Gripping force measurement in boring and turning operations using nanocomposite-based strain sensor

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1 7th International Conference on Virtual Machining Process Technology (VMPT), Hamilton, May 7 th May 11 th, 2018 Gripping force measurement in boring and turning operations using nanocomposite-based strain sensor M. Sanati, A. Sandwell, H. Mostaghimi, and S. S. Park * Dept. of Mechanical and Manufacturing Engineering University of Calgary, 2500 University Drive N.W. Calgary, T2N 1N4 *simon.park@ucalgary Abstract Monitoring gripping forces during boring and turning operations is a key factor in achieving high quality machined products with maximum productivity. In the case of insufficient clamping force, there is a high probability of the workpiece slipping and potentially even releasing from the chuck during machining operations, causing of damage to the machine and workpiece. The high rotational speeds of lathe cause increase in centrifugal forces which may loosen the gripping force. Excessive clamping force is also problematic as it increases the possibility of workpiece deformation during the machining process, especially in the case of thin-wall components. Therefore, an optimum gripping force is required during machining operations to mitigate these potential issues. In addition, the gripping force needs to be monitored continuously to react to any process variations that may occur during operations. Commercially available sensing systems for measuring clamping forces are not practical and cannot be used for concurrent measurements during machining using a lathe. In this study, a novel smart jaw is proposed for indirect measurement of gripping forces through strain measurements in real time. The in-house nanocomposite based strain sensors are used for measuring the strain on the smart jaws during machining operation. The performance and accuracy of the developed smart jaws in gripping force measurement are verified through static and dynamic tests and compared with the results of a commercial piezoelectric sensor. Keywords: Gripping force, Machining, Strain sensor, Smart jaw, Wireless communication 1. Introduction There is an increasing demand in the machine tools market for high productivity and accuracy of manufacturing process that can result in a better surface finish. Hence, the manufacturing processes need to be optimized through online monitoring of different parameters involving in the accuracy of machined components, such as cutting mechanism, cutting forces, gripping forces, chip formation, tool wear etc. Monitoring and controlling the gripping force of work holding devices during turning operations is of great importance in achieving accurately machined components. Spur and Metter reported that more than 10% of the accidents that occur in the metal working industry are due to the failure of the clamping systems [1]. Specifically, for slender and hollow parts, manufacturing tolerances of thin-wall cylindrical components can decrease by reducing the gripping force of the lath chuck [2]. However, the low gripping force might result in less friction and preload in the system which subsequently increases the risk of slippage during machining process that can lead to failures [3]. On the other hand, increasing clamping force might increase the chance of workpiece deformation especially in case of flexible rings and soft materials. Therefore, obtaining an optimum gripping force and monitoring this parameter during machining operations is a key factor M. Sanati, A. Sandwell, H, Mostaghimi, S.S. Park 2018

2 in controlling process and improving the quality of the final products. The gripping force of the lathe chuck is a function of different parameters including rotational speed. The increase of the rotational speed of the spindle decreases the gripping force of the lathe chuck due to centrifugal force, which in turn causes the loosening of the workpiece and a corresponding decrease in machining accuracy and productivity [4]. The lose of chucking force due to the increase of the rotational speed become critical in high speed cutting (HSC) [4]. High speed cutting (HSC) technology is widely used in machine tool centers and manufacturing processes due to the its advantages including low cutting force, high exciting frequency, low level of cutting heat transferred to the workpiece, which lead to high productivity and good surface finish [5]. There are different commercial sensors that can be used for monitoring the gripping forces during boring and turning operations however they are expensive and only useful for checking the relationship between the gripping force and rotational speed not online monitoring of the gripping force applied to the workpiece during machining; therefore, different researchers have tried to calculate and measure the lathe chuck gripping force through modeling and analytical calculations or indirect measurement using new sensing systems. Nyamekye and Mudiam presented a model for predicting the initial clamping forces in lathe chuck [6]. They also developed a new experimental process for measuring the gripping forces during machining using a strain gaugebased measuring system. Reviewing different studies performed for gripping force prediction shows that most of the studies in this area are analytical and computational that suffer from some limitations and assumptions that make them not accurate enough when it comes to machining applications. In addition, the developed stain gauge sensing techniques are not practical as they are mostly installed on the workpiece which limits the machining ability and requires that the process be repeated for each workpiece. On the other hand, among few available commercial sensors for this application, they generally have some limitations such as high cost. Moreover, these systems are only applicable for studying the effect of rotational speed on the gripping force not in-situ tests and real boring and turning operations; therefore, the objective of this study to develop a new type of sensing system for gripping force measurements in boring and turning operations that can be used in practical applications where the signals can be transferred wirelessly to monitor the state. The strain sensors used in this study are based on in-house developed nanocomposite sensors comprised of PVDF as a piezoelectric polymer matrix with carbon nanotubes (CNTs) to create a conductive network in different directions. Under loading and strains, changes in the geometry of the nanocomposite will result in contraction and expansion of the CNT network, and therefore the piezoelectric properties of the sensor changes that can be attributed to the strain applied to the system. Figure 1: Schematic of changes in MWCNT interactions due to the deformation M. Sanati, A. Sandwell, H, Mostaghimi, S.S. Park

In the case of negative strain compressing load, the number of connections among wavy CNTs may increase, as schematically shown in Figure 1, which results in more conductivity and less resistance in

The direct force measurements using piezo elements may hinder the dynamics and cause creep in the polymeric nanocomposite sensors when the applied force is high.

A new load cell/jaw design based on s-beams was chosen for its high strain sensitivity and stiffness.

3 In the case of negative strain compressing load, the number of connections among wavy CNTs may increase, as schematically shown in Figure 1, which results in more conductivity and less resistance in the sensor. 2. Development of Gripping Force Measurement Device Different scenarios were proposed for gripping force measurement in boring and turning operations mainly based on strain measurement. The direct force measurements using piezo elements may hinder the dynamics and cause creep in the polymeric nanocomposite sensors when the applied force is high. Therefore, a novel smart jaw that is capable of measuring strains was designed and the measured data are attributing to the gripping force in the chuck during the machining operation. A new load cell/jaw design based on s-beams was chosen for its high strain sensitivity and stiffness. This s-beam loadcell is connected to the end of the jaw where it grips the workpiece as shown in Figure 2. Also, some different finite element simulations were performed to find the optimal dimensions for the smart jaw. It concluded that the 25*25*34 mm can be considered as the optimal dimensions for the smart jaw due to the size limitations. The FEM results also show that the designed smart jaw can measure the gripping force till 30 KN. The numerical results are presented in Figure 2 as well. It shows that applying the gripping force to the designed jaw results in the maximum strain in the ring in the middle of the smart jaw where the strain sensors are mounted. Figure 2: Jaw S-beam design To study the relationship between the applied force to the smart jaw and the maximum strain experienced in the system, different forces applied to the structure and the maximum strain in the ring was measured. The results show that the smart jaw shows a linear strain behavior with applied clamping load up to a range of 30 kn as shown in Figure 3. Figure 3: Mechanical behavior of smart jaw M. Sanati, A. Sandwell, H, Mostaghimi, S.S. Park

The linear mechanical behavior of the smart jaw enables us to indirectly measure the clamping force applied to the jaw system during boring and turning operations through strain measurement.

4 The linear mechanical behavior of the smart jaw enables us to indirectly measure the clamping force applied to the jaw system during boring and turning operations through strain measurement. In order to increase the sensitivity of the developed smart jaw, four strain sensors are placed inside the ring of the jaw to build a full Wheatstone bridge as shown in Figure 4. Figure 4: Wheatstone bridge made of nanocomposite strain sensors Wheatstone bridge is comprised of four resistors with an excitation voltage, VEX, that is applied across the bridge. The configuration shown in Figure 4 results in two parallel voltage divider circuits. One voltage divider is composed of resistors 1 and 2 and the other voltage divider circuit is composed of resistors 3 and 4. The middle point of the two voltage dividers are used for measuring the output voltage of the Wheatstone bridge (V0) which is equal to: R 3 V o = [ R 2 ] R 3 + R 4 R 1 + R (1) 2 The above equation shows that the output voltage of the bridge is equal to zero when R 1 /R 2 = R 4 /R 3. Any changes in the resistance of each of the resistors will results in nonzero output voltage. Thus, if any other resistors shown in Figure 4 are replaced with a strain sensor, a nonzero output voltage will be produced from the bridge if any changes occur in the resistance of the strain sensor. In order to achieve higher sensitivity of the developed smart jaw, all resistors in Figure 4 are replaced with the strain sensors, however the mounting location of each sensor is of importance to achieve the highest sensitivity of the smart jaw. The smart jaw measurement system includes additional electronic devices for data capture, conditioning and transmission as shown in Figure 3. The bridge output from the strain sensor is first captured and filtered to remove high frequency noise. The resulting signal is digitized using a 10-bit analog-to-digital converter, sampling at 80 Hz. As the chuck rotates at high speeds, conventional wired transmission lines are not feasible. The proposed system communicates through Bluetooth Low Energy BLE due to its adequate speed and low power consumption. The data is then viewed wirelessly through a smart phone or other smart device. Figure 1: Smart Jaw monitoring system M. Sanati, A. Sandwell, H, Mostaghimi, S.S. Park

5 3. Future Works Preliminary theoretical research of the proposed chuck design has shown that according to their mechanical design, can be used for monitoring chuck gripping forces of workpieces during turning operations. Fabrication and testing of the prototype is in progress. Currently, the measurement of forces is of chief importance, but due to the high inertial loads imposed on the chuck during machining operations, it is also desirable to measure the centripetal force imposed on the chuck jaws. The use of an inertial measurement unit (IMU) capable of measuring both linear and angular acceleration may be used to estimate the inertial loads and add a correction to the direct force measurement of the smart jaw. Integration of this additional measurement may improve the performance of the smart jaw system. In addition, to monitor for workpiece loosening during high speed machining operations, it is necessary to sample at much higher frequencies to ensure that chuck loosening events are captured, and counter-measures initiated. Improving the electronics to allow for higher sampling rates is another goal and currently being investigated. 4. Acknowledgements This research was supported by Natural Sciences and Engineering Research Council of Canada (NSERC) under the CANRIMT Strategic Research Network Grant NETGP References [1] Spur, G, Mette, Clamping-force optimization allows high-speed turning, Production Engineering 5 (1) (1998) [2] Estrems, M, Arizmendi, M, Cumbicus W.E, Lopez, A, Measurement of clamping forces in a 3 jaw chuck through an instrumented aluminium ring, Procedia Engineering 132 (2015) [3] Slocum, A.H, Precision Machine Design, Prentice-Hall, Society of Manufacturing Engineers Dearborn (MI), USA, [4] Zhou, C & Yang, H & Yang, L, Qing, R. (2011). Piecewise model and experiment of power chuck s gripping force loss during high speed turning. Science Chinatechnological Sciences - SCI CHINA-TECHNOL SCI [5] Alquraan, T, Kuznetsov, Tsvyd, T, High seed clamping mechanism of the cnc lathe with compensation of centrifugal forces, Procedia Engineering 150 (2016) [6] Nyamekye, K, Mudiam, S.S, A Model for Predicting the Initial Static Gripping Force in Lathe Chucks, International Journal of Advanced Manufacturing Technology 7 (1992) M. Sanati, A. Sandwell, H, Mostaghimi, S.S. Park

Preliminary study of the vibration displacement measurement by using strain gauge

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