DRY WIRE ELECTRICAL DISCHARGE MACHINING OF THIN WORKPIECE

Transcription

1 DRY WIRE ELECTRICAL DISCHARGE MACHINING OF THIN WORKPIECE C.C. Kao, Jia Tao, Sangwon Lee, and Albert J. Shih Mechanical Engineering University of Michigan Ann Arbor, MI 4819 KEYWORDS Dry EDM, thin workpiece, MRR. ABSTRACT This study investigates the dry wire electrical discharge machining (EDM) on thin workpieces. Dry EDM experiments were conducted in air, which was used as the dielectric fluid. Effects of spark cycle (T), spark on-time (T on ), air flow rate, workpiece thickness, and type of work-material were studied under wet and dry EDM conditions. The material removal rate (MRR) was low in dry EDM and could be slightly improved by the use of air flow. The increase in workpiece thickness and work-material melting temperature had an adverse effect on the MRR. The reduction of MRR in dry EDM can be related to the rate and percentage of spark, arc, and short pulses. This study also observed the deposition of debris in the groove cut by dry wire EDM. For a thick workpiece, the groove was totally blocked. INTRODUCTION Dry EDM is a novel machining process that uses gas as dielectric fluid. This process was first presented by Kunieda et al. [1997] using a rotating copper tube as the electrode for dry diesink EDM. Dry EDM using air or oxygen flowing out from a tubular electrode was investigated. Experimental results showed that using oxygen as the dielectric fluid in dry EDM could achieve higher material removal rate (MRR) than that in wet EDM. The electrode wear rate was very low, which indicated the feasibility of using dry EDM for precision machining. Kunieda and his colleagues have further advanced the dry EDM process to wire EDM [Kunieda, 21; Wang, 24] and threedimensional EDM milling [Kunieda, 23; Yu, 24]. The gap between electrode and workpiece is narrow in dry EDM. The narrow gap, sometimes close to zero [Kunieda, 21], causes frequent short circuit and low MRR. Compared to water- and oil-based EDM dielectric fluids, gas has much lower viscosity. Therefore, dry EDM has lower energy density per pulse, which results in a lower MRR [Kunieda, 21; Li, 24]. To reduce the probability of short circuit and improve the MRR in dry EDM, a piezoelectric actuator was applied [Kunieda, 24]. In this study, the dry wire EDM cutting of workpiece with small thickness was investigated. A set of experiments was conducted on several Transactions of NAMRI/SME 253 Volume 34, 26

2 types of materials, including brass, aluminum alloy, carbon steel, and graphite bipolar plate to explore the feasibility of dry wire EDM. These materials have distinctly different melting temperatures, electrical conductivity, and machinability in dry EDM. It was observed that three key factors significantly influencing the machinability in dry EDM were the workpiece thickness, melting temperature, and heat capacity. A conventional wire EDM machine can be used for dry wire EDM of thin workpiece with low melting temperature. For thin workpiece, the debris can be efficiently removed by the air jet in dry EDM. The workpiece with low melting temperature allows low energy input without breaking the wire electrode. Dry wire EDM experiments were conducted in this study to quantify effects of the spark cycle, spark on-time, air flow rate, thickness, and type of workmaterial on the MRR. The wire EDM process monitoring [Rajurkar 1993; Ho, 23, 24] was applied to analyze the phenomena of dry wire EDM. By evaluating the measured gap voltage and current with respect to preset threshold values [Dauw, 1983], the spark, arc, and short EDM pulses can be identified. Measurement of the gap voltage and current and identification of types of pulse (open, spark, arc, and short) were conducted in this research for both wet and dry EDM processes. Under identical process parameters, the rate of each pulse type in wet and dry EDM were calculated and compared. In this paper, the dry EDM experimental setup and procedures are first presented. The experimentally measured MRR in dry EDM are reported and effects of work-material type and thickness are discussed. The rate of spark, arc, and short pulses for various dry EDM setups are compared. Finally, the groove width and deposition of debris are examined. EXPERIMENTAL SETUP AND PROCEDURES Wire EDM machine setup The EDM experiments were conducted on a Brother HS-51 wire EDM machine. A copper wire electrode of.254 mm diameter was used. Three cutting conditions were studied: wet, dry without air flow, and dry with air flow in the electrode-workpiece gap region. For wet EDM experiments, the workpiece was submerged in deionized water. Jets of water were applied at about 1 liter/min flow rate from both top and bottom to flush away the debris generated in the discharge gap between workpiece and wire electrode. No water was used in dry EDM experiments, which were conducted either in stationary air or using an optional air jet, as shown in FIGURE 1(A). The air jet was delivered at.17 MPa pressure via a 2 mm inner diameter plastic tube oriented with 45 angle to wire at 1 mm away for debris flushing. (A) Air tube (B) FIGURE 1. DRY WIRE EDM SETUP AND CUTTING OF 1.27 MM THICK AL 661: (A) THE ORIENTATION OF TUBE FOR AIR FLOW AND (B) DRY EDM WITHOUT AIR FLOW. For all wet and dry EDM experiments, the axial direction wire feed speed was set at 12 mm/s, the tension force of wire was 18 N, the servo voltage was 45 V, and the open voltage between the wire electrode and workpiece was about 72 V. and Al 661 were selected as the work-materials. Baseline dry wire EDM experiments were conducted on the.2 mm thick brass and 1.27 mm thick Al 661. FIGURE 1(B) shows the dry EDM of the 1.27 mm thick Al plate without the air flow. An odor of burning could be smelled during the dry EDM process. This environmental issue, not addressed in this study, needs to be resolved before the application of dry EDM in production. Experimental procedures In this study, wire EDM experiments were conducted to investigate: 1. MRR, 2. rate of spark, arc, and short EDM pulses, and 3. groove width and debris deposition. MRR. Spark cycle T, spark on-time duration T on, air flow rate, workpiece thickness, and type of work-material are five process variables selected. Effects of these variables on MRR in dry EDM were investigated and compared with that of wet EDM. A method to study effects of T and T on using envelopes of MRR has been Transactions of NAMRI/SME 254 Volume 34, 26

3 developed by Miller et al. [24,25]. This method was applied to identify characteristics of dry EDM in this study. Envelopes of feasible T and T on for wet and dry EDM with and without air flow on the.2 mm thick brass and 1.27 mm thick Al 661 workpiece were generated experimentally. For a given T on, T was varied to find the maximum achievable wire feed rate, which was converted to MRR, in each test setting. When T was increased to an upper limit, the short circuit occurred. At the other extreme, when T was decreased to a lower limit, the wire breakage occurred. For brass, T on was set at 2, 6, 14, and 18 µs for wet EDM and 3, 1, 14, and 18 µs for dry EDM. For Al, T on was set at 4, 1, 14, and 18 µs for both wet and dry EDM conditions. Very low T on, such as 2 µs, was not achievable in some EDM conditions. By connecting the upper and lower limits of each T on, the short circuit and wire breakage boundary lines of the envelope were obtained. In addition, specific machine limits of the maximum T (1 µs) and minimum T (6 µs) exist. To investigate effects of workpiece thickness and material in dry EDM, additional experiments were conducted at T = 25 µs and T on = 14 µs. Rate and percentage of spark, arc, and short pulses. An Agilent Infiniium 54833A digital oscilloscope was used to measure the gap voltage and current in the EDM process. The sampling rate of data acquisition was set at 1 MHz. Every 2 s, a 3 ms time period (3, data points) of voltage and current data were recorded. At least six sets of 3 ms data were gathered after the steady-state MRR had been achieved in the EDM process. The data formed several pulse trains which were used for further analysis of the pulse rate in each EDM setup. Three types of pulses (spark, arc, and short), as shown in FIGURE 2, were identified in the pulse train. To automatically determine the type and number of pulses with a computer, Dauw et al. [1983] developed an algorithm using preset threshold voltage values and rates of voltage change. A more elaborate pulse identification algorithm, which includes using the measured current data, is proposed in this study. For a spark pulse, the voltage has to be higher than a threshold value, designated as V h, and the current at discharge needs to be larger than another threshold value, designated as I h. The V h and I h used in this study, as marked in the FIGURE 2(A), are equal to 68 V and 2 A, respectively. For an arc pulse, as shown in FIGURE 2(B), both the voltage and current rise and drop quickly. The peak voltage is not as high as that in spark. To distinguish spark and arc, another threshold voltage, V l, as marked in FIGURE 2(A), is used. An arc pulse is defined when the voltage is between V h and V l and the current is larger than I h. In this study, V l = 2 V. For a short pulse, the voltage is low and current is high, although not as high as that in arc and spark pulses. As shown in FIGURE 2(C), when the voltage is below V l and current is above I h, this pulse is designated as a short. A Matlab program was developed to identify each pulse from measured voltage and current data. Voltage (V) V h V l Spark Votage Current Arc Votage 1 ms 1 ms Current (A) (B) (C) Short Votage 1 ms I h Current FIGURE 2. CHARACTERIZATION OF EDM PULSES: (A) SPARK, (B) ARC, AND (C) SHORT. Of all EDM pulses, the rates of spark, arc, and short pulses are three indices that determine the status or efficiency of the dry EDM process. A good EDM setup has low rates of short and arc pulses and a high rate of spark pulses. Groove width and debris deposition. The groove width and debris deposition of different EDM conditions were measured using an optical microscope at 1X. MATERIAL REMOVAL RATE IN DRY WIRE EDM FIGURES 3 and 4 show the experimentally measured MRR for brass and Al 661. Dry EDM process has a lower MRR than wet EDM Current (A) Transactions of NAMRI/SME 255 Volume 34, 26

4 FIGURE 3 shows envelopes of MRR for wet EDM, dry EDM with airflow, and dry EDM without airflow for cutting.2 mm thick brass Wet EDM A C Ton = 18 µs Ton = 14 µs Ton = 6 µs Ton = 2 µs Wire breakage T upper limit T lower limit Short circuit.2 mm Spark cycle, T (µs) Dry EDM with air flow (A) Ton = 18 µs Ton = 14 µs Ton = 18 µs Ton = 14 µs Ton = 1 µs Ton = 3 µs Ton = 1 µs Ton = 3 µs Wire breakage Short circuit Wire breakage Short circuit T upper limit A F D E Dry EDM without air flow Spark cycle, T (µs) (B) A.2 mm A FIGURE 3. ENVELOPS OF T ON AND T ON MRR FOR WIRE EDM CUTTING OF.2 MM THICK BRASS: (A) WET AND (B) DRY WITH AND WITHOUT AIR FLOW. Boundary lines of the envelopes are first identified. The upper boundary line is the 18 µs machine limit of T on. The left boundary line, in wet EDM (FIGURE 3(A)), is constrained by wire breakage and lower limit of T (6 µs); and, in dry EDM (FIGURE 3(B)), is determined by the wire breakage only. The bottoms of the envelopes are bounded by the lowest possible T on (2 µs for wet and 3 µs for dry EDM) and the short circuit limit, which is marked by the dashed line. The right boundary is the upper limit of T (1 µs). The maximum MRR in wet, dry EDM with airflow, and dry EDM without airflow are 14, 3.8, and 2.8 mm 3 /min, respectively. The low MRR in dry EDM is consistent with the observation of Wang and Kunieda [24]. This is caused by the low viscosity of air, which results in a smaller explosive force and less material removal for each spark. Low spark cycle T in wet EDM generates more frequent spark pulses and higher MRR, as illustrated in FIGURE 3(A). Dry EDM does not exhibit the same trend. The MRR drops when T reaches a threshold value. For T on = 18, 14, and 1 µs, such threshold values are about 25, 125, and 75 µs, respectively. The decrease of MRR at low T in dry EDM is due to the difficulty of expelling debris in the EDM region. The higher rate of debris generation at low T causes more frequent short pulses, which significantly reduce the MRR. For example, without air flow in dry EDM with T on = 18 µs, the MRR drops to only.3 mm 3 /min, marked by the circle F in FIGURE 3(B), when T = 6 µs. In dry EDM, the accumulation of debris in the gap between wire and workpiece results in frequent short circuiting and very low MRR. By introducing air flow into dry EDM to assist the debris removal, the MRR is increased by about 3% at T on = 18 µs and T = 25 µs, as shown in FIGURE 3(B). In wet EDM, high MRR can be achieved at low T. The problem changes from frequent short circuiting to wire breakage due to high energy input. The wire breakage boundary line is on the left side of the envelope. In dry EDM, due to the reduction of MRR at low T, the wire breakage boundary line shrinks significantly. Al 661 FIGURE 4 shows envelopes of MRR for wet and dry EDM of Al 661. The workpiece is 1.27 mm thick, compared to the.2 mm thick brass in FIGURE 3. Like in FIGURE 3, the upper boundary line is the machine limit (T on = 18 µs). The left boundary line is constrained by wire breakage. The bottoms of the envelopes are bounded by the lowest possible T on (4 µs) and the short circuit limit, marked by dashed lines. The right boundary line is the machine upper limit of T (=1 µs). In wet EDM, a low spark cycle T also generates higher MRR. Unlike in FIGURE 3, only when T on = 18 µs, the MRR of Al 661 in dry EDM show significant drop at low T. This is Transactions of NAMRI/SME 256 Volume 34, 26

5 also caused by the frequent short circuiting associated with a large volume of debris generation B Wet EDM G Ton = 18 µs Ton = 14 µs Ton = 1 µs Ton = 4 µs Wire breakage Short circuit Al mm Spark cycle, T (µs) Dry EDM with air flow (A) Dry EDM without air flow Ton = 18 µs Ton = 14 µs Ton = 18 µs Ton = 14 µs Ton = 1 µs Ton = 4 µs Ton = 1 µs Ton = 4 µs Wire breakage Short circuit Wire breakage Short circuit T upper limit B H I Spark cycle, T (µs) (B) B Al mm B FIGURE 4. ENVELOPES OF T ON AND T ON MRR FOR WIRE EDM CUTTING OF 1.27 MM THICK AL 661: (A) WET AND (B) DRY WITH AND WITHOUT AIR FLOW. The threshold value of T from which the MRR starts dropping is about 25 µs. With no air flow in dry EDM when T on = 18 µs, a slight change of T from 25 to 225 µs reduces the MRR from.78 to.44 mm 3 /min. The air flow can help remove the debris and increase the MRR by about 5 to 3% in dry EDM, as shown in FIGURE 4(B). In wet EDM, the 22 mm 3 /min maximum MRR of Al 661 is higher than that of brass (14 mm 3 /min). This is due to the lower melting temperature and heat capacity of Al 661. Dry EDM for Al 661 has a very low MRR. The maximum MRR in wet, dry EDM with air flow, and dry EDM without airflow are 22, 1., and.68 mm 3 /min, respectively. The maximum MRR achieved in dry EDM with air flow, marked as line BB in FIGURES 4(A) and 4(B), is less than 5% of that in wet EDM. In comparison, for the.2 mm brass in FIGURE 3, the maximum MRR in dry EDM is about 28% of that in wet EDM. The significant difference is most likely due to the thicker Al 661 workpiece. Effect of workpiece thickness on MRR The thickness of workpiece, t, has a significant effect on the efficiency of debris removal and MRR in wire EDM. FIGURE 5 shows the MRR of wet and dry EDM without air flow of Al 661 at seven levels of t, ranging from.2 to 1.27 mm Wet EDM Dry EDM without air flow Al 661 ln(mrr) = t ln(mrr) = t Workpiece thickness, t (mm) FIGURE 5. EFFECT OF THICKNESS ON MRR OF AL 661 FOR WET AND DRY EDM. All EDM tests were under the same T = 25 µs and T on = 14 µs. The MRR (in logarithmic scale) vs. t follows the trend of straight line for both wet and dry EDM conditions. This indicates the exponential decay of MRR versus the workpiece thickness. The slope of these two lines is the decay rate. The dry EDM condition has a more negative slope, i.e., a higher decay rate of MRR. When t =.2 mm, the dry EDM processes still have good MRR, about 6.5 mm 3 /min, which is about 8% of the 8 mm 3 /min in wet EDM. But when t is increased to 1.27 mm, the MRR in dry EDM is reduced to only.5 mm 3 /min, which is only about 25% of that in wet EDM. The frequent occurrence of short and arc pulses due to the agglomeration of debris is the likely cause. Effect of type of work-material on MRR FIGURE 6 shows the experimental results of MRR for three types of work-material, Al 661, brass, and AISI 12 carbon steel, all with the Transactions of NAMRI/SME 257 Volume 34, 26

6 same workpiece thickness t = 1.27 mm, T = 25 µs and T on = 14 µs for wet and dry, with and without air flow, EDM conditions. The wet EDM has higher MRR than that of dry EDM with airflow. The air flow in dry EDM always improves the MRR Wet Dry with air flow Al 661 AISI 12 Steel Dry without air flow FIGURE 6. EFFECT OF TYPE OF MATERIAL ON MRR (WORKPIECE 1.27 MM THICK). The correlation between the thermal properties of the work-material and MRR is consistent: the EDM of Al 661 has higher MRR than brass, and brass has higher MRR than AISI 12 steel. The heat capacity and melting temperature of Al 661, brass, and AISI 12 carbon steel are 2.42, 3.2, and 3.81 J/cm 3 -K and 652, 955, and 151 C, respectively [Davis, 1994, 21; Holt, 1996]. Compared to brass and AISI 12 steel, under all three EDM conditions, Al 661 has the lowest melting temperature and takes the least energy to reach its melting temperature per unit volume of workpiece. Therefore, the EDM of Al 661 has the highest MRR, as shown by the square symbol in FIGURE 6. For wet EDM, the MRR for Al 661 and brass are about the same, 2.3 mm 3 /min. For dry EDM, the MRR of brass is only 2% of that of Al 661. The lack of high energy density in dry EDM is the likely cause of such a phenomenon. PROCESS MONITORING RESULTS Pulse rate and pulse percentage The rates of the spark, arc, and short pulses, as shown in FIGURE 7, represent the effectiveness of EDM processes. Four representative EDM setups are presented. (a).2 mm thick brass at 5 mm/min wire feed rate, which is much lower than the maximum wire feed rate. (b).2 mm thick brass at maximum wire feed rate: 64, 38, and 28 mm/min for the wet, dry with air flow, and dry without air flow EDM conditions. (c) 1.27 mm thick brass at maximum wire feed rate: 5.,.55, and.42 mm/min for the wet, dry with air flow, and dry without air flow EDM conditions. (d) 1.27 mm thick Al 661 at maximum wire feed rate: 5.3, 1.7, and 1.2 mm/min for the wet, dry with air flow, and dry without air flow EDM conditions. Pulse rate (pulses/s) Pulse rate (pulses/s) Wet Dry with air flow Dry without air flow Maximum feed rate (mm/min) Wet: 64 (C in FIG. 3(a)) Dry with air flow: 38 (D in FIG. 3(b)) 5 mm/min feed rate Dry without air flow: 28 (E in FIG. 3(b)) 15.2 mm.2 mm 1 Spark Arc Short (A) Maximum feed rate (mm/min) Wet: 5. Dry with air flow:.55 Dry without air flow: mm 5 Spark Arc Short (C) Spark Arc Short (B) Maximum feed rate (mm/min) Wet: 5.3 (G in FIG. 4(a)) Dry with air flow: 1.7 (H in FIG. 4(b)) Dry without air flow: 1.2 (I in FIG. 4(b)) Al mm Spark Arc Short (D) FIGURE 7. RATE OF SPARK, ARC, AND SHORT PULSES IN WET AND DRY WITH AND WITHOUT AIR FLOW EDM CONDITIONS: (A).2 MM BRASS, 5 MM/MIN FEED RATE, (B).2 MM BRASS, MAX. FEED RATE, (C) 1.27 MM BRASS, MAX. FEED RATE, AND (D) 1.27 MM AL 661, MAX. FEED RATE. Under the low MRR with the wire feed rate much lower than the maximum possible value, as shown in FIGURE 7(A), the pulse rate is low: below 18 pulses/s for spark, below 2 pulses/s for arc, and essentially no short pulse. The data illustrates that, at wire feed rate below the maximum possible value, the EDM pulse condition is stable. For the same workpiece (.2 mm thick brass), under the maximum MRR condition, the wire feed rate and MRR can be Transactions of NAMRI/SME 258 Volume 34, 26

7 increased significantly. As shown in FIGURE 7(B), the rate of spark pulses has a significant jump to 133, 73, and 6 pulses/s for the wet, dry with air flow, and dry without air flow EDM condition, respectively. The rate of arc pulses also increases to about pulses/s under both wet and dry EDM conditions. The side effect of more aggressive removal of workmaterial is more frequent short pulses, which increased to about 14 pulses/s. The effect of increasing brass workpiece thickness from.2 mm to 1.27 mm on EDM pulses is illustrated by comparing FIGURE 7(C) and FIGURE 7(B). As shown in FIGURE 7(C), the rate of spark pulses increases to about 187 pulses/s for wet EDM and 98 pulses/s for dry EDM. The rate of arc pulses remains about the same for wet EDM but increases to over 54 pulses/s for dry EDM. The rate of short pulses has the most significant change, particularly for dry without air flow, which increases to 89 pulses/s. The dry EDM with air flow also has 65 short pulses per second. This shows the effect of a thick workpiece: more frequent short pulses, which significantly reduce the wire feed rate and result in lower MRR. is twice the gap distance between the wire electrode and workpiece during EDM. For wet EDM, the groove width is about.26 mm. For dry EDM with and without air flow, the width of groove is about the same,.21 mm, and w is equal to.4 mm. A better illustration of the severity of debris deposition after dry EDM is shown in FIGURE 9 for 1.27 mm brass workpiece. Under the maximum MRR, the groove is totally clogged (w =.254 mm). The air flow at.17 MPa pressure does not help to prevent the clogging of the groove in dry EDM. On the top view, only a hole of the wire is left at the end of wire EDM cut groove. Clogging is concentrated on the top of the groove, as shown in the cross-section view. The width of the groove can still be recognized near the bottom of the cross-section of the groove. For dry EDM with air flow, the groove width is.29 mm. Without airflow in dry EDM, the groove width slightly reduces to.28 mm. In comparison, wet EDM generates much wider groove,.34 mm, as shown in FIGURE 9(A). The w for wet EDM is about.9 mm, which is consistent with the gap width observed in most wire EDM processes. DEBRIS DEPOSITION AND GROOVE WIDTH The top view and width of three grooves in wet and dry EDM of the.2 mm thick brass under maximum MRR are shown in FIGURE 8. TOP VIEW.26 mm.21 mm.21 mm CROSS- SECTION VIEW.34 mm.29 mm.28 mm (A) (B) (C) Wet Dry with air flow Dry without air flow FIGURE 8. OPTICAL MICROGRAPHS OF WIRE EDM GROOVES FOR.2 MM THICK BRASS (USING.254 MM DIAMETER WIRE ELECTRODE). The floating debris has been reported by Kunieda et al. [21,24]. In wet EDM, the width of the machined groove is wider than the.254 mm diameter wire. Difference of the groove width and wire diameter, denoted as w, FIGURE 9. OPTICAL MICROGRAPHS ON THE GROOVES AND DEPOSITION OF DEBRIS GENERATED BY CUTTING 1.27 MM BRASS: (A) WET EDM, (B) DRY EDM WITH AIR FLOW, AND (C) DRY EDM WITHOUT AIR FLOW. CONCLUDING REMARKS In this study, the dry wire EDM of thin workpiece was proven to be possible. Effects of spark cycle (T) and spark on-time (T on ), air flow rate, workpiece thickness, and work-material type on the MRR for dry wire EDM of thin Transactions of NAMRI/SME 259 Volume 34, 26

8 workpiece were investigated. An EDM process monitoring system was set up to identify the spark, arc, and short EDM pulses. The rates of spark, arc, and short pulses were compared and discussed under the wet, dry with air flow, and dry without air flow EDM conditions. Experimental results showed that not all thin work-materials could be machined using dry EDM. For example, thin porous carbon foam and carbon bipolar plate [Miller, 24] have failed to be machined using the dry EDM process. The high melting temperature of carbon is the likely cause. The research in dry EDM is continuing to improve the precision, MRR, and environment issues. The use of a mist of deionzied water has been investigated to reduce the smoke and fumes generated during dry EDM and help collecting the debris in solid particulate form. ACKNOWLEDGMENTS This research is sponsored by the NIST Advanced Technology Program. Discussions with John MacGregor of Ann Arbor Machine are greatly appreciated. REFERENCES Dauw, D.F., R. Snoeys, and W. Dekeyser, (1983), Advanced Pulse Discriminating System for EDM Process Analysis and Control, Annals of the CIRP, Vol. 32, pp Davis, J.R. (Ed.), (1994), Aluminum and Aluminum Alloys, ASM International. Davis, J.R. (Ed.), (21), Copper and Copper Alloys, ASM International. Ho, K.H., and S.T. Newman, (23), State of the Art Electrical Discharge Machining (EDM), International Journal of Machine Tools and Manufacture, Vol. 43, pp Ho, K.H., S.T. Newman, S. Rahimifard, and R.D. Allen, (24), State of the Art in Wire Electrical Discharge Machining (WEDM), International Journal of Machine Tools and Manufacture, Vol. 44, pp Holt, J.M., H. Mindlin, and C.Y. Ho, (1996), Structural Alloys Handbook, CINDAS/Purdue University. Li, L., Z. Wang, and W. Zhao, (24), Mechanism Analysis of Electrical Discharge Machining in Gas, Journal of Harbin Institute of Technology, Vol. 36, pp (in Chinese). Kunieda, M. and S. Furuoya, (1991), Improvement of EDM Efficiency by Supplying Oxygen Gas into Gap, Annals of the CIRP, Vol. 4, pp Kunieda, M., and M. Yoshida, (1997), Electrical Discharge Machining in Gas, Annals of the CIRP, Vol. 46, pp Kunieda, M., and C. Furudate, (21), High Precision Finish Cutting by Dry WEDM, Annals of the CIRP, Vol. 5, pp Kunieda, M., Y. Miyoshi, T. Takaya, N. Nakajima, Z.B. Yu, and M. Yoshida, (23), High Speed 3D Milling by Dry EDM, Annals of the CIRP, Vol. 52, pp Kunieda, M., T. Takaya, and S. Nakano, (24), Improvement of Dry EDM Characteristics Using Piezoelectric Actuator, Annals of the CIRP, Vol. 53, pp Miller, S.F., A.J. Shih, and J. Qu, (24), Investigation of the Spark Cycle on Material Removal Rate in Wire Electrical Discharge Machining of Advanced Materials, International Journal of Machine Tools and Manufacture, Vol. 44, pp Miller, S.F., C. Kao, A.J. Shih, and J. Qu, (25), Investigation of Wire Electrical Discharge Machining of Thin Cross-Sections and Compliant Mechanisms, International Journal of Machine Tools and Manufacture, Vol. 45, pp Rajurkar, K. P., and Wang, W. M., (1993) Thermal Modeling and On-line Monitoring of Wire-EDM," Journal of Materials Processing Technology, Vol. 38, pp Wang, T., and M. Kunieda, (24), Dry EDM for Finish Cut, Key Engineering Materials, Vol , pp Yu, Z.B., T. Jun, and M. Kunieda, (24), Dry Electrical Discharge Machining of Cemented Carbide, Journal of Materials Technology, Vol. 149 pp Transactions of NAMRI/SME 26 Volume 34, 26