key: cord-0267791-c1qrickg
authors: Inada, Hidenori; Inada, Hiroshi; Yagishita, H.
title: Small Diameter-Deep Hole Drilling for Hard-to-Machine Metals ― Drilling of ϕ1.0×400 mm for Ti6Al4V ―
date: 2020-12-31
journal: Procedia Manufacturing
DOI: 10.1016/j.promfg.2020.05.066
sha: 660321bf42001afbc78b11aba72d784438f680c8
doc_id: 267791
cord_uid: c1qrickg

Abstract Small diameter-deep hole drilling is indispensable when several main parts of which airplane, automobile, machine tool and metal mould are manufactured. Especially, medical appliances, medical implants and surgical operation tools for brain and bone invariably need small diameter-deep hole drilling for hard-to-machine metals such as stainless steel and Ti6Al4V. The authors newly designed and manufactured a small diameter-deep hole drilling machine by applying technological developments which consist of a direct drive servo spindle motor controlled more precisely, removable steady rests to prevent for a slender fluted hollow shaft of gun drill to be deflected and a highly pressure circulation system of cutting fluid to prevent being clogged with chips. By using the small diameter-deep hole drilling machine and a special gun drill (specially made to order in BOTEK) a drilling of ϕ1.0×400 mm (L/D=400) for Ti6Al4V was carried out. As a result, it was ascertained that the drilled hole has high accuracy in the items of hole diameter, concentricity, roundness and inner surface roughness.

Deep hole drilling is tried by several machining methods by using a machining center, an electro-discharge machining, a gun drilling machine and others [1, 2] . The method by machining center is difficult to attain a desirable length-todiameter-ratio (L/D) and concentricity [3] . The method by electro-discharge machining requires long machining time and moreover it is difficult to attain desirable concentricity and good inner surface roughness [4] . On the other hand, fundamental studies on gun drilling of deep holes were executed in 1975 to 1977 [5, 6, 7, 8, 9] and furthermore, axial deviation of hole in deep drilling was researched in 1983 to 1992 [10, 11, 12, 13, 14, 15] in Japan. In 2018, study on superlong deep-hole drilling of titanium alloy is reported [16] . However, a maximum value of length-to-diameter-ratio (L/D), which is attained up to present, is up to 140 as far as we know.

Recently, the developers for not only medical appliances and medical implants but also surgical operation tools for brain and bone need a small diameter-deep hole drilling for hard-to-machine metals such as stainless steel and Ti6Al4V [17, 18, 19, 20] . Therefore, the authors were planning to develop a small diameter-deep hole drilling machine to enable a deep hole drilling of up to L/D=400 for hard-to-machine metals such as Ti6Al4V by applying the following technological developments.

First, a direct drive servo spindle motor has been developed and manufactured in Hi-TAK Co. Ltd. based on a lot of experiences stored until now so that a gun drill can be rotated at a highly power transmission rate and a constant rotational speed under load variation and moreover a chipping of gun drill's cutting edge may be detected at all times during drilling.

Second, in order to prevent for a slender fluted hollow shaft of gun drill to be deflected by a cutting load being applied to the tip of gun drill, removable steady rests, of which the number of active steady rest can be changed automatically by NC (Numerical Control) program according to the progress of drilling operation, have been developed.

Deep hole drilling is tried by several machining methods by using a machining center, an electro-discharge machining, a gun drilling machine and others [1, 2] . The method by machining center is difficult to attain a desirable length-todiameter-ratio (L/D) and concentricity [3] . The method by electro-discharge machining requires long machining time and moreover it is difficult to attain desirable concentricity and good inner surface roughness [4] . On the other hand, fundamental studies on gun drilling of deep holes were executed in 1975 to 1977 [5, 6, 7, 8, 9] and furthermore, axial deviation of hole in deep drilling was researched in 1983 to 1992 [10, 11, 12, 13, 14, 15] in Japan. In 2018, study on superlong deep-hole drilling of titanium alloy is reported [16] . However, a maximum value of length-to-diameter-ratio (L/D), which is attained up to present, is up to 140 as far as we know.

Recently, the developers for not only medical appliances and medical implants but also surgical operation tools for brain and bone need a small diameter-deep hole drilling for hard-to-machine metals such as stainless steel and Ti6Al4V [17, 18, 19, 20] . Therefore, the authors were planning to develop a small diameter-deep hole drilling machine to enable a deep hole drilling of up to L/D=400 for hard-to-machine metals such as Ti6Al4V by applying the following technological developments.

First, a direct drive servo spindle motor has been developed and manufactured in Hi-TAK Co. Ltd. based on a lot of experiences stored until now so that a gun drill can be rotated at a highly power transmission rate and a constant rotational speed under load variation and moreover a chipping of gun drill's cutting edge may be detected at all times during drilling.

Second, in order to prevent for a slender fluted hollow shaft of gun drill to be deflected by a cutting load being applied to the tip of gun drill, removable steady rests, of which the number of active steady rest can be changed automatically by NC (Numerical Control) program according to the progress of drilling operation, have been developed. 

Deep hole drilling is tried by several machining methods by using a machining center, an electro-discharge machining, a gun drilling machine and others [1, 2] . The method by machining center is difficult to attain a desirable length-todiameter-ratio (L/D) and concentricity [3] . The method by electro-discharge machining requires long machining time and moreover it is difficult to attain desirable concentricity and good inner surface roughness [4] . On the other hand, fundamental studies on gun drilling of deep holes were executed in 1975 to 1977 [5, 6, 7, 8, 9] and furthermore, axial deviation of hole in deep drilling was researched in 1983 to 1992 [10, 11, 12, 13, 14, 15] in Japan. In 2018, study on superlong deep-hole drilling of titanium alloy is reported [16] . However, a maximum value of length-to-diameter-ratio (L/D), which is attained up to present, is up to 140 as far as we know.

Recently, the developers for not only medical appliances and medical implants but also surgical operation tools for brain and bone need a small diameter-deep hole drilling for hard-to-machine metals such as stainless steel and Ti6Al4V [17, 18, 19, 20] . Therefore, the authors were planning to develop a small diameter-deep hole drilling machine to enable a deep hole drilling of up to L/D=400 for hard-to-machine metals such as Ti6Al4V by applying the following technological developments.

First, a direct drive servo spindle motor has been developed and manufactured in Hi-TAK Co. Ltd. based on a lot of experiences stored until now so that a gun drill can be rotated at a highly power transmission rate and a constant rotational speed under load variation and moreover a chipping of gun drill's cutting edge may be detected at all times during drilling.

Second, in order to prevent for a slender fluted hollow shaft of gun drill to be deflected by a cutting load being applied to the tip of gun drill, removable steady rests, of which the number of active steady rest can be changed automatically by NC (Numerical Control) program according to the progress of drilling operation, have been developed.

Deep hole drilling is tried by several machining methods by using a machining center, an electro-discharge machining, a gun drilling machine and others [1, 2] . The method by machining center is difficult to attain a desirable length-todiameter-ratio (L/D) and concentricity [3] . The method by electro-discharge machining requires long machining time and moreover it is difficult to attain desirable concentricity and good inner surface roughness [4] . On the other hand, fundamental studies on gun drilling of deep holes were executed in 1975 to 1977 [5, 6, 7, 8, 9] and furthermore, axial deviation of hole in deep drilling was researched in 1983 to 1992 [10, 11, 12, 13, 14, 15] in Japan. In 2018, study on superlong deep-hole drilling of titanium alloy is reported [16] . However, a maximum value of length-to-diameter-ratio (L/D), which is attained up to present, is up to 140 as far as we know.

Recently, the developers for not only medical appliances and medical implants but also surgical operation tools for brain and bone need a small diameter-deep hole drilling for hard-to-machine metals such as stainless steel and Ti6Al4V [17, 18, 19, 20] . Therefore, the authors were planning to develop a small diameter-deep hole drilling machine to enable a deep hole drilling of up to L/D=400 for hard-to-machine metals such as Ti6Al4V by applying the following technological developments.

First, a direct drive servo spindle motor has been developed and manufactured in Hi-TAK Co. Ltd. based on a lot of experiences stored until now so that a gun drill can be rotated at a highly power transmission rate and a constant rotational speed under load variation and moreover a chipping of gun drill's cutting edge may be detected at all times during drilling.

Second, in order to prevent for a slender fluted hollow shaft of gun drill to be deflected by a cutting load being applied to the tip of gun drill, removable steady rests, of which the number of active steady rest can be changed automatically by NC (Numerical Control) program according to the progress of drilling operation, have been developed.

Third, in order to prevent for a cutting edge of gun drill to be broken by clogging of chips a highly pressure (about 30 MPa) circulation system of cutting fluid has been developed.

The small diameter-deep hole drilling machine, which had been made by applying the three technological developments above-mentioned, enabled to machine a small diameter-deep hole up to L/D=400 for not only Al alloy and steel but also stainless steel and Ti6Al4V.

In this paper, a constitution of the small diameter-deep hole drilling machine developed is explained, after that the results of drilling test of φ1.0×400 mm (L/D=400) for Ti6Al4V are shown. The diameter, concentricity, roundness and inner surface roughness of drilled hole are shown and evaluated as a typical result from 30 drilled samples. Figure 1 shows the constitution of small diameter-deep hole drilling machine manufactured. As shown in Fig. 1 the drilling machine consists of a servo spindle motor, removable steady rests, a highly pressure circulation system of cutting fluid, seal bush, drill bush, chip box, a pair of servo motor and ball screw for feed and a servo motor to rotate a workpiece. A gun drill fixed to the main spindle of servo spindle motor is rotated clockwise at high speed. On the other hand, the workpiece is rotated anticlockwise at low speed by the servo motor. Simultaneously the gun drill is fed in the axial direction of the workpiece in a body with the servo spindle motor by the rotation of ball screw fixed to another servo motor.

Although four removable steady rests are shown in Fig. 1 between the main spindle of servo spindle motor and the seal bush to prevent a deflection of the slender fluted hollow shaft of gun drill, the number of removable steady rests can be increased as the depth of drilled hole becomes deeper. Especially, 11 removable steady rests are set when drilling of 400 mm depth. As the drilling operation proceeds, the removable steady rest can be got out of place in order from the left side so that the steady rest may not strike the main spindle of servo spindle motor. This action is automatically executed by a command of NC (Numerical Control) program set in the controller to drive the machine. Figure 2 shows the appearance of small diameter-deep hole drilling machine manufactured. Figure 3 (a) shows the whole view of 11 air cylinders to drive removable steady rest and Fig. 3 (b) shows working condition of six removable steady rests. In Fig. 3 (b) three removable steady rests from right side are supporting the fluted hollow shaft of gun drill and the other three steady rests from left side are getting out of place so that the steady rests may not strike the main spindle of servo spindle motor. The three supporting steady rests will get out of place in order from left side after the hand at the tip of removable steady rest is opened for the piston rod, of which the end is connected to the removable steady rest, to be moved into the cylinder by a command of NC program as the drilling operation proceeds. Blue small pieces attached to the tip of hands shown in Fig. 3 (b) are made by resin. Five electric current signals of servo spindle motor, servo motor to feed, servo motor to rotate workpiece, and both pressure and flow of cutting fluid pump are monitored and shown graphically on the display of controller during whole drilling operation. Therefore, when any trouble such as chipping of cutting edge or clogging of chips occurs during drilling operation, a warning is issued and the feed of gun drill is stopped based on an irregular electric current signal.

Drilling a deep hole of φ1.0×400 mm was carried out for a workpiece of Ti6Al4V having φ12×400 mm. UC-14KHP of oil-soluble type was used as cutting fluid and the pressure was confirmed at 32 MPa by a pressure gauge set at the main spindle of the machine. A machining time per a hole drilling is about 13 minutes. After drilling operation, hole diameter, concentricity, roundness and inner surface roughness were measured and recorded at the three positions of inlet, middle and outlet of the drilled hole of φ1.0×400 mm to confirm the accuracy. In this paper typical measured results of hole diameter, concentricity, roundness and inner surface roughness are shown in Section 3.2 although the same drilling operation could be done 30 times until regrinding of gun drill. Table 1 shows a summary of drilling conditions. Actual values of rotational speed of gun drill and workpiece, and feed rate are determined based on the data base in which a lot of drilling conditions are stored until now. The mechanical drawing of special gun drill used (specially made to order in BOTEK) is shown in Fig. 4 and its photograph is shown in Fig.5. In Fig. 4 , (a) shows whole shape, (b) shows shape of tip and (c) shows sectioned view of fluted hollow shank. The driver shown in Fig. 4 (a) is fixed rigidly to the main spindle of the machine shown in Fig. 1 . After the cutting fluid of 32 MPa supplied from the through hole of main spindle is poured into the oil hole of fluted hollow shank shown in Fig. 4 (c) , it blows out from the hole of tip at the end of shank with chips cut by the two cutting edges and it cools and lubricates the cutting edge of tip. After that the cutting fluid and chips go back to the chip box shown in Fig. 1 through the flute of shank. After the cutting fluid is returned to the cutting fluid tank shown in Fig. 1 , it is poured again to the through hole of main spindle by a highly pressure pump to be circulated.

Accuracy of drilled hole of φ1.0 ×400 mm Figure 6 shows the appearance of workpiece for a hole of φ1.0×400 mm to be drilled. After a short part was cut out at the inlet, middle and outlet of workpiece by a wirecut electric discharge machining, hole diameter, concentricity, roundness and inner surface roughness were measured and recorded for each short part of inlet, middle and outlet respectively. Fig. 6 Appearance of workpiece for a hole of φ1.0 ×400 mm to be drilled.

Hole diameter was measured by using a pin gauge as shown in Figs. 7 (a) (b) .

Hole diameter measured was 0.99 mm at inlet, 0.99 mm at middle and 0.99 mm at outlet respectively.

Concentricity was measured by using a dial gauge and both centers' supporting apparatus as shown in Figs. 8 (a) (b) (c). A maximum deviation of dial gauge's indicator was read as concentricity while the short part of inlet, middle and outlet supported by both centers was made one rotation slowly.

Concentricity measured was 0.04 mm at inlet, 0.25 mm at middle and 0.39 mm at outlet respectively.

Roundness was measured by a roundness measuring instrument at the short part of inlet, middle and outlet. Figures 9 (a) (b) (c) show roundness curve measured at the inlet, middle and outlet of workpiece respectively.

Roundness measured was 9.4 ㎛ at inlet, 9.3 ㎛ at middle and 13.9 ㎛ at outlet respectively. 

After the short part of the inlet, middle and outlet of workpiece were cut by a wirecut electric discharge machining in the axial direction along the center line of hole, inner surface roughness was measured by a contact probe profilometer. Inner surface integrity was observed by a microscope. Figures 10 (a) (b) (c) show the microscope photograph of inner surface at the inlet, middle and outlet respectively.

Inner surface roughness measured was Ra of 0.690 ㎛ at inlet, Ra of 0.770 ㎛ at middle and Ra of 0.580 ㎛ at outlet. The improvement of inner surface roughness Ra from 0.690 ㎛ at inlet to 0.580 ㎛ at outlet is thinkable that burnishing by the relief surface of gun drill's cutting edge affected more effectively at outlet than at inlet and middle. The burnishing effect can be observed from comparing the microscope photograph of inner surface of (c) Outlet with those of (a) Inlet and (b) Middle shown in Fig. 10 . Fig. 10 Microscope photograph of inner surface.

The accuracy of drilled hole of φ 1.0 × 400 mm for Ti6Al4V is summarized in Table 2 . From Table 2 it is known that the hole diameter is the same value of 0.99 mm at the inlet, middle and outlet of hole depth of 400 mm, the concentricity of 0.04 mm at inlet becomes larger to 0.25 mm at middle and 0.39 mm at outlet and the roundness of 9.4 ㎛ at inlet and 9.3 ㎛ at middle becomes larger to 13.9 ㎛ at outlet. The surface roughness Ra of 0.690 ㎛ at inlet and 0.770 ㎛ at middle are improved to 0.580 ㎛ at outlet. 

Chip morphologies are shown in Figs. 11 (a) (b). In Fig.  11 (a) there are two kinds of chips which consist of two tiny chips and the other six comparatively large chips. It is presumed that the two tiny chips were generated by the short outer cutting edge of gun drill's tip shown in Fig. 4 (b) and the other larger chips were generated by the long inner cutting edge shown in Fig. 4 (b) . The chip shown in Fig. 11 (b) is the expansion of typical large chip morphology generated by the long inner cutting edge. This chip consists of two parts; one looks like an arc like a belt and another lower part looks like a triangle which was bended upward so that the chip can go through the flute of hollow shaft having the lesser depth than 0.5mm. It is thought that this bending of chip could be caused by highly pressure cutting fluid of 32 MPa at the main spindle of the machine.

These chips shown in Figs. 11 (a) (b) evidence that cutting is done normally by both the inner cutting edge and the outer cutting edge of special gun drill's tip. 

In Fig. 9 (a) about 40 peaks are seen on the roundness curve.

Since the rotational speed of workpiece in anticlockwise is 300rpm/60s = 5 rps and the rotational speed of gun drill in clockwise is 12000rpm/60s = 200 rps, the cutting edges of gun drill rotate 200/5 = 40 times while the workpiece makes one rotation. On the other hand, the torque applying to the tip of gun drill might fluctuate slightly with a rotational period of gun drill since the tip of gun drill consists of a long inner cutting edge and a short outer cutting edge.

During the drilling at the inlet of workpiece a bending rigidity of gun drill is larger since the distance between the tip of gun drill and the drill bush is shorter as shown in Fig. 1 . As the drilling proceeds to the middle and the outlet of workpiece, the bending rigidity of gun drill becomes smaller since the distance between the tip of gun drill and the drill bush becomes longer. Therefore, the lateral deflection of tip of gun drill induced by the torque's fluctuation with the same period as gun drill's rotation becomes larger in order at the inlet, middle and outlet since the hollow shank of gun drill has a flute.

Considering the change of bending rigidity due to the progress of drilling and the lateral deflection of tip of gun drill induced by the torque's fluctuation with the same period as a gun drill's rotation, it is understood that the heights between peaks and valleys of about 40 shown in Fig. 9 (c) are larger than those shown in Fig. 9 (a) and the roundness of 13.9 ㎛ in Fig. 9 (c) is larger than 9.4 ㎛ in Fig. 9 (a) .

Furthermore, although the concentricity in Table 2 becomes larger in order at the inlet of 0.04 mm, middle of 0.25 mm and outlet of 0.39 mm, it would be caused by a reduction of bending rigidity to the thrust force applying to the tip of gun drill as the distance between the tip of gun drill and the drill bush becomes longer.

To enable deep hole drilling up to L/D = 400 a small diameter-deep hole drilling machine was manufactured. By employing the drilling machine and a special gun drill (specially made to order by BOTEK) a drilling of φ1.0×400 mm for Ti6Al4V was carried out and the accuracy of drilled hole was measured and evaluated. As a result, following conclusions are obtained.

(1) The three technological developments, which consist of a direct drive servo spindle motor controlled more precisely, removable steady rests to prevent for a slender fluted hollow shaft of gun drill to be deflected and a highly pressure circulation system of cutting fluid to prevent for a cutting edge of gun drill to be broken by clogging of chips, were thoroughly effective to manufacture a small diameter-deep hole drilling machine. The machine enabled a small diameter-deep hole drilling up to L/D=400 for Ti6Al4V. (2) By using the drilling machine and the special gun drill (specially made to order in BOTEK) a drilling of φ1.0× 400 mm for Ti6Al4V could be carried out successfully. The same drilling operation as φ 1.0 × 400 mm for Ti6Al4V could be done 30 times until a regrinding of the gun drill although the whole results could not be described in this paper. (3) The accuracy of the drilled hole of φ1.0 ×400 mm for Ti6Al4V is satisfied enough for a demanded value in the items of hole diameter, concentricity, roundness and inner surface roughness. Especially, it is a very desirable result that the hole diameter of 0.99 mm is constant at the inlet, middle and outlet of the drilled hole of φ1.0×400 mm. (4) The small diameter-deep hole drilling machine reported in this paper would be able to respond to the needs of a small diameter and feasibly deep hole drilling from the developer of medical appliances, medical implants and surgical operation tools and furthermore be able to assist to produce advanced medical appliances and surgical operation tools. The small diameter-deep hole drilling machine has enabled to produce deep holes of several products for hard-to-machine metals and the accuracy of deep hole drilled would be satisfied enough for the demand of several products.

A review of modrn advancements in micro drilling techniques

Deep hole drilling

A Study of Deep-Hole Machining of Sstainless Steel with a Small-Diameter Drill

High-speed Processing of High Aspect Ratio Fine Deep Hole by Electric Discharge Machining

Study on the Gun Drilling of Deep Holes (1st Report) -Measuring Method of Motion of Drill Tip

Measured Results of Motion of Drill Tip -, Jounal of the Japan Society of Precision Engineering

Influence of Drilling Conditions on Mortion of Drill Tip -, Jounal of the Japan Society of Precision Engineering

Oil-film Pressure around the Drill Tip -, Jounal of the Japan Society of Precision Engineering

Characteristics of the Journal Bearing composed of Drill Tip and Hole -Jounal of the Japan Society of Precision Engineering

Axial Deviation of Hole in Deep Drilling

Axial Deviation of Hole in Deep Drilling

Jounal of the Japan Society of Precision Engineering

Axial Deviation of a Hole in Deep Drilling -Influence of Tool Geometry (Comparison of Single and Multi

Axial Hole Deviation in Deep Drilling -The Influence of the Shape of the Cutting Edgw

The Influence of Workpiece Geometry on Axial Hole Deviation in Deep Hole Drilling

The Influence of Cutting Condition, Shank Rigidity, Shape of Cutting Edge and Tool Wear on Axial Hole Deviation in Deep Drilling Using a Gun Drill

Study on Super-Long Deep-Hole Drilling of Titanium Alloy

Shear Stress Ultrasonic Horn for Ultrasonic Surgical Aspiration

Surgical Instrument with Ultrasonic Tip for Fibrous Tissue Removal

This research has been executed based on the support of 2017~2018 strategically supporting project for basic technology ought to be developed in the Japanese Ministry of Economics & Industry and Shizuoka prefecture's Foundation to promote industry. The authors deeply appreciate the support and encouragement.