHAPTER VI
PRACTICE IN MAKING COMPONENT DRAWINGS
As a practical and specific illustration of the principles gov-
erning the dimensioning of component drawings set forth in
the preceding chapter, a small unit assembly showing the per-
cussion firing mechanism for a large cannon is taken as an
example. This particular mechanism is chosen because it is
composed of a small number of parts; also because it contains
several examples of comparatively unusual conditions. In
studying the various component or detail drawings to be re-
ferred to, the relation between the methods of dimensioning,
the tolerances and clearances specified, and the functional re-
quirements of each part should be carefully considered.
Drawing of Firing Mechanism Assembled. The assembly
of this mechanism is shown in Fig. i. The operation is as follows:
The firing mechanism container assembled must be withdrawn
before the breech of the cannon can be opened, and cannot be
replaced until the breech is closed. (The safety mechanism con-
trolling this is not shown on this drawing.) While this assem-
bled container is being withdrawn, a primer A is inserted in the
primer extractor. This primer is held in place by the pressure
of the firing pin guide spring acting against the firing pin guide
B. After the breech has been closed again, the container C with
the primer is inserted into the housing D and screwed home by
hand. The primer must seat tightly on the sharp taper in the
spindle plug E. A lanyard is attached to the striker F with a
connection that slips off when the end of the striker is withdrawn
beyond the end of the container cover G, thus allowing the
striker to move forward at the proper moment under the im-
pulse of the firing spring H. The firing pin transmits the blow
of the striker to the primer, thus detonating it and igniting the
charge in the cannon.
77
78 INTERCHANGEABLE MANUFACTURING
COMPONENT DRAWINGS 79
Functional Requirements of the Mechanism. The following
functional conditions must be maintained: The primer must be
seated in the spindle plug in such a manner that no gases can
escape when the gun is fired. Any leakage of these gases, which
are at a very high temperature and under high pressure, will
quickly erode or burn out the parts of the mechanism, thus
destroying its effectiveness. This requires that the surfaces of
the seat for the primer be smooth and that its dimensions be
maintained within close limits. The blow imparted by the
striker must be sufficient to insure that the primer will always
be detonated, since the sole object of the mechanism is to de-
tonate the primer. In order to insure this result, the firing pin
must always protrude, in operation, a certain minimum distance
(determined by experiments), while, in order not to pierce the
primer cup, it must never protrude beyond a certain maximum
distance (also determined by extensive experiments). The vari-
ous unit assemblies of the mechanism must be interchangeable
in order to allow quick replacements in service a vital re-
quirement. As far as proves economical, the various component
parts of the unit assemblies should be interchangeable to permit
ready repairs in service. Unless noted otherwise, the parts of
this mechanism must be interchangeable. These are the most
important of the functional requirements. Others will be dis-
cussed as they arise in connection with the details.
Drawing of Firing Pin Guide. A detail drawing of the firing
pin guide is shown in Fig. 2. The outside diameter is 0.782 inch
plus o.ooo, minus 0.003. This guide must be an easy slide fit in
the container. It has a minimum clearance of 0.002 inch, as
will be seen by a comparison with that part of the container
(see Fig. 7) which receives the guide. With a tolerance of 0.003
inch on each part, it has a maximum clearance of 0.008 inch.
With a reasonably smooth finish, such as that obtained by a
finishing cut on the guide and a finish-reaming operation on the
hole in the container, these clearances will maintain the condi-
tions required.
The firing pin must be an easy slide fit in the guide. The
diameter of the firing pin hole is 0.118 inch plus 0.003, minus
8o
INTERCHANGEABLE MANUFACTURING
o.ooo. The diameter of the firing pin is 0.117 inch plus o.ooo,
minus 0.003; hence the minimum clearance is o.ooi and the
maximum clearance (with tolerance of 0.003 on each part), 0.007
inch. With a reamed finish in the hole and a finishing cut on
the pin, these clearances will maintain the proper conditions.
The diameter of the large counterbore in the rear of the guide
is 0.584 inch plus 0.003, minus o.ooo. The flange of the firing
pin must be an easy slide fit in this counterbore. The diameter
of the flange is 0.582 inch plus o.ooo, minus 0.003; therefore the
minimum clearance is 0.002 and the maximum clearance, 0.008
0.118+8:8$
-\l
k^
Machinery
Fig. 2. Firing Pin Guide
inch. With a reamed finish in the counterbore and a finishing
cut on the flange of the firing pin, these clearances will maintain
the proper conditions.
The diameter of the smaller counterbore is 0.484 plus o.oi,
minus o.oo. This counterbore contains the firing pin guide
spring. The minimum clearance is 0.020 and the maximum
clearance 0.040 inch. It is apparent that this surface is of minor
importance. The only limit to the increase in diameter of this
surface is controlled by the width of shoulder at its mouth which
is needed to act as a stop for the firing pin. The basic width of
this shoulder is 0.05 inch; the tolerance on the counterbore
diameter is o.oi, thus reducing the effective width of the shoulder
by 0.005 inch. The tolerance specified should be sufficient to
enable this counterbore to be finished in one cut. No finishing
COMPONENT DRAWINGS 8 1
operations are necessary on this surface either for smoothness
or accuracy.
Exception to General Rule for Basic Dimensions. The
length of the guide (Fig. 2) is 0.525 inch plus 0.003, minus o.ooo.
This dimension is an exception to the general rule of making the
basic dimension represent the maximum metal conditions, be-
cause of the functional conditions which must be maintained in
this case. When the firing pin and the guide are seated solidly
in the container, the face of the guide and the end of the firing
pin should be as nearly flush as possible. Under no conditions
must the firing pin project, because any such projection makes
possible a premature explosion of the primer. The basic dimen-
sions on the firing pin and guide are identical for this point, thus
making these surfaces flush under basic conditions. The direc-
tion of the tolerances in each case is such that the pin can never
project. This method of dimensioning, therefore, adheres to
the principle of making the basic dimensions represent the danger
point, while the direction of the tolerance is such as to move
away from this danger point.
On the other hand, there is another danger point in the other
direction, although not as serious a one as the first. In such
cases, the basic dimension should always represent the more
dangerous point, while the tolerances should limit the extent of
the other. In this case, the second danger point is that the end
of the firing pin should be held as nearly flush with the face of the
guide as possible so as not to form a pocket into which the primer
cup might be forced under firing conditions. If this happens, it
is very difficult to remove the exploded primer. This may retard
the rate of fire and possibly put the gun out of action. The
tolerances on these dimensions limit the depth of this pocket to
0.006 inch which is as great as is considered safe. The front
face of the guide must be smooth; a polished surface is desir-
able, as this facilitates the insertion and removal of the primers.
The dimension from the bottom of the large counterbore to
the front face is 0.395 i nc h pl us o.ooo, minus 0.004. This di-
mension controls the protrusion of the firing pin and is, there-
fore, the dimension to be maintained. Experiments show that
82 INTERCHANGEABLE MANUFACTURING
the firing pin should protrude at least 0.026 inch in order to
insure detonation, while it should not protrude over 0.034 inch
or there will be danger of piercing the primer cup; therefore,
the corresponding length of the firing pin is made 0.421 inch
.which gives the minimum protrusion of 0.026 inch while the
tolerance of 0.004 inch applied to each part limits the maximum
protrusion to 0.034 inch.
Maintaining a Common Locating Point. Inasmuch as the im-
portant functional dimensions of length are given from the front
face, the bottom of the small counterbore is also located from
that surface so as to maintain one common locating point. This
surface is unimportant; a dimension of 0.136 inch plus o.oo,
minus o.oi is specified, which should give wide enough limits to
enable it to be machined in a single cut.
The bevel at the front of the guide is provided to assist in
the insertion of the primer. The diameter of the intersection
of this bevel with the front face is given as 0.551 inch plus o.oo,
minus 0.02. These limits should be wide enough to meet any
normal manufacturing conditions. The surface of the bevel
must be smooth; a polished surface would be desirable. The
angle of the bevel is given as 25 degrees. No tolerance is specified,
as the permissible variation is controlled by the tolerance given
for the face. The angle of the corresponding surface of the
primer extractor is 14 degrees. The angle on the guide is made
greater to insure that the forward corner of the guide will not
project above the bottom of the primer slot in the extractor.
Any such projection would interfere with the ready insertion of
primers.
No tolerances are given for the radii of the corners of the
guide. In the first place, a reasonable variation is already estab-
lished for them by the tolerances given on other dimensions.
In the second place, their exact contour is of no importance,
their purpose being to remove the sharp corners. A straight
bevel of the same dimensions would be as effective.
Drawing of Firing Mechanism Container Cover. The thread
of the container cover screws into the container and must be
set up as tightly as possible. The outside or major diameter
COMPONENT DRAWINGS
of the thread is 1.125 mcn pl us o.ooo, minus 0.008 (see Fig. 3).
The pitch diameter is 1.0979 inch plus o.ooo, minus 0.004. The
minimum clearance is o.ooo and the maximum clearance on
the pitch diameter, 0.008 inch, as will be seen by comparing
Figs. 3 and 7. This tolerance should be kept as small as normal
manufacturing methods will permit.
The diameter of the flange is 1.25 inch plus o.oo, minus o.oi.
This surface is an atmospheric fit and of little importance, as
regards either smoothness or accuracy; hence it should be com-
pleted in a single machining operation. The diameter of the
stem is i.io inch plus o.oo, minus o.oi. This surface is an
o' +0 - 00t >
w.o^u _ uoo
24-Thds. per in.- U.S. Form-R.H.
Pitch Dia. 1.0979 $$g('t /
Core Dia. 1.071 1 "
Dia. of Undercut.
^i.io'ia' H
0.494 i^"
I ft c v+-"
0.51_o.oi"
}* 1.544'i - 00 -"
Machinery
Fig. 3. Firing Mechanism Container Cover
atmospheric fit and should be machined in a single operation.
The distance across the flats on the stem is 0.945 inch plus o.oo,
minus 0.02. This surface is for the wrench used in assembling
and is of little importance. It should be machined in a single
operation by a straddle-milling tool.
The diameter of the hole and the width of slot is 0.520 inch
plus 0.006, minus o.ooo. These surfaces are for the striker and
lanyard connection. They should be as smooth as careful ream-
ing and finish-milling operations will leave them. The hole and
the slot must be matched so that no shoulder will be left at their
intersection, which would retard the striker in its action. This
INTERCHANGEABLE MANUFACTURING
03
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COMPONENT DRAWINGS 85
ing point. As the front surface is the logical working point,
this has been chosen.
The length of the thread is 0.394 inch plus o.oo, minus o.oi.
The width of the recess is 0.08 inch plus o.oi, minus o.oo. The
requirements of these dimensions are that there shall be suffi-
cient threads to hold the cover in position and that the length
of this stem shall be less than the depth of the recess in the con-
tainer, as the cover must always seat on its flange. The depth
of the recess into which the thread projects may extend about
o.oi inch below the bottom of the threads to give a suitable
clearance for threading. Also, the end of the stem and edge of
the recess may be beveled about thirty degrees to facilitate this
operation. The depth of the counterbore is identical with the
length of the threaded stem. This depth is of relatively small
importance.
The location of the rear face of the flange is 0.494 plus o.oo,
minus o.oi. This surface is an atmospheric fit and should be
finished in a single operation. The bottom of the wrench cuts
is 0.51 plus o.oo, minus o.oi. This is a clearance surface of little
importance. It is left above the rear surface of the flange to
eliminate all matching operations. The bottom of the slot is
located by these same dimensions, and its surface is of equal
unimportance.
The length over all -is 1.544 inches plus o.oo, minus 0.02.
The rear surface of the cover is an atmospheric fit of minor im-
portance. No tolerances have been specified for any of the
radii, as their exact contour is of no importance. Their purpose
is to remove the sharp corners. Furthermore, sufficient varia-
tions have been established for these radii by the tolerances on
other dimensions.
Drawing of Striker. The diameter of the front end of the
striker (see Fig. 4) is 0.591 inch plus o.oo, minus o.oi. This
surface must clear the counterbore in the container (see Fig. 7).
The minimum clearance is 0.019 inch and the maximum clear-
ance 0.039 inch. The radius of the air grooves is 0.07 inch plus
o.oi, minus o.oo. No finishing cuts are required on these sur-
faces. The stem which has a diameter of 0.38 inch plus o.ooo,
86
INTERCHANGEABLE MANUFACTURING
minus 0.005, must be a free sliding fit in the spring seat washers.
The minimum clearances are 0.002 inch and the maximum 0.012
inch. This stem must have a smooth surface, such as will be
secured by a finishing tool.
The rear shoulder which has a diameter of 0.517 inch plus
o.ooo, minus 0.005, must be a free sliding fit in the cover. The
minimum clearance is 0.003 mcn an< ^ tne maximum clearance
0.014 inch. This surface must be as smooth as a careful finish-
ing cut will leave it. The length of the front end (0.829 inch
plus o.ooo, minus 0.005) should be held within reasonably close
SOLID HEIGHT, NOT MORE THAN 0.621"
ASSEMBLED HEIGHT. 1.4&"
LOAD AT ASSEMBLED HEIdHT.NOT LESS THAN
3. 3 LBS.
SPRING TO BE COMPRESSED TO A HEIGHT 0.77l"
FOR 48 HOURS, AFTER WHICH IT SHALL SUPPORT
A LOAD OF 27.7 LBS. ATA HEIGHT OF 0.77l'0.03
FREE HEIGHT "'' V_ 0.092"
1.641' >
MarMncry
Fig. 5. Spring Seat Washer
Fig. 6. Firing Spring
limits in 'order to insure a uniform blow on the firing pin. Varia-
tions in this dimension will affect the force of this blow. The
front face of the striker must be as smooth as a finishing cut
will leave it. The length of the stem (1.70 inches plus o.oi,
minus o.oo) should also be held within reasonably close limits,
as it controls, to a certain extent, the force of the blow of the
striker. This stem could be machined with a form tool, the
roughing tool being made to form the neck for assembling the
washers. The neck is, therefore, located from the front end of
the stem, the dimension being 0.23 inch plus o.oi, minus o.oi.
The width is 0.145 mc ^ P^ us - OI > minus o.oo. These limits
should be adequate to permit this groove to be finished with a
roughing tool without any unnecessary refinements.
COMPONENT DRAWINGS 87
The location of the rear shoulder from the front end (2.73
inches plus o.oo, minus 0.02) and the width of the bottom of
the groove (0.40 inch plus o.oi, minus o.oo) are relatively unim-
portant. The surfaces, however, must be reasonably smooth
ones such as are obtained with a finishing tool. These limits
should be sufficient for all manufacturing purposes. The length
over all (3.30 inches plus o.oo, minus 0.02) is also relatively
unimportant. A sufficiently smooth surface for the rear end
will be obtained with a cutting-off tool.
No tolerances are given for the 45-degree bevel at the rear
end, nor for the radii, because none are required. A reasonable
variation is already established by tolerances given on other
dimensions. Sufficiently accurate radii will be obtained by
touching with a file the various corners which are not broken by
form tools, to remove the sharp edges.
Drawing of Spring Seat Washer. If a large number of spring
seat washers (see Fig. 5) were to be manufactured, they might
be made in a punch and die. The surface obtained in a well
made sub-press die would be sufficiently smooth, but the surface
obtained on the usual punch press in an open die would prob-
ably require some polishing. For a small number of parts, bar
stock could be used. The surface obtained with a finishing tool
would be satisfactory.
The hole must be a free sliding fit on the stem of the striker.
The minimum clearance is 0.002 inch and the maximum clear-
ance o.oi 2 inch. A surface equal to that obtained with a reamer
should be secured. The width of the assembling slot is unim-
portant. The faces of the washer are of but minor importance.
The original surface of flat stock, if that is used, or the surface
obtained with a cutting-off tool, if bar stock is used, will be
satisfactory.
Tolerances are not needed for the radius of the corners, but
enough of the corner must be removed to permit the washer to
seat properly in the counterbore in the cover. This corner may
be removed on a polishing wheel or with a file in the lathe.
Drawing of Firing Spring. No tolerances are given on the
dimensions of the firing spring (see Fig. 6), because the functional
88
INTERCHANGEABLE MANUFACTURING
COMPONENT DRAWINGS 89
requirements are covered by the weight specifications, and the
manufacturer is allowed reasonable latitude in these dimensions
as long as the weight requirements are maintained. The di-
mensions given are nominal. A variation of 0.005 mcn m the
diameter of the wire, or of 0.030 inch in the diameter of the coils
or free length of the spring will be of no moment. If these toler-
ances were expressed on the drawing, some manufacturer would
complain that the weight specifications would not allow him
to take full advantage of them, and would seek to have the
weight requirements altered or removed. These weight con-
ditions are the essential ones, as they control the force of the
blow on the primer. A minimum load of 3.3 pounds is required
at the assembled height of 1.45 inches. A load of 27.7 pounds is
required at a height of 0.771 inch plus 0.03, minus 0.03. By
thus specifying loads at two heights, the strength of the spring
is very closely controlled.
Drawing of Firing Mechanism Container. The container is
shown in Fig. 7. The housing thread (which has an outside
diameter of 1.417 inches plus o.ooo, minus 0.006) is a special
thread and will undoubtedly be milled. It must be a very free
fit in the housing, as the container is inserted and removed
every time the gun is fired. It must assemble readily, even if a
certain amount of dirt and grit is present. The minimum clear-
ance is 0.008 inch and the maximum clearance 0.020 inch. The
surfaces must be smooth. A finish- turning or milling cut will
be satisfactory. It will be necessary to match the turning and
milling cuts where the bottom of this thread matches the cylin-
drical portion of the container with a file after the part is
machined.
The thread for the primer extractor (outside or major diameter
1.024 inches plus o.ooo, minus 0.008) must be left hand to pre-
vent the primer extractor from unscrewing as the mechanism is
removed from the housing. The minimum clearance is o.ooo
and the maximum clearance on the pitch or effective diameter,
0.008 inch. The primer extractor must be screwed home as
firmly as possible, and the variations on these threads should
be kept as small as normal manufacturing methods will permit.
go INTERCHANGEABLE MANUFACTURING
The small counterbore in the rear end (diameter 0.6 10 inch
plus o.oi, minus o.oo) is for clearance and is unimportant. It
should be finished at a single operation of a counterbore. The
large counterbore in the rear end (diameter 0.950 inch plus 0.005,
minus o.ooo) must be a free sliding fit for the washer and should
be reamed. The minimum clearance between the hole having
a diameter of 0.35 inch plus o.oi, minus o.oo, and the firing pin
is 0.05 inch, while the maximum clearance is 0.07 inch. This
surface is unimportant and should be machined by a single
drilling operation. The diameter of the counterbore in the front
end is 0.784 inch plus 0.003, minus o.ooo. This surface must be
an easy sliding fit for the guide and requires a careful finish-
reaming operation. The length over all (3.271 inches plus o.oo,
minus 0.02) is relatively unimportant, yet both the front and
the rear faces must be smooth, as they form the seats for the
cover and primer extractor. The majority of the length di-
mensions are located from the front face. A few are given from
the rear end because of manufacturing considerations. The
remainder are given from intermediate points because of the
functional requirements of the mechanism.
The length of the thread for the extractor is 0.209 mc h pl us
o.oi, minus o.oo, and the width of the under-cut, 0.06 inch plus
0.005, minus o.ooo. The requirements of these dimensions are
that there shall be sufficient threads to hold the extractor prop-
erly and that the length of the stem be always long enough to
permit the extractor to seat on the front face of the container.
The bottom of the large counterbore, which is i.n inches
plus o.ooo, minus 0.005, from the firing pin seat, is an impor-
tant functional surface and should be finished by a special opera-
tion, locating from the firing pin seat. This dimension controls,
to a great extent, the force of the blow of the striker. The
location of the housing thread is also controlled from the firing
pin seat. This dimension (0.291 inch plus o.ooo, minus 0.005)
controls the angular position of the mechanism when it is
screwed home in the housing. A plus variation on this dimen-
sion might prevent the mechanism from locking. The width of
the housing thread is 0.405 inch plus o.oo, minus o.oi. The
COMPONENT DRAWINGS
rear surface or flank of this thread is a bearing surface and must
be smooth. A corner of this rear flank is beveled with a file to
facilitate entering the housing (see view showing development of
thread). The length of bevel is 0.158 inch plus 0.02, minus o.oo,
and the width of bevel 0.04 inch plus 0.02, minus o.oo. This
should give sufficiently liberal tolerances for all manufacturing
purposes. Double this tolerance could be given, however, if
necessary.
The latch-pin and spring holes are both located from the
front face of the container, as this is the logical locating point
for the drill jig. The distance from the latch-pin hole to the
24 Thds.U.S.F. L.H.
Pitch Dia.0.9969 ; S
Core Dia. 0.9698'+ o.oos ^
Dia. of Undercut+o.ois"
__ ___ 1.024^000^1
A A
^-
n "+ 0.004"
- 4 < 2 -o.ooo"
2 7 ? 0.02V
_L_l-;
0.06 8:151!};; -- -
o^s?;; Jr
Machinery
Fig. 8. Primer Extractor
center of the container is 0.625 inch plus 0.005, minus o.ooo.
A minus variation on this last dimension would develop inter-
ference between the bottom of the slot and the latch. The
latch-pin thread is a standard No. 5-44 A.S.M.E. thread. Taps
should be avalaible from stock from any reliable tap manu-
facturer. This is a fine thread and should be held as close to
size as normal manufacturing conditions will permit.
The distance to the cut on the left side of the flange is 0.30
inch plus o.oo, minus o.oi, and the height from the bottom of
the cut is 0.56 inch plus o.oo, minus o.oi. The requirements of
this cut are that the cutter shall not gouge the knurled handle
2 INTERCHANGEABLE MANUFACTURING
by cutting too deeply and that the tap shall not gouge the
bottom of this cut. The limits specified give the greatest per-
missible variations under maximum metal conditions and should
be great enough to allow this cut to be machined in a single
operation. The width of the latch slot is 0.300 inch, plus 0.006,
minus o.ooo. This surface must be reasonably smooth so as to
maintain an easy action of the latch. The minimum clearance
is 0.003 inch and the maximum clearance, 0.013 inch. With
the proper surfaces, these clearances will maintain the desired
conditions.
The location of the bottom of the slot from the center of the
container is 0.45 inch plus o.oo, minus 0.02. A plus variation
on this dimension would develop interference with the latch.
The surface of the bottom and ends of this slot is not important
and requires no finishing cuts. The cuts on the side of the slot
must be matched to provide a smooth bearing for the latch.
The ends of the housing thread are 0.02 inch plus 0.02, minus
o.oo, from the center of the container. These surfaces require
no finishing cuts. The length of the knurling is 1.60 inches
plus o.oo, minus 0.05. A scale measurement will be sufficient
to check this dimension. No tolerances are given on the vari-
ous radii or angles, as none are required. Tolerances on other
dimensions establish liberal variations for these surfaces.
Drawing of Primer Extractor. The outside diameter of the
primer extractor (see Fig. 8) which is 1.102 inches, plus o.ooo,
minus 0.006, should approximately match the corresponding
diameter on the container shown in Fig. 7. The surface should
be reasonably smooth. The diameter of the recess for the firing
pin guide (0.791 inch, plus o.oi, minus o.oo) is clearance and is
unimportant. The depth (0.264 inch, plus 0.005, minus o.ooo)
is more important, as it controls the amount of surface which
engages the head of the primer. A finish cut will be required
on this surface.
The width of the extractor slot (0.472 inch, plus 0.004, minus
o.ooo) is an important dimension and the surface must be
smooth. A finish cut will be required. The bevel at the end
of this slot is at an angle of 30 degrees and is located from the
COMPONENT DRAWINGS 93
center of the extractor at a distance of 0.236 inch, plus o.oi,
minus o.oi. The exact dimensions of the bevel are unimpor-
tant, as it is provided merely to facilitate assembling the primer.
The surfaces must be smooth, however, even if an extra filing
operation is needed to match the cuts.
The distance across the flats (0.945 inch, plus o.oo, minus
o.oi) is for the wrench used in assembling and is unimportant.
No finish cuts are required. The length of the extractor is
0.394 inch, plus o.oo, minus o.oi. The front face must clear the
rear face of the spindle plug when the primer is seated; there-
fore, no plus variation is permissible. Any great minus varia-
tion will weaken the extractor. The tolerance given should be
liberal enough for all normal manufacturing purposes. Both
the front and rear surfaces should be reasonably smooth. This
will require finishing cuts.
The depth of the tapped hole is 0.209 inch, plus o.oo, minus
o.oi, and the width of the thread under-cut, 0.06 inch, plus
0.005, minus o.ooo. Enough threads must be secured to hold
the extractor firmly in position, yet the depth must be shallow
enough to permit the extractor to seat on the front face of the
container. It is permissible to make the depth of the thread
under-cut not over 0.005 inch below the bottom of the threads
to provide clearance for the tap. A greater diameter of under-
cut than 1.042 inch (maximum outside or major diameter of
thread or 1.032 inch plus o.oi in diameter allowed for tap
clearance) would weaken the extractor to such an extent that
it would not be safe to use it in service.
The location of the bottom of the primer slot from the rear
face is 0.248 inch, plus o.ooo, minus 0.005. This surface should
never come below the corner on the firing pin guide, for if it
did, it would be difficult to insert the primer. The surface
should be smooth and all corners about this slot must be care-
fully broken.
Dimensioning to Prevent Compound Tolerances. The coun-
tersink which merges into the beveled surface on the under side
of the primer head is located from a theoretical point, where its
angle of 35 degrees intersects the center line of the extractor.
94
INTERCHANGEABLE MANUFACTURING
The distance from this intersecting point to the front face is
0.306 inch, plus 0.005, minus o.ooo. Such a method of dimen-
sioning is necessary to prevent compound tolerances. It will
be noted that no dimensions are given for the intersections of
this angle with the primer slot or bottom of the recess. Such
dimensions are unnecessary and could not be measured directly
in any event. The dimensions given locate this surface defi-
nitely and completely. It will be necessary for the manufacturer
to compute the diameters on this countersink to suit his own
ENDS GROUND SQUARE
6 COILS NO. 10 MUSIC WIRE
No.5 44-U.S.Form-R.H.
Pitch Dia. 0.1102 +$&''
Core Dia. 0.0955 +0.000;;
Machinery
Fig. 9. Locking Latch, Spring and Pin
particular needs. This surface must be smooth and will require
a careful finishing operation.
No tolerances are given on any of the angles or radii because
none are needed. Sufficient variation on these surfaces is per-
mitted by the tolerances given on other dimensions.
Drawing of Locking Latch, Spring, and Pin. The surfaces of
the locking latch (Fig. 9) must be smooth, as they bear on the
sides of the slot in the container. This part has a tolerance of
minus o.oi inch on the entire contour. This means that the
COMPONENT DRAWINGS
95
S3 I 38 33
e>o - d oo oo
+ 1 +1+1 +1
96 INTERCHANGEABLE MANUFACTURING
piece may vary o.oi inch normal to the profile at any point in
the direction that will make the piece smaller, or, in other words,
any variation from the normal dimensions must remove more
metal. The diameter of the pin hole (0.092 inch, plus 0.004,
minus o.ooo) corresponds with the pin hole in the container.
The diameter of the spring hole (0.175 inch, plus o.oi, minus
o.oo) also corresponds with the spring hole in the container.
The locking latch spring (Fig. 9) is a part of minor importance.
It is made of No. 10 music wire, and no tolerance is specified for
its diameter. This means that commercial music wire bought
in the open market will be satisfactory. No difficulty should
be experienced in maintaining the limits given.
The thread of the locking latch pin is a No. 5-44 U. S. form.
This is a standard A.S.M.E. thread, and dies should be avail-
able in stock at any reliable die manufacturer's. After this
pin is assembled into the container, the end thread of the tapped
hole in the container should be upset slightly with a punch to
prevent this pin from falling out. It should be understood that
it is permissible to bevel at both ends of the thread to facilitate
the threading. The surface of the stem forms a bearing for the
latch and should receive a finish cut. This part should be com-
pleted in a single operation on a screw machine.
Drawing of the Housing. The outside diameter of the hous-
ing (Fig. 10) is 2.638 inches, plus o.oo, minus o.oi. This is an
atmospheric fit and requires no finishing cut, which also applies
to the shoulder, the diameter of which is 1.925 inches plus o.oo,
minus o.oi. The Whitworth thread, the full or major diameter
of which is 2.100 inches, plus 0.012, minus o.ooo, must assemble
on the hinged collar (Fig. n). The Whitworth form of thread
is used to suit other types of firing mechanisms now in service;
otherwise the U. S. form of thread would be preferable.
The counterbore in the front end (diameter 1.502 inches,
plus 0.02, minus o.oo) must clear the spindle plug. No finish-
ing cut is required. The counterbore in the rear end of the same
size must clear the flange on the container. The minimum
clearance is 0.007 i ncri an d the maximum clearance, 0.047 inch.
No finish cut is required. The full or major diameter of the con-
COMPONENT DRAWINGS
97
tamer thread is 1.425 inches, plus 0.006, minus o.ooo. A sector
of this thread is removed to permit the assembly of the con-
tainer. All of these surfaces require finishing cuts. The rear
flank is the bearing flank and the most essential. The re-
quirements were previously given in connection with the firing
mechanism container. The front face of the housing should be
reasonably smooth, as it seats against the hinged collar and is
the most important working point for other machining operations.
The depth of the Whitworth thread is 0.80 inch, plus 0.02,
minus o.oo. The hole must be deep enough for the hinged collar,
0.160
Machinery
Fig. 11. Hinged Collar assembled
and enough threads must be maintained to hold the firing
mechanism in position during the firing of the gun. This thread
is subjected to a considerable strain at this time. The depth
of the counterbore in the front end is 1.08 inches, plus 0.02,
minus o.oo. This surface must clear the end of the spindle plug.
No finish cut is required.
The location of the thread for the container is 1.987 inches,
plus 0.005, minus o.ooo from the front end. This is an impor-
tant functional dimension and must be carefully watched, as
it controls the angular position of the firing mechanism when
98 INTERCHANGEABLE MANUFACTURING
it is screwed home. A minus variation on this dimension would
prevent the mechanism from locking. A slot is shown in the
lower right-hand side of the housing for the safety mechanism.
The safety bar should be a very free fit in this slot. The slot
is 1.59 inches, plus o.oi, minus o.oi from the front end; the
length, 0.75 inch, plus 0.02, minus o.oo; and the width, 0.375
inch, plus 0.02, minus o.oo. The surface in the slot should be
reasonably smooth. A tapped hole, having a full or major
diameter of 0.50 inch, plus o.oi, minus o.oo, is shown in the
bottom of the housing for the firing mechanism pin. It is per-
missible to run the tap drill below the thread, provided that
this drill does not break through into the hole in the center of
the housing.
A recess is milled in the rear face of the housing to engage
the locking latch. This recess allows for a possible variation
in the locked position of the container of 90 degrees. The toler-
ances given on the various controlling dimensions will permit a
variation of approximately 30 degrees. The variations on the
primer plug and the primer will permit approximately 30 degrees
more. This leaves the remaining 30 degrees to allow for wear.
The depth of the recess is o.io inch, plus 0.02, minus o.oo and
the ends of the recess are located 0.15 inch, plus o.oi, minus
o.oo from the center lines through the rear face, 90 degrees
apart.
The hole and counterbores for the collar-catch (shown in
Fig. 12) are located from the center of the housing because these
holes will be drilled in a jig which should locate the housing
centrally. The counterbores are 1.088 inches, plus o.ooo, minus
0.005, and the hole, 1.183 inches, plus 0.005, minus o.ooo from
the center line (see end view Fig. 10). The diameter of the
hole is o.i 6 inch, plus 0.006, minus o.ooo. This hole should be
a free sliding fit for the stem of the collar-catch. The mini-
mum clearance is 0.002, and the maximum clearance, 0.014 inch.
The full or major diameter of the tapped hole is 0.375 inch,
plus 0.008, minus o.ooo, and the pitch diameter, 0.3417 inch,
plus 0.004, minus o.ooo. No core or minor diameter is given,
as the small counterbore, which is a few thousandths inch larger
COMPONENT DRAWINGS
99
then the theoretical core diameter, limits the height of the
threads. The V-form of thread is used to obtain as great an
area of contact as possible. After assembly of the collar-catch
screw, the metal should be upset slightly with a cold chisel into
the slot of the screw to prevent disassembling. The bottom of
the counterbore is 0.125 inch, plus 0.02, minus o.oo from the
rear face of the body. This counterbore receives the head of
the collar-catch screw and also forms a groove in the stem
which provides clearance for drilling and tapping.
The width of the thread for the container is 0.41 inch, plus
o.oi, minus o.oo. A 45-degree bevel (0.03 inch, plus 0.02, minus
o.oo wide) is required on the corner of this thread to facilitate
TOLERANCE 'n/ AS SHOWN
Machinery
Fig. 12. Collar-catch
the insertion of the container. This bevel may be made with a
rile; the tolerance should be great enough to cover this method
of manufacture. The ends of the thread sector are located from
the 45-degree center line of the housing (see end view) at 0.02
inch, plus 0.02, minus o.oo. This sector is at an angle so as to
always insure a minimum contact on this thread of 135 degrees.
No tolerances are given on the various angles and radii, as none
is required.
Drawing of Hinged Collar. The hinged collars and housings
are not interchangeable and must be furnished in pairs. To
make these parts interchangeable and insure that the housing
would be screwed tightly against the shoulder on the hinged
collar when the locking holes in each part were in correct align-
IOO INTERCHANGEABLE MANUFACTURING
ment, would require very expensive manufacturing methods.
In such a case, the position of the start of the Whitworth thread
in the housing would have to be held very closely in relation to
the position of the locking hole. The same would be true on
the hinged collar. Some variation must of necessity be allowed.
This would introduce a further variation longitudinally of the
position of the firing mechanism. The effect of such a variation
would be an additional angular variation in the locked position
of the mechanism. If the original pairs of housings and hinged
collars become separated, an additional locking hole will have
to be drilled in the flange of the collar, Fig. n, transferring it
from the housing which is to be used. The diameter of the lock-
ing hole is 0.160 inch, plus 0.006, minus o.ooo. It should be
drilled in one operation by using its companion housing as a jig.
Drawing of Collar-catch. For convenience of manufacture,
the collar-catch (see Fig. 12) is made in two parts which are
permanently assembled. The stem and the finger piece may
be made interchangeable, or a system of selective assembly may
be employed. This is matter to be determined by the manu-
facturer to suit his own convenience. Therefore no tolerances
will be given on the dimensions of the riveted end. The stem
must be a snug fit in the finger piece, and the two parts must be
solid after riveting. Any parts made within o.oi inch of the
nominal dimensions and which meet the above conditions will
be acceptable. The rear face of the finger piece will be finished
after riveting.
As the diameter of the stem is 0.158 inch, plus o.ooo, minus
0.006, it should be possible to secure drill rod well within these
limits; no further machining will be required on this surface.
The length of the stem is of minor importance. The surface left
by the cutting-off tool will be satisfactory. The length of the
finger piece (0.525 inch) is an atmospheric fit; no tolerances are
given, because the note in regard to the profile gives a tolerance
of minus 0.04 inch.
The thickness of the flange (0.125 inch plus o.oo, minus o.oi)
is of minor importance; hence the flange can be completed in
a single operation after the stem is riveted. The width of the
COMPONENT DRAWINGS
101
flange (0.245 inch, plus o.oo, minus o.oi) must be free in the
slot in the housing. The minimum clearance is 0.005 mcn an d
the maximum clearance, 0.035 inch.
The dimensions of the profile of the finger piece are given
without tolerances, but a note is added: "Tolerance plus o.oo,
minus 0.02, as shown by dotted line." The entire upper part of
the finger piece is an atmospheric fit. It must be reasonably
smooth because the finger operates this part. The note permits
a minus variation of 0.02 on this profile where no other toler-
ances are given. This variation is measured as normal to the
profile at any point. If a clean drop-forging is secured, this
!
a
>0 + |
0.04 R // ^-1 5t-
-0.04 R .*en *H
,77 \
1 \_
y-T-
0.30lo.oo;;
*A / ^i f\
S^ ^r=i:t^-^
arhincry
.frig. 13. Firing Pin Guide Spring and Firing Pin
surface may be finished by removing all rough scale, flash, and
other rough spots with a file or on a polishing wheel. The con-
tour of this surface is not important enough to require expen-
sive form-milling cuts.
Drawing of Firing-pin Guide Spring and Firing Pin. The
diameter of the wire for the firing-pin guide spring (Fig. 13) is
0.04 inch, plus o.ooo, minus 0.002 ; the outside diameter of the
coils, 0.464 inch, plus o.oo, minus o.oi; and the free height, 0.69
inch, plus 0.02, minus o.oo. These limits should be readily
maintained under normal manufacturing conditions.
The firing-pin flange is 0.582 inch in diameter, plus o.ooo,
minus 0.003. This surface must be a free sliding fit in the
102
INTERCHANGEABLE MANUFACTURING
firing-pin guide. The surface will require a careful finishing cut.
The diameter of the front end (0.117 inch, plus o.ooo, minus
0.003) must be a free sliding fit in the firing pin guide. This
surface requires a careful finishing cut.
The surface of the rear end clears the hole in the container
by 0.050 inch and requires no finishing cut. The diameter of
the large end of the taper is an unimportant clearance surface.
The taper is provided to strengthen the end of the firing pin.
No finishing cut is required on this tapered surface. The length
over all is an important functional dimension and, in part,
26 V-Thds. per in. R.H.
Pitch Dia. 0.3417'g;9$>,'/'
Core Dia. 0.3804'+ J}-{gjj
Dia.of Undercut 0.3804' t-9?o:
3 COILS NO. 15 MUSIC WIRE
MacMnerv
Fig. 14. Collar-catch Screw and Spring
controls the force of the blow on the primer. The front face of
the pin must be as smooth as possible, and a polished surface is
desirable. The rear face should be as smooth as a careful finish-
ing cut will leave it.
The location of the rear face of the flange (0.525 inch, plus
o.ooo, minus 0.003) * s important, as this dimension controls
the location of the end of the firing pin. The surface should
receive a finishing cut. The distance to the front face of the
flange is an important functional dimension which controls
the protrusion of the firing pin, as noted in connection with the
firing-pin guide. A finishing cut is required on this surface. The
COMPONENT DRAWINGS 103
location of the beginning of the taper is relatively unimportant.
This dimension maintains clearance with the bottom of the
counterbore in the firing-pin guide.
No tolerances are given for the radii because none are needed.
Attention is called, however, to the radius of 0.04 inch at the
front end. This must not be exceeded. The purpose of this
radius is to remove the sharp corner, but care must be taken to
remove as little material as possible.
Drawing of Collar-catch Screw and Spring. The collar-
catch screw is shown in Fig. 14. The diameter of the head must
enter the counterbore in the housing. The thread, which is a
sharp V-form, must assemble into the tapped hole in the hous-
ing. Sufficient threads must be secured to hold the screw in
position. It is permissible to bevel both under-cut and end to
facilitate threading. This screw should be completed in a
single operation on a screw machine.
No. 15 music wire is specified for the collar-catch spring.
Commercial wire of this number will be satisfactory. The func-
tion of this spring is to hold the collar catch in its locked posi-
tion. The limits given should be maintained readily under
normal manufacturing conditions.
All dimensions and tolerances given on these drawings repre-
sent limit gage sizes. If a hole is given as 1.25 inches, plus o.oi,
minus o.oo, this means that the hole must be made so that a plug
gage 1.25 inches in diameter will always enter, while a plug
gage 1.26 inches in diameter will not. In general, the extent of
the tolerances allowed on any surface is a good index of the char-
acter of the finish required. All burrs, fins, etc., and unneces-
sary sharp corners must be removed. All cuts and surfaces,
whether rough or finished, must show no evidence of careless-
ness. All cuts must be made with clean and sharp tools. Gouges,
tears, and unnecessary scratches produced by dull or improper
tools and careless workmanship or carelsss handling should be
sufficient cause for rejection.
Unless noted otherwise, common manufacturing practices,
such as under-cutting and beveling for threads, extending the
tap drill a reasonable amount below the threads in tapped holes,
104 INTERCHANGEABLE MANUFACTURING
countersinking to guide the tap, providing reasonable grinding
clearances where necessary, burr-beveling corners on screw
machine parts, etc., are permissible. Whenever any differences
exist between the dimensions and tolerances expressed on the
drawing and the above specifications, the figures on the drawing
should be used. The dimensions and tolerances given on the
component drawings should be strictly maintained. If modi-
fications are possible which will relieve the situation, they should
be made. No deviations from the specified requirements are
permissible, however, until definite modifications are authorized.
CHAPTER VII
ECONOMICAL PRODUCTION
WHEN certain manufacturing methods are to be decided upon,
the decision made in this connection should be recorded, to-
gether with the reasons for it. This practice tends to eliminate
many expensive, unnecessary refinements which are often arbi-
trarily specified, because, instead of baldly specifying the vari-
ous requirements, the necessity for adding sufficient reasons
therefor demands a careful analysis of the mechanism and its
purpose; and a careful analysis of almost any mechanism will
soon make it apparent that only a small proportion of the di-
mensions and other requirements are exacting. This and many
other subjects bearing upon the attainment of economical prac-
tice in interchangeable manufacturing are dealt with in the
present chapter.
Principal Elements in Economical Production. There are
three principal elements in the economical and successful pro-
duction of a commodity. Stated briefly, they are as follows:
(i) A thorough knowledge of the object (function) of the article
and of all the conditions essential in attaining it. (2) The de-
velopment of manufacturing methods and facilities that will
most economically produce a satisfactory product. (3) The
development of testing methods and apparatus to determine in
an economical manner, at any stage, whether or not the desired
results are being achieved.
Duplication of work never results in economy. Therefore,
a record should be made of any solution reached in regard to
these questions. Almost every problem has more than one
satisfactory method of solution. The multiplication of solu-
tions, however, particularly in manufacturing, is a hindrance
to team work. For example, if the foreman of one department
uses one solution of a problem, while the foreman of another
department who performs succeeding operations on the same or
105
106 INTERCHANGEABLE MANUFACTURING
companion parts arrives independently at another solution and
uses it, the final results may be chaos; whereas, if each solution
is recorded, whenever or wherever made, an opportunity is
created to check these solutions against each other, thus making
possible the elimination of inconsistencies at an early stage of
the work. This practice will aid greatly in promoting team-
work, and thereby eliminate many misunderstandings.
Specifications. Specifications, in their broadest sense, in-
clude the solutions of all the three problems mentioned. This
information may be compiled and recorded in one place or it
may be scattered throughout the plant. In general, if the entire
control of the design and manufacture of a commodity is held
in one plant, the compiling and assembling of much of this
information may be of doubtful value. As long as it is on record
somewhere and available when needed, that' is sufficient. On
the other hand, if the control of the design rests with one organi-
zation, the control of the production with another, while the
control of the final inspection is distinct from either of the two
foregoing establishments, reasonably complete specifications are
imperative if economical and expeditious production is to be
obtained.
Specifications thus defined include component drawings.
For purposes of discussion, however, component drawings and
specifications will be considered as distinct. In this case the
specifications are supplementary to the component drawings
and include all information which is not given on these drawings.
Function and Essential Requirements of Product. The com-
ponent drawings consist of pictures of the parts, statements of
the physical dimensions required, and usually specifications of
the material to be employed. By themselves they only partially
solve the first of the major problems noted previously. They
tell little or nothing of the object of the commodity. They state
requirements, but give no reasons therefor. Thus, the first
function of the specifications is to state briefly the purpose of
the mechanism and its functional requirements. The preceding
chapter, " Practice in Making Component Drawings," indicates
the lines which specifications should follow.
ECONOMICAL PRODUCTION 107
A second function of the specifications is to indicate the
quality of workmanship desired. The extent of the tolerances
given on the drawings indicates, to a certain degree, the proper
character of the finished surfaces. The specifications should
supplement this information by stating not only desired results,
but also reasons therefor. The preceding chapter previously
referred to illustrates this practice. To a certain extent, per-
haps, many of the conditions discussed there are so obvious as
to need no mention, yet no harm is done by being explicit.
Another subject to be included in the specifications is the
matter of the materials to be employed, and their nature, com-
position, and ultimate use. When standardized material is
used, this can be called for directly on the component drawing,
together with the proper heat-treatment. It is of interest to
note that the Society of Automotive Engineers has done much
valuable work in establishing standard specifications and
methods of heat- treatment for nearly every kind of material
used in automobile construction. The adoption of such stand-
ards greatly simplifies the provision of proper component draw-
ings and specifications. In those cases where standardized
material cannot be used, the specifications should give all perti-
nent information to enable the proper material to be secured.
For preservative finishes the drawings or specifications should
give complete information as to nature, need, and use.
When the component drawings have been completed and the
specifications have reached this stage, the first important ele-
ments of the work are established. Until this is accomplished,
those responsible for the manufacturing design have not done
all in their power to secure the economical production of dupli-
cate mechanisms in large quantities. Undoubtedly, many
minor revisions will be required before the proper solutions of
the succeeding problems will be found. But without the fore-
going information, such revisions cannot be made intelligently.
Furthermore, this information, in most cases, will point the
way to a simple and direct solution of the succeeding problems.
"Well begun is half done" was never nearer the truth than in
connection with interchangeable manufacturing.
108 INTERCHANGEABLE MANUFACTURING
Specific Manufacturing Data. We will now consider the
solution of the second major problem the development of
suitable manufacturing methods and facilities. The first step
to be taken is to make up the operation lists for every compo-
nent, noting in detail the type of machine, fixture, and tool
required. It is of great assistance in many cases to develop
concurrently the operation drawings, indicating on them the
work to be performed at each operation. In addition, it is a
good plan to include in this part of the work an estimate of the
production time on each operation. This information is neces-
sary to establish the amount of equipment required and also to
make a comparison, when desired, of the economy of several
methods.
This information should be revised and kept up-to-date after
production is under way. This furnishes valuable data for
estimating on new commodities and facilitates comparison be-
tween the costs of different methods of manufacturing. Such
information is invaluable when it becomes necessary to call on
outside plants to assist in obtaining greater production. It
need not be bound together with other parts of the specifica-
tions, but it should be in such shape that it can be quickly
found and readily applied. The foregoing information serves
as the basis for designing the special manufacturing equipment
necessary as well as for arranging the manufacturing depart-
ments so that the component parts can be produced rapidly and
efficiently.
This second problem is seldom or never fully solved. Im-
proved methods are being devised constantly, and these intro-
duce new factors into old problems. Even greater care must
be exercised in adopting a new method on work already in
process of production than is required in adopting the original
methods, because in these cases, ultimate economy requires
that such changes result in a saving which will pay for the dis-
carded equipment as well as for the new. The effect of a possible
interruption in production must also be carefully considered.
The production records of the manufacturing equipment
should be so kept that it will be always possible to trace back
ECONOMICAL PRODUCTION 1 09
through every change in equipment and make direct compari-
sons between the results obtained by each method. At the
same time, these records should be so simple as not to entail
unnecessary clerical expense. Whenever a change is made, the
reason for making it should be on record. All these data furnish
information which cannot be secured in any other way. As a
matter of fact, many plants keep a complete record of changes,
but do not provide this class of information when the produc-
tion of an entirely new mechanism is undertaken.
General Manufacturing Data. In addition to the specific in-
formation required for each individual part and each assembled
mechanism, there is a vast amount of general data which must
be had before decisions as to the economy of different methods
of manufacture can be made with certainty. Much of this
information should be available from the cost department
records, but, in most cases, these records are kept merely for
accounting purposes and their use as engineering data often
gives incorrect results.
Factory Cost of Production. It is not the purpose to outline
here a new system of cost-accounting. A discussion of some of
the factors entering into the factory costs of production, how-
ever, is necessary to indicate the character of the information
needed to promote economical production. For the purpose of
simpler accounting, it is often customary to prorate the entire
amount of indirect or overhead charges against the total output
of the plant, distributing them according to the direct labor
costs. From the accountant's viewpoint, this method is correct.
If the product of the plant consists of one simple specialized
article, such a method of accounting undoubtedly gives suffi-
cient data for general engineering purposes. On the other
hand, if the products are varied, or if the productive operations
are subdivided into elementary operations, performed in vari-
ous departments, the data so collected are incomplete and mis-
leading for engineering purposes, because the direct labor cost
alone will be the determining factor in selecting the apparent
economical methods of production. As a matter of fact, this
direct labor cost is but a small percentage of the total cost of
IIO INTERCHANGEABLE MANUFACTURING
production. It seldom amounts to 25 per cent. Furthermore,
as the volume of business increases, the percentage of direct
labor charges decreases. Thus, as the quantity of production
increases, the data so obtained become more and more unreliable.
Another method of distributing the indirect expense consists
in establishing overhead rates for each department, prorating
these charges in proportion to the direct labor cost as before.
If the departments are arranged to contain only one type of
equipment, and to perform similar operations, the data so ob-
tained are valuable, but such an arrangement of machines and
operations is seldom possible or desirable. Different types of
work creep into a department. When this condition exists, the
information obtained from the use of a departmental overhead
will again lead to false conclusions. Such a condition will
cause manufacturing methods that are not economical to be
accepted.
Certain types of equipment are always duplicated to some
extent in several departments. All other things being equal,
the cost of duplicate operations on duplicate equipment is
identical regardless of the physical location in the plant. But
with the use of departmental overhead charges, the book costs
will show otherwise. For example, in one plant a sheet-metal
part required a foot-press operation between two power-press
operations. Foot presses were available in two departments:
the power-press department with an overhead charge of 150 per
cent, and a sub-assembly department in a distant part of the
factory with an overhead charge of only 50 per cent. The
original operation list assigned all three operations to the power-
press department to eliminate unnecessary trucking and transfer.
This was changed so that the second operation would be per-
formed in the other department because of the lower overhead
there. Actually, this last method cost more than the first
because of the trucking and transfers back and forth, but be-
cause the book records showed a higher cost for the first method,
it was disapproved despite all arguments. This is not an ex-
treme case. Similar conditions exist in the majority of manu-
facturing plants.
ECONOMICAL PRODUCTION III
It is realized that book costs and actual costs are not identical.
To obtain such accuracy would entail a system so complex and
elaborate that its cost alone would overbalance all other ex-
penses. Yet some simple way must be found to give more nearly
true costs of production in order to promote true economy of
manufacture. The direct labor and direct material costs are
readily obtained. Most accounting methods apply these
charges directly against the individual parts, which is the
proper distribution. But this, in most cases, disposes of less
than half of the total cost of production. Indirect expenses are
not only the most difficult to distribute equally, but also involve
the larger amount of the costs. The total amount of these
charges can be easily determined. This is purely a matter of
bookkeeping. Their equitable distribution, however, is more
an engineering than an accounting problem.
Distribution of Indirect Factory Expenses. There are three
main factors to which most of these indirect expenses can be
logically applied: First, the direct labor; second, the general
productive equipment; third, the component parts themselves.
There are also a number of other indirect expenses which must
be charged to the general factory expense. As these are rela-
tively few, they can be arbitrarily distributed over the entire
product without affecting the value of the data sought. Even-
tually, of course, the product must carry them all. The great
problem is to distribute them simply and properly.
If the attempt is made to apply all indirect charges to any
one of the above factors, many economic errors will result.
Attempts have been made to carry them all on the direct labor
factor with far from satisfactory results. Neither can they all
be applied to the equipment factor with any better results.
Each factor must bear only its own indirect expenses. In order
to determine where each indirect expense belongs, a process of
elimination should be adopted. Without such a factor, would
this expense exist? If the expense remains after direct labor,
general productive equipment, and individual components are
eliminated, it belongs to the general factory expenses or factory
management.
112 INTERCHANGEABLE MANUFACTURING
Expenses Due to Direct Labor. Let us first consider the
indirect expenses due to direct labor. Hereafter, for the sake
of brevity, direct labor will be referred to as labor.
Cost of Supervision. One charge against labor is the cost
of supervision as represented by the salary of the foremen and
their assistants. The number of these depends chiefly on the
number of men to be controlled. This charge could be prorated
against the number of men employed. Such a method might,
in extreme cases, be erroneous because the higher-priced men
should require less supervision. Although, in such cases, it
might be more logical to proportion this charge in an inverse
ratio to the wages of the men, the clerical work necessary to
accomplish this would cost more than the information would
be worth.
Making up Payroll. The cost of time-keeping and making
up the payroll logically belongs to the labor factor. Eliminate
labor and there is no payroll to make up. This should also be
distributed on the basis of the number of men employed, as it
costs as much to make up the pay account of a man getting
fifteen dollars a week as it does to make up the pay account of
a man getting thirty dollars. Here again, the indirect expense
of high-priced labor is proportionately less than that of lower-
priced labor.
Employment Department. The cost of the employment de-
partment also belongs to the labor factor. Eliminate labor and
there is no further need of an employment bureau. These
charges should also be distributed on the basis of the number of
men employed, for it costs as much to hire one man as another.
In many cases the higher-priced men are the most reliable
not always, of course, as so many other conditions enter into
this and thus it is possible that a closer result would be
obtained by distributing the cost in an inverse proportion to
the wages of the men.
Educational Department. Wherever personnel or educational
departments are established or other similar departments the
objects of which are to promote cooperation between the em-
ployer and employe to their mutual advantage, all expense
ECONOMICAL PRODUCTION 113
incurred should be charged against labor. It is extremely diffi-
cult to analyze such charges accurately so much depends upon
the nature of the activities of such departments and upon the
character of the persons with whom they deal. Therefore it
might be best to distribute these charges also on the basis of
the number of men employed so as to simplify the accounting
by prorating all labor charges in a uniform manner.
Maintaining Health and Safety of Employes. All charges for
installing safety devices, fire escapes, improved sanitary equip-
ment, for heating and lighting, and other similar expenses neces-
sary for maintaining the health and safety of the employes as
required by law or promoted by the dictates of humanity, belong
to the labor factor. However, some of these expenses might be
included in other general items fire escapes may be included
in the cost of the buildings, for example and it may not be
possible or feasible to isolate them. Those few cases where it
is not practicable to apply expenses directly where they logi-
cally belong will not affect the values of the final data much.
It should offer no great accounting difficulties to isolate the
majority of the expenses enumerated and all other kindred
items. This is all that the accountant would necessarily do.
The total amount so determined could be prorated on the basis
of the average number of men engaged in actual production,
and thus give the labor overhead. This would be used as a
constant for a predetermined period in a similar manner to the
usual overhead charges, and would be close enough for all prac-
tical purposes. Of course, it is evident that this labor overhead
fluctuates constantly, but as these data are for the use of engi-
neers and not accountants, an exact balance is not essential.
As a matter of fact, the use of a general overhead burden does
not give an exact balance. A comparison between estimated
results and the accountants' records will show how wise a use
has been made of this information. This direct labor overhead
is a valuable factor in determining economical methods of pro-
duction. If the general type of labor employed differed to any
great extent in the various departments, a separate labor over-
head could be established for each department.
114 INTERCHANGEABLE MANUFACTURING
Machine-hour Rate. Next will be considered the indirect
charges that belong to the general productive equipment which,
prorated on an hourly basis, will be called the machine-hour rate.
Interest on Investment, Depreciation and Insurance. The first
items are the interest on the investment, depreciation, and
insurance. These are relatively simple to determine. Their
maintenance charges belong here also. When possible, these
should be applied to the particular types of machines. The
many petty items would be distributed over the entire equip-
ment. Having the machinery, there must be a place to put it.
The majority of the factory space is utilized by productive
equipment. It would seem, therefore, that all the fixed plant
charges would be best distributed here. This could be done
proportionately to the average floor space required for operat-
ing each type of machine.
Power Charges. The power charges would also be included
in the machine-hour rate. One cannot go into the exact dis-
tribution of this factor without incurring great expense in
power tests tests that become valueless as the machining
cuts vary from light to heavy, short to long. An approximation
could be made which would determine the value of this factor
close enough for practical purposes. A few power tests could
be made to advantage for the purpose of securing data to assist
in the proper distribution of all power costs. The cost of belting
and lubricating oils, etc., could also be included in the total
power charges. Such a plan would insure an equitable and
simple method of distributing these.
Non-productive Time of Machines. There is always a certain
amount of idle time for any machine, no matter how well the
work is planned. The amount of this normal non-productive
time varies for different types of machines. For example, an
automatic screw machine as efficient a productive machine
as there is is idle on an average of one hour in five. This idle
time is caused by the necessity of adjusting tools, oiling, chang-
ing bars, etc. Therefore, the actual productive time of this
machine is 80 per cent of its total operating time. In the print-
ing trades, where a complete system of machine-hour costs has
ECONOMICAL PRODUCTION 115
been developed, 65 per cent is considered as a normal average of
productive time. The normal percentage of non-productive
time should be estimated as closely as possible and included
in the machine-hour rate.
Lack of Work or of Labor. There is, however, another source
of idle time to be considered. This may be lack of work or lack
of labor. If it is lack of labor, it should logically be charged
against the employment department. Lack of work may, of
course, be due to one of several causes. If due to lack of busi-
ness, it could be charged against the sales department, while if
due to poor planning, it belongs to the general factory expense.
For simpler accounting, abnormal idle time might best be
included in the general factory expense.
Keeping of Records. The shop office carries records of the
machinery, such as inventory lists giving the original value,
depreciation, locations, etc. The results of using machine-hour
rates would be, to all intents and purposes, the creation of a
payroll for the various machines. All this would require a
certain amount of clerical work, the cost of which should be
prorated against the machines.
The accountant would carry accounts showing only the
total amounts spent for power, belts, lubricating oil, fixed plant
charges, fixed charges on general manufacturing equipment,
labor costs for machine records, etc. These totals would be
prorated as just discussed to establish a constant machine-hour
rate for each type of equipment, which would be simple to apply.
Such values would be adjusted periodically as required. Here,
again, the successive reports of the accountant would make
apparent how wise a use had been made of the information so
gathered.
The mechanical manufacturing industries would find it well
worth their while to follow, in one respect at least, the example
of the printing trades. The problem of the actual cost of produc-
tion has been studied by them on a cooperative basis. For
several years, one of their trade journals has been collecting cost
data from many plants throughout the country. The informa-
tion so willingly given has been compiled and analyzed for the
Il6 INTERCHANGEABLE MANUFACTURING
benefit of all. As a result, a complete series of average machine-
hour costs has been developed, the use of which gives results
very close to the actual balances of the accountants. This
data is kept up to date and corrected values are distributed
periodically. This information is invaluable for estimating and
other planning purposes.
Product Overhead. The indirect expenses due to the prod-
uct itself will now be considered. The following method of
distributing indirect costs is a radical departure from any estab-
lished practice, yet their logical distribution leaves no other
path open. There are two classes of these charges; those which
can be applied to specific component parts and those which
apply to the whole product in general. For the sake of brevity,
those of the first type may be called specific product charges
and those of the second, general product charges.
Specific Product Charges. The most important specific prod-
uct charge and the one most readily isolated is that for jigs,
fixtures, special tools, and gages. These certainly belong to
specific parts. The adoption of this practice will soon develop
a valuable record. Tt will then be possible to obtain and
apply directly where it belongs the cost of making changes in
the design and methods of manufacturing of the various parts.
Then only will it be possible to determine whether or not such
changes result in an economic gain. The writer believes that
frequent changes of this sort are altogether too common in
American manufacturing practice. Judging from personal ex-
perience during the past ten years, the majority of such changes
are the results of mental laziness. When the first important
difficulty appears, the issue is avoided instead of being carried
through to its logical conclusion. It is the easiest thing in the
world to try to make a part in a different way from the one
originally planned; but by so doing one set of difficulties with
which we are somewhat familiar is merely substituted by others
with which we have had no previous experience, and, in the
end, no progress has been made. The writer feels safe in stating
that not less than seventy-five per cent of the changes made
are attempts to avoid trouble which is not eventually escaped,
ECONOMICAL PRODUCTION 1 17
and that this percentage of the cost of changes represents a
total economic loss. A method of distributing indirect expenses
along the lines indicated will expose such conditions as nothing
else will.
Another specific product charge is the cost of constructing
special machines for specific parts. This is logically included in
the jig and fixture costs. When the special machine is used on
several parts, these expenses, of course, will be classified with
other general machinery items. Its normal non-productive
time, however, is usually large.
The loss resulting from scrapped work is another specific
product charge. Some parts are delicate and difficult to ma-
chine and this results in a high percentage of scrap. Others are
simple and more rapidly produced with little or no scrap. It
is manifestly unfair to distribute this item of expense over the
product as a whole because this will give an entirely wrong idea
in regard to the economy of the simpler and sturdier designs.
Often, when the design is changed, a large number of finished
parts are scrapped or reworked. All such expenses should be
charged to specific pieces when possible. There will, of course,
be some credit items, due to salvage. If it should be difficult
and expensive to distribute these salvage credits specifically,
they could be credited to a general scrap account and applied
proportionately to the cost of the scrap charged against each
part. Any inaccuracy resulting from this procedure would have
little effect on the value of the final result.
General Product Charges. Most of the other expenses in-
curred by the product are general product charges. Among
them would be the cost of trucking, including the interest on
the investment of the trucks, elevators, etc., depreciation, and
their maintenance costs. The expenses of shop rearrangements
also belong with the general product charges, as these altera-
tions are made to facilitate production. If, however, they can
be charged directly to specific component parts, they should be
so distributed. The majority of the factory office expenses
would be included in the general product charges. This would
include the cost-keeping, production records, engineering and
Il8 INTERCHANGEABLE MANUFACTURING
experimental work, etc. All these general product charges could
be distributed in conjunction with the general factory expenses.
This method would be as equitable as any.
Clerical and Accounting Work. The clerical and accounting
work which such a procedure would entail will now be con-
sidered. First, the accountant must arrange his books so as to
separate all expenses due to direct labor. This would be only
a single account. Next he would open another account to
carry all the expenses making up the machine-hour rate of the
general productive equipment. Another account would be
opened for the indirect charges due to the product. The in-
direct charges caused by the purchase and handling of the
material have not been previously mentioned. These would be
handled in the same manner as the other indirect expenses, and
distributed over the direct material. The accountant would
carry tjiis account, and one to cover the general factory expenses
which are not distributed elsewhere. These would be all of the
indirect factory expense accounts which he would require. He
would also carry direct labor and material accounts. The sum
of these accounts would represent the factory cost of production
of the work produced during a given period. This, balanced
against the value of the output would represent the gain or loss
during the specified period. This is the essential information in
which the stockholders and directors of a plant are interested.
The factory cost department would carry independent ac-
counts along entirely different lines. These should give detailed
information in regard to the cost of each component. They
need not necessarily check exactly with the accountant's records.
The constants which are used by the factory should be so estab-
lished as to make the total of these records a small percentage
higher than the accountant's. A factor of safety of, say, 5 per
cent, should be included in the various constants. The direct
labor and material charges are the only ones which should check
absolutely in both sets of records, while all the others need
check only within the limits of the factor of safety established.
The various constants would, necessarily, be adjusted from
time to time to bring this result.
ECONOMICAL PRODUCTION 119
The factory cost department would carry an account for
each separate component, including sub-assemblies and com-
plete mechanisms. Records must be kept on each of these
accounts, showing the direct labor, direct material, machine-
hours, special equipment, and scrap charges. To this would
be added the proper direct labor overhead, direct material
overhead, machine-hour rates, general product overheads and
general factory expense. These last would be specified con-
stants. The comparison of these shop records with the account-
ants' records would prove, first, the relative accuracy of the
established constants, and, second, how effective a use had
been made of the data that was so collected.
If it were possible for several manufacturing concerns to adopt
a method of factory cost-keeping along these lines and to com-
pare from time to time, not the details but certain of the result-
ant factors, much valuable information would be secured
information which would not in the least reveal any of the con-
fidential facts of the business but would uncover many economic
truths as yet undiscovered. For example, it is of the utmost
value to determine what the normal direct labor burden should
be. The different plants involved in such an undertaking need
not be engaged in the manufacture of the same products, and
their methods of manufacturing might differ in many particu-
lars, yet from data so obtained the normal cost of this factor
could be determined closely. These plants would thus estab-
lish a standard with which to compare their costs, giving them
either the gratifying knowledge that their labor burden was
normal or else a warning of conditions that should be corrected.
The same is true in regard to the machine-hour rates. With
cooperation between a large number of plants, an accurate series
of values could be established for these rates. Accuracy or
precision in manufacturing requires established standards. Why
should not the same principle be applied to other problems?
If relative standards for machine-hour rates and direct labor
overhead are established, for example, the same progress can
be expected in these respects as has been made in mechanical
work since the establishment of physical standards. It may
120 INTERCHANGEABLE MANUFACTURING
be a long and slow process, but there is sure to be improvement
during its development. The product factors would be of little
value for general comparisons. Their nature prevents their
economic use for this purpose.
Inspection and Testing. The third major problem the
development of inspection methods and facilities should be
solved to a great extent in conjunction with the development
of the other manufacturing equipment. The inspection opera-
tions and necessary gages should appear in their proper place
on the operation lists. In some cases, these lists should be
supplemented by a more detailed description of the methods of
inspection. This is unnecessary for limit gages measuring ele-
mentary surfaces. It is required, however, in connection with
the use of functional and other gages for many composite sur-
faces. Such information should be included in the specifications
as an integral part of the description and requirements of each
component part.
Specific and General Information. It is obvious, then, that
specifications may be divided into two main divisions. The
first division contains the specific information required in the
production of particular commodities. The preceding chapter
gives a good illustration, as far as the requirements of dimen-
sions and surfaces are concerned, of the nature of specifications
of this class. Operation lists, material specifications, inspection
requirements, etc., are needed to make them complete. In
this way, the efforts of all will be directed along similar lines,
the exacting requirements will receive the greatest attention
and this they should always receive while those of lesser
importance will be treated accordingly.
The second division consists of that vast amount of general
information that is derived from cost and production records,
" traditions of the shop," and all other general data gleaned
from every possible source which applies equally to every com-
modity. This should not be duplicated in the written specifica-
tions of every individual commodity, but should be gathered
independently in usable form.
CHAPTER VIII
EQUIPMENT FOR INTERCHANGEABLE MANUFACTURING
IN the preceding chapters an idea was given of the informa-
tion required for interchangeable manufacturing, desirable lines
to be followed in obtaining these data were indicated and proper
methods of recording this information were described and illus-
trated. Even with the exercise of the greatest care in this
preliminary work, many petty details will be overlooked; but
the design and construction of the manufacturing equipment
can be carried through with expedition and confidence as soon
as this preliminary information is obtained. If time is impor-
tant, the work of designing and constructing this equipment
can be turned over to a large number of persons or to several
different plants to develop independently. It is, of course, essen-
tial that a uniform method of interpreting drawings and toler-
ances be used, and that complete operation lists be furnished.
These operation lists should map out the plan of action for
the selection or design of the equipment. Fig. i shows a form
which has proved satisfactory in service. ,The preliminary lists
should give such descriptions of the special tools and gages to
be made that their design can be readily developed. The final
lists may refer to these tools and gages by number only. If
these lists are kept up to date, they become a valuable key to
all production.
Selection of Machine Tools. In order to insure ultimate
economy, the proper choice must be made between standard or
special machine tools. The amount and type of available equip-
ment affects this decision. In general, standard equipment is
advisable, although occasionally the reverse is true. For ex-
ample, in the case of an extremely high rate of production,
special automatic machines built to serve one specific purpose
prove more economical. In other cases, a small special, single-
122
INTERCHANGEABLE MANUFACTURING
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EQUIPMENT 123
typewriters; hence, to meet this demand, new features have been
introduced on the machines, such as tabular stops, etc. The
possibility of such modifications must never be overlooked.
Another characteristic of the machine tool equipment, there-
fore, is its adaptability. All other factors being equal, that
machine which is the most adaptable to other operations should
be chosen. In this connection, it is of interest to note that
single-purpose machines are usually the more adaptable, and
furthermore, that they often prove more economical in other
Fig. 2. Early Type of Manufacturing Milling Machine
ways. For instance, the writer knows of a watch factory which
was engaged in making fuse bodies during the war. Some were
made on semi-automatic turret lathes and others were machined
on small bench lathes. The turret lathes practically completed
the parts in two operations, while more than a dozen operations
were required on the bench lathes. The parts produced on the
bench lathes were not only more nearly identical than those
produced on the turret lathes, but also cost less to manufacture.
124 INTERCHANGEABLE MANUFACTURING
The turret lathes were purchased particularly for this job while
the bench lathes had been formerly used in turning watch cases.
The machines selected must be sufficiently rigid to perform
their task. The introduction of high-speed steels for cutters
has created more severe conditions than formerly existed, and
the improvement of the cutters in this respect has caused a
great increase in the rigidity of many machine tools. It is inter-
Fig. 3. Modern Type of Manufacturing Milling Machine
esting to compare the general construction of an earlier type
of manufacturing milling machine which is shown in Fig. 2,
with that of a more recent type shown in Fig. 3.
From the production standpoint, the most important factor
of the machine tool equipment is the ease and facility with
which it can be set up, adjusted, and operated. Several suc-
cessive operations on simple single-purpose machines are often
EQUIPMENT
more economical than a single operation on a more complicated
machine. The actual production time of the latter may be
much less than that of the former, but when it is stopped for
adjustment and setting-up, the loss of production is correspond-
ingly greater. Furthermore, the multiplicity of adjustable parts
makes it necessary to stop for readjustments more frequently.
It should not be assumed from this that single-operation ma-
chines are always the most economical, because on parts hav-
ing liberal tolerances the reverse is true.
Design of Jigs and Fixtures. Jigs and fixtures are provided
to assist in machining specific surfaces on specific parts, and
171
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Machinery
Fig. 4. Drawing of Stud machined in One Operation on a Screw
Machine
their design depends to a large extent upon the design of the
parts to be machined. There are, however, a number of general
principles which apply equally- to all types of fixtures. The
holding or register points of tools and fixtures for all finishing
cuts should be identical with /those surfaces from which the
dimensions are given on the component drawings. On roughing
cuts, these holding points are of lesser importance, yet it is
good practice to maintain, as far as possible, the same register
points on both roughing and finishing cuts. The stud shown in
Fig. 4 is presented as a simple example to illustrate this prin-
ciple. It will be noted that the dimensions of length, except the
126
INTERCHANGEABLE MANUFACTURING
depth of the counterbore, are all given from one end, that the
depth of the counterbore is given from the opposite end, and
that the part is to be machined all over. This part can be
machined in a single operation on a screw machine by means of
the tools illustrated in Fig. 5.
Surface Y of the cutting-off tool A establishes the register
point for forming tool B and facing tool C. Surface X of the
forming tool is adjusted in relation to surface Y of the cutting-off
tool so that the length of the stem on the work will be as given
on the component drawing. Surface Z of the facing tool is also
adjusted in relation to surface Y of the cutting-off tool in order
E
Machinery
Fig. 5.
Diagrammatic Illustration of Tools used in machining
Stud
to maintain the proper over- all length of the piece. Counter-
bore D is provided with an adjustable stop E which registers
against the forward end of the stud. The contact surface of
this stop is adjusted in relation to surface W of the counterbore
so as to maintain the prescribed depth.
If the bottom of this counterbore were located on the com-
ponent drawing from the end of the stem, the stop on the turret
would be adjusted so that surface W of the counterbore would
keep its proper position in relation to surface Y of the cutting-off
tool. Stops on the cross-slide carrying the forming tool control
the outside diameters.
EQUIPMENT 127
The next question that arises is the proper limit at which to
set the tools. There are four main sources of variation to con-
sider: First, the errors resulting from imperfections in the
machine; second, those caused by wear on the cutting edges of
the tool; third, those due to errors in the tools; and fourth,
those caused by improper setting of the raw material in their
holding devices. In the following, only those variations which
affect the lengths of the stud will be dealt with.
Results of End Play in the Machine Spindle. Any end play
in the spindle of the machine will reduce the distance between
the surfaces machined by tools A and C, Fig. 5. If the forming
and cutting-off tools are held rigidly together, this end play
will not affect the distance between the surfaces machined by
tool A and surface X on tool B. If, however, these two tools
are independent of each other, as shown in the illustration, this
end play may cause a variation in either direction. In the first
case mentioned, a male dimension is dealt with and inaccuracies
in the action of machine tools cause a minus variation in such
a length whether the cutting tools are independent of each other,
or combined in a single tool. Similarly in the case of a female
dimension, it is evident that the variation will be plus. End
play in the machine will not affect the depth of counterbore
when the tool is self-registering as shown; thus a self -registering
tool often produces more accurate results than one controlled
by stops on the machine itself.
Wear on the cutting edges of tools A and C will increase the
distance between surfaces Y and Z, but if the wear on surfaces
Y and X is uniform, no variation will develop from this source.
If it is not uniform, the variation may be in either direction.
As surface W becomes worn, the distance between this surface
and the contact surface of stop E decreases, that is, of course,
assuming that there is no appreciable wear on the contact sur-
face of the stop.
Considering only the first two factors, that is, imperfections
in the machines and wear on the cutting edges of the tools, the
original setting of facing tool C should be as near to the mini-
mum limit as the accuracy of the machine permits; the setting
128 INTERCHANGEABLE MANUFACTURING
of forming tool B should be at the mean dimension until actual
practice demonstrates a tendency to vary more in one direc-
tion than the other; while the setting of stop E on counterbore
D should be as near to the maximum limit as possible. This
will give the maximum time between readjustments. The con-
ditions created by 'errors in the tools themselves will be dealt
with later in this chapter.
Results of Wear on Cutting Edges of Tools. It is thus evi-
dent that the effect of imperfections in the operation of machine
tools, when the cutting tools are located by stops on the machine,
is to cause a plus variation on female dimensions, a minus varia-
tion on male dimensions, and a plus and minus variation on
neuter dimensions, such as the horizontal distance between
surfaces Y and X, which cannot be strictly classified as either
male or female. In other words, imperfections in machine tools,
such as end play, etc., cause additional metal to be removed.
In a similar manner, the wear on the cutting edges of the tools
causes a minus variation on female dimensions, a plus varia-
tion on male dimensions, and a plus or minus variation on neuter
dimensions. In the last case, if the cutting edge of one tool
consistently wears faster than that of the other, the variation
will run in only one direction. Assuming that surface Y wears
faster than surface X, the variation will be plus. If the wear
is equal, no variations develop from this cause. In other words,
the wear on the cutting edges of the tools usually causes a varia-
tion in the reverse direction from that caused through the
presence of imperfections in the machine tools.
The same principles apply equally to all types of operations
such as turning, milling, planing, grinding, profiling, shaping,
boring, etc. In order to establish the proper dimensions for
the holding and registering points on tools and fixtures, it is
necessary to analyze each particular surface carefully and pro-
ceed accordingly.
In the above example, errors due to the improper setting of
the stock in the holding device are not present. In the case
of milling, drilling and other similar operations, where the parts
are handled singly instead of being machined from bar stock,
EQUIPMENT I2Q
this is one of the most serious problems. Chips are apt to re-
main on the locating points or the operator may fail at times
to seat the piece properly before clamping. In the case of drill-
ing, this results in the improper location of the holes. In the
case of other operations, it will result in removing additional
stock. Proper training of the operator is the only cure for this
fault. Correctly designed fixtures greatly reduce the chances
of these errors, yet, as in other matters, it is not possible to
make everything so that it will be entirely fool-proof.
Important Factors in Designing Fixtures. The second im-
portant factor of the design of fixtures is their operation in serv-
ice. The selection of the proper locating points controls in a
large measure the uniformity of the product. The facility with
which these fixtures may be operated determines to a great
extent the rate of production. The direct labor cost of produc-
tion is also greatly reduced with quick-acting jigs and fixtures.
On the other hand, such equipment is often very expensive, and
therefore the total output involved determines the amount of
money which may be economically expended on the equipment.
In the case of a very small total output, little or no special equip-
ment need be provided.
When continuous production is involved, every effort should
be made in designing the equipment to have it operate rapidly.
It should operate easily and with a single motion of the opera-
tor's hand if possible. Second, the fixture should open so that
the part to be removed is accessible. Third, the position of
such openings in relation to the cutting tools should be such
that there is no danger to the operator. Whenever the operator
is required to place his hand close to the cutting tool, he nor-
mally moves slowly and cautiously, thus reducing the rate of
production. Fourth, all exposed sharp corners and edges on
the fixtures should be eliminated to prevent injury to the opera-
tor. Fifth, the locating points should be accessible, to facilitate
cleaning and the proper insertion of the work. Sixth, liberal
chip clearances should be provided to facilitate cleaning the
fixture and also to prevent marring the machined surfaces of
the parts under the process of manufacture.
130 INTERCHANGEABLE MANUFACTURING
Examples of Efficient Fixture Design. Except for relatively
large parts, careful study will make apparent a simple and
effective means of clamping the work with a single motion of
the operating handle. Loose nuts and screws should be avoided
for clamping purposes whenever possible, as these are very slow
to operate. A milling fixture which requires only a single
motion to clamp is shown in Fig. 6. An assembly view of this
Fig. 6. Design of Milling Fixture in which Three Pieces of Work
are clamped by a Single Motion
fixture is shown in Fig. 7, and the details of its construction
will be understood by reference to this illustration.
This fixture is used when milling the bottom, right- and left-
hand sides of a small forging in a single operation. The forging
is first placed in the position indicated at A by dot-and-dash
lines, where one side is milled. This piece is then advanced to
position B, where the opposite side is milled. It is then inserted
at C in which position the bottom is milled. In actual opera-
tion, when the original piece is placed in position B, a second
EQUIPMENT
piece is placed at A, and when the original piece is moved to
C } the second piece is moved to B and a third piece is placed
at A. Thus three pieces are being machined at the same time.
All three parts are clamped by one motion of the lever, D and E
being fixed jaws and F and G clamping jaws. The cam which
is attached to the operating lever draws back the jaw F and
pushes forward the jaw G. The jaw F is designed so that it can
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Machinery
Fig. 7.
Assembly Drawing of the Milling Fixture illustrated in
Fig. 6
rock, thus securing the parts at A and B with equal pressure
regardless of the variations in size between the two pieces. This
particular construction permits clamping in three places simul-
taneously with a single motion of the operating lever. Modifica-
tions may be made in this design which will permit clamping in
three or more different directions when this is desirable or
necessary.
132
INTERCHANGEABLE MANUFACTURING
Inaccessibility is one of the most common faults found with
jigs and fixtures. Sufficient clearance should always be allowed
to enable the operator to remove the completed work readily
and also to insert a new part and hold it in position while clamp-
ing. Careful attention to this point will greatly promote rapid
production. Fig. 8 .shows a spline milling fixture which affords
a simple example of this point. The construction of this fixture
is shown in Fig. 9. Two pieces are held at one time, the pieces
Fig. 8.
Machine provided with Fixture used in Spline Milling in
which the Work is Readily Accessible
being clamped in position by a leaf. It will be noted that each
piece is clamped in two places by a single operating lever. The
leaf is hinged so that it may be thrown back out of the way when
the work is being removed or inserted. The rapidity with which
this can be accomplished is a means of increasing the production.
Protection from Cutting Tools and Sharp Corners. In many
cases, standard machine tools are so designed that the table is well
away from the cutting tool in its loading position. In the case of
EQUIPMENT
133
drill jigs, the operator withdraws them from beneath the spindle of
the machine before he unloads them. Sometimes, however, the
design of the machine tool or fixture is such that the fixture cannot
be removed from under the cutting tools. In such cases, if the fix-
ture cannot be designed to open on the side away from the cutting
tool, provision should be made for rolling or rocking the fixture
away from the tool. If this cannot be done, some safety device
Machinery
Fig. 9. Construction of Spline Milling Fixture
should be designed which will stop the cutting tool and prevent it
from starting while the operator's hand is in any position of danger.
Safety guards and automatic stopping devices on punch presses
are good examples.
It is evident that the operator will handle a jig or fixture without
sharp edges much more quickly and surely than one on which he
is continually tearing his hands. It is the standard practice of
134 INTERCHANGEABLE MANUFACTURING
most .tool-rooms to remove all such corners carefully, whether
the drawing of the fixtures specifies it or not.
Accessibility of Locating Points. The locating points in the
jigs and fixtures should be so placed that they can be readily
reached. This not only facilitates the cleaning of the fixture
but enables it to be more accurately made. It also allows any
necessary corrections due to wear or change in dimensions to
be quickly and economically made. These locating points
should stand clear and be as nearly self-cleaning, as regards
chips and dirt, as it is possible to make them.
Fig. 10. Jig designed to permit Easy Removal of Chips during
Various Operations on the Same Piece
Necessity for Proper Chip Clearances. Careful considera-
tion should always be given to the provision of suitable chip
clearances. If this point is neglected, the operator will often
spend more time in removing the chips from the fixture than
he does on any other operation. On the other hand, if he does
not remove them, inaccurate work will result. Many ingenious
devices have been developed for cleaning. A jet of compressed
air or a stream of oil or soda water properly directed often
accomplishes this task quickly and well, and this demands that
EQUIPMENT
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136 INTERCHANGEABLE MANUFACTURING
obtaining chip clearances. The construction drawing of this
fixture is shown in Fig. n. The part to be drilled is located in
one place by the tapered tongue B shown in section A- A, and
is clamped by a leaf. The body of the jig is a skeleton frame
which contains no pockets to catch the chips. The leaf swings
open, thus allowing all chips to be easily brushed or blown out.
Results Obtained with Proper Chip Clearances. The follow-
ing example shows the results actually obtained in production
by attention to these details. A certain firm contracted to
make 200,000 small pinions as shown in Fig. 12. It will be
noted that three of the six teeth are partly removed. The
Machinery
Fig. 12. Pinion having Three Teeth partially removed
pinions were made from brass pinion stock being extruded to
form, after which they were drilled and cut off in an automatic
screw machine, and counterbored in a drilling machine to re-
move the stock from three of the teeth.
The drill jig first employed in the last operation was one
with a leaf in which several pinions were held. It was designed
and built without much thought, as the job seemed simple and
unimportant. The parts were so small and difficult to handle
rapidly and the chips were so troublesome that the best rate of
production possible with this jig was about 160 an hour. This
was so far under the estimated production that a serious loss
on the contract seemed inevitable. After a little study, a new
EQUIPMENT
137
jig incidentally a cheaper one than the original was de-
signed and built. This jig is shown in Fig. 13. A pair of paral-
lel jaw pliers A was used as a basis. These had special jaws
B inserted to hold the pinions, one of them carrying plate C
which had three holes that served as drill bushings. Spring
D which was attached to one jaw, opened the pliers, while the
grip of the operator closed them and held the pinion in position.
A cast-iron parallel E about two inches high was clamped to
the table of the drilling machine, and two rods F which served
Machinery
Fig. 13.
Jig developed for handling Pinions in
Counterboring Operation
as feet to level the device, were riveted to the handles of the
pliers. The bottom of the pinion rested on the cast-iron parallel,
while the top rested against the drill plate C.
The operator handled this jig as follows: About half a dozen
pinions were placed on the cast-iron parallel approximately an
inch apart. The open pliers were placed over the first piece
and then closed. The shape of the jaws insured that the piece
when clamped would be in the proper position for the operation.
The pliers were then slid under the spindle of the drilling ma-
chine and the pinion was counterbored. To unload, the operator
138 INTERCHANGEABLE MANUFACTURING
moved the jaws away from the cast-iron parallel as he released
his grip. The completed parts and all chips then dropped out,
and he proceeded to pick up the next pinion. The rate of pro-
duction with this jig was over 500 an hour, and nearly double
the estimated output.
Simplicity and Standardization of Jigs and Fixtures. Need-
less to say, the greater the rigidity of the tools the greater the
accuracy of the product. Whenever possible, the pressure of
the cutting tools should be withstood by a solid part of the
fixture and not by a clamp. Fixtures which are permanently
fastened to the machine should be sufficiently rugged to with-
stand all use and abuse. Independent jigs which must be lifted
or turned over in operation should be as light as they can safely
be made. The design in all cases should be as simple as possible
because simplicity is a primary source of economy. This sim-
plicity, however, is seldom attained spontaneously. It is the
result of constant study and careful and painstaking develop-
ment. It may be safely asserted that designs which are not
simple are incompletely developed.
Economy in the construction of jigs and fixtures offers a field
for standardization. Not only the various drill bushings,
operation levers, drill jig feet, vise jaws, etc., but also the base-
plates for jigs and fixtures, various clamping devices, leaves,
etc., can be standardized to advantage. This is now common
practice in many up-to-date tool-rooms.
Methods Employed in Manufacturing Fixtures. The in-
formation required on fixture drawings depends to a large extent
upon the methods to be employed in the construction of these
fixtures. One of two general methods is usually employed.
First, the complete fixture may be made by one man or a small
group of men working in close cooperation. Second, the work
on all fixtures may be resolved into its elements and one man
or one group of men may do only the work of a certain type,
such as planing, milling, boring, etc.
Each method has certain advantages and certain disadvan-
tages. Among the disadvantages of the first method are the
following: When one man performs all types of operations,
EQUIPMENT 139
either the tool-room must be over-equipped with machinery or
a considerable amount of time will be lost by the various men
in waiting their turn to use certain machines. Again, few men
are equally expert in operating all types of machinery. The use
of this method prevents the specialization of the men on those
operations in which they are most expert. In order to overcome
the foregoing disadvantages, the second method has been used.
This often results in reducing the amount of tool-room equip-
ment and increasing the total output. On the other hand, the
elimination of the disadvantages of the first method has resulted
in the loss of many of its advantages. With the first method,
the toolmaker takes a personal interest in the development and
completion of a certain fixture. It is a complete mechanism
in which he can take personal pride, and this advantage is lost
by the adoption of the second method. Furthermore, a careful
distinction must be made between working time and elapsed
time. The adoption of the second method, while it often re-
duces the total working time required to complete tools, inevi-
tably increases the elapsed time between the receipt of the
order and the delivery of the completed fixture.
To operate successfully under the second method, a certain
amount of work must be kept ahead of each operation. This
means that although the machines and men may be kept busy,
each piece of work must wait its turn at each operation. Thus,
in many cases, the elapsed time required to complete the fix-
tures under the second method is more than double the elapsed
time required under the first. For replacement work, which
can be anticipated and started well in advance, and for new
work when time is not essential, the second method is often the
better. In other cases, if ultimate economy is desired, the first
method must be employed. Take for example an emergency
repair. If we balance the decreased cost in the tool-room under
the second method against the increased cost of the idle time
of productive machinery and labor, we shall find that we have
lost more than we have gained. The same holds true for the
construction of equipment for a new product in a case where
time is essential.
140 INTERCHANGEABLE MANUFACTURING
Tolerances on Fixture Drawings. The drawings required in
the tool-room under the first method of production need be
functional ones only. Except on important dimensions of
functional locations, etc., tolerances will be of doubtful value.
Detailed drawings of the individual parts of fixtures must be
supplied, yet tolerances and clearances on these details can be
safely omitted. On the other hand, drawings to be used with
the second method should include tolerances on all but atmos-
pheric fits. The direction of the tolerances on those surfaces
which are fitted at assembly should leave surplus metal. Those
parts which assemble without fitting should have their dimen-
sions and tolerances expressed in the same manner as on the
component drawings of the interchangeable product itself.
The establishment of the tolerances on the functional locations
is identical in all cases. Variations in these dimensions will be
reproduced in the product itself. The effect of these will be to
reduce the manufacturing tolerance. It is obvious that these
variations must be in that direction which will hold the product
within the established tolerances. Naturally, then, these toler-
ances should be kept as small as possible. The fixtures are usually
made but once, while the product must be reproduced many
times over. The amount of this tolerance on the jigs and fix-
tures should be a fair percentage of the component tolerance
and 20 per cent should be sufficient in most cases. It is clear
that the location of drill bushings for holes which may vary
0.020 inch is not so important as for holes which can vary but
0.002 inch.
It should be kept in mind that in many cases adjustment is
provided on the machine tools to align the work-holding fixture
correctly in relation to the cutting tool. Such is the case on
milling, planing, and other similar machine tools. In the case
of jigs for drilling and boring, however, no such adjustment is
possible. Such equipment must, therefore, be constructed with
greater accuracy than milling fixtures. It is thus apparent that
the original tolerances and clearances for those surfaces ma-
chined on this type of equipment must be designated on the
component drawings with great care. It is obvious that if
EQUIPMENT 141
needlessly severe tolerances are required by the component
drawings, the cost of the equipment will be greatly increased
and no commensurate benefit will be derived from it.
Checking and Testing Jigs and Fixtures. The most effective
method of checking jigs and fixtures is to set them up and make
the required cuts on sample pieces. This, however, is not
always possible and so model parts are a good substitute. Such
parts are invaluable for checking purposes during the construc-
tion of any of the special manufacturing equipment. The final
inspection of the completed fixtures should be a functional
inspection only. The operating parts must function properly
and the functional locations must bear the proper relation to
one another. The component drawings, as well as the fixture
drawing, should be consulted. Any fixture which will insure
the proper machining of the component part is satisfactory as
regards its accuracy. If the fixture drawing has been carefully
completed, the information given there will indicate reasonable
limits. If the fixture, however, exceeds those limits, but does
insure the completion of the component part within the estab-
lished tolerances, and without imposing over-severe conditions
in the production, the fixture should not be rejected because of
this technicality. It is well to keep a record of such deviations
for reference. The purpose of this inspection is not to see how
much fault can be found with the work accomplished, but to
determine as surely as possible whether or not it will fulfill the
purpose for which it is intended. This is never definitely de-
termined until the fixture has actually produced satisfactory
work.
Tolerances to be Allowed on Cutting Tools. In connection
with Figs. 4 and 5, several sources of variations in the product
were mentioned. Some of these errors are fixed quantities,
others are variable. In discussing the cutting tools, we will
consider two sources of variations the first variable, the
second practically fixed. The first source of error, which has
been previously mentioned, is that due to the wear on the
cutting edges of the tool. This, as we have seen, results in
leaving more metal on the piece. Disregarding all other factors
142 INTERCHANGEABLE MANUFACTURING
of error, and assuming that the wear is equal on all surfaces,
we should make the tools to the minimum metal sizes of the
component drawings, in order to insure their longest life. When
faces are neither male nor female, the initial size of the tool
should be the mean dimension. But if a tolerance of, say, 20
per cent of the component tolerance has been permitted on the
jigs and fixtures, the cutting tools should be made approxi-
mately the same percentage inside the minimum metal sizes of
the component, except those made to the mean dimensions.
Tools made to such dimensions should produce work within
the established limits. If, however, sample parts made by
such tools are beyond the established limits, they will vary
outside of the minimum metal sizes. In such cases the tools
can be salvaged, as metal must be removed from the tools to
correct this fault.
The second source of variation in the product is initial error
in the size or form of the tool itself. Tolerances for tools should
be established from experience gained in actual practice as
carefully as the tolerances are established for the components
themselves. Cutting tools must be replaced over and over
again in the course of production. On one hand, in order to
reduce the first cost of these tools, their tolerances should be
as liberal as possible. On the other hand, to lengthen their life
and thus reduce the number of replacements, they should be
held as close to the minimum metal limits as other conditions
will permit. The economical balance between these two factors
establishes the proper tolerances. In practice, the fixed error
due to inaccuracies in machine tools and fixtures can readily be
determined after production is under way. This error, properly
added to or subtracted from the minimum metal limit of the
component, establishes the maximum metal limit of the cutting
tool. This maximum metal limit should be the basic dimen-
sion of the cutting tool. The application of the tolerance then
establishes the minimum limit of new cutting tools. Whenever
the established tolerance on the component is exacting, the
tool should be made adjustable if possible, thus enabling wider
limits to be established on the tools, reducing their initial cost
EQUIPMENT 143
and prolonging their life. Such, for example, is the purpose of
interlocking milling cutters.
Maintaining Tolerances on Tools Machining Several Surfaces.
It is well to note that the more surfaces machined by a single
tool, the more difficult it is to maintain close tolerances, either
on the tool itself or on the product. Take, for instance, the
cutting of a thread. If a die is used, the three main elements
are carried on one tool; namely, the form, the lead, and the
diameter. Adjustment for the diameter is possible, but the
form and lead are fixed. Under present conditions, variations
develop in the lead and shape when the tool is hardened and
these are difficult and expensive to correct. As one die is re-
placed with another, these variations are different. If the
tolerances on the component are sufficiently liberal, the use
of dies is satisfactory. If they are severe, satisfactory dies
are very expensive, although the direct production costs are
low. In these cases, if the threads are milled or chased, the form
alone is carried by the tool. Such a tool is readily and accu-
rately made and maintained. The diameter is controlled by the
setting of the tool, and is readily adjusted and readjusted. The
lead is controlled by the lead-screw of the machine and is prac-
tically constant. Lead-screws can be obtained within any
reasonable degree of accuracy.
Sufficient chip clearances must be provided on all cutting tools
if a free cutting tool is to be obtained. The proper rate of the
cutting edge should be determined as early as possible and
carefully maintained. The design should be as simple and
rugged as conditions will permit. The individual design and
requirements of the cutting tools, of course, depend on the
nature of the material to be machined and the methods of
machining employed. The drawings of the cutting tools should
be made with as much care and in conformity with the same
basic principles as the component drawings themselves. When
continuous production is involved, it is necessary to provide a
constant supply of cutting tools. This supply of cutting tools
must be available for instant use with as little adjusting or
correction as possible.
144
INTERCHANGEABLE MANUFACTURING
O
60
EQUIPMENT
145
Special Equipment for Machining Automobile Transmission
Cases. In order to make clear the application of some of the
principles stated, examples of properly designed jigs and fix-
tures will now be presented. For the first example, part of the
special equipment required to manufacture the transmission
case shown in Figs. 14 and 15 will be considered. As over forty
operations in all are required to machine this case, space will
SECTION A-A, FIG. 1
Machinery
Fig. 15. Sectional View of Transmission Case
not permit a detailed discussion of each, but as many are either
practically duplicate or are handled in well-known conventional
ways, only the most instructive operations will be dealt with.
The drawings shown in Figs. 14 and 15 are not complete, many
dimensions and projections which do not affect the operations
to be discussed having been purposely omitted. The first opera-
tion is to snag and chip the casting. This is a bench job and
requires no special equipment. The second operation is to drill
i 4 6
INTERCHANGEABLE MANUFACTURING
the main bearing hole A, Fig. 15, as shown in the operation
drawing, Fig. 16.
Operation drawings of this sort are of great value to the
tool designer and also to the shop foreman and machine opera-
tor. Only the information required for a single operation
appears on each drawing, thus making it handy for reference in
the shop and also preventing any possibility of using a wrong
dimension. They may be drawn to a much smaller scale than
the component drawings and still contain information that
cannot always be placed on the component drawing without
danger of misuse. In practice, they are readily made. Small
drawings of each projection of the work are made, and then the
LOCATING
P
^^ EQUALIZING
/ CLAMPS
(F)CLAMP
LOCATING
'-II
Mashinery
Fig. 16. Operation Drawing for drilling Main Bearing Hole in
Small End
section or projection required for any particular operation draw-
ing is traced and the required dimensions and notes added. Free-
hand tracings are often sufficient. No great amount of detail
need be shown in these drawings. All that is required is enough
to indicate the machined surface in question, the required regis-
ter or locating surfaces, and the clamping points. On opera-
tions where little or no special equipment is required, no opera-
tion drawings are necessary. When these operation drawings
are developed in conjunction with the operation lists described
earlier in this chapter, a further advantage is gained. The work
of designing the tools and fixtures can be readily and safely
distributed among several designers, either in the same organi-
EQUIPMENT 147
zation or in independent shops. When time is essential in
commencing production, this factor becomes of great importance.
Jigs for Drilling Holes in Transmission Case. The first
machining operation is important in several ways. As the
surface machined at this time becomes the register point for
many of the succeeding operations, it is necessary that the
casting be carefully centralized in the jig, which is shown in
Figs. 17 and 18. A study of these illustrations and the opera-
tion drawing shown in Fig. 16, will make clear the general con-
Fig. 17. Front View of Jig for drilling Main Bearing Hole in Small End
struction of the jig. The locating points and clamps are lettered
alike on these three illustrations.
The operation of this jig is as follows: Leaf E, Fig. 17, being
open, the transmission case is slid along the locating rails H
until the shifter housing face B, Fig. 15, comes in contact with
bar A, Fig. 17, and the locating lugs come in contact with the
buttons B. This locates the case in one plane and also squares
it up. Clamp-screw C operates on an angle against the fillet
on boss C, Fig. 15, and holds the case down on rails H and also
148
INTERCHANGEABLE MANUFACTURING
against bar A (Fig. 17). Handwheel D (Fig. 18) operates the
equalizing clamps G (Fig. 17) which both locate and clamp
the case in another plane. The leaf E is then closed and the
clamp-screw F which it carries is used to complete the rigid
clamping of the case. Thus, the case is located and clamped
in three planes. As this transmission case is a relatively large
piece, the design of jigs and fixtures to clamp it with only one or
two motions of the operator's hands would be a complicated and
difficult task unless some type of hydraulic or pneumatic clamp-
Fig. 18 Rear View of Jig, showing Mechanism for operating
Equalizing Clamps.
ing mechanism were employed. This has been successfully
accomplished in a simple and effective manner, but is not yet
common practice. An example of such a jig will be presented
later.
A drawing of the jig just described is shown in Fig. 19. It
will be seen that stop A is pivoted to allow for inequalities in
the casting. The equalizing clamps G are operated by levers
which are actuated by nuts, one threaded right-hand, and the
EQUIPMENT
149
other left-hand. These nuts are operated by means of similar
threads on the handwheel spindle. It will be noted that this jig
is rugged and simple, that all functional locations and parts are
accessible, and also that the chip clearances are liberal, thus making
Machinery
Fig. 19. Assembly Drawing of Jig illustrated in Figs. 17 and 18
a fixture that is readily cleaned and operated. It is also one that
requires little attention in service.
The third operation consists of drilling and rough-counterboring
the main bearing hole D, Fig. 15, and facing and turning flange E
and boss F. This is done in a large Porter & Johnston lathe. An
attachment to the spindle of the machine which runs in a cat-
head at its outer end, locates the case on an arbor through the hole
INTERCHANGEABLE MANUFACTURING
drilled in the preceding operation and from the back of the
flange. The case is also aligned and clamped around the out-
side of the flange. Two locating lugs cast on the bell of the case
assist in locating and driving the work. These two locating
lugs are removed after the machining is completed . The face of
flange E and the center line of the main bearing holes A and
Machinery
Fig. 20.
Operation Drawing for counterboring the Main Bearing
Hole in the Small End and facing the Boss
D now become the primary locating points for most of the suc-
ceeding operations.
Jigs and Tools Used in Various Operations. The fourth
operation is performed on a boring mill and consists of facing
the case to length and rough-counterboring hole A, Fig. 15.
The operation drawing for this operation is shown in Fig. 20.
The case is located in the jig shown in Fig. 21, by flange E and
boss F, Fig. 15, and also on an arbor which passes through the
EQUIPMENT
main bearing hole D. This arbor is hollow to receive the pilot
on the boring-bar, thus helping to align it. The case is squared
up by two studs A, Fig. 21, which bear on the shifter housing
face B, Fig. 15, and is supported centrally and clamped by two
Fig. 21
Jig and Boring-bar used in machining Case to Length and counter-
boring Main Bearing Hole in Small End
clamp-screws B, Fig. 21. The bearing in the end of the jig is
large enough to permit the boring-bar C to enter with the cut-
ting tools assembled.
The boring-bar for this operation is shown in Fig. 22. Pilot
Fig. 22. Boring-bar disassembled from the Jig illustrated in Fig. 21
A enters the hollow arbor on the jig. Surface B carries a ream-
ing tool which corrects the alignment of hole A, Fig. 15. Slot
C carries a combined boring and facing tool which faces surface
152 INTERCHANGEABLE MANUFACTURING
C, Fig. 15, and so machines the case to length, and rough-
counterbores hole A, Fig. 15. This makes a self -registering
tool for the depth of the counterbore. It will be noted that
this depth of counterbore is given in a different way on the
operation drawing from the way it is shown on the component
drawing. This is only a roughing operation, and it will be noted
that stock is left for finishing. A later operation will bring the
dimensions for this surface in accordance with the component
drawing. Surface D on the tool, Fig. 22, runs in the large bear-
ing in the fixture, while the lock-nut E is adjustable and acts
as a stop for regulating the position of the facing tool. A little
Fig. 23. Fixture employed in reaming both Main Bearing Holes
of the Transmission Case
study will show that this arrangement will maintain the length
of the case in accordance with the component drawing. The
face of the flange is located against fixed points on the fixture.
The face of the shoulder of the large bushing D is held in a fixed
position in relation to the locating blocks on the fixture. There-
fore, it makes a reliable registering point for a tool which must
maintain a specified relation to these locating blocks.
The fifth operation is to mill the shifter housing face B, Fig.
15, and the clutch hand-hole face G. This is a straight milling
operation performed on a large planer-type milling machine
which machines a large number of castings at one time. The
EQUIPMENT
153
sixth operation consists of hand-reaming the counterbores of
holes A and D, Fig. 15, both for diameter and depth. The
stand shown in Fig. 23 is provided to hold the case and to pilot
the tools while hand- reaming. The case is placed on the arbor
shown at the left to ream counterbore A, Fig. 15, with the face of
flange E resting on the locating blocks. The pin A, Fig. 23,
enters a hollow lug at the large end of the case to prevent it
Mnchinerv
Fig. 24. Operation Drawing showing Methods of locating and
clamping Work for Seventh Operation
from rotating. The end of the arbor acts as a stop for the ream-
ing tool, thus maintaining the conditions called for on the com-
ponent drawing. The case is then inverted and placed on the
arbor shown at the right in Fig. 23 to ream counterbore D,
Fig. 15. The block B rests against the shifter housing face B,
Fig. 15, to prevent rotation of the case. The bottom of the
counterbore finished on the other arbor rests on shoulder C,
154
INTERCHANGEABLE MANUFACTURING
while the end of the arbor acts as a stop for the reamer, thus
maintaining the location of the bottom of this counterbore in
accordance with the information on the component drawing.
Operations on Vertical Milling and Profiling Machines. The
seventh operation consists of milling surfaces H and 7, Fig. 15,
and the corresponding surfaces around the idler shaft hole M ,
Fig. 14. This operation is performed on a vertical milling
machine equipped with an auxiliary cutter-head to reach down
into the case. The operation drawing for this job is shown in
Fig. 25. Fixture used in Milling Operation on Case, and Parts
for supporting Auxiliary Cutter-head
Fig. 24. The fixture used for this operation is shown at the left
in Fig. 25. The case is located in one plane on the flange, squared
up and located in the second plane on the shifter cover face B,
Fig. 15, and located in the third plane by being centralized on
the body. The center line of holes A and D, Fig. 15, is not used
as a locating point for this operation, for two reasons: First,
the requirements of the surface milled in this operation are
that they be parallel to the flange and that they be maintained
at the specified distance from the flange and from each other.
Therefore, the location of the case in relation to the main bearing
holes is unimportant. The second reason is that the use of an
arbor in this fixture would greatly increase the amount of time
required to load and unload the work. The case is clamped on
EQUIPMENT 155
surface /, Fig. 15, by the leaf and on surface C by the clamp-
screw.
The construction of the auxiliary cutter-head is shown in Fig. 26.
The driving spindle E fits into the spindle of the vertical milling
machine and drives the cutters F and G through a train of gears.
Fig. 26. Auxiliary Cutting Head used in milling Surfaces on Inside of Case
The drawing of this cutter-head should be self-explanatory. The
cutter-head is supported by a bridge composed of pieces A, B,
and C, shown at the right, Fig. 25, which is clamped to the milling
machine, as shown in Fig. 27. Support B is clamped at the proper
height on the dovetail of the machine column. Support A is clamped
to the dovetail on the knee of the machine, while bridge C is sup-
156
INTERCHANGEABLE MANUFACTURING
ported on the tops of the two supports. The auxiliary cutter-
head is then fastened on surfaces D. Vertical adjustment is
provided for the outer end of the bridge which rests on A,
while none is required at the other end.
The knee of the milling machine is adjustable up and down
to control the position of the cuts, and as support B holds a
fixed position in relation to the machine while A moves with
the knee, it is necessary to provide the adjustment for the outer
Machinery
Fig. 27. Assembled View of Parts clamped on Milling Machine to
support the Auxiliary Cutting Head
end at A. The fixture, Fig. 25, is attached to the table and fed,
with the table, to the cutters. Attention is called to the fact
that the loading side of the fixture is on the side farthest from
the cutters. This arrangement enables the feed of the table to
be kept to a minimum.
The eighth operation consists of milling the outside boss
around the idler shaft hole M, Fig. 14, on the small end of the
case. Ordinarily, this operation would be performed on a verti-
cal milling machine, or with a facing tool on a drilling machine.
In this particular case, however, profiling machines were avail-
able, while vertical milling machines were not. Furthermore,
this cut is a relatively light one, which a profiling machine can
handle satisfactorily, and so this type of machine is used. This
EQUIPMENT 157
indicates to a certain degree how the design of the equipment
is determined by the machine tools available. The fixture for
this operation is shown to the right in Fig. 28. The case is
located on the face of flange E, Fig. 15, and by boss F, and is
clamped on the back of the flange. This is an instance where a
pneumatic or hydraulic clamping device could be used to great
advantage and save fully half the setting-up time.
The fixture not only consists of a work-holding device, but
also acts as the stand for the working gage. Lug A is used for
registering the position of the cutters and also for registering
the flat step-gage D which is used to test the finished height of
the boss. Because the maximum distance between the table
Fig. 28. Equipment provided to adapt a Profiling Machine for a
Milling Operation on the Case
of the profiling machine and its cutter-spindles was not great
enough, raising blocks C were provided to lift the heads of the
machine sufficiently to permit the transmission case to be
machined. Bracket B was also required to support the end of
the operating handle shaft in its raised position.
Drilling, Reaming, and Milling Operations. The ninth and
tenth operations, respectively, consist of drilling and reaming
the countershaft holes K and Z,, Fig. 15. Both of these opera-
tions are performed on horizontal radial drilling machines, and
the work-holding fixtures employed in each case are practically
identical in design. The jig for the tenth operation is shown
in Fig. 29. The case is located on the face of flange E, Fig. 15,
158 INTERCHANGEABLE MANUFACTURING
and on arbors through the main bearing holes A and D, and is
squared up by two equalizing plungers which bear on surface
B. The case is clamped by the end of the hollow arbor B,
Fig. 29, which is drawn back against the bottom of the counter-
bore A, Fig. 15. Arbor A, Fig. 29, which passes through B
contains a groove which receives leaf F. Thus, when arbor A
is drawn up by the clamping nut C, this leaf is drawn up against
the end of arbor B, clamping the case between the face of the
flange and the bottom of the counterbore previously referred to.
When leaf F is swung aside, arbor A is withdrawn. Bracket D
is provided to support the end of the arbor A in its loading
position. Handles G and H are used to wring the arbors into
Fig. 29. Work-holding Fixture for Drilling and Reaming Opera-
tions on Radial Drilling Machine
the case, as these arbors are a very close fit in the counterbores
which are held to a tolerance of o.ooi inch. Handle E operates
the equalizing plungers which bear against surface B, Fig. 15,
and square up the case. The eleventh operation consists of
drilling and reaming the idler shaft hole M, Fig. 14. A very
similar jig is used in this operation, the principal difference
being that the case is squared up by a plug in countershaft hole
K, Fig. 15, instead of by equalizing plungers acting against
surface B.
The twelfth and thirteenth operations, respectively, con-
sist of milling the surfaces of the tire pump face N and pedal
support boss 0, Fig. 14. Both of these operations are performed
EQUIPMENT 159
on a profiling machine, and the same fixture is used. This
fixture is shown in Fig. 30. The case is located on holes A and
D, Fig. 15, by arbor A, Fig. 30, and hollow plug B, and on
surface C, Fig. 15, by shoulder C. It is squared up by the pin
D projecting into hole K, Fig. 15. Two screws E are adjusted
to suit surface B, Fig. 15, to support the case against the clamp-
ing device F, Fig. 30. The spring plunger G is locked by the
clamp-screw H and supports the case against the thrust of the
cutters. Arbor A has a bayonet cam- slot which engages a stud
in the hollow plug B, clamping the case between the bottom
Fig. 30. Another Fixture provided for a Milling Operation on a
Profiling Machine
of counterbore D and surface C, Fig. 15. Stud K is the register
point for the tool when milling the pedal support boss, while
stud L is the register point for the tool when milling the tire
pump face. Due to the position of this fixture on the machine,
all of the operating handles must be on the front or on the sides.
This is the reason for the extensions to the two clamps which
are carried through the side of the fixture.
In the fourteenth operation, the clutch shaft hole P, Figs.
14 and 15, is drilled and reamed. This operation is performed
on a boring mill. The jig, with slip bushings for the reamers,
l6o INTERCHANGEABLE MANUFACTURING
Fig. 31. Jig employed in Drilling and Reaming Operations on
a Boring Mill
Fig. 32. Simple Stand provided for Supporting Case during
Operations on Drain Plug Hole
EQUIPMENT l6l
is shown in Fig. 31. The case is located by the usual register
points, on the main bearing bore and counterbore D, Fig. 15,
and on the flange E. It is squared up by the countershaft hole
L, and clamped from the small end against the face of the flange.
A groove at the end of arbor A, Fig. 31, receives leaf B. Clamp-
nut C draws the arbor back, thus clamping the case between
surface C, Fig. 15, and the face of the flange. Leaf B is swung
aside when unloading, thus permitting arbor A to be withdrawn.
Fig. 33. Drill Jig used for drilling Seven Small Holes in Trans-
mission Case at One Time
Bracket G supports the shoulder of the large arbor in its with-
drawn position. Handle D is provided to assist in wringing the
large arbor into the case. Lug F enters the case through the
opening in the shifter housing face B, Fig. 15. Plug E, Fig. 31,
wrings into the countershaft hole L and into the lug F to align
the case.
The fifteenth operation is performed on a drilling machine,
and consists of drilling, tapping, and counterboring drain plug
l62
INTERCHANGEABLE MANUFACTURING
hole /, Fig. 15, and facing the surface of its bore. The exact
location of this drain plug hole is of no importance. A conical
spot is cast in the boss which is used to locate and center the
drill point. The simple stand shown in Fig. 32, unprovided
with bushings or clamps, supports the case on the drilling ma-
chine table. Further elaboration of this simple design would
not improve its effectiveness in any way. The drilling machine
is fitted with a quick-change collet to permit the operator to
change the drill, tap and facing tools rapidly.
Jigs for Drilling a Number of Holes at One Time. The six-
teenth operation on the transmission case consists of drilling
Fig. 34. Cradle Jig and Cluster Plate employed in drilling Twelve
Holes on a Multiple-spindle Drilling Machine
seven small holes Q (see Fig. 14) in the small end of the case.
This operation is performed on a single-spindle drilling machine
equipped with a special multiple drill head. The jig used is
illustrated in Fig. 33. The face of flange E, Fig. 15, rests on
rails B, while the large stud C enters the main bearing counter-
bore A, Fig. 15, and the small stud D enters the idler shaft hole
M, Fig. 14, thus centering and squaring the case. The operat-
ing handle E is pulled forward and down as indicated by the
arrow, thus lowering the head by means of the crank and con-
EQUIPMENT 163
necting-rod A, and clamping the case during the drilling opera-
tion. The operator holds handle E down with one hand while
he operates the drilling machine with the other. Attention is
called to the commendable features of this jig, which include
simplicity of design, accessibility of all functional surfaces, chip
clearances, and rapid operation.
The seventeenth operation is performed on a multiple-spindle
drilling machine provided with a lifting table, and consists of
drilling eight holes R, Fig. 14, and four holes around hole D,
Fig. 15. The jig used for this operation, together with the
cluster plate for guiding the drills, is shown in Fig. 34. These
parts are shown assembled in Fig. 35. The jig is in the form
of a cradle to permit it to be tipped for removing and inserting
the product. Otherwise the table of the machine would have
to be lowered an excessive amount to accomplish the same
result.
The case is located by an arbor through the main bearing
holes A and D, Fig. 15, and rests on surface C, Fig. 15. The
jig is rocked to its drilling position and locked there by the
spring plunger A, Fig. 35. Four spring plungers B rest against
the back of the flange, and are locked in position by clamp-screws.
These support the flange against the thrust of the drills. The
case is located radially by stud E which enters countershaft
hole K, Fig. 15. The cluster plate slides on the column of the
drilling machine, and is held down by the springs on the three
supporting rods F, while the washers at the end of these rods
limit its downward travel. As the table of the machine is
raised, the arbor in the jig enters hole C in the cluster plate,
Fig. 34, thus aligning the case, while blocks D press against the
face of the flange of the transmission case, clamping it in posi-
tion. This makes a simple and quickly operated jig. As the
table is raised toward the drilling position, the work is clamped,
and as the table is lowered, the work is automatically undamped.
This automatic clamping feature effects a considerable saving
in time when setting up work.
The eighteenth operation consists of drilling six holes in sur-
face B, Fig. 15, and two holes in bosses S, Fig. 14. This opera-
164
INTERCHANGEABLE MANUFACTURING
I
*
EQUIPMENT
165
Fig. 36. Another Drill Jig for drilling a Number of Holes at One
Time on a Single-spindle Drilling Machine having a Special Head
Fig. 37,
Jig used for drilling and Reaming Operations performed
on Boring Mill
tion is performed on a single-spindle drilling machine equipped
with a special multiple-spindle drill head. The jig is of the box
type, as shown in Fig. 36. The case is located on an arbor
i66
INTERCHANGEABLE MANUFACTURING
through the main bearing holes A and D, Fig. 15, radially by a
stud in countershaft hole K, and is clamped from the bottom
of counterbore D against surface C, Fig. 15. The hollow plug
A, Fig. 36, fits over the arbor into the main bearing counterbore
D, Fig. 15. Leaf B is then swung under the washer and bears
against the end of the hollow plug as clamp C is tightened. This
leaf has the same function as a slotted washer and is employed
in place of such a washer to eliminate an additional loose piece
on the jig.
The nineteenth and twentieth operations are similar drill-
ing operations on small holes, performed on single-spindle drill-
Fig. 38. Drill Jig provided for drilling Three Small Holes in Pedal
Support Boss
ing machines equipped with special multiple-spindle drill heads.
The transmission case is held in simple jigs for both of these
operations. The twenty-first operation consists of drilling and
reaming hole T, Fig. 14, and is performed on a boring mill.
The fixture for this operation is shown in Fig. 37. The holding
points and methods of clamping are identical to those of the
fixture shown in Fig. 31. The twenty-second operation con-
sists of drilling holes U, Fig. 14, on a single-spindle drilling
EQUIPMENT 167
machine provided with a special multiple-spindle drill head.
The jig shown in Fig. 38 is similar to the one shown in Fig. 33.
The transmission case is located from the main bearing holes
A and Z>, Fig. 15, by arbor A and hollow plug B. Surface V,
Fig. 15, is held against shoulder C by the shoulder of the hollow
plug acting against face C, Fig. 15. The plug is held by a stud
which engages the bayonet cam-slot in arbor A. The case is
located radially by stud D which enters countershaft hole L,
Fig. 15. The operating handle E is moved as indicated by the
arrow, which lowers the head F carrying the drill bushings and
also a stud which enters hole T, Fig. 14.
Fig. 39. Equipment employed in performing Various Operations
on Filler Hole
Operations on Filler Hole. The twenty- third operation con-
sists of drilling, chamfering, spot-facing, and tapping filler hole
W, Fig. 14. This operation is performed on a single-spindle
drilling machine by means of the jig shown in Fig. 39. The case
is located on an arbor through the main bearing holes A and
D, Fig. 15, and clamped against surface C, Fig. 15, in the usual
manner. It is located radially by a stud which enters the
countershaft hole K, Fig. 15. The spring plunger A, which is
locked by the clamp-screw B, supports the under side of the
filler boss against the thrust of the cutting tools. The drilling
i68
INTERCHANGEABLE MANUFACTURING
Fig. 40. Fixture equipped with Trunnioned Roller Bearings, used
in tapping Holes
Fig. 41. Pivoted Ball-bearing Fixture used in Tapping Operation
EQUIPMENT
169
machine is equipped with a quick-change collet attachment to
promote the speedy change of tools.
The next six operations are all minor drilling operations
requiring simple jigs. The thirtieth operation consists of coun-
tersinking all the holes to be tapped in succeeding operations
with a small electric hand drill, equipped with the proper coun-
tersinks. No special work-holding device is required. The
thirty-first and thirty-second operations are tapping operations
performed on a tapping machine, without the use of special
Machinery
Fig. 42.
Drawing of Tapping Fixture illustrated in Fig. 40, equipped with
Trunnioned Roller Bearings
fixtures. The eight remaining machining operations are tapping
operations for which fixtures are provided. Some of them are
simple stands, while others are mounted on ball or roller bear-
ings to make lighter work for the operator in shifting the jig
from position to position. Two of these jigs will be described.
Jigs Provided for Tapping Operations. The thirty-fourth
operation consists of tapping the six holes previously drilled in
face B, Fig. 15, and is performed on a tapping machine. The
fixture is shown in Fig. 40. The transmission case is located
on an arbor through the main bearing holes A and Z), Fig. 15,
170
INTERCHANGEABLE MANUFACTURING
and is squared up by a stud which enters the countershaft hole
K, Fig. 15. The fixture is mounted on two sets of roller bear-
ings which permit it to be readily rolled in two directions. The
construction is shown in Fig. 42. In order to make the travel
of the rolls as short as possible, they are made with two small
trunnions as shown at A. These trunnions roll on the bottom
rails, while the large periphery of the rolls is in contact with
the upper rails. The effect of this construction is that the
upper part of the fixture can move about six inches while the
Machinery
Fig. 43. Pivoted Ball-bearing Fixture shown in Fig. 41
rolls have moved only about one inch. The lower rails are
shorter than the upper ones in the lower sections of the fixture.
The thirty-seventh operation consists of tapping holes U,
Fig. 14, and is performed on a tapping machine by means of
the fixture shown in Fig. 41. The case is supported on an arbor
through the main bearing hole D, Fig. 15, and is clamped and
squared up against the face of flange E, Fig. 15, by a slotted
washer which bears against face X, Fig. 15. This washer is
drawn back by a clamp-nut on the plunger through the arbor.
The case is located radially by the stud seen in Fig. 41, which
enters countershaft hole L, Fig. 15. The upper part of the
EQUIPMENT 171
fixture is pivoted and revolves on a large ball race, as shown in
Fig. 43-
The foregoing descriptions of some of the special manufac-
turing equipment for an automobile transmission case have
not been given for the purpose of illustrating how such a part
may be machined, but rather to indicate in some degree the
many factors which must be considered in the design of any
special manufacturing equipment. These descriptions are in-
complete, yet a careful study of the component drawing and
the illustrations will supply many of the omissions in the de-
scriptions. With few exceptions, no mention has been made
of the cutting tools, as these have been for the most part stand-
ard mills, boring-bars, drills, taps, reamers, or similar standard
tools.
Attention is called to the general grouping of the opera-
tions. First, the boring and facing, next the milling, then the
drilling and reaming of the larger supplementary holes, fol-
lowed by a few profiling operations. The final operations
consist of drilling and tapping the many smaller holes. For
large production, the machine tools required would usually be
so grouped that the parts would pass consecutively from
one operation to the next. For smaller rates of production
where the machine tools are not kept set up constanly for one
operation, they are often grouped together by types of ma-
chines. In either case, the foregoing general arrangement of
operations would be satisfactory. A few minor changes in
sequence might be made to advantage, but the general lay-out
would remain unchanged. In order to cover some points which
the foregoing examples have not touched upon, a few examples
of tools and several additional fixtures will be presented. These
will indicate possibilities in design, which effect savings in
direct labor costs.
Pneumatic Clamping Devices. It has been pointed out that
the use of pneumatic or hydraulic clamping devices is necessary
on many fixtures for large parts, if the clamping is to be per-
formed by a single movement or two of the operator. The best
known examples of pneumatic clamping devices are air chucks.
172
INTERCHANGEABLE MANUFACTURING
Fig. 44 shows a drill jig, together with the part that is to be
drilled; this is clamped by air pressure. A flexible hose, at-
tached to the air line, is connected with this fixture by part A .
The part to be drilled is slid into the jig from the right, with
the machined face down. The opening of a valve operates four
pistons, one built into leaf B, one in leaf C, and the others on
the back of the fixture which operates clamps D and E. These
hold the part firmly in position while the holes are being drilled.
Sight-holes are provided so that any mislocation of the part is
Fig. 44. Drill Jig equipped with Air-operated Clamping Devices
readily detected. As a further safeguard against movement of
the work, the large hole F is first drilled, and a plug is then
inserted in it through the bushing. This jig is easily and
rapidly operated, and the design of the clamping pistons is as
simple as that of any other clamping device would be.
There are ocasions when a convenient air line is not present,
or where the attachment of a flexible hose would interfere with
EQUIPMENT
o S
* ~?
a I g-l
3
174
INTERCHANGEABLE MANUFACTURING
end in the illustration and B at the left. This bar carries four
tool -holders C, D, E, and F, and is used to face the bosses on
the spindle head of a machine tool. The head to be machined
Fig. 46. Close-up View of One of the Facing Bars in Fig. 45, show-
ing Grooves used in feeding Cutter across the Surface
Fig. 47. Typical Example of Fixtures used in Continuous Milling
Operations for producing Parts in Large Quantities
is clamped in position on the table of a boring mill with the
facing bar in position, and the bearing caps are then assem-
bled. Bracket G is also clamped to the table of the machine
EQUIPMENT 175
to prevent endwise movement of the outer spindle. As the
facing bar revolves, the inner bar is fed in, which feeds the
facing tools carried in the tool-holders into the faces of the
bosses. This is accomplished by means of angular tongues (such
as may be seen at end B on the inner bar) sliding in angular
grooves (such as shown at A, Fig. 46), which are cut on the
tool-holders. The outer spindle is relieved to permit the facing
tools to pass beyond the inside edges of the faces of the bosses.
The tongues and grooves are so cut that when one tool has
Fig. 48. Continuous Drilling Fixture having Three Work-holding Stations
traveled its full distance, it stops close to this relief until all
the other tools have finished cutting. The cutting edge of the
tool has then traveled beyond the face of the boss so that it
does not score the face by dwelling there. After all the tools
have completed their cuts, the work is removed from the ma-
chine and the facing tools returned to their starting position by
withdrawing the inner bar. The tools are adjustable, which en-
ables the lengths and relative positions of the bearings to be
accurately maintained. With this tool, several bosses can be
faced in little more time than one.
Milling and Drilling Fixtures. Fixtures and machines for
continuous milling operations are chiefly used where the produc-
tion of any large quantity of parts is required. An example of
such a milling fixture is shown in the illustration Fig. 47. When
these fixtures are used, the machine is working constantly in-
176 INTERCHANGEABLE MANUFACTURING
stead of being idle during the time the operator is engaged in
removing the finished parts and inserting new work in place in
the fixture.
While continuous milling fixtures are quite common, similar
drilling fixtures are not so well known. Fig. 48 shows such a
fixture designed to be used on a single-spindle drilling machine;
the fixture is mounted on the machine in a vertical position.
The special multiple-spindle drill head A is driven by the spindle
of the machine and also raised and lowered in the usual manner
by the machine spindle. The bars B guide the head and also
prevent it from rotating. The two brackets C and D are bolted
to the column of the machine, the bracket D containing bushings
to assist in the alignment of the drills. The special table E is
also bolted to the column of the drilling machine, in place of
the standard table. The jig is pivoted and supported by the
bracket D and table E. This jig contains three work-holding
stations, so that while drilling one piece, the operator removes
and inserts parts in the other stations, thus obtaining a much
higher production rate.
All the foregoing examples of special manufacturing equip-
ment are used for relatively large parts, but the same general
principles apply equally to small pieces. Certain detailed
practices which are sometimes followed for one class of work
are not always feasible when the size of the parts becomes much
larger or smaller. The fundamental problems, however, are
the same, and economical solutions require the careful use of
the same basic factors.
CHAPTER IX
GAGES IN INTERCHANGEABLE MANUFACTURING
A GAGE is an instrument or apparatus for measuring a specific
dimension. Every manufactured part is measured during its
production and after its completion, in one way or another. This
applies equally to a single piece made for a special machine or as
a repair part, or to a hundred thousand duplicate parts manu-
fuctured for an interchangeable product. The mere removing
or shaping of the raw material in itself is seldom a difficult or
exacting task. The critical point is in stopping this process at
the proper moment. The approach to this point can be watched
only by some form of measurement. The most elementary
method of measuring a part is to try it with the companion
parts with which it is to operate. Such was common practice
in the early stages of mechanical industry. This practice neces-
sarily continues to a great extent with repair work and also in
the construction of small numbers of special machines, jigs, and
fixtures.
A later method consisted of measuring the parts individually,
with standard measuring tools such as scales, calipers, verniers,
and micrometers. In many cases, these measurements were
merely preliminary to the fitting together of the parts at as-
sembly. Fitting at assembly is expensive. It takes time and
requires a relatively large amount of space and highly skilled
labor. Most of the metal removed at this time is done by hand.
If any great amount of metal is to be removed, the part must
be taken back to the machine shop and relocated on the machine,
thus interrupting other work. Under such conditions, the eco-
nomic production of any great quantities of duplicate mechanisms
is impossible.
Gages an Economical Necessity. Interchangeable manu-
facturing was developed primarily to eliminate these conditions.
If parts could be made close enough to some uniform size so that
177
178 INTERCHANGEABLE MANUFACTURING
most, at least, of this fitting could be eliminated, it was evident
that larger production could be secured with the expenditure
of the same effort. Furthermore, many parts could be machined
in advance and carried in stock, thus making earlier deliveries
possible in many cases. Clearly, one of the most essential factors
of such a plan is a reliable means of measuring each part as it
is made. This measuring, to be effective, must insure uniformity
and be economical. The use of standard measuring instruments
such as micrometers is not always reliable in measuring large
numbers of duplicate parts. In the first place, for many ex-
acting conditions, measurements hurriedly made by several dif-
ferent men do not prove sufficiently uniform. In the second
place, many of the surfaces to be measured are not readily ac-
cessible by standard measuring tools. And, in the third place,
while both of the preceding conditions may often be satis-
factorily met, the time consumed by this method would be too
great to be economical. In order to meet all these conditions,
special measuring tools known as gages have been developed.
Gages are an integral part of interchangeable manufacturing
equipment. They comprise that part of the equipment the
purpose of which is to measure the product, as distinguished
from that part of the equipment the purpose of which is to change
the form of the material or to hold the part during a manufac-
turing operation. Under this broad definition of a gage, it is ap-
parent that some of the manufacturing equipment may be not
only a holding device, but also a gage. In fact it is good practice
to make fixtures so that an unserviceable part cannot be inserted.
It often happens that when the normal manufacturing variations
of certain machining processes are small and within known
limits, a gage may be employed to test the size or form of the
cutting tool, and not be applied directly to the product. At
other times, a gage in the form of a setting block for the position
of the cutting tool is made as an integral part of the fixture.
Therefore, to determine the character of the gages that are re-
quired for the production of any particular part, it is necessary
to consider both the requirements of the part in question and the
other manufacturing equipment that is provided.
GAGES 179
Classification of Gages According to Use. In general, there
are three purposes for which gages may be needed: First, in the
manufacture of large numbers of duplicate pieces, it is a measure
of economy to detect and discard all unserviceable parts as soon
as possible, thus saving the expenditure of additional effort on
such parts. The gages provided for the purpose are commonly
known as working gages. These are often limit gages, placed
in the hands of the machine operator to check each individual
machining operation as it is performed.
Second, it is necessary to check the parts as they are trans-
ferred from one manufacturing department to another, and also
before the finished parts are sent to the stock-room or assembling
floor, so as to prevent unserviceable parts from proceeding
farther. It is also customary to inspect the parts in process
after certain groups of operations have been performed. The
gages used for these purposes are commonly known as inspection
gages. Some of these are limit gages which are often duplicates
of some of the working gages, while others are functional gages
which check the results of several operations at one time. These
inspection gages are generally used by a force of inspectors who
are independent of the production department.
Third, when gages are used to any extent, it is necessary to
have reliable standards as a means of checking the working and
inspection gages and to establish the sizes of new gages as the
old ones wear out. Such standards are variously known as
checks, reference gages, standards, master gages, and model
parts. These gages are usually kept in the tool-room or in the
hands of a gage inspector, and their purpose is to test the working
and inspection gages so as to insure a suitable degree of preci-
sion in them.
Required Accuracy of Gages. Extreme refinement in gages
is expensive and is unwarranted by the functioning of the ma-
jority of component parts. The accuracy required by a gage
depends in a great measure upon the extent of the manufacturing
tolerances. If these tolerances have been properly established,
only a small percentage of them will be exacting. It is evident
that a gage used to measure a dimension which has a tolerance
i8o
INTERCHANGEABLE MANUFACTURING
of 0.002 inch must be made closer to size than one which meas-
ures a dimension having a tolerance of 0.020 inch. It is common
practice to establish the tolerance on a gage at 10 per cent of
the component tolerance. A tolerance of less than 0.0002 inch
should seldom be specified unless the conditions are unusually
exacting and economy is no object.
With variations in gages, no matter how slight, and with
parts passing through successive inspections, many misunder-
standings are inevitable unless precautions are taken to guard
against them. The most common method of meeting this con-
10 PER CENT OF
Machinery
Fig. 1. Diagrammatic Illustration showing Differences between
Working and Inspection Gages and Tolerances on these
Gages
dition is to establish the limits of the working gages inside the
limits of the inspection gages. Fig. i is a diagrammatical il-
lustration showing the differences between working and inspec-
tion gages and the tolerance on these gages. The full lines
represent the maximum size of a part while the dotted lines
represent the minimum size. The maximum size of the "go"
inspection gage is identical with the maximum size of the com-
ponent. The minimum size of the "go" inspection gage is 10
per cent of the component tolerance smaller than its maximum
size. The maximum size of the "go" working gage is identical
GAGES l8l
with the minimum size of the "go" inspection gage, while its
minimum size is 10 per cent of the component tolerance smaller.
In a similar manner, the minimum size of the "not go" inspec-
tion gage is identical with the minimum size of the component
while its maximum size is 10 per cent of the component tol-
erance larger. The minimum size of the "not go" working gage
is identical with the maximum size of the "not go" inspection
gage, while its maximum size is 10 per cent of the component
tolerance larger. As the tolerance on the component increases, it
is often advisable to reduce this percentage. Thus, for plain plug
and snap gages a tolerance greater than from o.ooi to 0.002 inch
is seldom necessary.
Relation between Working and Inspection Gages. It is evi-
dent that if the sizes of the working gages are always kept inside
of the sizes of the inspection gages, few questions should arise
due to parts passing the working gages and being rejected by the
inspection gages. This arrangement may be secured by making
and maintaining the gages as outlined in the foregoing or by a
process of selection and grading. If all the gages used at the
same time for the same surface are checked concurrently, those
permitting the widest variation in the product should be used
as inspection gages, while the others should be used as working
gages. In all cases the nominal sizes of the inspection limit
gages should be identical with the limits of the component, and
all tolerances should keep them within the limits of the com-
ponent. Thus, the maximum gage may be smaller than its
nominal size but never larger, while the minimum gage may be
larger but never smaller.
Such a practice brings up two age-old arguments: First, that
a i-inch plug will not enter a i-inch hole, and second, that the
tolerances on the gages rob the manufacturer of some of the
tolerances given on the drawing. The answer to these argu-
ments depends upon the interpretation of the drawing. If this
interpretation is that the dimensions and tolerances given on
the drawing represent the extreme sizes of the limit gages, and
all variations of whatever source must come within these limits,
neither of the above arguments has any weight; and this is the
182 INTERCHANGEABLE MANUFACTURING
only logical interpretation that can be used definitely and con-
sistently. With this interpretation, it does not matter whether
the hole is ever exactly one inch or not. As for the second
argument, if the shop does not attempt to maintain its product
within slightly smaller tolerances than the extreme tolerances,
too large a percentage of parts will inevitably run outside of the
tolerances and be rejected.
Gage Requirements Controlled by Ultimate Economy. A
limit gage is one that measures both the maximum and mini-
mum sizes of the component. Such gages usually check ele-
mentary surfaces, although they are at times provided for check-
ing profiles and other composite surfaces. The most common
types of limit gages are snap gages, plug gages, ring gages, depth
gages, and length gages. A functional gage is one that checks
primarily the functional operation of a component without
strict adherence to its exact physical dimensions. Several types
of these gages were discussed in Chapter V. The purpose of
such a gage is to insure, as far as possible, the proper assembling
and operation of all parts.
The extent to which gages should be employed depends on
the product and the rate of production. If the rate of produc-
tion is low, it is often possible to control the accuracy of the
product with standard measuring instruments. As it increases,
the time spent in using standard instruments reaches a point
where the time saved by the use of gages more than pays for their
cost. Gages should be provided for only those surfaces which
it is essential to maintain within certain dimensions. Each
gage should have its definite purpose just as any other piece of
manufacturing equipment has some definite duty to perform.
A gage is a preventive and not a cure. Gages are required
wherever their use will tend to prevent the production of faulty
work. Thus a more complete system of gages is necessary in a
shop that employs a large percentage of semi-skilled labor than
in a shop employing highly skilled operatives.
Interchangeability between Parts Made in Different Shops.
Experience has shown it to be difficult to obtain interchange-
able parts from several independent plants producing a common
GAGES 183
product unless great precaution is taken at the outset to insure
this result. Under these conditions the most certain method is
to maintain identical working and gaging points at the various
plants for all functional surfaces. Component drawings, properly
dimensioned, assist greatly in accomplishing this end. This
does not necessarily mean that the design of the gages must
be identical. The exact design of a gage is never in itself a matter
of great importance. The effectiveness and economy of the re-
sults obtained are the important considerations. Usually the
gaging equipment must be very complete to meet successfully
the requirements of interchangeability between independent
plants.
For the further discussion of gages, they will be classified ac-
cording to their type, such as snap gages, ring gages, plug gages,
profile gages, thread gages, flush-pin gages, functional gages,
etc.
Snap Gages. Gages were first developed as part of the equip-
ment necessary for manufacturing large numbers of duplicate
parts. Now gages are used to a large extent in the production
of smaller numbers of parts. In this case, however, many modi-
fications in the design, such as adjustable features, have been
developed to keep the cost within reasonable limits. Snap gages
for use in the manufacture of large numbers of interchangeable
parts will be discussed first. The earliest form of snap gage
was the "one-size" type; that is, a gage to measure one flat
dimension only. This type is still used to a large extent in tool-
rooms and machine shops when limits are not expressed on the
drawings and when the clearances for the different fits are left
to the judgment of the workmen.
The limit gage with two steps was later developed, one step
being provided for measuring the maximum limit, and the other
for measuring the minimum limit. For small parts produced
in large quantities the non-adjustable gages are most satisfactory.
Formerly a number of gage slots were cut in one piece of steel to
permit a combination of gages in one piece, but the disadvantage
of this design was that when one gage became worn the whole
gage was lost. One method of overcoming this disadvantage is
184
INTERCHANGEABLE MANUFACTURING
to have a filler block inserted on one side of the gage jaw which
can be replaced when the gage becomes worn. Sometimes a com-
bination of gages is mounted on a ring similar to a key ring. In
a later snap gage construction, individual gages are assembled in
convenient units and held together by clamping strips and screws.
This construction permits the
easy removal of a gage when
necessary.
Various Other Types of
Snap Gages. One type of
snap gage has an intermediate
step between the two limit
steps, to aid the machine
operator in setting up and
adjusting the tools. In set-
ting up a machine for repeti-
tion work, the object is to set
the tools so as to have the
maximum time between ad-
justments. When a circular
part is machined with a form
tool or a box-tool, the piece
becomes larger as the tool
wears. Therefore, the initial
setting should be as near the
Fig. 2. Snap Gage with Shallow . . ,, ,. .,
Throat for measuring Lengths minimum or not gO limit
as other conditions will per-
mit. The intermediate steps on these working snap gages is
made to approximately the mean dimension. Thus, if the
operator sets the machine to produce work between the mini-
mum limit of the gage and the intermediate step, the life of the
tool, as regards wear at the particular setting, is equal to at least
half of the working tolerance. These intermediate steps are not
used on inspection gages, as they would serve no purpose there.
There are two general types of snap gages, those with deep
throats for measuring diameters and those with shallow throats
as illustrated in Fig 2, for measuring lengths. When the gage
GAGES 185
slot is very narrow, snap gages are frequently made with a re-
movable strip serving as one side of the gage slot. This con-
struction permits the gaging surfaces to be readily ground.
For larger pieces and for smaller rates of production, adjustable
snap gages have been developed. Gages of this type are shown
in Fig. 3. In common with other types of adjustable tools,
these should be adjustable in the tool-room and fixed in the
manufacturing departments. This result is obtained by pro-
viding a place for a seal which must be broken before the gage
Fig; 3. Adjustable Snap Gages employed for Comparatively Large Pieces or
when the Rate of Production is Small
can be adjusted, thus preventing the adjustment from being
tampered with. These gages may be readily adjusted in the tool-
room to any desired limits, so that a few sets of them provide a
flexible and economical equipment of gages for checking ele-
mentary dimensions, such as external diameters, thicknesses,
and lengths.
Ring Gages. Under some conditions, the use of a snap gage
for testing diameters is not sufficient, and in such cases ' ring
gages are employed. Wherever possible, however, snap gages
i86
INTERCHANGEABLE MANUFACTURING
should be employed, as they are more economical to use. A
snap gage can be used more rapidly than a ring gage. Further-
more, on many parts, a machine operator cannot use a ring
gage without removing the work from the machine. The ex-
tent of the tolerance required to manufacture a part economically
depends in a large measure on the type of gage employed. For
example, if a ring gage is used in place of a snap gage, any de-
parture from rotundity or from size affects the acceptance of the
part by the gage. Thus, in effect, a snap gage checks an ele-
mentary surface while a ring gage checks a composite one.
Fig. 4.
A Ring Gage of the Ordinary
Type
Fig. 5. Counterbore
Plug Gage
The severest possible inspection of a cylindrical surface is
obtained by the use of "go" ring gages and "not go" snap gages.
It is therefore evident that a description of the gaging and in-
spection methods is essential in the specifications to avoid mis-
understandings. The larger ring gages are made as individual
gages, as shown in Fig. 4, and are sometimes provided with
handles. Several small ring gages are often inserted into a soft
holder which keeps them together.
Plug Gages. Plain plug gages are old and simple forms.
Standard plug gages, as with standard snap gages, are largely
GAGES
I8 7
used in tool-rooms and general machine shops. A "not go"
gage was a later development and is attached either to the other
end of the same handle as the "go" gage or is a separate gage.
It is common practice to make standard handles and standard
plug gage blanks, later finishing these blanks to the size required
and assembling them into the standard handles. Solid double-
ended plug gages are open to the same objections as solid com-
bined snap gages. If one end becomes unserviceable, the whole
gage must often be discarded. In standard limit plug gages
the minimum or "go" ends are made longer than the maximum
Fig. 6.
Two-step Plug Gages used for the Inspection of
Through Holes
or "not go" ends; this practice is followed in order to make the
"go" end readily distinguishable from the "not go" end, and,
furthermore, as the "not go" end is subject to little wear, there
is no necessity for making it very long.
When a through hole is to be gaged, it is customary to make
a two-step plug gage such as shown in Fig. 6. This permits
rapid inspection, but the gage cost is greater than when two
separate ends are used. Often, however, the saving in inspec-
tion costs will greatly exceed the additional expense of this type
of gage, so that the practice is economical in the long run.
1 88
INTERCHANGEABLE MANUFACTURING
The Pratt & Whitney Co. manufactures a gage known as the
"star" gage, which is of the expansion type, having four movable
measuring ends. This gage is used for measuring the bores of
tubes and jackets for large guns, the bores of engine cylinders,
etc. Plug gages made from flat stock are often used to measure
the width or length of slots or grooves. These gages are fre-
quently rounded at the end and used for measuring the length
of a splined slot.
Plug Gages for Several Surfaces and Taper Surfaces. Thus
far only gages for elementary surfaces have been considered.
Fig. 7. Taper^lug Gage
mKsSSSSSmm
Fig. 8. Combination Plug and Snap Gage
The dimensions for such gages are readily determined from the
limits expressed on the component drawings. To test concen-
tricity, however, the assembly requirements of the mating parts
must be considered. A gage of this type for testing the con-
centricity of the bore and counterbore of the main bearing of an
automobile transmission case is shown in Fig. 5. Plug gages are
often made with steps on the end to gage both the diameter
and the depth of a hole at the same time. At other times a
sliding collar is provided which saves the use of a straightedge
if the hole to be gaged is either countersunk or counterbored.
GAGES
189
Taper plug gages are usually provided with either lines or
steps to gage not only the diameters of the tapered hole, but
also their locations. A taper gage is shown in Fig. 7. A groove
is cut near the large end of this gage and the width of this groove
indicates the limits. The gage must go into the work until one
edge of the groove is flush with or below the face of the part,
while the other edge must never go in beyond the face of the part.
Sometimes steps are provided to indicate the limits and at other
times, lines are graved to serve the same purpose. If the tapered
hole is properly dimensioned on the component drawing so
T
+0-000.
_ o.oio
|< 0.745 "MAX.
_L
.50o"l;V
T
f;
^ ;/
1.500
;
i
(
T
t
Machinery
Fig. 9. Illustration showing how Compound Tolerances are
involved when testing the Concentricity of a Hub
with a Hole
that no compound tolerances exist, the correct dimensions of
the gage are readily determined. If compound tolerances exist
on the drawing, however, some arbitrary method of interpreta-
tion must be promulgated as otherwise endless arguments will
ensue about the proper gage sizes.
Application of Combination Plug and Snap Gages. A com-
bination plug and snap gage is illustrated in Fig. 8. Such gages
may be required for several purposes. They may be used to test
the concentricity of a hub with a hole, the location of a hole
from the edge of a part, the depth of a slot in relation to a hole
INTERCHANGEABLE MANUFACTURING
etc. In determining the dimensions of such gages, compound
tolerances are almost inevitably present. Therefore, some ar-
bitrary method of interpreting the drawing must be established.
A gage for testing the concentricity of the hub with a hole will
be considered first.
Assume that the hub and hole represented at A in Fig. 9 must
be gaged. The diameter of the plug in this case will be taken
as the minimum size of the hole or 1.500 inches. If it is con-
sidered that the limits given for the hole and hub establish parallel
zones of permissible variations, as shown at B, there will be a
; |< 0.620 WIN.
0.628 MAX.
Machinery
Fig. 10. Application of a Combination Plug and Snap Gage in
testing the Location of a Hole from the Edge of a Part
minimum distance of 0.738 inch between the gaging parts of the
combination gage shown at C, and a maximum distance of 0.745
inch. The use of this gage will then permit the extreme condition
of eccentricity which is shown at D. If the diameter of the hole
is maximum and the eccentricity is at this extreme, the size of
the hub will be 2.987 inches. If the diameter of the hole is mini-
mum, the size of the hub will be 2.983 inches. The more nearly
concentric the hole and hub are maintained, the greater the
amount of tolerance which remains for their diameters. The
full tolerance on these diameters becomes available only when
GAGES
IQI
they are concentric with each other. It may be pointed out that
the condition shown at D does not keep the parts within the
parallel zones shown at B. This is true, and will be found to
be true wherever compound tolerances are involved. It is this
condition that makes it necessary to establish some arbitrary
interpretation of the drawings.
The next example will be of a combination plug and snap
gage used to test the location of a hole from the edge of a part.
The procedure to determine the gage sizes is identical with the
foregoing. A part having a hole which is to be gaged from an
Fig 11. Simple Type of Contour Gage for checking the Uniformity
of Contours or Profiles
Fig. 12. Matching Gage used in testing the Positions of Gradua-
tions on a Part
edge is shown at A , in Fig. 10. The parallel zones of variation
given on the drawing are shown at B, and the gage is shown at C.
The diameter of the plug is shown as the minimum size of the
hole. As a matter of fact, the diameter of the plug may be any
size smaller than the hole in these cases, as the gaging dimen-
sion is controlled by the gap between the edge of the plug and
the steps of the arm. A plug of minimum size is generally used
so that it may also be employed as the "go" gage for the hole.
IQ2 INTERCHANGEABLE MANUFACTURING
A modification of this gage is used to test the position of a hole
that must be carefully located between two edges. One side of
the snap gage part is made longer than the other to detect the
side at fault in case the gage does not go on.
Contour or Profile Gages. Contours or profiles are among the
most difficult surfaces to gage properly. A contour gage of the
earliest type is shown in Fig. n, but this type should be used only
when accuracy is not important. The main objection to this
form is that it measures only the shape of the work and not the
Fig. 13. Receiving Gages having Holes or Openings Corresponding to the
Shape of the Part inspected by them
location of the contour. A gage designed to overcome this ob-
jection has guides for the contour and locating points for the
work.
Contour gages of the matching type are sometimes used when
great accuracy is not required. The part is placed on the gage
and its outline compared, either visually or by a straightedge,
with the outline of the gage. In Fig. 12 is shown one type of
matching gage. This is used to test the position of graduations
on a part. The work is inserted in the gage and the graduations
are compared visually. Similar gages are often used for check-
ing the shape of springs made from flat stock and also for check-
ing the graduations on dials, etc.
GAGES
193
Receiving Gages. The simplest form of receiving gage is a
flat templet in which a hole or opening is cut corresponding to
the form of the part to be inspected. Such gages do little more
than insure inter changeability. If the part enters, it is not too
large, but it is impossible to determine from such a gage if the
piece is too small. If the piece does not enter, it is too large.
It is difficult to find the exact location and amount of the error.
Gages of this type are shown in Fig. 13. In an improved type
Fig. 14. Profile Gage which checks the Contour of the
Work by Flush Pins
of receiving gage the work is inserted in the opening of the gage
and properly located there. This opening is a uniform distance
from the work so that maximum and minimum plug gages may
be inserted between the work and the gaging surfaces.
This same principle may be applied to the gaging of irregular
openings by making a male profile a uniform amount under
size and using limit plug gages to check for errors. Fig. 14 shows
another type of profile gage which checks the contour to definite
limits. In this case the contour consists of several flat surfaces
cut at different angles. The part is located by block A. The
flush pins B, C, D, and E check the various faces on the piece.
Dial Indicator Contour Gages. The highest development in
gages for formed surfaces is doubtless the dial indicator contour
194
INTERCHANGEABLE MANUFACTURING
gage, a simple example of which is shown in Fig. 15. This type
of gage consists of a baseplate which has mounted on it means
for holding the piece to be gaged as. well as the master form with
Fig. 15. Dial Indicator Contour Gage
Fig. 16. Another Type of Dial Indicator Contour Gage
which the piece being gaged is compared. The piece to be gaged
is shown in position at A , and the master form is directly beneath
it. The stud B is used to locate the work. The dial indicator C
is mounted on a baseplate of its own and slides on the baseplate
GAGES
195
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196 INTERCHANGEABLE MANUFACTURING
of the dial indicator follows around the master form, while a
projection on the baseplate registers against the work. Such a
gage is shown in Fig. 16. Otherwise its operation is the same
as in the previous example. This type of gage lends itself readily
to the inspection of work having many difficult and exacting
requirements. A modification is shown in Fig. 17. The piece
to be gaged is shown below the gaging fixture. This gage is
used for inspecting the cam surface, A . In operation, the pin B
follows the cam surface A, while the point C on the dial indicator
follows the master cam D. Any variations are thus readily and
accurately detected. This type of gage is not limited to the gag-
Fig. 18. Simple Form of Flush-pin Gage, consisting of a Plunger
which slides in a Sleeve
ing of contours, but may also be used for testing depths, steps,
recesses, etc., much more readily than a great number of snap,
plug and depth gages.
Flush-pin Gages. Flush-pin gages are generally used for
tolerances over 0.002 inch, especially in cases where snap gages
cannot be applied conveniently. It is possible to use them for
smaller tolerances, but it is seldom practicable in such cases to
depend on the sense of touch. The flush-pin gage in its simplest
form, as shown in Fig. 18, consists of a plunger which slides in
a sleeve. Steps are provided, sometimes on the top of the plunger
GAGES
197
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198 INTERCHANGEABLE MANUFACTURING
be applied in a great variety of ways. Fig. 14 shows its applica-
tion to a contour gage.
The advantages of flush-pin gages may be briefly summa-
rized as follows: The flush-pin gage is the simplest form of gage
for measuring the position of one surface with reference to a
locating point, when the relation is such that a snap gage cannot
be used. Flush-pin gages are subject to a comparatively small
amount of wear, and repairs are simple. Mistakes in reading the
indications on them are rare.
Fig. 21. Gages with Three Sets of Double Flush Pins
Sliding-bar Gages. Among the gages made with sliding mem-
bers, the sliding bar gages occupy an important place. In prin-
ciple, they are similar to flush-pin gages, but differ in the method
of taking the readings or indications. A common type is similar
to a micrometer in general construction. On the sliding bar
is engraved a line, which must be between two steps on the frame
if the part being measured is acceptable. Another similar ex-
ample is shown in Fig. 19. Two lines are engraved on the cylin-
drical part of the frame, while a single line is engraved on the
slotted surface of the sliding arm. The arm swings out of the
way to allow the gage to enter the work. This gage is used to
measure the thickness of the bottom of a shell, and is made light
GAGES
IQ9
for ease of operation, as the work itself is too heavy to be handled
rapidly.
Fig. 21 shows a gage with three sets of double flush pins for
measuring the irregular slot in the piece shown in A. On sliding-
bar gages where the tolerance is too small to be read from lines
engraved on the plunger, the movement of the bar is multiplied
by a lever which points to a graduated scale on the side of the
gage. In this way it is possible to note quickly whether the
work is machined within the requirements or not.
Flat Depth and Length Gages. A simple type of templet gage
to measure the depth of a counterbored hole is shown in Fig. 22.
Fig. 22. Simple Type of Templet Gage
One step is used as a "go" gage, while the other is used as a
"not go" gage. A similar gage can be used for measuring the
length of a shoulder. Fig. 20 illustrates a length gage, on which
the limits are indicated by scribed lines. This type is really a
special scale for measuring certain fixed dimensions.
Hole gages. A "hole gage" is a gage for testing the location
of holes relative to each other or to a specified register point.
The gages measure the distances between the outer and inner
points of the peripheries of the holes. They are, in effect, func-
tional gages. While the gage does not actually measure the
center distances, it will insure that the accepted parts may be as-
sembled properly. When designing such gages, the functional
conditions of the mating parts must be carefully checked and
200
INTERCHANGEABLE MANUFACTURING
analyzed to insure that the results desired will be secured
without imposing unnecessary hardships on the manufacturing
departments. Attention is called to the previous discussion of
these conditions in the paragraph " Dimensioning of Holes" in
Chapter V.
This type of gage comprises an almost infinite number of
designs. Some may be simple templets with studs or bushings
and plugs, while others may be almost duplicates of the drill
jigs used in producing the holes, using plugs through the bush-
ings instead of drills and reamers. In fact, if the drill jig is not
Fig. 23. Hole Gages and Plugs used in testing the Locations of Holes in an
Automobile Transmission Case
in constant use, and is kept properly checked, the addition of
suitable plugs will make it an extremely effective gage. A few
examples of gages of this type will be illustrated to indicate their
wide variety. In Fig. 23 are shown two hole gages with their
plugs. These are used for testing the locations of the holes in
the end of the transmission case dealt with in Chapter VIII.
They are examples of the simplest type of hole gages.
A more elaborate gage for the same transmission case is shown
in Fig. 24. The holes in the shifter cover face of the work are
tested with this gage for their relation to each other and to the
main shaft. The transmission case is centered on the arbor and
clamped by the hollow plug A. The stud B locates the case
radially. The plate C is placed on the shifter cover face, and is
GAGES
201
located from the central arbor by forks D on the plate fitting in
the grooves E on the arbor. The small plug F is then used to
test the locations of the six small holes in the shifter cover face.
Fig. 24. Gage used in testing Holes in a Transmission Case, the
Work being mounted on the Arbor for the Purpose
Fig. 25. Another Type of Hole Gage which is a Duplicate of the
Drill Jig used in machining the Holes in the Work
At the left in Fig. 25 is shown a hole gage unloaded, and at the
right, the same gage is shown with the work in the gaging posi-
tion. This gage is a duplicate, in .its general design, of the jig
that is used when drilling the holes in the shifter cover face.
202
INTERCHANGEABLE MANUFACTURING
Factors Involved in Gaging Threads. At the present time, a
wide difference of opinion exists as to the proper method of test-
ing threads. Owing to the fact that a thread is a complicated
composite surface, and that any composite surface is difficult
to measure readily, and also because threads are employed in
so many places for a wide variety of purposes, this condition is
one to be expected. There are three main elements in a threaded
surface: The form of the thread, the lead or pitch of the thread,
-M
Machinery
Fig. 26. Diagram illustrating the Meaning of the Terms used in
the Discussion on Thread Gages
and its diameter. The form of the thread itself is composed
of several surfaces. The form of a sharp V-thread is composed
of two angular flanks, but this form is theoretical only. A certain
amount of rounding or flattening is inevitable at the crest and
root. The form of most other standard threads is composed of
four surfaces the crest, the root, and the two flanks. Thus a
thread is a composite surface, the accuracy of which depends
upon the interrelation of the following: The diameters and form
of the crest and root, the form of the two flanks, and the lead.
There is a broad general principle in regard to limit gages
which should always be kept in mind. Where compound tol-
erances are not involved, a "go" gage with fixed measuring
surfaces may check as many dimensions at one time as desired,
GAGES 203
and effective inspection will be secured. On the other hand,
an effective "not go" gage can check only one dimension. By
effective inspection is meant assurance that specified require-
ments in regard to size are not exceeded. The above principle
must be applied with common sense, as there are a great many
requirements that drawings fail to express clearly. This is es-
pecially true in the case of surfaces that are threaded.
The gaging of an externally threaded component will now
be considered. A diagram of such a surface, illustrating the
terms that will be used in the discussion, is shown in Fig. 26.
The outside, or largest diameter, will be called the "major diam-
eter." The smallest diameter will be called the "minor diam-
eter." The top of the thread will be called the "crest," and
the bottom, the "root." The sides of the thread will be called
the "flanks." The dimension taken square with the axis of
the thread from flank to flank at any point, will be called the
"pitch diameter." The "included angle" is the angle between
the flanks of two threads, and the "lead" is the distance from a
certain point on one thread to a similar point on the next thread.
Method of Expressing Tolerances on Threaded Parts. The
correct method of gaging this thread consistently depends on the
manner in which the tolerances are expressed. In any event,
the major and minor diameters should be treated as independent
elementary surfaces. They may be gaged, when necessary,
either independently or in conjunction with the other elements
on the "go" gage. The tolerances on the other elements may
be expressed in one of two ways either as a total cumulative
error expressed in terms of the pitch diameter, which will elimi-
nate compound tolerances, or as individual tolerances on each
element, which will introduce compound tolerances with all their
resulting annoyances and inconsistencies.
If the tolerances are expressed in the second manner, the
only consistent method of gaging would be to provide suitable
gages for each element and to test each of them independently.
Any gage to test all of the elements at one time would need to
be a functional gage and its design would involve a careful study
of each set of companion threads to establish the proper dimen-
204
INTERCHANGEABLE MANUFACTURING
sions. If the tolerances are expressed in the first manner, the
gaging problem is simpler. The "go" gage may be a ring thread
gage checking the "go" dimensions of all elements. As a matter
of economy, in making the gages, it is necessary to provide clear-
ance at that part of the ring gage which would check the major
diameter, so as to facilitate lapping or grinding. This dimension
on the part may readily be checked, when desired, by a simple
snap or ring gage.
A decided difference of opinion exists as to the proper length
of engagement for thread gages. From an academic viewpoint,
Fig. 27.
Standard Ring Thread Gages and a Standard Plug Thread
Gage
the gage should be as long as the effective length of the thread
on the component. The effect of the use of longer or shorter
gages is to hold the error in the lead of the thread to a lesser or
greater amount. Many conditions exist where a relatively
large error in lead is not important. In such cases, a shorter
gage will make the manufacturing conditions much easier, and
at the same time pass only satisfactory parts to meet such con-
ditions. For example, if the length of a gage is reduced to one-
half the effective length of the thread on the component, it will
permit an error in lead of double the amount which would be
GAGES 205
allowed by a gage which is as long as the full effective length of
the thread on the component.
Theoretically, individual "not go" gages should be provided
for the major diameter, minor diameter, and pitch diameter.
Practically, the most severe requirements will usually be met
by providing a "not go" gage for the pitch diameter only. This
would be a ring thread gage, made to clear both the major and
minor diameters of the component. Its length must be such that
it will not engage over one or two turns on the component. As
a matter of fact, the strength of the engagement of a screw and
nut depends primarily on the amount of the engagement area
between the threads.
In many cases suitable inspection will be secured by the use of
a "go" ring thread gage which has clearance at the major diam-
eter of the component, and a "not go" ring or snap gage for the
major diameters of the component. At A in Fig. 27 is shown a
standard ring thread gage. The gaging of internally threaded
components involves the same problem as the gaging of those
threaded externally. A standard plug thread gage for this pur-
pose is shown at C. In this case the "go" gages are made to
clear the minor diameter of the component, while the "not go"
gages are made to clear both the major and minor diameters.
Types of Thread Gages. Standard plug and ring pipe thread
gages are tapered, and on this account one gage acts both as a
"go" and as a "not go" gage. A notch on the gage must be
flush with the end of the part within a specified number of turns.
These gages are made to clear both the major and minor di-
ameters. In Fig. 28 is shown a "qualifying" gage for the breech
mechanism of a large gun. This is a case where the thread must
start in a certain specified position. A line is graved on the
flange of this gage which must coincide within specified limits,
when the gage is screwed home, with another line on the work.
The gage measures the relationship between flanks on the work
and surface A on the gage. This gage is cleared at all points
except at surface A and on the thread flanks, the angle of the
threads being sufficient to center the gage when it is screwed
home.
206
INTERCHANGEABLE MANUFACTURING
Wing and Indicator Gages. Wing gages are used in some in-
stances where snap gages cannot be conveniently employed.
These gages are of the limit type and have two projections or
wings. The principle is that one wing must pass the surface of
the work to be gaged, while the other must not. This construc-
tion permits work to be rapidly and accurately gaged.
Many effective gages can be made simply by providing suit-
able stands or holding blocks for dial indicators. In one of the
prominent watch factories, a large percentage of the gaging
Fig. 28.
Qualifying Gage for inspecting the Breech Mechanism
of a Large Gun
equipment is constructed in this manner. These indicators can
be set up not only to measure lengths, diameters, etc., but also
profiles and locations. Several standard indicating gages are
now on the market which require very little in the way of hold-
ing blocks, etc., to adapt them to measure a great variety of
surfaces. In Fig. 29 is shown an amplifying gage, which is very
GAGES
207
rapid in operation. In Fig. 30 is shown an indicator used in con-
nection with bench centers for the purpose of testing the con-
centricity of a rifle barrel that is assembled with a receiver.
Fig. 29. Amplifying Gage which permits the Rapid Inspection of
a Large Variety of Surfaces
Functional Gages. A functional gage is one that tests the
functional operation of a component without strict adherence
208
INTERCHANGEABLE MANUFACTURING
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GAGES 209
Functional Gaging of Gears. The satisfactory gaging of gear
teeth is a complicated and difficult problem, if each element is
tested independently, because of the many factors involved.
If the testing is reduced to a functional inspection, however, the
problem becomes simpler. The prime object of gears is to trans-
mit a uniform motion, and if this result is accomplished it does
Fig. 31. Machine for testing the Uniformity of Motion between Two Gears
not matter what the exact contour or dimension of any of the
operating surfaces may be. On the other hand, if the gears do
not accomplish this result, the only good that knowledge of the
various discrepancies does, is to indicate the possible causes for
the error. This is useful information in the making of the gears,
but has little value to the inspector who must decide whether or
not the gears are acceptable.
2IO
INTERCHANGEABLE MANUFACTURING
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Fig. 2. Information placed on Drawings used in Selective As-
sembly Manufacturing to facilitate the Grading of Parts
part only is sorted according to size. In such cases, instead of
defining grades for the major part, a note to the following effect
is substituted, "Select stud to suit at assembly."
In many cases, two separate drawings are made of a part
which is to be graded before assembly. One shows the manu-
facturing tolerances only, so as not to confuse the machine op-
erator, while the other gives the proper grading information.
In Chapter V, five laws of dimensioning were given for inter-
changeable parts. All of them, except the third, apply equally
to parts which are selectively assembled. The Jaws which apply
to this method of manufacture will be given again.
228 INTERCHANGEABLE MANUFACTURING
Laws of Dimensioning for Selective Assembly, i. In manu-
facturing, there is only one dimension (or group of dimensions)
in the same straight line which can be controlled within fixed
tolerances. This is the distance between the cutting surface of
the tool and the locating or registering surface of the part being
machined. Therefore, it is incorrect to locate any point or sur-
face with tolerances from more than one point in the same
straight line.
2. Dimensions should be given between those points which
it is essential to hold in a specific relation to each other. The
majority of dimensions, however, are relatively unimportant in
this respect. It is good practice to establish common location
points in each plane, and to give, as far as possible, all such di-
mensions from these common location points.
3. This law relates to the proper basic dimension to be given
on the component drawing. In selective assembly the conditions
which must be met are so different, that no general rule in this
respect can safely be given. Each case requires special con-
sideration.
4. Dimensions must not be duplicated between the same
points. The duplication of dimensions causes much needless
trouble, due to changes being made in one place and not in the
others. It causes less trouble to search a drawing to find a di-
mension than it does to have them duplicated and more readily
found but inconsistent.
5. As far as possible, the dimensions on companion parts
should be given from the same relative locations. Such a pro-
cedure assists in detecting interferences and other improper
conditions.
Similarity of Specifications, Equipment, Gages, and Inspec-
tion Methods. The general principles of specifications for inter-
changeable manufacture, which were given in Chapter VII,
hold true for manufacturing on a selective assembly basis. Par-
ticular care should be taken to specify clearly the parts to be so
manufactured and the method of grading to be followed. If the
component drawings are properly made, there is no difference
in the actual productive operations between manufacturing on
SELECTIVE ASSEMBLY 2 29
an interchangeable and on a selective assembly basis. In both
cases, the task is to produce parts within specified tolerances.
Therefore, the conditions governing the design of the manu-
facturing equipment are constant.
The working and shop inspection gages for either interchange-
able parts or those made for selective assembly are similar. Ad-
ditional gages for the purpose of grading, however, are required
for the final inspection. Often these are indicator gages, which
promote the rapid sorting of the product. In other cases, gages
with successive steps or with slightly tapered measuring surfaces
are used.
The detailed shop inspection differs in no particular from
that employed in interchangeable manufacturing. The only
difference is the addition of the selection and grading of the
parts after completion. Sometimes the actual selection takes
place at the assembly itself. If the first part tried is too large
or too small, another is chosen which assembles properly. If
the rate of production is relatively low, this procedure is often
satisfactory. In fact, it is often observed in the assembly of
parts which are supposed to be interchangeable. But if the pro-
duction is high, too much time will be lost by the assembler to
make the practice economical.
In general, manufacturing on an interchangeable basis will
be found more economical than manufacturing on a selective
assembly basis, provided the design permits sufficient clearances
to allow reasonable manufacturing tolerances. In the first place,
a larger stock of parts is required for selective assembly to in-
sure that companion parts of suitable sizes will always be avail-
able. In the second place, the additional expense of sorting,
whether done by an inspector or by the assembler, is involved
in this method of manufacture. In its actual operation, the main
difference between selective assembly and interchangeable manu-
facturing is that overlapping tolerances are required in selective
assembly while such tolerances are absolutely wrong in inter-
changeable manufacturing.
CHAPTER XII
SMALL-QUANTITY PRODUCTION METHODS
INTERCHANGEABLE manufacturing methods, as considered in
previous chapters, relate to a comparatively high rate of con-
tinuous production for which the expense of a complete equip-
ment of special tools, fixtures, and gages is justified, and for which
the time and constant study required to keep the component
drawings in proper shape is essential to prevent any break in the
continuous flow of production. But, when any commodity is
manufactured intermittently in small lots, the cost of such pro-
cedure is often greater than the results justify. Nevertheless,
many of the principles involved in interchangeable manufactur-
ing can be applied with economical results to the production of
small quantities.
When comparatively few machines of one type are manu-
factured, few parts are duplicated in great numbers, and so,
similar surfaces, rather than similar parts, should receive at-
tention. This requires a thorough analysis of the four following
factors: (i) The possibilities of standardizing the nominal sizes
so as to have the smallest possible number; (2) the possibilities
of standardizing the minimum clearances between companion
parts for each standard size to meet the various functional con-
ditions; (3) the possibilities of standardizing the tolerances for
the various standard sizes and conditions; and (4) the deter-
mination of the best surface to be maintained as a standard
size; that is, whether it should be the maximum male surface
or the minimum female surface. Until these factors are deter-
mined, it will be difficult to lay out a simple and consistent pro-
cedure that will result in economical production.
One caution must be given before further discussion is made
of the subject of standardization. In order to obtain the best
results, all known conditions involved must be given due weight;
but the consideration given to any factor should depend on the
230
SMALL-QUANTITY PRODUCTION 231
frequency of its occurrence. One usual condition will far out-
weigh several exceptional conditions. An unusual condition
will always require special consideration regardless of attempts
at standardization. If an established standard will not meet
the required condition, it should not be used. Regardless of the
extent of standardization, exceptions will always occur and must
be dealt with. Thus, a standard is theoretically the best con-
struction that will satisfy the majority of the known conditions.
In practice, however, all existing conditions must be met. There-
fore, if an established standard is unsatisfactory for any par-
ticular service, unusual conditions are present and must be met.
Standardization of Nominal Sizes. It is evident that if the
number of nominal sizes employed is reduced, the number of
standard tools and gages required in the production department
will be correspondingly reduced. As an example, the matter of
reducing the number of nominal sizes of shafting was recently
taken up by a committee of the American Society of Mechani-
cal Engineers, and their recommendations are well worth follow-
ing. Two distinct but closely related problems were considered.
First, the standardization of the diameters of shafting used for
the transmission of power, such as lineshafts and countershafts,
etc. This usually consists of cold-rolled shafting which is used
without machining. The following fourteen sizes have been
adopted as standard for this type of shafting: Y!~? I T 3 6"? I T 7 6? I tt'
iyih 2 rV 2^, 2f| , 3^, 3 ff , 4re, 4H 5r 7 6 and 5^ | inches.
The second problem confronted by the committee was the
standardization of the diameters of machined shafting used by
machine-tool builders in making their product. For this pur-
pose, the following have been adopted as standard: Sizes up to
2\ inches, increasing by intervals of sixteenth inches; from
2 \ inches to 4 inches inclusive, by eighth inches; and from
4 to 6 inches by quarter inches. The foregoing sizes are sufficient
to meet the majority of conditions. If proper attention is given
to this point in the design of a mechanism, the use of unneces-
sary intermediate sizes will be eliminated.
Standardization of Minimum Clearances. The amount of the
minimum clearance between companion parts depends on many
232 INTERCHANGEABLE MANUFACTURING
factors. Among them are the size of parts, the length of the
bearing, the class of fit required, and the conditions, such as
temperature, etc., under which they must operate. The classes
of fits which apply to cylindrical parts, for example, may be ap-
proximately summarized as follows: (i) Running fits, where
one part must revolve freely; (2) sliding fits, where one part
must slide freely; (3) push, or dowel fits, where neither part is
required to revolve but where both parts must assemble readily,
and be held in alignment; (4) force, driving, or shrinkage fits,
which are made with pressure or by shrinkage, and used in as-
sembling parts which must be held in fixed positions.
The amount of the minimum clearance for a running fit is
dependent, to some degree, on the length of the bearing. A
long bearing, for example, may have a somewhat greater clear-
ance than a short one. The proper length of the bearing depends
on the load and the material used in the bearing. The load con-
trols, to a large extent, the diameter of the bearing. Thus, the
first step toward standardizing the minimum clearances is to
determine the most common material employed in making the
bearings, and to establish standard lengths of bearings for the
various diameters of shafts. The exceptions which will inevit-
ably develop must, of course, be treated on their own merits.
Take, for example, a long feed-screw or a long bending roll which
is supported on the ends. Regardless of the diameter or length
of the bearing, greater minimum clearances than the established
standard would be required. If the number of similar excep-
tions is appreciable, supplementary standards can be developed
to meet them.
Another factor which must be considered before the stand-
ards can be safely established, relates to the conditions under
which the parts must operate. Thus, if the parts must operate
when subjected to higher or lower temperatures than normal
shop temperatures, due allowance must be made. On the other
hand, if such temperatures are the exceptions, the corresponding
clearances must be exceptions. A good example of this occurred
with a concern that manufactures power presses, several of which
were ordered by a plant in Alaska. The shed in which the presses
SMALL-QUANTITY PRODUCTION 233
were set up was unheated; for this reason the lubricating oil
became very heavy, and the presses would not work properly
until the clearances in the bearings had been increased sufficiently
to permit the heavier oil film.
In a similar manner, standards for all other classes of fits
desired should be developed, not only for cylindrical but also
for all other common surfaces. Every attempt should be made
to standardize the more common surfaces and conditions first,
and the others as it proves advisable.
Standardization of Tolerances. When manufacturing inter-
changeable parts in large quantities, the tolerances should be as
great as the functioning of the mechanism permits, in order to
secure the greatest economy of manufacture. As the functional
conditions will vary so much, this practice seldom permits any
great standardization of tolerances. On the other hand, when
manufacturing in small quantities, using standard tools and
equipment wherever possible, the tolerances should represent, as
far as possible, results which may be consistently obtained with
the use of standard tools and which will insure that the parts
will function properly. The first step, therefore, toward stand-
ardizing the tolerances is to determine the accuracy of parts pro-
duced by the various manufacturing methods. In this way,
standard tolerances for each method will be developed, such as
tolerances for grinding, reaming, drilling, boring, finish-turning,
rough- turning, milling, planing, etc. The next step is to es-
tablish a practice for machining the various functional surfaces
according to the requirements which they must meet. For ex-
ample, on shafts, all running fits should be ground, all bearings
should be reamed, etc. In general, the extent of the tolerances
will increase as the sizes of the parts increase. Thus, for each
method of manufacturing, a standard tolerance should be de-
termined for each standard size.
Maximum Male or Minimum Female Size as Standard. In
considering whether the maximum male or minimum female
basic size should be the standard, shafts and their corresponding
holes will be dealt with. In general, the tools for making the
holes, such as drills, reamers, etc., are nonadjustable, while the
234
INTERCHANGEABLE MANUFACTURING
tools for machining the shafts are either adjustable in themselves
or are carried on adjustable members of the manufacturing
machines. This makes a strong argument in favor of main-
taining the basic size of the holes as standard. This practice is
now becoming quite universal. Of course, an exception to this
will always occur when cold-rolled shafting is to be used without
machining.
A little study will show that this practice can be applied to
advantage on other surfaces. The basic dimension of the width
Fig. 1. Four-speed 10-horsepower Speed-box attached to the
Rear of a Radial Drilling Machine
of a slot or groove can be kept standard, and the necessary clear-
ance can be provided by reducing the size of the mating member.
Effecting Economy by Using Standard Parts. The develop-
ment and use of standard parts offers one of the greatest op-
portunities for economy in the manufacture of small lots of com-
modities. The greatest difficulty in the way of accomplishing
this is the necessity of training the designers to use -them. There
seems to be a fear among these men that the extensive use of such
standard parts will limit their initiative and curtail their origi-
nality. The fact is the extensive use of standard parts will
eliminate a large amount-of the designer's drudgery, thus freeing
much of his time and thought for creative work.
In order to promote the use of standard parts, records relating
to them should be made in a simple and convenient manner.
SMALL-QUANTITY PRODUCTION 235
Certain types of parts, such as levers, gears, bushings, studs,
pulleys, etc., should always be considered as potential standard
parts, even if certain of them are used merely in a single place,
and should be tabulated or indexed for ready reference. In this
way a series of standard parts is readily begun. After the start,
a study should be made and a balanced series laid out to cover
possible future needs. Otherwise the series will be unbalanced,
that is, the differences between some of them will be very small,
Fig. 2. Speed-box applied to a Horizontal Boring Machine
while between others they will be very great. Such a series would
eventually contain an excessive number in order to cover a given
range. The essence of standardization is to reduce the number
of standards to a minimum.
Standardizing Unit Assemblies to Suit Several Machines.
The design of the commodity which is to be manufactured in
small lots should be carefully studied and every opportunity
taken to incorporate smaller unit assemblies. As with many
of the component parts, each unit assembly should be considered
as a potential standard. If this is done, many of them, such as
oil-pumps, speed- and feed-boxes, reversing mechanisms, etc.,
will be. found applicable to several machines.
236
INTERCHANGEABLE MANUFACTURING
Fig. 3.
Application of Four- speed 10-horsepower Speed-box to
Another Horizontal Boring Machine
Fig. 4.
Large Blotter which is equipped with Same Speed-box as
shown on Other Machines
SMALL-QUANTITY PRODUCTION 237
An interesting example of what is possible along these lines
is shown in the accompanying illustrations. Here a standard
four- speed, lo-horsepower speed-box is illustrated on various
types of machine tools. In Fig. i, this speed-box is shown at A
attached to the rear of a radial drilling machine. In Figs. 2 and
-3, it is attached to different types of horizontal boring machines,
its position again being indicated by A. This speed-box is also
illustrated in Fig. 4, at A, as part of a large slotter. The various
machines themselves are made in small lots as required, but
these speed-boxes, and other common standard parts, are made
in relatively large lots and carried in stock. This example indi-
cates to a small degree the many possibilities along these lines.
It is of interest to note in this connection that one large cor-
poration which controls several plants building many different
kinds of machine tools, has been carrying out a standardization
program for several years. Certain types of parts and some unit
assemblies which have been developed at the different plants
have been compared and discussed.
In most cases, this discussion has led to the adoption of a
certain series of them as standards for all plants. In addition,
the plant best fitted for that particular work has been selected
to manufacture all of such parts or unit assemblies for all the
plants. In this way, the economies resulting from producing
in large quantities are secured, where no one of the plants in-
volved has a very large production of any one size and type of
machine tool. As with standard parts, if any extensive use is
to be made of standard unit assemblies, they must be recorded
and tabulated in a simple and convenient form for ready reference.
Component Drawings for Small-quantity Production. The
component drawings of a mechanism which is to be manufactured
in small lots will vary considerably from those used when the
production is large. As a rule relatively few of the operating
clearances and also few of the manufacturing tolerances will be
specified. Notes, such as " force fit," "running fit," etc., will
be the usual method of noting this information.
To determine properly the correct clearances and tolerances
for any surface, much time is required for studying the design,
238 INTERCHANGEABLE MANUFACTURING
checking results obtained on the various surfaces in production,
etc. When parts are made in small quantities, there is little or
no opportunity to do this work, although to specify these re-
quirements without such study is generally useless. At best
they are only a guess, and are often established by some one who
knows little of the actual working conditions.
When a part becomes standard, or when elementary surfaces
as in holes and on shafts are standardized, the production of
these parts and surfaces is large enough to permit the necessary
study and tests to be made. Here the component drawings
should specify the maximum and minimum sizes exactly as in
the case of component drawings for large-quantity production.
A full discussion of the requirements of such drawings is given in
Chapters V and VI.
Manufacturing Equipment. Relatively little special manu-
facturing equipment is provided for manufacturing in small lots.
Generally nothing more than boring jigs and planer templets
is necessary. The machine operators are usually skilled machin-
ists and perform most of the machining cuts on standard ma-
chine tools with the use of standard cutting tools. Each piece
of work is set up and clamped to the bed of the machine with
only the aid of standard measuring tools to test its position.
With such a type of operator, the component drawings do not
actually require the same amount of detailed information as is
necessary when the work is performed by less highly skilled labor.
In many cases, not even boring jigs or planer templets are
provided ; the work is first laid out, and then the machining cuts
are taken to match the lines drawn. As the production increases,
however, more and more special manufacturing equipment can
be used to advantage. As the quantities become large enough
to pay for the cost of this equipment, its provision and use will
greatly promote economical production. The essential require-
ments of this equipment, whether much or little is provided, are
identical with the requirements of equipment for manufacturing
large quantities.
Gages and Methods of Inspection. The gages used in this
type of manufacturing consist principally of standard measuring
SMALL-QUANTITY PRODUCTION 239
instruments and plug, ring, and snap gages of standard sizes.
Thread gages for standard threads are also used to an appreciable
extent, as well as adjustable snap and plug gages which may be
set with the aid of standard measuring instruments or standard
size blocks. Where adjustable gages are used, it is very desir-
able to have means for sealing them so that they may be adjusted
in the tool-room but not promiscuously in the shop.
When boring jigs are provided, these form in themselves ef-
fective gages for testing the location of holes by using suitable
plug gages and bushings in place of the boring tools. When
planer templets are employed, these also make effective gages,
an indicator being substituted for the planer tool. As with
other manufacturing equipment, gages should not be provided
until the volume of production is great enough to make their
use economical.
The inspection of parts made in small quantities, where stock
is left on many pieces for fitting at assembly, and where the
component drawings give incomplete information, is quite dif-
ferent from the inspection of parts made in large quantities.
The extent of the inspection required depends to a large extent
on the methods of paying the workmen. For example, when
the wage is paid on a time basis alone, this inspection is relatively
slight. On the other hand, if piecework prices or bonuses are
paid, a more complete inspection is required, as a bonus should
not be given for spoiled work.
Most of this inspection requires a skilled workman, as little
special gaging equipment is available, and this necessitates the
use of standard measuring instruments in many cases and also
many special set-ups. In addition, with the incomplete com-
ponent drawings, the inspector must be sufficiently experienced
to tell whether or not the parts as completed will function
properly when no fitting is to be done at assembly. When fitting
is required, he must also be able to determine whether or not the
amount of stock left for this purpose is suitable.
On large pieces, the inspection should be made while the part
is set up on the machine used in finishing it, so that if corrections
are necessary, they can be made without an additional set-up.
240 INTERCHANGEABLE MANUFACTURING
When no special locating fixtures are employed and a part is re-
moved from the machine, it is almost impossible to relocate it
in order to correct one surface and yet keep the proper align-
ment with the other finished surfaces. The inspector should be
capable, not only of detecting errors, but also of convincing the
workman of them without antagonizing him. The inspection
of standard parts should be carried on in the same. general man-
ner as the inspection of parts produced on a large-quantity basis.
The assembling of small lots of machines usually involves a
considerable amount of fitting. For example, all the small holes
are not drilled in the larger pieces until assembly. Small brackets
and similar parts are then clamped in position and the holes for
their holding screws, dowels, etc., are located from them. Slid-
ing members are scraped to fit each other, and to correct their
alignment. This requires a certain amount of machinery on the
assembling or erecting floor and also the services of skilled me-
chanics. However, as much of the machining as possible should
be completed before the parts reach the assembling department.
In most cases, this requires the provision of special manufactur-
ing equipment and gages. Thus as the quantity of the produc-
tion increases and more and more special equipment is furnished,
less fitting at assembly is necessary. After the machines are as-
sembled, they should be carefully tested for alignment, back-
lash, etc., and when possible, they should be actually tried out
on work of the character they are made to perform. This last is
the crucial test because upon its results the success or failure
of the mechanism is judged.
CHAPTER XIII
SERVICE FACTOR IN INTERCHANGEABLE
MANUFACTURING
IN the final analysis, no manufactured machine or device is
ever purchased for itself alone, but is acquired for the purpose
of securing the service which it is supposed to render. Thus,
for example, the purchase of a reamer is the purchase of reamed
holes of a desired quality or standard. Consequently it follows
that the reamer which produces the most reamed holes of the re-
quired accuracy at the least ultimate cost is the best reamer
and is finally recognized as such. Its first cost may be higher
than others, yet if it produces more holes during its life, or pro-
duces them more quickly or with less power, the average cost of
the reamed holes may be much less than those produced with a
cheaper tool. In like manner, the purchase of a machine tool
represents the purchase of machined surfaces; the purchase of
a typewriter, typed letters; the purchase of a sewing machine,
sewn seams; of an automobile, transportation, etc.
The ultimate test of any manufactured article is the test of
service. The component parts may be absolutely interchange-
able, the manufacturing processes may be developed to produce
large quantities economically, and the inspection may be as
rigid as possible; yet if the required service is not rendered, all
of this work is useless.
Preparing Functional and Manufacturing Designs. If a com-
modity is to give satisfaction, this service must be built into it
at every stage of its development and manufacture. The first
conception of a new mechanism develops from the realization
of some service to be performed. The functional design of this
mechanism is solely the development of some mechanical means
of performing this service; this thought is paramount and every
other consideration is subordinate to it. It is only after this re-
241
242 INTERCHANGEABLE MANUFACTURING
suit has been obtained that any great thought is given to the
matter of producing the mechanism commercially.
The primary purpose of the manufacturing design is to de-
velop the functional design into one which can be economically
manufactured; yet, at the same time, the greatest care must
be exercised to maintain all the serviceable qualities of the origi-
nal design. The factor of economical manufacture must never
be the controlling one when economy is secured at the expense of
service rendered. The customer is purchasing this service, and
any action which may rob him of some part of it is unjustifiable.
The development of the correct manufacturing design is a long
process. There are no laboratory tests which will show all the
requirements and results of service. The largest part of this
information must necessarily come from the study of the results
obtained from the commodity when in actual service.
A large amount of this information can be readily secured
if proper attention is given to every complaint from customers.
Too often, information from such sources is treated as an an-
noyance to be smoothed over rather than as a definite problem
to be solved or as a matter which is of far more value and im-
portance to the producer than to the customer. In general,
a complaint from a customer results from one of three causes:
First, some faults in design, workmanship, or material may
exist in the mechanism which prevent it from giving the service
which is due the customer. If this is the case, prompt steps
should be taken to correct the trouble at its source. Obviously
this matter is of more importance to the producer than to the
user if he hopes to remain long in business. Second, the cus-
tomer may not thoroughly understand the handling and care
which the mechanism requires. In such cases it is of the great-
est importance to the manufacturer that the customer obtains
the needed information, or else the reputation of his product
will inevitably suffer.
The third complaint is usually due to the customer's attempt
to perform work for which the product was not intended or
which is beyond its capacity. It is essential that the manu-
facturer know the limitations of his product. Furthermore,
SERVICE FACTOR 243
information derived from complaints of this sort often leads to
modifications of the product which greatly increase its field of
usefulness. Complaints of all sorts should be carefully checked
and acted on accordingly. Several manufacturing concerns have
a man or division in their engineering department that in-
vestigates all complaints from customers, using the information
so gained in the improvement of their product. Only by such
knowledge of actual results obtained under many conditions can
the maximum service be built into a product.
Keeping Specifications up to Date. The specifications should
include all information which is needed to produce a commodity
capable of giving the desired service. In whatever form they are
kept, they should be constantly revised to keep abreast to the
needs of service. For example, if the material specified for a
certain part proves too weak in actual use, it must be altered.
Thus, the part may require a stronger material, a different kind
of heat-treatment, or a strengthened design. Often it may be
found that the original requirements of many surfaces or parts
are not the correct ones. All of this information, if kept in such
form that it is always available, will be found invaluable in the
development of future products; products which will contain
from the start a higher quality of service than any of the pre-
ceding ones.
Planning Production to Obtain Requisite Service. Every
part of the manufacturing equipment provided should be selected
or designed with the object of producing parts capable of render-
ing the required service. The design of the commodity itself,
if properly recorded on the drawings, will emphasize these points;
yet a careful check should be made to insure that no vital factor
has been overlooked.
The constant care which must be exercised in every stage of
the actual production determines in a large measure the character
of the service delivered. No operation is too unimportant to be
neglected. This care, however, must be taken by each individual
workman. To obtain the necessary cooperation, every effort
must constantly be made to develop in each workman the spirit
of true craftsmanship. A craftsman, in the opinion of the writer,
244 INTERCHANGEABLE MANUFACTURING
is a man who takes pride in the work and skill of his hands and
brain; who feels that each result of his labor is a monument to
himself; and whose enthusiasm and consciousness of power pre-
vent him from doing any work but his very best. No man can do
justice to his own capabilities unless he is interested in and proud
of the results of his labor. The manufacturer must realize that
he should have a vital interest in the proper training of each
one of his workmen, and should use every means in his power
to foster true craftsmanship in all branches of his establishment.
No part of any work is too elementary to justify such an attitude.
Inspecting Parts to Insure Service. Every inspector must
keep in mind at all times the requirements of service which the
parts under inspection must render. This service is the sole pur-
pose for which the parts are made. If they will render it, the
parts are correct; if not, they are incorrect. In a well-balanced
organization, the inspection is not carried on to discover the
faults which others have committed, but rather to protect the
customer and the firm's good name as well, by guarding against
the possibility of faulty work going out despite all precautions
taken in the productive departments. Yet, even with the most
rigid inspection, some flaws remain hidden and are not discovered
until the commodity is in the hands of the customer. With an
honest inspection, such occurrences will be the exception, but
without proper safeguards, these occurrences are apt to be the
rule, and the customer will soon learn it, to the disadvantage of
the manufacturer.
The majority of mechanical products are tested on work of
the type they are built to perform, before they are shipped.
Needless to say, no attempt should be made to favor the com-
modity in such tests. Every effort should be made to detect
any faults, and each fault detected should be permanently cor-
rected. The interest of the manufacturer in the commodity
should not cease when it reaches the customer. It is of more
interest to the manufacturer than to the purchaser to see that
his product is employed on the work it can best perform, and to
see that it performs its maximum service. By so following up
his product, he not only makes a satisfied customer but also
SERVICE FACTOR 245
creates new markets for his product. Furthermore, as noted
previously, the information gained by observing his mechan-
isms in service under many varying conditions will be invaluable
to him in developing and improving his product, as well as often
pointing the way to the development of new products.
Manufacturers in a number of different lines have established
well-organized service departments, with a view to insuring that
the machines or devices that they build will give the highest
possible service and satisfaction to their customers. Such serv-
ice departments are well known in the automobile and type-
writer fields, but similar departments, somewhat different in
their nature, on account of the varying conditions under which
the product is used, are also found in the machine tool field,
where some manufacturers have highly organized service de-
partments for determining the best conditions under which
the customer's work may be performed. Through such service
departments it has often been found possible to increase greatly
the output of the machines built.
INDEX
PAGE
Accuracy, definition 27
required, of gages 1 79
Alignment and concentricity 74
Assembled mechanisms, testing 223
Assembly, selective, manufacturing for 224
Atmospheric fits, definition 24
Basic dimensions, exception to general rule for 81
Basic size, definition 20
Chip clearances, proper, necessity for 134
results obtained with 136
Clamping devices, pneumatic 171
Clearances 4
maximum, definition 22
minimum, definition 21
minimum, in small-quantity production, standardization of 231
proper chip, necessity for 134
proper chip, results obtained with 136
Clearances and tolerances 35
in selective assembly manufacturing 225
Clearance surfaces, definition 24
Component drawings 6
definition 27
dimensions and tolerances on 226
examples illustrating practice in making 77
for small-quantity production 237
principles in making 46
Compound tolerances 58
definition 26
dimensioning to prevent 93
Composite surfaces, definition 25
dimensioning 56
Concentricity and alignment 74
Contour or profile gages 192
Costs, distribution of, in economical production 109
clerical and accounting work 118
educational department 112
employment department 112
247
248 INDEX
PAGE
Costs, factory 109
general product charges 1 16
health and safety of employes, maintaining 113
inspection and testing of product 1 20
interest on investment, depreciation and insurance 114
lack of work or of labor 115
payroll, making up 112
power charges 114
records, keeping of 115
specific product charges 1 16
supervision 112
Cutting tools and sharp corners, protection from 132
Cutting tools, tolerances allowed on 141
Data, manufacturing, general 109
specific 108
Definitions of terms in interchangeable manufacturing 18
accuracy 27
atmospheric fits 24
basic size 20
clearance, maximum 22
clearance, minimum 21
clearance surfaces 24
component drawings 27
composite surfaces 25
compound tolerances 26
elementary surfaces 25
function ,. 19
functional surfaces 23
interference 22
limit 19
maximum metal size ' . . . 21
minimum metal size 21
model size 20
operating surfaces 23
operation drawings 28
precision , 26
register or working points 26
selective assembly 18
tolerances 19
tolerances, compound 26
unit assembly 26
Design, classes of .- 31
effect of, on successful interchangeability 3
functional and manufacturing, preparing 241
function of 30
manufacturing, functioning tested by manufacturing model 40, 44
INDEX 249
PAGE
Design, of fixtures, efficient, examples of 130
of jigs and fixtures 125
simplifying 32
Designing fixtures, important factors in 1 2O
Designing for assembly and service 38
Dial indicator contour gages I9 3
Dimensioning, careless, possibility of draftsman's errors increased by 53
component drawings, examples illustrating practice 77
composite surfaces 56
force fits 61
for selective assembly, laws of 228
holes 65
in interchangeable manufacturing, laws of 48
laws of, violations 49
profile surfaces 64
to prevent compound tolerances 93
Dimensions, basic, exception to general rule for 81
on component drawings 8
Dimensions and tolerances on component drawings 226
Drawings, component 6
discrepancy between part and 218
functional 47
manufacturing 47
Drawings and specifications, incomplete 217
Drilling and milling fixtures 175
Drilling, reaming and milling operations, examples of, in interchangeable
manufacture 157
Economy, in interchangeable manufacturing i, 105
in production 105
Educational department, cost of " 112
Elementary surfaces, definition 25
Employment department, cost of 112
Equipment for interchangeable manufacture 14, 121
Examples, illustrating practice in making component drawings 77
of efficient fixture design 130
of special equipment for interchangeable manufacture 145
Expenses, due to direct labor 112
indirect, belonging to general productive equipment 114
indirect, due to product "6
indirect factory, distribution of 1 1 1
Experimental model, function of 3
Facing bar, example of use of, in interchangeable manufacture 173
Factory cost of production 109
Factory expenses, indirect, distribution of in
250 INDEX
PAGE
Fixtures and jigs, accessibility of locating points 134
checking and testing I4I
simplicity and standardization of 138
Fixtures, designing, important factors in 1 29
design of, in interchangeable manufacturing 14
design of jigs and I2 5
efficient design of, examples 130
methods of manufacturing I3 8
milling and drilling 175
tolerances indicated on drawings 140
Flat depth and length gages 199
Flush-pin gages 196
Force fits, dimensioning 61
Functional designs, preparing 241
Functional drawings, purpose of 47
Functional gages : 207
Functional surfaces, definition 23
Function, definition 19
Function and essential requirements of product, indicated in specifications. 106
Gage requirements controlled by ultimate economy 182
Gages, classified according to use 1 79
combination snap and plug, application of 189
dial indicator contour 193
flat depth and length 199
flush-pin 196
for checking parts in interchangeable manufacture 12
functional 207
hole 199
in interchangeable manufacturing 177
inspection, requirements of " 54
location of holes checked by 67
master and reference 215
plug 186
profile or contour 192
receiving 193
required accuracy of 179
ring 185
sliding-bar 198
snap 183
special, for rapid inspection 212
thread, types of 205
wing and indicator. 206
Gages and material, inspection of 223
Gages and methods of inspection in small-quantity production 239
Gaging of gears, functional 209
machine for.. 211
INDEX 251
PAGE
Gaging threads, factors involved in 202
Gears, functional gaging of 209
machine for gaging 211
tolerances, specifying 76
Hole gages 199
Holes, dimensioning 65
expressed tolerances on location of 70
location of, gages for checking 67
tolerances for "group" location of 72
Indicator gages 206
Inspection, of gages and material 222
of parts to insure service 244
of product, final 16
of product, in the shop 16
of work, final 222
rapid, special gages for 212
shop, personnel of 221
Inspection and testing 217
of product 1 20
Inspection and working gages, relation between 181
Inspection department, position of 219
Inspection gage requirements 54
Inspection methods and gages in small-quantity production 238
Inspection methods, shop 219
Interchangeable manufacturing, compared with selective assembly manu-
facturing 228
economical production in 105
equipment for 121
examples of special equipment for 145
machine design 29
service factor in 241
specifications for 10
Interchangeable principle, applying 35
Interchangeability, between parts made in different shops 182
definition 18
desirable extent of 2
effect of design on 3
Interference, definition 22
Jigs, examples of use of, in interchangeable manufacture .... 147, 162, 169, 175
Jigs and fixtures, accessibility of locating points 134
checking and testing 141
design of 1 25
simplicity and standardization of i3 8
252 INDEX
PAGE
Labor, direct, expenses due to 112
lack of, effect on cost of production 115
skilled and unskilled, value of 15
Laws of dimensioning, in interchangeable manufacturing 48
violations of 49
Length gage 199
Limit and tolerance, definitions 19
Machine design in interchangeable manufacture 29
Machine-hour rate 114
Machine tools, selection of 121
Manufacturing, causes of variations in 66
data, general 109
data, specific 108
design, preparing 241
drawings, purpose of 47
equipment for small-quantity production 238
model, functions of 4
model, to test functioning of manufacturing design 40
tolerances 5
Manufacturing fixtures, methods of 138
Master gages and reference gages 215
Materials, factors governing choice of 32
cost 33
machining qualities 33
service required 34
source of supply 33
weight of finished product 34
Maximum metal size, definition 21
Milling and drilling fixtures 175
Milling and profiling machines, example of use of, in interchangeable manu-
facture : 154
Milling operations, example of, in interchangeable manufacture 158
Minimum metal size, definition 21
Models, for standard of precision 43
in interchangeable manufacturing, purpose of 40
tolerances tested by 42
Model size, definition 20
Operating surfaces, definition 23
Operation drawings, definition 28
Payroll, cost of making up 112
Plug and snap gages, combination, application of 189
Plug gages 186
Pneumatic clamping devices 171
INDEX 253
PAGE
Precision, definition 26
model for standard of 43
Production problems 15
Product overhead 116
Profile surfaces, dimensioning 64
Reaming, drilling and milling operations, example of, in interchangeable
manufacture 157
Receiving gages 193
Reference gages r 215
Ring gages 185
Safety and health of employes, cost of maintaining 113
Selective assembly, definition 18
manufacturing for 224
Selective assembly manufacturing compared with interchangeable manu-
facturing 228
Service factor in interchangeable manufacturing 241
Service, inspection of parts to insure 244
requisite, planning production to obtain 243
Shop inspection, methods 219
personnel of 221
Sliding-bar gages 198
Small-quantity production methods 230
Snap and plug gages, combination, application of 189
Snap gages 183
Specifications, for interchangeable manufacture 10
function and requirements of product indicated in 106
keeping up to date 243
specific and general information 1 20
Specifications and drawings, incomplete 218
Standardization, of minimum clearances in small-quantity production 231
of nominal sizes, in small-quantity production 231
of parts 37
of tolerances in small-quantity production 233
Standardization and simplicity of jigs and fixtures 138
Standardizing unit assemblies to suit several machines 235
Standard parts, economy effected by use of, in small-quantity production .... 234
Standard sizes, based on maximum plug or minimum hole diameters 233
Supervision, cost of 112
Terms used in interchangeable manufacture, definitions . . . 18
Testing and checking jigs and fixtures 141
Testing and inspection 217
Testing assembled mechanisms 223
Threaded parts, tolerances on, method of expressing 203
Thread gages, types of 205
254 INDEX
PAGE
Threads, gaging, factors involved in 202
Tolerance, allowed on cutting tools 141
compound 58
compound, definition 26
for "group " location of holes 72
maintaining, on tools machining several surfaces 143
manufacturing 5
manufacturing, reduced by careless dimensioning 53
on fixture drawings 140
on threaded parts, method of expressing 203
specifying, for gears 76
standardization of, in small-quantity production 233
tested by models 42
Tolerance and limit, definition 19
Tolerances and clearances 35
in selective assembly manufacturing 225
Tolerances and dimensions on component drawings 226
Tools, cutting, and sharp corners, protection from 132
cutting, tolerances allowed on 141
for machining several surfaces, maintaining tolerances on 143
machine, selection of 121
wear on cutting edges of, results of 128
Unit assembly, construction, advantages of 37
definition 26
standardizing, to suit several machines 235
Wing and indicator gages 206
Working and inspection gages, relation between 181
Work, inspection of, final 222
Working or register points, definition 26
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