Development of a micro-punching machine and study on the influence of vibration machining in micro-EDM

Journal of Materials Processing Technology 180 (2006) 102–109

Development of a micro-punching machine and study on the influence of vibration machining in micro-EDM Gwo-Lianq Chern ∗ , Ying-Jeng Engin Wu, Shun-Feng Liu Department of Mechanical Engineering, National Yunlin University of Science and Technology, 123 University Rd., Sec. 3, Touliu, Yunlin 640, Taiwan, ROC Received 16 September 2004; received in revised form 3 March 2006; accepted 9 May 2006

Abstract This paper describes the development of a novel micro-punching machine that is capable of producing precision micro-holes. A significant feature of this machine is to fabricate the micro-punch and then the micro-die in the same machine, totally eliminating the eccentricity between the punch and the die when punching is proceeded. By applying vibration machining technique, we can decrease the possibility of electric short-circuiting during the micro-EDM process. The utilization of a proportional solenoid as the power unit of the micro-punching machine and as the source of vibration is found to be a successful attempt. Experiments to punch micro-holes with diameters of 0.1 and 0.2 mm on an SUS 304 stainless steel strip with 0.1 mm in thickness were carried out. The results show that the performance of this machine and the geometry of punched micro-holes are satisfactory. © 2006 Elsevier B.V. All rights reserved. Keywords: Micro-EDM; Micro-punching; WEDG; Proportional solenoid; Vibration machining

1. Introduction From the past to nowadays, machining of micro and accurate holes is a key point of developing new products in various fields of industry. It is well known that the electrical discharge machining (EDM) is capable of producing small holes because cutting force involved during the process is very small, as compared with conventional machining methods. The key question is how to obtain small-diameter electrodes. This problem was solved when Masuzawa et al. [1] invented the micro-EDM technique by introducing the wire electro-discharge grinding (WEDG). Electrical discharge takes place between the traveling wire and the electrode to be machined. Wire guides are used to constrain the wire motion to minimize the vibration and deviation of the wire, so as to maintain the gap distance between the electrode and the wire throughout the micro-EDM process. Miniature electrodes and micro-structures [2], such as series-pattern micro-disk [3] and micro-nozzles [4] could be fabricated precisely with the help of this WEDG technique.

∗

Corresponding author. Tel.: +886 5 5342601x4145; fax: +886 5 5312062. E-mail address: [email protected] (G.-L. Chern).

0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.05.010

Since the metal removal rate is very small in micro-EDM, ultrasonic machining has been employed to increase the efficiency during the micro-EDM process. Gao and Liu [5] presented a new combined method of ultrasonic and EDM with workpiece vibration. They found that the efficiency of material removal rate increased noticeably with the help of ultrasonic vibration, being eight times greater than that of conventional micro-EDM. Zhao et al. [6] employed ultrasonic vibration on the EDM process to produce deep and small hole on titanium alloy. Holes with a diameter of less than 0.2 mm and a depth-to-diameter ratio of more than 15 can be drilled successfully. Others have combined micro-EDM with ultrasonic vibration for producing micro-holes on ceramic or brittle materials. Yan et al. [7] obtained highly accurate micro-holes with diameters of about 150 ␮m and depth of 500 ␮m in borosilicate glass. Thoe et al. [8] drilled small-diameter (of less than 1 mm) cooling holes in ceramic coated nickel alloys. They found that using ultrasonic vibration during EDM greatly increased the workpiece material removal rate. But micro-EDM is a time-consuming process. Tool wear is another problem if the electrode is to create many micro-holes. In order to solve these problems, a micro-punching machine is thus needed. Fujino et al. [9] developed a micro-punching

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system based on the WEDG technique. They were able to produce micro-holes of 50 ␮m on phosphorus bronze strip and on polyamide (PA) plastic strip with a thickness of 50 ␮m. But micro-holes had not been punched successfully on steel material due to its high strength and toughness. The purpose of this study is to develop a new micro-punching machine and to investigate the effect of vibration EDM technique on machining punch and die, and to clarify the mechanism for the improvement of material removal rate. We have found that the vibration machining technique increases the disturbance of coolant flow within the EDM gap and thus decreases the possibility of electric short-circuiting during the micro-EDM process. This results in better die-hole roundness and larger machining feed. Consequently, efficiency of micro-EDM is improved. The utilization of a proportional solenoid as the power unit of the micro-punching machine and as the source of vibration is a brand new attempt. Design considerations for the machine elements are indicated. The experimental results and discussions are presented. 2. Experimental equipment and method The principles used in this study are the vibration machining and the microEDM techniques to fabricate circular punch electrodes. Then we used the electrodes to micro-EDM the punching die. Finally, the electrode, which worked as a punch, was used to punch the stainless steel strip. Fig. 1 shows the micro-EDM procedure for this research, based on our previous work [10]. The tungsten-steel punch was manufactured to the desired dimension by the micro-EDM technique first, as shown in Fig. 1(a) and (b). We then used the punch as an electrode

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Fig. 2. Schematic layout of the machine.

to machine the die. The tip of the punch might have been worn when electrodischarge machining the die. So the punch end was sharpened again by the micro-EDM, similar to that shown in Fig. 1(a). Since both the punch and the die were fabricated on the same machine, no further assembly and alignment are needed and the eccentricity can be totally avoided. Now the punch and the die are ready to punch strip materials, as shown in Fig. 1(c). Fig. 2 shows the schematic layout of the micro-punching machine. Fig. 3 is the photo of the machine. The whole machine is placed on a passive isolator to avoid disturbance from the environment. Basically this machine is composed of seven sub-systems. Each sub-system is described as follows.

2.1. Proportional solenoid incorporated with dither signal The proportional solenoid (SMC, VEF2121) is located on the top of the machine, as shown in Fig. 2. The proportional solenoid is a popular electromechanical transducer used in the design of hydraulic or pneumatic proportional valves [11,12]. It is a widely used component in fluid-technical industry and thus the price for a usual proportional solenoid is low. It possesses the features of short working stroke and the linearly controllable output magnetic force. The power needed for this machine is relatively small. Thus the chosen proportional solenoid may provide an approximate output power of 20 W, which is quite suitable for the development of the micro-punching machine.

Fig. 1. Micro-EDM procedure for making the punch and the die.

Fig. 3. Photo of the whole machine.

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Table 1 Technical data of the proportional solenoid Max excitation current (A) Max output force (N) Linear working range (mm) Linearity deviation (%) Hysteresis (%) Rated power (W) Rated voltage (V)

0.81 65 3 2 3 13.8 24

The proportional solenoid is connected to the control box (VEA250). Once the exciting current is given, the spindle and the punch move downwards. This provides the punching force needed to punch the micro-holes. An extra voltageto-current transducer with built-in dither signal generator is combined in this control system for two purposes. Firstly, the dither signal incorporated into the control signal improves the precision of the machining because it reduces the effect of nonlinear stick-slip [13]. Secondly, the dither signal causes the vibration of the punch to a maximum of 250 Hz, which serves the purpose of an actuator of the vibration EDM technique. Some technical data of the proportional solenoid are summarized in Table 1. An inductive displacement sensor (Keyence, AS440) is employed and placed near the proportional solenoid to measure the displacement of the punch.

2.2. Rotation and positioning of spindle Rotation of the spindle is required during micro-EDM the punch. The spindle (ST20-NBS, 6-100) is employed for its small size. The rotation of the spindle is provided by the DC servo motor (Maxon) and the belt. The collet of the spindle can hold diameters in the range of 0.25–6 mm. Eccentricity between the collet and the spindle is limited to within 3 ␮m. The servo motor is chosen for the purpose of angular positioning and ease of control. Motion of the motor is transferred to the spindle through the belt, as shown in Fig. 4 which is viewed from the opposite side of Fig. 3. Top of the spindle is connected to the proportional solenoid by the positioning component set, as shown in Fig. 5. It contains the bearing, the spring and the cover. The main function of the positioning component set is to let the spindle: (a) rotate during micro-EDM the punch, and (b) move vertically during punching. The spring helps the spindle return to its original position after punching.

Fig. 5. Photo of positioning component set. diameter) must travel smoothly to avoid short-circuiting and to machine the punch to the desired dimension. To achieve this, the wire must be coiled by two rolls, and be stretched by the four pulleys and the guiding block, as shown in Fig. 6. To reduce the friction between the wire and the v-shaped pulley and to avoid the vibration of the wire, a thin layer (of about 3 ␮m) of brass and nickel is coated on the surface of the pulley. The guiding block is made of SK2 tool steel with a hardness of HRC55 after heat treatment and shows great corrosion resistant behavior during the micro-EDM. A tiny slit is created on the guiding block for two purposes: (a) to stretch the wire to a straight line in order to increase the efficiency of the micro-EDM, and (b) to allow the punch to pass over the centerline of the wire when electro-discharge machining the end surface of the punch.

2.4. Positioning tables

The main principle of WEDG in this research is to use the moving wire to micro-EDM the punch. RC EDM circuit is employed in this research. The dielectric coolant used is BP DI 180 water. The copper wire (of 0.1 mm in

Two positioning tables (Aerotech, ATS50-25-M) are employed in this research, as shown in Fig. 7. Resolution and reliability of the table are 0.05 and 0.3 ␮m, respectively. These tables are incorporated with a controller (Aerotech, U500) for their motion control. Both tables are arranged horizontally in the machine. These tables provide two kinds of linear motion for the machine: (a) radial direction and (b) axial direction. Radial-direction motion is needed for electro-discharge machining the punch to the desired diameter. This is achieved by the X table. Axial-direction motion is used when vertical movement is required. This motion is provided by the Y table. Two wedge blocks with built-in linear guides are utilized to convert the horizontal motion of the Y table to the

Fig. 4. Photo of motor, spindle and pulley.

Fig. 6. Photo of guiding block and pulley.

2.3. Wire guides for the WEDG

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are made of aluminum which possesses the characteristics of light weight and good wear resistance. The feeding device is composed of the motor and the torque controller. The torque controller can provide a torque from 0 to 1.7 kg f cm. The strip can be stretched by adjusting the torque controller and be fed by the motor.

2.7. Software controller The control of the punching machine as well as the acquisition and processing of the measured data are all integrated in a PC-based software controller we developed. The software language is Visual Basic 5.0 which provides a userfriendly environment, as shown in Fig. 9. It shows the absolute and relative coordinate data of X and Y positioning tables in real time. It can allow the user to run or edit a CNC program, or switch to manual data input (MDI) module.

3. Experimental results and discussions 3.1. Modeling, simulation and calculation of the system

Fig. 7. Positioning table and wedge block. axial direction we need. The wedge angle is 20◦ and thus 1 mm movement of the Y table creates an axial movement of 0.369 mm, i.e. tan 20◦ .

2.5. Die and tooling set The die and tooling set is composed of die, die holder, die stopper, pressure plate and on-off valve, as shown in Fig. 8. The die, die holder, die stopper and pressure plate are made of SKD11 alloy steel which possesses enough strength and shows well behavior of wear resistance. Their hardness can reach HRC58 after heat treatment. They are manufactured by wire-EDM and grinding. The die holder is fastened to the machining plate by screws. The die is securely positioned by the wedge-shaped die stopper. This design is based on the ease of die changing. The strip is clamped on the die by the pressure plate and the on–off valve. By doing this, the punching force can be reduced and thus the life of the punch can be prolonged.

When punching the micro-holes, the movement of the punch is basically a reciprocated motion. The modeling of this dynamic system is described as follows. First, the proportional solenoid is modeled as a first-order element [12]. The corresponding transfer function is given by Kip Fm (s) = I(s) 1 + Tp s

(1)

where Fm is the magnetic output force, I the input current, Kip the static gain of the proportional solenoid, Tp the time constant of the first-order element, and s is the Laplace operator. Fig. 10 shows the simplified model of the micro-punching machine. Basically it is a second-order mass-spring-damper oscillation system, following the equation of motion: Fm = m¨y + b˙y + ky

(2)

2.6. Strip feeding device

where y is the displacement of the punch, b the viscous damping coefficient, k the spring constant, and m is the mass of the punch. After Laplace transformation, we obtain:

The strip feeding device, as shown in Fig. 3, serves two purposes: (a) allowing the strip to move and feed to the desired position; (b) stretching the strip to an appropriate tension. The strip is coiled and stretched by two rolls and four pins. The rolls and pins are fixed to the holding block. The holding block and the rolls

1 Y (s) = 2 Fm (s) ms + bs + k

(3)

Thus, the overall transfer function between the input and output can be described as KUI Kip Y (s) = V (s) (1 + Tp s)(ms2 + bs + k)

Fig. 8. Photo of die and tooling set.

(4)

where KUI is the gain of the driver. The output response of the punch can be easily obtained from Eq. (4) by using a MATLAB-based computer program. The block diagram of the control system is shown in Fig. 11. Fig. 12 shows the simulated and the experimental step responses of the control system. The displacement of the punch was measured by an inductive displacement sensor (Keyence, AS440), located near the proportional solenoid. In addition, a CCD camera was also utilized to monitor the punching process. These step responses give support to the design and can be used to represent the actual displacement of the punch while punching a micro-hole. The maximum punching speed achievable is about 50 mm/s. It can be seen that the simulation agrees well with the experimental result.

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Fig. 9. Software controller.

Fig. 10. Simplified model of the micro-punching machine.

The equation that can be used to describe the shearing force needed to punch a micro-hole is given as P = Ltτ

(5)

where P is the shearing force, L the circumferential length of the punched hole (L = πd, where d is the diameter of the punched hole), t the material thickness, and τ is the shear strength (τ = 52 kg/mm2 for SUS 304). The thickness of the strip is 0.1 mm. The micro-electrode is made of tungsten steel. The maximum diameter of the micro-electrode diameter in this study is 0.2 mm. Thus the maximum required P is 32 N, determined

Fig. 12. Simulated and experimental step responses of the control system.

by Eq. (5). The punching force is provided by the proportional solenoid, as shown in Fig. 3, which possesses a maximum output force of 65 N. Thus the micro-punching machine should have sufficient load capacity to carry out the punching experiments.

Fig. 11. Control block diagram of the micro-punching machine.

G.-L. Chern et al. / Journal of Materials Processing Technology 180 (2006) 102–109 Table 2 Experimental conditions for micro-EDM

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Table 4 Time consumed for micro-EDM the die

Supply voltage EDM circuit Capacitance Resistance Spindle speed Vibration frequency Wire diameter Wire travel speed Dielectric coolant

120 V RC 5500 PF 700  1300 rpm 0 and 250 Hz 0.1 mm 24 mm/min BP DI 180 water

Punch diameter (mm)

Time consumed for 0 Hz (min)

Time consumed for 250 Hz (min)

0.1 0.2 0.3 0.4

1.2 1.8 3.9 3.9

1.2 1.4 2.9 3.2

Table 5 Roundness of the die opening Table 3 Time consumed for micro-EDM the punch Final diameter (mm)

Time consumed for 0 Hz (min)

Time consumed for 250 Hz (min)

0.1 0.2 0.3 0.4

16 14 12 6

14 12 6 3.5

3.2. Micro-EDM with vibration machining technique Table 2 shows the experimental conditions for micro-EDM. To investigate the effect of vibration on micro-EDM, we machined the circular punch from its original diameter of 0.5 to 0.4, 0.3, 0.2 and 0.1 mm, respectively, with and without vibration. Machining length of the punch is 1 mm for all tests. Feed is adjusted and kept constant to the maximum value during microEDM while short-circuiting does not occur. Time consumed for each machining condition is listed in Table 3. When the vibration of 250 Hz is imposed on the punch during micro-EDM, it causes a rapid disturbance in the dielectric coolant. The tiny machining debris between the punch and the wire can be carried away by the coolant circulation more effectively. The occurrence of short-circuiting can thus be suppressed. This allows us to increase the feed and to reduce the machining time, as can be seen in Table 3. We obtained a maximum of 50% reduction of the machining time, from 12 to 6 min, when electro-discharge machining the punch from 0.5 to 0.3 mm. We then used the punch (with diameter of 0.4, 0.3, 0.2 and 0.1 mm, respectively) as an electrode to machine the die. Thickness of the die is 0.3 mm. Again, the feed is chosen with a maximum value while not causing short-circuiting. Time con-

Punch diameter (mm)

Roundness for 0 Hz (␮m)

Roundness for 250 Hz (␮m)

0.1 0.2 0.3 0.4

3 4 3 3

1 2 2 3

sumed for micro-EDM the die is listed in Table 4. The influence of vibration machining can be seen by comparing the time cost with and without vibration. For a punch diameter of 0.3 mm, the time consumed for electro-discharge machining the die can be reduced from 3.9 to 2.9 min by imposing vibration machining, showing a 25% reduction of the machining time. The shape of the die-opening produced on SKD11 alloy steel is also investigated. Under the limitation of the measuring equipment available, we measured the diameters of the die-opening D in four directions (0◦ , 45◦ , 90◦ , 135◦ ) and used the maximum difference among the data (Dmax − Dmin ) to represent the roundness. This method had been employed in our previous research on micro-drilling [14] and had been proven to be an easy approach to analyze the form error of a drilled hole. We found that this simple method can also be applied to analyze the roundness of die-opening on SKD11 alloy steel and the roundness of punched micro-holes on SUS 304 stainless steel in this study. The results are listed in Table 5. Roundness of the die-opening is improved in most of the tests with vibration machining, especially for small diameter holes. For a punch diameter of 0.1 mm, the roundness of the die-opening is improved from 3 to 1 ␮m. When electro-discharge machining electrode of that small size, the effect of vibration machining becomes prominent since tiny machining debris is hard to be conveyed out of the EDM gap.

Fig. 13. Photo of a 0.1 mm hole on an SUS 304 stainless steel strip with a thickness of 0.1 mm: (a) entrance side and (b) exit side.

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Fig. 14. Photo of a 0.2 mm hole on an SUS 304 stainless steel strip with a thickness of 0.1 mm: (a) entrance side and (b) exit side.

Fig. 15. Photo of two 0.2 mm holes on an SUS 304 stainless steel strip with a thickness of 0.1 mm: (a) entrance side and (b) exit side.

For a larger punch electrode, 0.4 mm for example, the rotation of the punch might be sufficient to cause enough disturbance of the coolant flow and thus the effect of vibration machining is not outstanding. 3.3. Punching of micro-holes and tool-wear analysis Figs. 13 and 14 show the experimental results of micropunching on an SUS 304 stainless steel strip with a thickness of 0.1 mm for punch diameters of 0.1 and 0.2 mm, respectively. The roundness of micro-hole on the entrance side is about 2 ␮m in Fig. 13 and is below 5 ␮m in Fig. 14. Fig. 15 shows the other experimental result of punching successive 0.2 mm holes on the similar strip. From Figs. 13–15, it can be seen that the punching of micro-holes in this study is successful and the geometry of the punched micro-holes is satisfactory. Fig. 16 shows the photo of a punch with diameter of 0.2 mm after punching 50 micro-holes, viewed from its end. Basically the punch is still intact. But some minute damages (edge chipping) on the punch can be observed, as marked by a dashed oval in Fig. 16. The reasons for the occurrence of premature progressive tool wear might be those stated as follows: (1) The surface condition on the punch after micro-EDM is not good enough. (2) The strip material is not very flat, or is not stretched sufficiently, causing uneven resistance force on the punch. (3) The strip is too thick, as compared with the dimension of the micro-punch. (4) The SUS 304 stainless steel is too tough and strong, as compared with the punch material which is made of tung-

Fig. 16. Photo of a punch with diameter of 0.2 mm after punching 50 microholes.

sten steel. Brass and copper are much easier to be punched since they possess less strength and show more ductile behavior. Nevertheless, micro-punching on SUS 304 stainless steel has been realized successfully on the self-developed micropunching machine in this study. 4. Conclusions 1. In this paper, we have successfully developed a micropunching machine that is capable of producing precision micro-holes. Design of each sub-system of the machine is proposed and described in detail.

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2. The utilization of a proportional solenoid as the power unit of the micro-punching machine and as the source of vibration is found to be a successful attempt. By applying the vibration machining technique, efficiency of micro-EDM is enhanced, which can be shown in the reduction of machining time and in the improved roundness of the die opening and the punched micro-holes. 3. Experiments to punch micro-holes with diameters of 0.1 and 0.2 mm on an SUS 304 stainless steel strip with 0.1 mm in thickness were carried out successfully. Experimental results also show that the performance of the developed novel micropunching machine is satisfactory. Acknowledgements The author would like to thank Dr. J.C. Renn and C.L. Kuo of National Yunlin University of Science and Technology for providing some helpful suggestions for this study. References [1] T. Masuzawa, M. Fujino, K. Kobayashi, Wire electro-discharge grinding for micro-machining, Ann. CIRP 34 (1) (1985) 431–435. [2] H.S. Lim, Y.S. Wong, M. Rahman, M.K.E. Lee, A study on the machining of high-aspect ratio micro-structures using micro-EDM, J. Mater. Process. Technol. 140 (2003) 318–328. [3] C.L. Kuo, J.D. Huang, Fabrication of series-pattern micro-disk electrode and its application in machining micro-slit of less than 10 ␮m, Int. J. Mach. Tools Manuf. 44 (2004) 545–553.

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[4] T. Masuzawa, C.L. Kuo, M. Fujino, A combined electrical machining process for micronozzle fabrication, Ann. CIRP 43 (1) (1994) 189– 192. [5] C.S. Gao, Z.X. Liu, A study of ultrasonically aided micro-electricaldischarge machining by the application of workpiece vibration, J. Mater. Process. Technol. 139 (2003) 226–228. [6] W.S. Zhao, Z.L. Wang, S.C. Di, G.X. Chi, H.Y. Wei, Ultrasonic and electrical discharge machining to deep and small hole on titanium alloy, J. Mater. Process. Technol. 120 (2002) 101–106. [7] B.H. Yan, A.C. Wang, C.Y. Huang, F.Y. Huang, Study of precision microholes in borosilicate glass using microEDM combined with microultrasonic vibration machining, Int. J. Mach. Tools Manuf. 42 (2002) 1105– 1112. [8] T.B. Thoe, D.K. Aspinwall, N. Killey, Combined ultrasonic and electrical discharge machining of ceramic coated nickel alloy, J. Mater. Process. Technol. 92–93 (1999) 323–328. [9] M. Fujino, M. Yamamoto, T. Masuzawa, Micro-punching System as an Application of WEDG, vol. 39, no. 6, Institute of Industrial Science, University of Tokyo, Seisan-Kenkyu, 1987, pp. 277–280 (in Japanese). [10] S.F. Liu, A development and research of the multi-functional micropunching and micro-forming machine, Master Thesis, National Yunlin University of Science and Technology, Taiwan, ROC, 2001 (in Chinese). [11] J.C. Renn, G.L. Chern, Design of a novel micro-punching machine using proportional solenoid, J. CSME 25 (1) (2004) 89–93. [12] J.C. Renn, C.J. Chen, A new open-loop test device and a nonlinear model for the proportional solenoids, J. Sci. Technol. 8 (3) (1999) 185– 191. [13] J.C. Renn, Position control of a pneumatic servocylinder using fuzzysliding surface controller, Int. J. Fluid Power 3 (3) (2002) 19– 25. [14] G.L. Chern, H.J. Lee, Using workpiece vibration cutting for micro-drilling, Int. J. Adv. Manuf. Technol. 27 (7) (2006) 688–692.