key: cord-0318990-cbluabwu authors: Rezvani, Sina; Nikolov, Nicholas; Kim, Chang-Ju; Park, Simon S.; Lee, Jihyun title: Development of a Vise with built-in Piezoelectric and Strain Gauge Sensors for Clamping and Cutting Force Measurements date: 2020-12-31 journal: Procedia Manufacturing DOI: 10.1016/j.promfg.2020.05.143 sha: aefb76ead407fd64c43622b73c43d8351f58ede7 doc_id: 318990 cord_uid: cbluabwu Abstract Accurate measurement of static and dynamic forces during machining operations is important in the process monitoring, optimization and prediction of machining quality. Cutting and clamping forces are typically measured separately, for example, through hydraulic clamping force devices and piezoelectric table dynamometers. Since cutting and clamping forces interact throughout machining, both forces need to be measured simultaneously to accurately predict machining quality. This paper proposes a novel force measuring device in the form of a vise with built-in piezoelectric sensors and strain gauges to measure multi-axial clamping and cutting forces simultaneously. A strain concentrating design based on a cross-shaped groove was utilized to attach three strain gauges to measure clamping forces. These locations were chosen for their ability to measure clamping forces in real-time and minimize cross-talks. Piezoelectric sensors were embedded between layers in the vise jaw to measure cutting forces. A prototype of the proposed apparatus was fabricated and used to experimentally investigate sensitivities, bandwidth, clamping forces and cutting forces. The results showed that the proposed device has good performance in measuring clamping and cutting forces in milling operations. Accurate measurement of forces during machining operations is essential for enhancing productivity and accuracy in modern manufacturing. Cutting force is used to indirectly predict the quality of the workpiece's surface [1, 2] , detect chatter vibration [3] , prevent excessive deflection of tools, and monitor tool wear and breakage [4] [5] [6] . Moreover, measurement of the clamping force is critically important to ensure that the workpiece is held in its position properly during the machining operation. An insufficient clamping force would cause workpiece slippage; whereas excessive clamping force would cause workpiece distortion. Hence, the measured clamping forces can be utilized to increase the accuracy of the workpiece location [7] and minimize workpiece deformation [8, 9] . The accuracy of measured static and dynamic forces can be improved by attaching piezoelectric sensors or strain gauges as close as possible to where force is applied. Challenges such as manufacturing complexity, dynamic cross-talks, noise and cost still exist. The deformation of the workpiece caused by the clamping and cutting forces cannot be ignored during the machining operations. Many studies [10] [11] [12] [13] have focused on developing devices for measuring either cutting or clamping forces separately. Altintas et al. [10] has integrated piezoelectric force sensors with a spindle for cutting force measurement. Varghese et al. [11] has investigated embedding piezoceramic force sensors in a grinding wheel to monitor cutting force in-process. Denkena [12] proposed to integrate strain gauges with a hydraulic clamping system for the measurement of clamping force. Takao [13] developed a machine vise which includes a strain detector device located between the support and flange of the feed screw for detecting an axial force. However, Zhou [14] pointed out that the unevenness of the cutting force and the clamping force during the machining processes can lead to the deformation of the workpiece. Based on the FE analysis, he investigated the effect of the superposition of cutting force and clamping force on the deformation of the workpiece. Accurate measurement of forces during machining operations is essential for enhancing productivity and accuracy in modern manufacturing. Cutting force is used to indirectly predict the quality of the workpiece's surface [1, 2] , detect chatter vibration [3] , prevent excessive deflection of tools, and monitor tool wear and breakage [4] [5] [6] . Moreover, measurement of the clamping force is critically important to ensure that the workpiece is held in its position properly during the machining operation. An insufficient clamping force would cause workpiece slippage; whereas excessive clamping force would cause workpiece distortion. Hence, the measured clamping forces can be utilized to increase the accuracy of the workpiece location [7] and minimize workpiece deformation [8, 9] . The accuracy of measured static and dynamic forces can be improved by attaching piezoelectric sensors or strain gauges as close as possible to where force is applied. Challenges such as manufacturing complexity, dynamic cross-talks, noise and cost still exist. The deformation of the workpiece caused by the clamping and cutting forces cannot be ignored during the machining operations. Many studies [10] [11] [12] [13] have focused on developing devices for measuring either cutting or clamping forces separately. Altintas et al. [10] has integrated piezoelectric force sensors with a spindle for cutting force measurement. Varghese et al. [11] has investigated embedding piezoceramic force sensors in a grinding wheel to monitor cutting force in-process. Denkena [12] proposed to integrate strain gauges with a hydraulic clamping system for the measurement of clamping force. Takao [13] developed a machine vise which includes a strain detector device located between the support and flange of the feed screw for detecting an axial force. However, Zhou [14] pointed out that the unevenness of the cutting force and the clamping force during the machining processes can lead to the deformation of the workpiece. Based on the FE analysis, he investigated the effect of the superposition of cutting force and clamping force on the deformation of the workpiece. Accurate measurement of forces during machining operations is essential for enhancing productivity and accuracy in modern manufacturing. Cutting force is used to indirectly predict the quality of the workpiece's surface [1, 2] , detect chatter vibration [3] , prevent excessive deflection of tools, and monitor tool wear and breakage [4] [5] [6] . Moreover, measurement of the clamping force is critically important to ensure that the workpiece is held in its position properly during the machining operation. An insufficient clamping force would cause workpiece slippage; whereas excessive clamping force would cause workpiece distortion. Hence, the measured clamping forces can be utilized to increase the accuracy of the workpiece location [7] and minimize workpiece deformation [8, 9] . The accuracy of measured static and dynamic forces can be improved by attaching piezoelectric sensors or strain gauges as close as possible to where force is applied. Challenges such as manufacturing complexity, dynamic cross-talks, noise and cost still exist. The deformation of the workpiece caused by the clamping and cutting forces cannot be ignored during the machining operations. Many studies [10] [11] [12] [13] have focused on developing devices for measuring either cutting or clamping forces separately. Altintas et al. [10] has integrated piezoelectric force sensors with a spindle for cutting force measurement. Varghese et al. [11] has investigated embedding piezoceramic force sensors in a grinding wheel to monitor cutting force in-process. Denkena [12] proposed to integrate strain gauges with a hydraulic clamping system for the measurement of clamping force. Takao [13] developed a machine vise which includes a strain detector device located between the support and flange of the feed screw for detecting an axial force. However, Zhou [14] pointed out that the unevenness of the cutting force and the clamping force during the machining processes can lead to the deformation of the workpiece. Based on the FE analysis, he investigated the effect of the superposition of cutting force and clamping force on the deformation of the workpiece. Accurate measurement of forces during machining operations is essential for enhancing productivity and accuracy in modern manufacturing. Cutting force is used to indirectly predict the quality of the workpiece's surface [1, 2] , detect chatter vibration [3] , prevent excessive deflection of tools, and monitor tool wear and breakage [4] [5] [6] . Moreover, measurement of the clamping force is critically important to ensure that the workpiece is held in its position properly during the machining operation. An insufficient clamping force would cause workpiece slippage; whereas excessive clamping force would cause workpiece distortion. Hence, the measured clamping forces can be utilized to increase the accuracy of the workpiece location [7] and minimize workpiece deformation [8, 9] . The accuracy of measured static and dynamic forces can be improved by attaching piezoelectric sensors or strain gauges as close as possible to where force is applied. Challenges such as manufacturing complexity, dynamic cross-talks, noise and cost still exist. The deformation of the workpiece caused by the clamping and cutting forces cannot be ignored during the machining operations. Many studies [10] [11] [12] [13] have focused on developing devices for measuring either cutting or clamping forces separately. Altintas et al. [10] has integrated piezoelectric force sensors with a spindle for cutting force measurement. Varghese et al. [11] has investigated embedding piezoceramic force sensors in a grinding wheel to monitor cutting force in-process. Denkena [12] proposed to integrate strain gauges with a hydraulic clamping system for the measurement of clamping force. Takao [13] developed a machine vise which includes a strain detector device located between the support and flange of the feed screw for detecting an axial force. However, Zhou [14] pointed out that the unevenness of the cutting force and the clamping force during the machining processes can lead to the deformation of the workpiece. Based on the FE analysis, he investigated the effect of the superposition of cutting force and clamping force on the deformation of the workpiece. 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to Consequently, the development of a system with built-in force sensors to measure both clamping and cutting forces simultaneously during machining operations is necessary. This study proposes a novel force measuring device integrated with a vise. The force measurement apparatus is comprised of piezoelectric lead zirconate titanate (PZT) sensors and strain gauges to measure clamping and cutting forces. Piezoelectric force sensors are not appropriate to measure the static forces because the generated electrostatic charge will decay over time. On the other hand, the frequency bandwidth of the strain gauge system is not sufficient to measure the high rotational cutting forces in milling. Hence, in this paper, strain gauges were predominantly utilized to measure clamping forces and the milling forces were predominantly measured by PZT piezoelectric sensors. A strain concentration design based on a cross-shaped groove was used to attach the strain gauges. The locations of the strain gauges were chosen for their ability to minimize cross-talks and measure multi-axis forces in real-time. The jaw on the other side of the vise was embedded with six PZT piezoelectric sensors to measure cutting forces. The piezoelectric sensors were fabricated and poled in shear and axial directions to measure the cutting forces in three different directions. The outline of this paper is as follows: Section 2 depicts the prototype of the vise with built-in piezoelectric sensors and strain gauges and shows its FEM results. In Section 3, the static and dynamic performances of the prototype have been analyzed in the time domain and frequency domain. Sensitivity, bandwidth, and measuring cutting forces have been investigated in this section. Discussion has been presented in Section 4. Conclusion and future work have been discussed in Section 5. The jaws of the vise are integrated with strain gauges and piezoelectric sensors to measure clamping and cutting forces together as described in Fig. 1 . The strain gauge jaw on the vise has three strain gauges (Vishay Electric C4A-06-060SL-350-39P) which measure X, Y, and Z forces. The layered jaw on the other side of the vise has six piezoelectric sensors consisting of lead zirconate titanate (PZT) piezoceramic elements which measure X, Y, and Z axes dynamic forces. The axial piezoelectric sensors measure forces in X direction and the shear piezoelectric sensors measure forces in Y and Z directions. They are fastened to the rest of the vise assembly through the two counterbored holes. An even preload is applied to each piezoelectric element using eight screws between every two layers of the jaw and a torque of 1 N.m is applied to each screw. The cross-shaped groove with three attached strain gauges was designed to maximize the strain of each strain gauge while maintaining sufficient stiffness. This cross design is chosen to minimize cross-talk between the three axes of forces. Ideally, the static forces should be measured independently; however, due to the inherent design challenge, the strain gauges experience cross-talks in other directions. The positions of strain sensors were selected through calculating strain values in finite element simulations. With an applied clamping force in the simulation, the strain of the cross was evaluated for cross groove designs ranging from 4×4×4 mm to 9×9×9 mm. The optimized design from FE simulations was chosen as 9×9 mm with the depth of 8 mm. The elements with maximum strain in the desired direction were chosen as the sensor location, as shown in Fig. 2 which illustrates the simulation results using a finite element software (ABAQUS/CAE 2019). Z axis 21% <1% -Finally, the cross was machined on a mild steel plate and the strain gauges were attached to the chosen locations, as shown in Fig. 3 . The jaw located at the fixed end of the vise embeds six piezoelectric (PZT) sensors in four separate layers. Z axis shear piezoelectric sensors lie between the first and second layers. Y axis shear piezoelectric sensors are inserted between the second and third layers. Finally, X axis axial piezoelectric sensors lie between the third and fourth layers. Both shear and axial piezoelectric sensors are the same dimensions, 17 × 17 × 2.5 mm 3 . Flexible Copper-Kapton laminate was used as the electrodes on both sides of the piezoelectric sensors. Figure 4 shows the design of the layered jaw, which was manufactured using layers of mild steel. A thickness of 10 mm is chosen to ensure that the layers remain parallel during clamping and machining. Figure 5 shows the schematic diagram of a workpiece and two piezoelectric sensors. Moments result from the force being applied at different points along the workpiece. Since the end mill moves along the workpiece during operation, the piezoelectric sensors will be measuring moments as well as forces. The moments and forces need to be extracted from the measured signals of the piezoelectric sensors. Through the schematic diagram in Fig. 5 and equations (1) -(4), cutting forces in X, Y, and Z axes and moment in X direction can be correcting. Cutting forces in different directions can be obtained by calculating the average of measured signal from piezoelectric sensors described in equations (1), (2), and (3) and the moment in X direction can be calculated by equation (4) . The Y distance to sensors from the center of gravity of the workpiece, a, is multiplied to the measured signals, then the moment in X direction can be calculated. , = � , 1 + , 2 �/2 (1) The distance between the piezoelectric elements from the center to the center is 56 mm. Consequently, signals measured from the piezoelectric sensors in this vise can calibrate the cutting forces and X axis moment by using the distance between piezoelectric sensors. Furthermore, the stiffness of the vise assembly after modification is compared with the stiffness of the system with normal jaws, using the finite element method. Minimal stiffness reduction is desired when the normal jaws are replaced with the newly designed ones. With the modified jaws, the stiffness is decreased by approximately 14%. The performance of the strain gauge system was evaluated by conducting some clamping tests. Moreover, in order to find the sensitivities of piezoelectric sensors and acquire the frequency response functions of the system, impact hammer tests were performed. The clamping test and hammer test were carried out based on the experimental setup described in Fig. 6 . A workpiece (Aluminum Al6061 130×55×55 mm 3 ) was clamped in the vise and its clamping force and dynamics were analyzed in the time domain and frequency domain. A threeaxis force sensor (Kistler 9251A) was used as a reference. For the dynamic analysis, impact hammer tests were performed using a modal hammer (PCB 208A03) and a data acquisition system (NI 9234 DAQ) to obtain frequency response functions. Charge amplifiers (Kistler 5010) were used for piezoelectric sensors. As the impact hammer hits the workpiece, depending on the sensitivity, a small charge is generated by the piezoelectric element. This charge is amplified and converted to a voltage by the charge amplifier and sent to the data acquisition system. Finally, a digital signal is transferred to the computer and can be analyzed using the appropriate software. Figures 7 and 8 show the results of impulse impact hammer tests conducted on the entire vise assembly. Frequency response functions (FRFs) in Fig. 7 represent the relationship between the measured output forces in X, Y, and Z directions and the input impulse force by a modal hammer in X, Y, and Z directions, respectively. To compare with the results, the double integral of acceleration signals in X, Y, and Z axes measured by accelerometer are also shown in Fig. 8 . As extracted from the experimental hammer testing results measured by piezoelectric sensors, the lowest natural frequency is at approximately 400 Hz because of the dynamics of the workpiece. In Fig. 7 (c) , there is a high peak around 700 Hz, which could also have negative influence on the bandwidth. This natural frequency of the workpiece restricts the frequency bandwidth, which is directly related to the performance of the sensor. Therefore, the workpiece dynamics should be compensated to extend the bandwidth of the vise. The sensitivities of piezoelectric sensors are depicted in Table 2 . Theses sensitivities can be used to evaluate the performance of the PZT sensors to measure the clamping and cutting forces, as explained in the following sections. Monitoring clamping forces is critical to the final quality of the workpiece. If excessive clamping force is applied, the workpiece may deform simply from the clamping load; conversely, if inadequate clamping force is applied, the workpiece may slip during machining operations. Both scenarios have an adverse effect on the quality of the machined workpiece. In order to calibrate and evaluate the performance of the embedded strain gauge, the output signal of the X axis strain gauge was compared with the signals measured by the reference force sensor in X axis and the axial PZT piezoelectric sensors, as shown in Fig. 9 . The clamping force was applied by tightening the vise slowly such that the piezoelectric sensor can also calibrate the clamping force under quasi-static environment. From the figure, it is obvious that the strain gauge can measure the clamping force similar to the reference signal. Although, the signal of the PZT piezoelectric sensor is also similar to the reference signal, the generated electrostatic charge will leak to zero after a while and the signal cannot be used to monitor the clamping forces. In order to investigate the feasibility of the force sensing vise, some milling tests were conducted using a Bridgeport (Series 1) milling machine and the cutting forces captured based on the setup shown in Fig. 10 . The milling tests were performed parallel to X axis on an aluminum Al6061 workpiece using a 4-flute carbide end mill tool with a diameter of 12.7 mm. The depth of cut, X-axis feed rate and the spindle speed were 1 mm, 5 mm/s and 1500 rev/min, respectively. The spindle speed of 1500 rev/min was selected to be within the frequency bandwidth of piezoelectric sensors. The generated electrostatic charge was reset before each test. Thus, the PZT sensors measured only the cutting forces during the milling tests. To evaluate the performance of the proposed setup, the same cutting tests were performed on a sample mounted on a table dynamometer (Kistler 9257B) and the results were compared as shown in Fig. 11 . Figure 11 shows the results of the milling cutting force measurement in X and Y directions and their Fast Fourier transform (FFT). The results show that there is a good agreement between the forces measured by the piezoelectric force sensors and the reference dynamometer. The FFTs reveal that the dominant frequency in the tests is the spindle frequency (25 Hz) due to a high runout of the cutting tool. The effect of the runout is also obvious in time domain signals; the magnitudes of the cutting forces are significantly different for each flute. Another dominant peak is at 100 Hz which is the tooth passing frequency (considering the spindle speed and number of flutes). The output signals of the strain gauges are measured while the clamping force and cutting forces are applied. The axial force, measured by the strain gauge system, slightly declines when the cutting process starts, as shown in Fig. 12 . However, the decrease is not significant considering the magnitude of the clamping force. Due to the low bandwidth and low sensitivity of the strain gauges used in this study, the magnitude of the cutting forces measured by the strain gauge setup was much lower than the force measured by PZT sensors. Thus, the strain gauge system predominantly measures the clamping forces during the milling process. In the design of the vise prototype, it was assumed that the piezoelectric sensors are symmetrically embedded in the jaws. The vise with the embedded sensors in this paper has limitations about bandwidth which degrades the performance of the embedded sensors. As the impact hammer tests showed in Fig. 7 , the vise has a natural frequency of approximately 400 Hz. This frequency is function of size and material of the workpiece as well as the boundary conditions. For instance, higher clamping force will lead to higher natural frequency. In order to increase the frequency bandwidth of sensor system, a dynamic compensation algorithm has to be implemented. There are some methods such as the expanded Kalman filter for dynamic compensation [10] . In order to implement the filter, first, the transfer function of the structure between the cutting forces acting on the workpiece and the measured forces by the embedded PZT sensors should be identified. Using the cutting force signals measured by the PZT sensors, the Kalman filter will be designed to filter the influence of structural modes on the force measurements. Furthermore, the thermal effects on the response of the PZT sensors have not been considered in this paper. As the Curie temperature of the fabricated PZT sensors is about 130 °C, the proposed setup has limitation about maximum temperature during the cutting tests. The work on this layered jaw assembly continues with the above-mentioned future work. More experiments are needed to verify the proposed design. As future work, a dynamic compensation scheme will be employed to increase the frequency bandwidth of the sensor and the temperature effect will also be investigated. This paper proposed a new force measuring device integrated with a vise developed to measure both clamping and cutting forces simultaneously using PZT piezoelectric sensors and strain gauges. A layered machine jaw assembly has been designed to embed these sensors inside. The setup can be easily mounted on every vise with the same screw patterns (the distance between the two screws should be about 10 cm) and the only modification needed is replacing the normal jaws with the newly designed ones. FE analysis was performed to find the optimal design and location where strain gauges were attached. Cutting forces and X axis moment during machining operations can be obtained using a simple mathematical model. The prototype with embedded sensors was examined using experimental results. Clamping tests showed that the embedded strain gauge can measure static clamping force in X direction. The impact hammer tests showed that the bandwidth of the vise prototype is limited due to the first natural frequency at approximately 400 Hz. Cutting force measurements with a spindle speed of 1500 rev/min for a 4-flute end mill showed that the embedded piezoelectric sensors can measure dynamic milling forces accurately. The proposed force measurement system can be used to monitor the cutting forces during the milling process to predict the quality of the workpiece's surface and detect chatter vibration. Clamping forces can be measured simultaneously in order to prevent workpiece deformation or slippage. However, more work is needed to enhance the performance of the system in terms of dynamic compensation and thermal effects. Analytical prediction of chatter stability in milling -part II: Application of the general formulation to common milling systems Effect of cutting-edge geometry and workpiece hardness on surface generation in the finish hard turning of AISI 52100 steel Multisensor approaches for chatter detection in milling Tool wear monitoring of micro-milling operations Tool condition monitoring (TCM) -the status of research and industrial application Development of a tool wear monitoring system for hard turning Fixture clamping force optimisation and its impact on workpiece location accuracy Novel workpiece clamping method for increased machining performance A new approach for force measurement and workpiece clamping in micro-ultrasonic machining Dynamic compensation of spindle-integrated force sensors Development of a sensor-integrated "intelligent" grinding wheel for in-process monitoring Sensor integration for a hydraulic clamping system Machine Vise with Clamping Force Detector Influence of cutting and clamping forces on machining distortion of diesel engine connecting rod This work is sponsored by the Korea Institute of Machinery and Materials in South Korea and the Natural Sciences and Engineering Research Council of Canada (NSERC).