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Some Effects on EDM Characteristics by Assisted Ultrasonic Vibration of the Tool Electrode
Available online at www.sciencedirect.com
ScienceDirect Procedia CIRP 68 (2018) 76 – 80
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
Some Effects on EDM Characteristics by Assisted Ultrasonic Vibration of the Tool Electrode Atsutoshi HIRAOa* Hiromitsu GOTOHb, Takayuki TANIb b
a Faculty of Education, Niigata University, Niigata, Japan Department of Industrial Information, Tsukuba University of Technology, Tsukuba, Japan
* Corresponding author. Tel.: +81-25-262-7086; fax: +81-25-262-7086. E-mail address: [email protected]
Abstract It is well known that electrical discharge machining (EDM) is a machining method that does not affect the hardness of the material being processed. The EDM method is widely used for the finish machining of metal molds, for which materials having a high degree of hardness is used. However, the EDM method has a disadvantage in that the machining speed is remarkably slow compared to other machining methods. However, if the discharge energy is increased, the quality of surface finish is decreased. For this reason, it is difficult to machine a finished surface. In order to improve machining characteristics while using the same discharge energy, it is important to homogeneously disperse the discharge, so as to increase the rate of normal discharge, and to discharge the machining dust that accumulates between the electrode and workpiece. However, it is difficult to generate a constantly normal discharge in EDM. What is worse, the more deeply a hole is machined, the more difficult it is to remove the machining dust that accumulates between the electrode and the workpiece. A concentrated discharge tends to occur locally because of the presence of machining dust between the electrode and workpiece. This concentrated discharge has an adverse effect on the surface finish. Also, it creates a big problem, namely a bridging contact is generated between parts. In order to avoid generating such a concentrated discharge or abnormal discharge, it is necessary to disperse the discharge appropriately. In this study, we investigate EDM characteristics by using assisted ultrasonic vibration of the tool electrode as a method of overcoming the problem. The result showed that the machining speed was increased several times by using assisted ultrasonic vibration of the tool electrode. The method proved to remarkably effective in increasing machining speed even if the amplitude of the ultrasonic vibration was 1 μm. Furthermore, the rate of normal discharge increased by applying ultrasonic vibration. In particular, this method yielded a beneficial effect in finishing conditions. This is expected to produce a better effect under conditions of finish machining when there is a small gap between the electrode and workpiece. In particular, occurrences of abnormal discharges, such as a concentrated discharge, decreased since the electrode tool was forcibly separated from the workpiece. Machining wastes are positively removed as a result of the tool electrode working with a "pumping action." In particular, when applied to deep-hole machining, this method is extremely effective in machining deeper holes. © 2018 2018The The Authors. Published by Elsevier B.V. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining
Keywords: EDM, Ultra sonic vibration, Machining characteristics, Amplitude effect, Dispersion of discharge
1. Introduction General processes that are assisted by ultrasonic vibration include ultrasonic vibration-assisted polishing and ultrasonic welding[1-2]. Recently, ultrasonic vibration has been extensively studied for application to removal processes such as ultrasonic vibration cutting, ultrasonic vibration grinding, and ultrasonic elliptical vibration cutting [3–6]. Ultrasonic vibration cutting involves applying ultrasonic vibration to a turning tool with an amplitude of several microns and is
effective for improving the accuracy of low-speed and precision cutting of a special workpiece. Research has also been conducted on applying ultrasonic vibration to a tool electrode or workpiece for electric discharge machining (EDM) [7–9]. When finishing steel materials with copper electrodes, it has been shown that stable machining is realized even under minute electrical conditions [10-11]. With EDM, tool electrodes of various shapes and lengths are used, so the effective application of ultrasonic vibration is difficult. A method has been devised in which ultrasonic vibration is not
2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.025
77
Atsutoshi Hirao et al. / Procedia CIRP 68 (2018) 76 – 80
applied directly to an electrode or workpiece. Instead, ultrasonic cavitation is generated in the EDM oil. This approach can effectively discharge machining dust and air bubbles between gaps to improve machining efficiency [5]. The speed of EDM can be improved by increasing the discharge energy, such as by increasing the current or pulse duration. Applying ultrasonic vibration may affect the processing characteristics. In this study, ultrasonic vibration was applied to a tool electrode. EDM was performed while varying the frequency and amplitude of the ultrasonic vibration, and the EDM characteristics were investigated. 2. Ultrasonic vibration assistance The machine was equipped with a z-axis motorized stage, which was a stepping motor drive on the main axis. The stepping motor drive made high-resolution control of the main axis possible for the processing machine. A Langevin-type ultrasonic vibrator was equipped on the main axis. Previous studies have reported a high discharge effect for a tool electrode assisted by ultrasonic vibration, such as an increased processing speed. However, there have been no reports on how the frequency of the tool electrode affects the machining characteristics. Ultrasonic vibrations at frequencies of 20–80 kHz and an amplitude of as small as several microns were considered. This is because it is difficult to precisely measure the amplitude and vibration frequency. The relationship between the vibration frequency of the ultrasonic vibration applied to the tool electrode and the removal rate was measured in detail. 2.1. Langevin-type ultrasonic vibrator A Langevin-type ultrasonic vibrator was used for the transducer in the EDM. Table 1 presents the specifications of the transducer. The natural resonance frequency of the transducer was 40 kHz. A screw was machined against the copper of the tool electrode. The electrode was firmly fixed by using a nut on the transducer. When the copper was fixed to the transducer, the natural resonance frequency changed. In order to actually vibrate the tool electrode, the apparatus shown in Fig. 1 was constructed. First, a sine wave matching the natural resonance frequency was output from the wave generator. The sine wave was amplified by the amplifier and input to the transducer. The tool electrode vibrated with the input sine wave. The natural frequency of the transducer was determined when the current value was measured by the current transformer.
Sine wave
Waveform generator
Current transformer
Amplified sine wave
Amplifier
Electrode (Cu)
Fig. 1. Langevin-type ultrasonic vibrator system.
2.2. EDM equipment Fig. 2 shows the EDM machine that was developed in the present study. The machine was equipped with an xyz motorized stage that was capable of moving in increments of 1 μm. The discharge was from a transistor that completed the circuit. The Z axis was controlled from the PC by a program using the field programmable gate array (FPGA). Table 2 gives the experimental conditions. A copper wire with a diameter of 3 mm and fixed to the transducer was used as the starting electrode. High-tension steel (NAK 55) with an area of 20 mm × 20 mm was used as the workpiece. Z axis motorized stages
Langevin type ultrasonic vibrator Electrode
Work tank XY axis motorized stages
Current transformer (Discharge current ) Current transformer (Ultrasonic vibration) Fig. 2. Overview of experimental device. Table 2. Experimental conditions.
Table 1. Langevin-type ultrasonic vibrator.
Langevin type ultrasonic vibrator (Al)
Open circuit voltage, E
100 [V]
Discharge current, I
1.6 [A]
Type
1540P2BF
Reference voltage
60 [V]
Diameter
φ15 [mm]
Pulse duration
10 [μsec]
Height
67 [mm]
Pulse interval time
20 [μsec]
Frequency
40 [kHz]
Electrode (+)
Cu φ3 [mm]
Workpiece (-)
NAK 55
Dielectric working fluid
oil
Atsutoshi Hirao et al. / Procedia CIRP 68 (2018) 76 – 80
3. EDM characteristics with ultrasonic vibration assistance
2.3. Frequency and removal rate
Surface roughness [μmRa]
Removal rate [mg/min]
Natural frequency [kHz] Fig. 3. Relationship between vibration frequency, removal rate and surface roughness.
Based on the results for the relationship between the frequency and removal rate, the relation between the amplitude of the tool electrode and the machining characteristics was examined in detail. Fig. 4 shows the amplitude measurement method. The amplitude and frequency of the tool electrode side (copper) and transducer side (aluminum) were measured with different instruments. The eddy current displacement sensor was used for the tool electrode side (copper), and a laser displacement meter was used for the transducer side (aluminum). Fig. 5 shows the measured vibration frequency and amplitude with the applied ultrasonic vibration. The output voltage from the waveform generator was 1.2 V. The current input to the transducer was measured with the current transformer (red line). The waveform measured by the current transformer was used to estimate that the transducer was vibrating at a frequency of 50.6 kHz. The green waveform is based on the vibration frequency and amplitude of the transducer measured by the laser displacement meter. The results confirmed that the amplitude on the transducer side was 5.37 μm. The green waveform is stepped because the vibration frequency was 50.6 kHz, and the response speed of the laser displacement meter was 500 kHz. For this reason, 10 steps were confirmed in the waveform of one cycle. The blue waveform is based on the vibration frequency and amplitude of the copper tool electrode that were measured with the eddy current displacement sensor. This waveform indicates that the amplitude was 5.76 μm. Current transformer current Eddy displacement Laser displacement meter 6 μm
Laser displacement meter
3.1. Amplitude measurement method
100 mA
The effect of the natural frequency of the electrode due to the ultrasonic vibration on processing characteristics such as the removal rate and surface roughness was investigated. The natural frequency was varied by changing the length of the copper electrode. The vibration of the tool electrode was confirmed by direct touching of the electrode. The electrode was assumed to vibrate according to the current waveform detected by the current transformer. Fig. 3 shows the relationship between the natural frequency, removal rate, and surface roughness. The frequency was 0 kHz without vibration assistance for the electrode. The removal rate was low without ultrasonic vibration assistance. The results confirmed that the removal rate was improved with ultrasonic vibration assistance at any frequency. In addition, the removal rate improved with increasing frequency up to 66.5 kHz, after which the removal rate gradually decreased. The removal rate did not change much from 27.6 kHz to 57.3 kHz. This may be due to the difference in electrode wear and magnitude of the amplitude. The surface roughness increased with ultrasonic vibration assistance to about 1 μm Ra. The difference in roughness with frequency was not large. 1.3 4 removal rate surface roughness 3 1 2 0.7 1 0.4 0 0 20 40 60 80 100
5 μm
78
10 μsec Langevin type ultrasonic vibrator Electrode
Fig. 5. Result of displacement measurement.
3.2. Amplitude characteristics of the ultrasonic transducer Eddy current displacement
Fig. 4. Measurement method of displacement measurement.
This system amplified the sine wave output from the waveform generator and inputted it to the transducer. The measurement with the laser displacement meter indicated that the amplitude of the transducer was limited to 6 μm even when the output voltage was increased. When the output voltage of the waveform generator was 1 V, the transducer became saturated with an amplitude of 6 μm.
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Atsutoshi Hirao et al. / Procedia CIRP 68 (2018) 76 – 80
Tool electrode (Eddy current displacement) Transducer (Laser displacement meter) 6 5 4 3
4 φ6 mm (50.6 kHz) φ3 mm (36.8 kHz)
3
2 1 0 0
2
1
2
3
4
5
6
Amplitude [μm]
1
Frequency : 50.6 kHz
0 0
1 2 3 Waveform generator output voltage [V]
Fig. 7. Comparison of removal rate for each electrode.
4
Fig. 6. Difference in amplitude between tool electrode and transducer.
Open circuit voltage
100 V
Amplitude [μm]
7
Fig. 9 shows the relationship between the vibration amplitude of the tool electrode and the surface roughness. The surface roughness increased with ultrasonic vibration assistance for the tool electrode. The surface roughness increased with the amplitude. When an amplitude of 6 μm assisted the tool electrode, the surface roughness increased by 0.2 μm Ra or more compared to the case without ultrasonic vibration assistance. In EDM, the surface roughness largely depends on the processing conditions. The ultrasonic vibration assistance for the tool electrode increased the effective discharge number. However, the processing conditions did not change. It is necessary to verify whether the surface roughness increases with the application of ultrasonic vibration.
Removal rate [mg/min]
A tool electrode was attached to the transducer, and the amplitudes of the transducer side and tool electrode sides were measured. Fig. 6 shows the relationship between the output voltage from the waveform generator and the amplitudes of the transducer and tool electrode. Both the transducer and tool electrode showed the same amplitude with respect to the output voltage. Slight variations were observed in the amplitude on the transducer side. This may be due to the response speed of the laser displacement meter. Increasing the output voltage increased the amplitudes of both the vibrator and tool electrode. The amplitude was linearly amplified with an increasing output voltage up to 1.5 V. At higher voltages, the amplitude was limited to 6 μm. The results confirmed that the amplitudes of the copper on the electrode side and aluminum on the transducer side were the same.
3.3. Effect of amplitude on the EDM characteristics
1A
Discharge current
500 μs (a) Without ultrasonic vibration assistance.
100 V
Open circuit voltage
Discharge current
1A
Fig. 7 shows the relationship between the vibration amplitude of the tool electrode and the removal rate. In this experiment, tool electrodes with diameters of 6 and 3 mm were used for comparison. Similar to the results given above, assisting the amplitude of the tool electrode increased the removal rate. Moreover, the removal rate was confirmed to increase in proportion with the amplitude. In the experiment with the tool electrode having a diameter of 6 mm, the removal rate was increased twofold with an assisting vibration amplitude of 1 μm and increased sevenfold with an assisting vibration amplitude of 6 μm. In this system, the amplitude for the assistance to the tool electrode had a limit of 6 μm. Therefore, it is necessary to investigate the removal rate for an amplitude of 6 μm or more. The processing speed was generally increased by increasing the number of effective discharges. The waveform with the discharge was observed with an oscilloscope. Fig. 8(a) shows the voltage waveform and current waveform with no ultrasonic vibration assistance, and Fig. 8(b) shows the waveforms with assistance. The amplitude for the tool electrode was 5.2 μm. Pulse waves were observed for the current waveform, which indicated when a discharge occurred. The results in Figs. 8(a) and (b) confirmed that the number of discharge occurrences increased more than twofold with ultrasonic vibration assistance.
500 μs (b) With ultrasonic vibration assistance. Fig. 8. Discharge waveforms during EDM.
Atsutoshi Hirao et al. / Procedia CIRP 68 (2018) 76 – 80
Surface roughness [μmRa]
80
1.2
Education, Culture, Sports, Science and Technology, Japan, and the Japan Science and Technology Agency.
1
References
0.8 0.6
0.4 0.2 0
Frequency : 50.6 kHz
0
2 4 Amplitude [μm]
6
Fig. 9. Relationship between vibration frequency and surface roughness.
4. Conclusions We applied ultrasonic vibration assistance to a tool electrode and investigated the effect of different amplitudes on the EDM characteristics in detail. The obtained results are summarized below. (1) When the tool electrode was vibrated, the maximum amplitude was 6 μm. Applying ultrasonic vibration assistance with an amplitude of 6 μm to EDM increased the removal rate up to sevenfold. If an amplitude of 6 μm or more can be imparted to the tool electrode, this may increase the removal rate further. (2) Under the same conditions, the surface roughness increased with ultrasonic vibration assistance. The surface roughness increased with the amplitude. The relationship between the gap distance and surface roughness must further be investigated. Acknowledgements This study was financially supported by a Grant-in-Aid for Young Scientists (B) (16K17994) from the Ministry of
[1] H. Suzuki, T. Moriwaki, T. Okino, Y. Ando: Development of Ultrasonic Vibration Assisted Polishing Machine for Micro Aspheric Die and Mold, Annals of the CIRP, Vol. 55/ 1, 2006, pp. 385–388. [2] H. Suzuki, S. Hamada, T. Okino, M. Kondo, Y. Yamagata, T. Higuchi: Ultraprecision finishing of micro-asphric surface by ultrasonic two-axis vibration assisted polishing, CIRP Annals–Manufacturing Technology, 59 , 2010, pp. 347-350. [3] M. FujimotoˈY. WuˈM. NomuraˈH. KanaiˈM. Jin, Wear Behavior of Mini-size Diamond Wheel in Ultrasonic Assisted Grinding, The 15th International Conference on Precision Engineering, 2014. pp. 147-152. [4] E. Shamoto, T. Moriwaki: Ultraprecision Diamond Cutting of Hardened Steel by Applying Elliptical Vibration Cutting, Annals of the CIRP, Vol. 48/ 1, 1999, pp. 441-444. [5] T. Moriwaki, E. Shamoto: Ultrasonic Elliptical Vibration Cutting, Annals of the CIRP, Vol. 44/ 1, 1995, pp. 31-34. [6] N. Suzuki, Z. Yan, M. Haritani, J. Yang, S. Hamada, R. Hino, E. Shamoto: Ultraprecision Machining of Tungsten Alloy by Applying Ultrasonic Elliptical Vibration Cutting (in Japanese), Journal of the Japan Society for Precision Engineering, Vol. 73, No. 3, 2007, pp. 360-366. [7] A. Hirao, T. Tani, N. Mohri, N. Saito: Some Effects of Ultrasonic
Vibration on combined Electrical Discharge Machining and its Practical Use (in Japanese), Journal of the Japan Society for Precision Engineering, Vol. 73, No. 7, 2007, pp. 781-785. [8] G. S. Prihandana, Y. Fukuzawa, S. Nagasawa and K. Yamashita: Effect of Vibrated Electrode in Electrical Discharge Machining, Proceedings of the First International Conference and Seventh AUN/SEED-Net Fieldwise Seminar on Manufacturing and Material Processing, 2006, pp. 133-138. [9] G. S. Prihandana, M. Mahardika, M. Mamdi, K. Mitsui: Effect of lowfrequency vibration on workpiece in EDM processes, Journal of Mechanical Science and Technology, Vol. 25, No. 5, 2011, pp. 1231-1234. [10] H. Ogawa, K. Bando, Y. Tsunemoto: Effect of Applying Ultrasonic Vibration to Machining Fluid in EDM (1st Report) (in Japanese), Journal of the Japan Society of Electrical Machining Engineers, Vol. 41, No. 98, 2007, pp. 163-168. [11] T.Ichikawa, W. Natsu: Investigation of Machining Characteristics of Micro-EDM Ultrasonically Vibrated Machining Fulid under Ultrasmall Discharge Energy (in Japanese), Journal of the Japan Society of Electrical Machining Engineers, Vol. 47, No. 116, 2013, pp. 155-162.
ScienceDirect Procedia CIRP 68 (2018) 76 – 80
19th CIRP Conference on Electro Physical and Chemical Machining, 23-27 April 2018, 2017, Bilbao, Spain
Some Effects on EDM Characteristics by Assisted Ultrasonic Vibration of the Tool Electrode Atsutoshi HIRAOa* Hiromitsu GOTOHb, Takayuki TANIb b
a Faculty of Education, Niigata University, Niigata, Japan Department of Industrial Information, Tsukuba University of Technology, Tsukuba, Japan
* Corresponding author. Tel.: +81-25-262-7086; fax: +81-25-262-7086. E-mail address: [email protected]
Abstract It is well known that electrical discharge machining (EDM) is a machining method that does not affect the hardness of the material being processed. The EDM method is widely used for the finish machining of metal molds, for which materials having a high degree of hardness is used. However, the EDM method has a disadvantage in that the machining speed is remarkably slow compared to other machining methods. However, if the discharge energy is increased, the quality of surface finish is decreased. For this reason, it is difficult to machine a finished surface. In order to improve machining characteristics while using the same discharge energy, it is important to homogeneously disperse the discharge, so as to increase the rate of normal discharge, and to discharge the machining dust that accumulates between the electrode and workpiece. However, it is difficult to generate a constantly normal discharge in EDM. What is worse, the more deeply a hole is machined, the more difficult it is to remove the machining dust that accumulates between the electrode and the workpiece. A concentrated discharge tends to occur locally because of the presence of machining dust between the electrode and workpiece. This concentrated discharge has an adverse effect on the surface finish. Also, it creates a big problem, namely a bridging contact is generated between parts. In order to avoid generating such a concentrated discharge or abnormal discharge, it is necessary to disperse the discharge appropriately. In this study, we investigate EDM characteristics by using assisted ultrasonic vibration of the tool electrode as a method of overcoming the problem. The result showed that the machining speed was increased several times by using assisted ultrasonic vibration of the tool electrode. The method proved to remarkably effective in increasing machining speed even if the amplitude of the ultrasonic vibration was 1 μm. Furthermore, the rate of normal discharge increased by applying ultrasonic vibration. In particular, this method yielded a beneficial effect in finishing conditions. This is expected to produce a better effect under conditions of finish machining when there is a small gap between the electrode and workpiece. In particular, occurrences of abnormal discharges, such as a concentrated discharge, decreased since the electrode tool was forcibly separated from the workpiece. Machining wastes are positively removed as a result of the tool electrode working with a "pumping action." In particular, when applied to deep-hole machining, this method is extremely effective in machining deeper holes. © 2018 2018The The Authors. Published by Elsevier B.V. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining. Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining
Keywords: EDM, Ultra sonic vibration, Machining characteristics, Amplitude effect, Dispersion of discharge
1. Introduction General processes that are assisted by ultrasonic vibration include ultrasonic vibration-assisted polishing and ultrasonic welding[1-2]. Recently, ultrasonic vibration has been extensively studied for application to removal processes such as ultrasonic vibration cutting, ultrasonic vibration grinding, and ultrasonic elliptical vibration cutting [3–6]. Ultrasonic vibration cutting involves applying ultrasonic vibration to a turning tool with an amplitude of several microns and is
effective for improving the accuracy of low-speed and precision cutting of a special workpiece. Research has also been conducted on applying ultrasonic vibration to a tool electrode or workpiece for electric discharge machining (EDM) [7–9]. When finishing steel materials with copper electrodes, it has been shown that stable machining is realized even under minute electrical conditions [10-11]. With EDM, tool electrodes of various shapes and lengths are used, so the effective application of ultrasonic vibration is difficult. A method has been devised in which ultrasonic vibration is not
2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 19th CIRP Conference on Electro Physical and Chemical Machining doi:10.1016/j.procir.2017.12.025
77
Atsutoshi Hirao et al. / Procedia CIRP 68 (2018) 76 – 80
applied directly to an electrode or workpiece. Instead, ultrasonic cavitation is generated in the EDM oil. This approach can effectively discharge machining dust and air bubbles between gaps to improve machining efficiency [5]. The speed of EDM can be improved by increasing the discharge energy, such as by increasing the current or pulse duration. Applying ultrasonic vibration may affect the processing characteristics. In this study, ultrasonic vibration was applied to a tool electrode. EDM was performed while varying the frequency and amplitude of the ultrasonic vibration, and the EDM characteristics were investigated. 2. Ultrasonic vibration assistance The machine was equipped with a z-axis motorized stage, which was a stepping motor drive on the main axis. The stepping motor drive made high-resolution control of the main axis possible for the processing machine. A Langevin-type ultrasonic vibrator was equipped on the main axis. Previous studies have reported a high discharge effect for a tool electrode assisted by ultrasonic vibration, such as an increased processing speed. However, there have been no reports on how the frequency of the tool electrode affects the machining characteristics. Ultrasonic vibrations at frequencies of 20–80 kHz and an amplitude of as small as several microns were considered. This is because it is difficult to precisely measure the amplitude and vibration frequency. The relationship between the vibration frequency of the ultrasonic vibration applied to the tool electrode and the removal rate was measured in detail. 2.1. Langevin-type ultrasonic vibrator A Langevin-type ultrasonic vibrator was used for the transducer in the EDM. Table 1 presents the specifications of the transducer. The natural resonance frequency of the transducer was 40 kHz. A screw was machined against the copper of the tool electrode. The electrode was firmly fixed by using a nut on the transducer. When the copper was fixed to the transducer, the natural resonance frequency changed. In order to actually vibrate the tool electrode, the apparatus shown in Fig. 1 was constructed. First, a sine wave matching the natural resonance frequency was output from the wave generator. The sine wave was amplified by the amplifier and input to the transducer. The tool electrode vibrated with the input sine wave. The natural frequency of the transducer was determined when the current value was measured by the current transformer.
Sine wave
Waveform generator
Current transformer
Amplified sine wave
Amplifier
Electrode (Cu)
Fig. 1. Langevin-type ultrasonic vibrator system.
2.2. EDM equipment Fig. 2 shows the EDM machine that was developed in the present study. The machine was equipped with an xyz motorized stage that was capable of moving in increments of 1 μm. The discharge was from a transistor that completed the circuit. The Z axis was controlled from the PC by a program using the field programmable gate array (FPGA). Table 2 gives the experimental conditions. A copper wire with a diameter of 3 mm and fixed to the transducer was used as the starting electrode. High-tension steel (NAK 55) with an area of 20 mm × 20 mm was used as the workpiece. Z axis motorized stages
Langevin type ultrasonic vibrator Electrode
Work tank XY axis motorized stages
Current transformer (Discharge current ) Current transformer (Ultrasonic vibration) Fig. 2. Overview of experimental device. Table 2. Experimental conditions.
Table 1. Langevin-type ultrasonic vibrator.
Langevin type ultrasonic vibrator (Al)
Open circuit voltage, E
100 [V]
Discharge current, I
1.6 [A]
Type
1540P2BF
Reference voltage
60 [V]
Diameter
φ15 [mm]
Pulse duration
10 [μsec]
Height
67 [mm]
Pulse interval time
20 [μsec]
Frequency
40 [kHz]
Electrode (+)
Cu φ3 [mm]
Workpiece (-)
NAK 55
Dielectric working fluid
oil
Atsutoshi Hirao et al. / Procedia CIRP 68 (2018) 76 – 80
3. EDM characteristics with ultrasonic vibration assistance
2.3. Frequency and removal rate
Surface roughness [μmRa]
Removal rate [mg/min]
Natural frequency [kHz] Fig. 3. Relationship between vibration frequency, removal rate and surface roughness.
Based on the results for the relationship between the frequency and removal rate, the relation between the amplitude of the tool electrode and the machining characteristics was examined in detail. Fig. 4 shows the amplitude measurement method. The amplitude and frequency of the tool electrode side (copper) and transducer side (aluminum) were measured with different instruments. The eddy current displacement sensor was used for the tool electrode side (copper), and a laser displacement meter was used for the transducer side (aluminum). Fig. 5 shows the measured vibration frequency and amplitude with the applied ultrasonic vibration. The output voltage from the waveform generator was 1.2 V. The current input to the transducer was measured with the current transformer (red line). The waveform measured by the current transformer was used to estimate that the transducer was vibrating at a frequency of 50.6 kHz. The green waveform is based on the vibration frequency and amplitude of the transducer measured by the laser displacement meter. The results confirmed that the amplitude on the transducer side was 5.37 μm. The green waveform is stepped because the vibration frequency was 50.6 kHz, and the response speed of the laser displacement meter was 500 kHz. For this reason, 10 steps were confirmed in the waveform of one cycle. The blue waveform is based on the vibration frequency and amplitude of the copper tool electrode that were measured with the eddy current displacement sensor. This waveform indicates that the amplitude was 5.76 μm. Current transformer current Eddy displacement Laser displacement meter 6 μm
Laser displacement meter
3.1. Amplitude measurement method
100 mA
The effect of the natural frequency of the electrode due to the ultrasonic vibration on processing characteristics such as the removal rate and surface roughness was investigated. The natural frequency was varied by changing the length of the copper electrode. The vibration of the tool electrode was confirmed by direct touching of the electrode. The electrode was assumed to vibrate according to the current waveform detected by the current transformer. Fig. 3 shows the relationship between the natural frequency, removal rate, and surface roughness. The frequency was 0 kHz without vibration assistance for the electrode. The removal rate was low without ultrasonic vibration assistance. The results confirmed that the removal rate was improved with ultrasonic vibration assistance at any frequency. In addition, the removal rate improved with increasing frequency up to 66.5 kHz, after which the removal rate gradually decreased. The removal rate did not change much from 27.6 kHz to 57.3 kHz. This may be due to the difference in electrode wear and magnitude of the amplitude. The surface roughness increased with ultrasonic vibration assistance to about 1 μm Ra. The difference in roughness with frequency was not large. 1.3 4 removal rate surface roughness 3 1 2 0.7 1 0.4 0 0 20 40 60 80 100
5 μm
78
10 μsec Langevin type ultrasonic vibrator Electrode
Fig. 5. Result of displacement measurement.
3.2. Amplitude characteristics of the ultrasonic transducer Eddy current displacement
Fig. 4. Measurement method of displacement measurement.
This system amplified the sine wave output from the waveform generator and inputted it to the transducer. The measurement with the laser displacement meter indicated that the amplitude of the transducer was limited to 6 μm even when the output voltage was increased. When the output voltage of the waveform generator was 1 V, the transducer became saturated with an amplitude of 6 μm.
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Atsutoshi Hirao et al. / Procedia CIRP 68 (2018) 76 – 80
Tool electrode (Eddy current displacement) Transducer (Laser displacement meter) 6 5 4 3
4 φ6 mm (50.6 kHz) φ3 mm (36.8 kHz)
3
2 1 0 0
2
1
2
3
4
5
6
Amplitude [μm]
1
Frequency : 50.6 kHz
0 0
1 2 3 Waveform generator output voltage [V]
Fig. 7. Comparison of removal rate for each electrode.
4
Fig. 6. Difference in amplitude between tool electrode and transducer.
Open circuit voltage
100 V
Amplitude [μm]
7
Fig. 9 shows the relationship between the vibration amplitude of the tool electrode and the surface roughness. The surface roughness increased with ultrasonic vibration assistance for the tool electrode. The surface roughness increased with the amplitude. When an amplitude of 6 μm assisted the tool electrode, the surface roughness increased by 0.2 μm Ra or more compared to the case without ultrasonic vibration assistance. In EDM, the surface roughness largely depends on the processing conditions. The ultrasonic vibration assistance for the tool electrode increased the effective discharge number. However, the processing conditions did not change. It is necessary to verify whether the surface roughness increases with the application of ultrasonic vibration.
Removal rate [mg/min]
A tool electrode was attached to the transducer, and the amplitudes of the transducer side and tool electrode sides were measured. Fig. 6 shows the relationship between the output voltage from the waveform generator and the amplitudes of the transducer and tool electrode. Both the transducer and tool electrode showed the same amplitude with respect to the output voltage. Slight variations were observed in the amplitude on the transducer side. This may be due to the response speed of the laser displacement meter. Increasing the output voltage increased the amplitudes of both the vibrator and tool electrode. The amplitude was linearly amplified with an increasing output voltage up to 1.5 V. At higher voltages, the amplitude was limited to 6 μm. The results confirmed that the amplitudes of the copper on the electrode side and aluminum on the transducer side were the same.
3.3. Effect of amplitude on the EDM characteristics
1A
Discharge current
500 μs (a) Without ultrasonic vibration assistance.
100 V
Open circuit voltage
Discharge current
1A
Fig. 7 shows the relationship between the vibration amplitude of the tool electrode and the removal rate. In this experiment, tool electrodes with diameters of 6 and 3 mm were used for comparison. Similar to the results given above, assisting the amplitude of the tool electrode increased the removal rate. Moreover, the removal rate was confirmed to increase in proportion with the amplitude. In the experiment with the tool electrode having a diameter of 6 mm, the removal rate was increased twofold with an assisting vibration amplitude of 1 μm and increased sevenfold with an assisting vibration amplitude of 6 μm. In this system, the amplitude for the assistance to the tool electrode had a limit of 6 μm. Therefore, it is necessary to investigate the removal rate for an amplitude of 6 μm or more. The processing speed was generally increased by increasing the number of effective discharges. The waveform with the discharge was observed with an oscilloscope. Fig. 8(a) shows the voltage waveform and current waveform with no ultrasonic vibration assistance, and Fig. 8(b) shows the waveforms with assistance. The amplitude for the tool electrode was 5.2 μm. Pulse waves were observed for the current waveform, which indicated when a discharge occurred. The results in Figs. 8(a) and (b) confirmed that the number of discharge occurrences increased more than twofold with ultrasonic vibration assistance.
500 μs (b) With ultrasonic vibration assistance. Fig. 8. Discharge waveforms during EDM.
Atsutoshi Hirao et al. / Procedia CIRP 68 (2018) 76 – 80
Surface roughness [μmRa]
80
1.2
Education, Culture, Sports, Science and Technology, Japan, and the Japan Science and Technology Agency.
1
References
0.8 0.6
0.4 0.2 0
Frequency : 50.6 kHz
0
2 4 Amplitude [μm]
6
Fig. 9. Relationship between vibration frequency and surface roughness.
4. Conclusions We applied ultrasonic vibration assistance to a tool electrode and investigated the effect of different amplitudes on the EDM characteristics in detail. The obtained results are summarized below. (1) When the tool electrode was vibrated, the maximum amplitude was 6 μm. Applying ultrasonic vibration assistance with an amplitude of 6 μm to EDM increased the removal rate up to sevenfold. If an amplitude of 6 μm or more can be imparted to the tool electrode, this may increase the removal rate further. (2) Under the same conditions, the surface roughness increased with ultrasonic vibration assistance. The surface roughness increased with the amplitude. The relationship between the gap distance and surface roughness must further be investigated. Acknowledgements This study was financially supported by a Grant-in-Aid for Young Scientists (B) (16K17994) from the Ministry of
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