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ScienceDirect Procedia CIRP 42 (2016) 601 – 605
18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII)
Clarification of Gap Phenomena in Wire EDM Using Transparent Electrodes Azumi Moria,*, Masanori Kuniedaa and Kohzoh Abeb a Department of Precision Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan b Hamada Heavy Industries Ltd., Hukuoka 804-0053, Japan * Corresponding author. Tel.: +81-3-5841-6463; fax: +81-3-5841-1952 E-mail address:
[email protected]
Abstract In machining process of WEDM, the gap phenomena are intricately changed by pulse conditions, materials or thickness of workpieces, and the types of wire electrodes. Moreover, the wire vibrates in narrow gap because of electrostatic force, electromagnetic force, and reaction force by rapid expansion of bubbles generated by discharge, and jet flow of machining fluid. Hence, it has been difficult to observe the gap phenomena directly. On the other hand, SiC single crystal has been developed as a promising material for semiconductor power devices in recent years. Since SiC is optically transparent and electrically conductive, it can be used as the workpiece electrode of WEDM. Hence, WEDM was performed using SiC as the workpiece, and the gap phenomena between wire and workpiece were investigated by direct observation of the gap. © 2016 2016 The The Authors. Authors. Published Publishedby byElsevier ElsevierB.V. B.V. This is an open access article under the CC BY-NC-ND license Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining XVIII). (ISEM XVIII) Keywords: Wire electrical discharge machining (WEDM), Gap phenomena, SiC single crystal wafer, Transparent electrode, Direct observation, High speed video camera
1. Introduction Detection of discharge position is one of the most active research topic in wire electrical discharge machining (WEDM) in order to elucidate the WEDM phenomena. There are mainly two detection methods reported, one an electrical method [1, 2] and the other a visualized method [3]. As an example of the first method, Shoda et al. [1] used divided feeding method which measured the current flowing through the upper and lower feeding brush of the wire electrode respectively by current sensors, and then identified the discharge position from the ratio of the flow current. Using this method, it was possible to detect the distribution of consecutive discharge positions and discharge concentration before wire breakage. However, detection accuracy was low as about ± 1 ~ 2mm, and it could not be detected when several discharges occurred at the same time. On the other hand, Okada et al. [3] directly observed the discharge point from the rear side of the wire electrode by using a high-speed video camera. They conducted the experiments
with various machining conditions, and showed that discharge position was more dispersed as the servo voltage was higher. However, in these methods, only the discharge position in the wire axis direction could be detected. The discharge position in the circumferential direction of the wire, which may cause the wire vibration and influence the machining accuracy of the kerf width, could not be recognized because of its low detection accuracy. On the other hand, wire vibrations have been one of the important subjects of the WEDM process. Yamada et al. [4] simulated the wire vibration in the feeding direction considering the electrostatic force and discharge reaction force acting on the wire electrode. However, not much research on measuring the wire vibration in the kerf width direction was reported so far. In this study, it is aimed to measure the discharge position in the circumferential direction of the wire and the wire vibration and to clarify the relationship between the discharge position and the wire vibration by using a high-speed video camera. In order to observe the discharges clearly, Kitamura et al. [5, 6] employed SiC single crystal as an electrode. Since SiC has a
2212-8271 © 2016 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 organizing committee of 18th CIRP Conference on Electro Physical and Chemical Machining (ISEM XVIII) doi:10.1016/j.procir.2016.02.219
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high optical transmissivity and electrical conductivity, it can be used for the observation of gap phenomena in EDM. Therefore in this study, we applied SiC to WEDM to observe the gap phenomena directly. Discharge positions in the wire circumferential direction were detected with high accuracy through SiC, and the relationship between the discharge position and wire vibration was investigated. Moreover, machining experiments were conducted using brass wire and zinc-coated wire electrode respectively, and the differences in the gap phenomena and machining properties between brass and zinc-coated wires were discussed.
Table1 Machining conditions Peak current [A] Discharge duration [µs] Discharge interval [µs]
5 0.6 11
Table2 Settings of the high-speed video camera Resolution [pixel] Frame rate [fps] Shutter speed [s]
320h232 100,000 1/100,000
2. Experimental methods
3. Observation of discharge locations and wire vibration
Fig. 1 shows a schematic view of the experimental setup. Wire electrode with a diameter of 200μm and SiC wafer (Nippon Steel & Sumikin Materials Co., Ltd.) with a thickness of 380μm were used as the cathode and anode respectively. Machining experiments were conducted in deionized water. SiC wafer was cut by the wire electrode, and the discharge locations and bubble generation were taken by a high-speed video camera through SiC during machining. In addition, the waveforms of discharge current and discharge voltage were recorded at the same time. Machining conditions and highspeed video camera settings are shown in Table 1 and Table 2, respectively. The frame rate of the high speed camera was set as 100,000 [fps] which was higher than the discharge frequency, therefore only one discharge could be taken in one frame.
3.1. Relationship between discharge position and wire displacement
Stainless steel frame
Precision vise
Machining table Brass wire(-) Front door of work tank
Acrylic viewing window Water surface
High-speed video camera
To clarify the reason for the wire vibration, wire displacement when discharge was generated at a certain position in the wire circumferential direction was measured. Here, it should be noted that the transparency of SiC was decreased after machining as discharge crater was formed on SiC surface. Consequently the position of the wire electrode cannot be observed from the other side of SiC. Therefore, in order to observe the discharge point and the wire vibration simultaneously, shooting range was set within the square as shown in Fig. 2. The measurement results are shown in Fig. 3. Here, the wire center is defined as the zero position all the time. The horizontal axis indicates the discharge position relative to the wire center. After occurrence of a certain discharge, the wire electrode was displaced, and the vertical axis indicates the wire displacement ΔX which is defined as the relative distance of the wire before and after the occurrence of the discharge. Here, the wire displacement was measured at a point right beneath the edge of the SiC plate. Although the wire displacement could not be measured near the discharge position, it is considered that the difference can be ignored as the shooting range is narrow enough; about 1mm in the axial direction. The value of +ΔX and - ΔX represents the wire was displaced to the right and left side, respectively. From Fig. 3, it was found that the wire electrode was displaced towards the side opposite to the discharge position due to discharge. Discharge position
Brass wire(-)
SiC (+)
Machining table 0.2m
Fig. 1 Experimental setup (above: top view, below:side view)
Fig.2 Shooting range
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Wire displacement ΔX [μm]
wire electrode
-100
-50
8 6 4 2 0 -2 0 -4 -6 -8
A right㻌
A㻌 left㻌 50
100
ο ܣൌ ܣ௧ െ ܣ௧ Fig.5 Projected area of expanded bubble 10000
Discharge position [µm]
3.2. Relationship be tween discharge position and bubble size The wire displacement is caused by the discharge reaction force due to the bubble expansion. In order to observe the behavior of bubbles clearly, in these experiments, the gap phenomena at the beginning stage of machining when the discharge crater has not yet been formed on SiC surface, were observed. Fig. 4 shows an image of the bubble expansion when the discharge occurred at the position of -45μm from the wire center. Since the bubble expanded larger on the left side, the impact force acting on the wire electrode from the left side was larger than that from the right side, which probably caused the wire displacement to the right. Bubble Brass wire Discharge point SiC
5000 ΔA [μm ]
Fig.3 Relationship between discharge position measured perpendicular to wire axis and wire displacement
0 -100
-50
0
50
100
-5000
-10000 Discharge position[µm] Fig.6 Relationship between discharge position and ǼA
Here, the projected area of the expanded bubble at the left and right side of the wire are defined as Aleft and Aright, respectively, and the difference between them is defined as ΔA (Fig. 5). Fig. 6 shows the relationship between discharge position and ΔA. It is found that the shape of the expanded bubble was changed according to the discharge position. Therefore, it is concluded that the discharge distribution in the circumferential direction influences the wire vibrations.
4. Comparison of machining speed between different wire electrodes It is generally known that machining speed of zinc-coated wire is higher than that of brass wire. In this section, we aim to clarify this reason by comparing the gap phenomena of WEDM between brass wire and zinc-coated wire electrode. 4.1. Difference in machining characteristics Impulsive force (a) Discharge ignition
(b) Bubble expansion 40µs after discharge ignition
Fig.4 Impulsive force caused by the difference of bubble size
Cold tool steel (SKD11) with a thickness of 5mm was used as the workpiece, and the brass wire and the zinc-coated wire were used as the wire electrode, respectively. Machining speed with both two wire electrode materials was compared. The machining conditions are shown in Table 3 and the results are shown in Fig. 7. Vzn here refers to the machining speed with
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zinc-coated wire and Vbr is that with brass wire electrode. The feeding speed of the wire under stable machining conditions was used to represent the machining speed. The discharge duration was varied as 0.5, 0.6, 0.68, 0.8, 0.88 [μs], and the discharge interval was changed as 5.2, 7.6, 10.8, 20.8, 25.6 [μs] respectively. From Fig. 7, it can be seen the value of (Vzn – Vbr) is positive under all of the machining conditions. It indicates that the machining speed of zinc-coated wire was always faster than that of brass wire. Since the difference of the machining speed became larger with decreasing the discharge interval and increasing the discharge duration, it was considered that the zinc-coated wire has more superior properties than brass wire under unstable machining conditions. In addition, it was impossible to conduct stable machining with the condition of discharge duration longer than 0.8μs when brass wire was used because of high probability of wire breakage. On the other hand, when zinc-coated wire was used, wire breakage hardly occurred even with discharge duration longer than 1μs. Therefore it was concluded that stable machining could be performed at higher discharge energy with using a zinc-coated wire. The difference in the surface temperature of the wire electrode is considered as the reason for this. Since the melting point of zinc (692K) is much lower than that of brass (1478K), the removal volume of zinc-coated wire is usually larger than that of brass wire by the discharge during machining. The debris of the wire electrode can absorb the discharge heat and take it out from the discharge gap as a result of removal. Therefore it is considered that the dissipation efficiency of discharge heat on the wire electrode will be higher with zinc-coated wire because it generates more debris and consequently the temperature of the wire surface can be decreased more easily.
4.2. Distribution of discharge positions Lower probability of discharge concentration is considered as one of the reasons for the superior machining performance of zinc-coated wire electrode. In order to clarify this, the distribution of discharge point was investigated by observing the discharge positions during machining using the high speed video camera [3]. SiC of 5mm height in the wire axial direction was used as the workpiece. Machining conditions are shown in Table 4. Fig. 8 shows the frame of the (i)th discharge and the (i + 1)th discharge during machining. The distance between the (i)th and (i + 1)th discharge positions was defined as the relative distance. The relative distance for 400 discharges was measured when using brass wire and zinc-coated wire, respectively. Fig. 9 and Fig. 10 show the histogram of the relative distance of discharges with using brass wire and zinc-coated wire, respectively. The vertical axis stands for the frequency of the relative distance. It can be seen that although the frequency decreases as the relative distance becomes larger with both wire electrodes, the peak of the frequency differs. With brass wire, the frequency peaks at a relative distance of about 0.2 mm, and it is about 0.8 mm with zinc-coated wire. Therefore, it was concluded that the relative distance between two consecutive discharges with zinc-coated wire is larger than that of brass wire, indicating that consecutive discharges are more likely to be dispersed and discharge concentrations are more hardly to occur with zinc-coated wire. This is considered to be another main reason for stable machining process of zinc-coated wire.
Table4 Machining conditions Peak current [A] 18 Discharge duration [µs] 0.75 Discharge interval [µs] 20 Servo voltage [V] 40 Wire tention [N] 11.8
Vzn-Vbr [m/min.]
Table3 Machining conditions Workpiece SKD11(t=5mm) Peak current [A] 5 0.5, 0.6, 0.68, Discharge duration [µs] 0.8, 0.88 5.2, 7.6, 10.8, Discharge interval [µs] 20.8, 25.6 Servo voltage [V] 35 Wire tention [N] 11.8 Flushing [L/min.] 8 Discharge duration
1.4 1.2 1 0.8 0.6 0.4 0.2 0
5mm Relative distance
0.5µs 0.6µs 0.68µs 0.8µs 0
10
20
30
Discharge interval [µs]
i
i+1
Fig.8 Discharge distribution with zinc-coated wire Fig.7 Difference of machining speed between brass wire and zinc-coated wire
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Acknowledgements 10.00
This research was supported by the Japan Society for the Promotion of Science (Grant-in-Aid, no. 15H03899Generic Research B). The authors are thankful to Sodick Co., Ltd. and HODEN SEIMITSU KAKO KENKYUSHO CO., LTD. for the support to perform the experiment. Dr. Zhao Yonghua is greatly appreciated for his help in preparing the manuscript.
Frequency
8.00 6.00 4.00 2.00 0.00 0.0 0.4 0.9 1.3 1.8 2.2 2.7 3.1 3.6 4.0 4.5 4.9
Relative distance in axis direction [mm] Fig.9 Relative distance of brass wire
Frequency
10.00 8.00 6.00 4.00 2.00 0.00 0.0 0.4 0.9 1.3 1.8 2.2 2.7 3.1 3.6 4.0 4.5 4.9
Relative distance in axis direction [mm] Fig.10 Relative distance of zinc-coated wire
5. Conclusions The discharge gap phenomena in WEDM was directly observed by a high speed video camera using SiC single crystal and the following conclusions were obtained: ࣭Discharge position in the wire circumferential direction and wire vibration were observed simultaneously. It was found that the wire electrode tended to displace to the opposite side to the discharge position. ࣭The bubble generated by a certain discharge expanded to both the left and right side of the wire electrode. The size of the expanded bubble on the left and right side varied according to the discharge position in the circumferential direction. Therefore, the impact force acting on the wire on the left and right sides was different, causing the wire vibration. ࣭ Under unstable machining conditions such as a long discharge duration or a short discharge interval, the machining speed with zinc-coated wire electrode was higher than that with brass wire electrode. ࣭Observation of discharge locations through a transparent electrode showed that discharge locations were more dispersed in consecutive discharges with zinc-coated wire than brass wire.
References [1] Shoda K, Kaneko Y et al., Adaptive control of WEDM on-line detection of sparc locations, ISEM10, 1992, pp. 410-416. [2] H. Obara, Y.Okuyama, M. Komeya, T. Ioka, Study on detection of EDM discharge position, IJEM, Vol.24, No.47, 1990, pp.12-22 [3] A. Okada, Y.Uno, M. Nakazawa, T. Yamauchi, Evaluation of spark distribution and wire vibration in wire EDM by high-speed observation, CIRP Annals, 59-1, 2010, pp.231-224. [4] H. Yamada et al, Transient Response of Wire Vibration System in Wire Electrical Discharge Machining, JSPE, Vol.63, No.11, 1997, pp.1548-1552. [5] T. Kitamura, M. Kunieda, K. Abe, Observation of relationship between bubbles and discharge locations in EDM using transparent electrodes, Precision Engineering, vol.40, pp.26 - 32, 2014. [6] T. Kitamura, M. Kunieda, K. Abe, Observation of Gap Phenomena in EDM in Oil and Deionized Water Using Transparent Electrodes, Autumn Meeting of JSPE, Session Q47, 2013.
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