Electrodischarge Machining

Electrodischarge machining (EDM) is a thermoelectric process that removes material from the workpiece by a series of discrete sparks between a work and tool electrode immersed in a liquid dielectric medium. The method of removal of material from the workpiece is by melting and vaporizing minute amounts of electrode material, which are then ejected and flushed away by the dielectric (Ref 1, 14).

The two major types of EDM are die sinking EDM and wire EDM (WEDM). Die sinking EDM is traditionally performed vertically, but it may also be conducted horizontally. WEDM is a special form in which the electrode is a continuously moving conductive wire. In the WEDM process, spark discharges are generated between a small wire electrode (usually smaller than 0.5 mm diameter) and a workpiece with deionized water as the dielectric medium. The electroerosion process is used to produce complex two- and three-dimensional shapes, even in harder materials. According to the most agreed-on process mechanism, when a voltage is applied through a dielectric medium across the gap between the tool and the workpiece, an electric field builds along the path of least resistance. This causes a breakdown of the dielectric and initiates the flow of current. In the second stage, electrons and ions migrate toward the anode and cathode at high current density, forming a column of plasma and initiating the melting of the workpiece. When the application of voltage is stopped, the column collapses, a portion of the molten metal is ejected from the workpiece, and a crater is formed. The debris remaining on the workpiece is flushed away by the dielectric.

In EDM the erosion rate and tool wear, and the resulting surface integrity and geometry, depend on the current, voltage, on-time, off-time, polarity, pulse shape, work and tool material properties, dielectric flushing conditions, dielectric properties, electrode geometry, and machine characteristics. A general overview of the EDM process is given in Volume 16 of the ASM Handbook.

EDM Equipment

All EDM systems include the machine (including the frame, ram, worktable, tool and workpiece holders, and clamping devices), pulse-power supply, tool electrode, dielectric system, and servo control system. Figure 8 is a schematic of an integrated die sinking EDM system. The EDM power system transforms the utility ac power into pulsed dc power with 30 to 300 V and from several milliamperes to 100 A of peak current. Various types of power supply systems exist today. Among those that are suitable for EDM are the relaxation power system, which consists of a charge loop and a discharge loop, and the independent power system, which consists of a dc power source, pulse controller, and a power controller. The pulse controller in this type of power supply sets a time basis and controls the "on" and "off" states of the power controller. The power controller delivers the pulse to the gap with the required power. Some EDM machines are equipped with power supplies that combine the relaxation and independent power supplies in order to improve surface roughness.

Fig. 8 Schematic of die-sinking electrodischarge machining system

Five-axis computer numerical control WEDM is now routinely employed in complex three-dimensional contour machining jobs (Ref 1). Another application of EDM is electrical discharge grinding, which is used for precision machining of electrically conductive workpieces. Electrical discharge grinding uses a rotating electrically conductive grinding wheel as the "electrode" or cutting tool. (See Volume 16 of the ASM Handbook.)

Tool Material. The basic requirements for a tool material are high electrical conductivity, high melting point, and high thermal conductivity. The tool materials should be easy to machine and inexpensive. Some of the most frequently used tool materials include graphite and bronze for machining steels and copper-tungsten for machining carbides. Bronze and copper tungsten are often used for producing smooth surfaces and for high-precision EDM.

Dielectric Fluid. The main functions of the dielectric fluid are to insulate the gap between the tool and the workpiece before high energy is accumulated, to concentrate the discharge energy to a tiny area, to recover the gap condition after the discharge, and to flush away the discharge products. The two most commonly used dielectric fluids are petroleum-based hydrocarbon mineral oils and deionized water. Dielectric flushing is very important in EDM operations. The commonly used flushing methods are immersion, spray, or jet.

Controls. A servo control is used to keep the interelectrode gap within a small range of variations around a desired setting during machining. Typical values of the gap that are used during EDM are 0.010 to 0.050 mm, though gaps as small as a few microns or as large as several hundred microns can be used, depending on the voltage, current, dielectric media, and surface finish requirements. In order to maintain a constant gap size, the tool feed rate should equal the material removal rate in the feed direction. However, because the removal rate is often not constant, a servo control is used that takes the gap signals (the average voltage) as the measure of the gap size and compares them with the servo reference voltage. Action as to whether to retract the tool or to move it faster toward the workpiece is taken, depending on this comparison.

EDM Process Characteristics

In the case of EDM, the metal removal rate and the surface roughness depend on the peak current, pulse on-time, peak voltage, frequency of pulses, and the flow rate of the dielectric. The surface roughness obtained by EDM can range from 2.5 to 30 /''m Ra for rough machining. The metal removal rate achieved by EDM can range from 50 to 200 cmVhr for rough cuts. As the removal rates increase, the surface finish deteriorates.

Electrodischarge polishing (EDP) uses exactly the same principle as that of EDM; however, the objective is to produce smooth, lustrous surfaces. Hence EDP is carried out with very low discharge energy. EDP can be used to produce smooth surfaces (0.2 ^m Ra) that exhibit extremely thin, homogeneous, crack-free surface layers of uniform width (Ref

15). In this process, the high metal removal rate associated with deep craters is undesirable. The aim is to achieve melting of roughness peaks with ensuing resolidification, while avoiding molten metal spinoff. It has been found that EDP with a positive workpiece electrode produces flat craters with a smooth surface and a crater rim raised only slightly above the surrounding workpiece surface (Ref 15). EDP makes use of a gradual reduction of the discharge energy of the individual pulses of less than 3 ^J and a simultaneous increase of the frequency to around 100 kHz. It has been found that the surface roughness rises sharply with increasing discharge current and is found to decrease with increasing polishing depth. The higher smoothing of the surface achieved as a result of the increased polishing depth increases the processing time drastically, however. Because gap widths in EDP are extremely small and difficult to control, flushing is another significant factor influencing the variables for physical conditions in the working gap. With continuous flushing, the workpiece roughness decreases with a decreasing flow rate. In EDP, planetary motion is necessary to avoid undulating workpiece surfaces, because this technique entails relative movement of the electrodes, producing a surface that is microscopically as well as macroscopically smooth.

Applications, Process Capabilities, and Limitations. EDM is capable of machining difficult-to-cut materials such as hardened steels, carbides, high-strength alloys, and even ultrahard conductive materials such as polycrystalline diamond and some ceramics. The process is particularly well suited to sinking cavities and drilling irregularly shaped holes. The only limit in machinability is the electrical conductivity of the workpiece material. The other problems in EDM include tool wear and the irregularity of the tool wear, and limitations of EDM to machine very sharp corners because of the existence of the gap between the tool and the workpiece. A recently developed EDM process called micro-EDM expands the capabilities of EDM with respect to fine part fabrication. This process can achieve a surface roughness of 0.1 ^m Rmax and a high accuracy (roundness of 0.1 ^m, and the straightness of some fine parts as small as 0.5 ^m).

Recent Advances in EDM as a Surface Finishing Process. As described earlier, EDM and WEDM are performed by passing a dc pulse anywhere between several tens to several hundreds of volts between the tool (the negative wire in case of WEDM) and the positive workpiece. In the case of WEDM, water is generally used as the machining fluid. During WEDM, there is some electrolysis during machining. Consequently, besides being affected by the heat at the time of electric discharge, the electrolysis results in a drop in the machined surface quality. In a recent development, engineers at Mitsubishi Electric have developed a WEDM with an antielectrolysis (AE) power source (Ref

16). Experiments using AE power supply show that the corrosion of the workpiece surface is totally preventable. One of the problems of EDM is the formation of microcracks, which must be avoided, and the use of AE power supply with uniform pulses has also been found to reduce the number of microcracks in EDM surfaces (Ref 16).

In an attempt to achieve extremely fine surface finishes by EDM, different kinds of powders such as silicon, aluminum, and graphite have been suspended in the working fluid (Ref 17). In the case of conventional EDM, during finishing operations, the gap distance between the electrode and the work is very small, resulting in frequent abnormal discharges. Use of a suspended powder in the working fluid results in an increase in the working gap distance. The effective working gap distance depends on the concentration of the powder and the type of the powder (Ref 17, 18). In fact, experiments show that the working gap distance increases ten times compared to that of the conventional working fluid. The powder suspension is also found to disperse the electrical discharge very well. A good dispersion of the electrical discharge is extremely important for a fine surface finish. The work surface machined using a fluid with suspended powder also has strong corrosion resistance. The surface finish achieved by the use of a fluid with suspended powder was in the range of 0.6 to 1.8 ^m ^max.

Because EDM is becoming a key process for die manufacturing, it has also become necessary to realize full automation of the EDM process. Engineers at the Toyota Technological Institute have developed a technique to eliminate manual polishing after EDM. During die manufacturing, cusps are left on the workpiece after milling, and in this research, the cusps were removed by EDM under high-wear conditions with planetary motion of the electrode (Ref 19). In order to realize a mirror-like surface on the workpiece, electrically resistive material such as silicon was used as an electrode. This improved the surface finish remarkably; surface roughnesses in the range of 2 to 3 ^m were achieved. However, because silicon is hard and brittle, forming an electrode in a complicated shape was extremely difficult. Hence silicon powder was suspended in the working fluid and EDM was carried out. EDM carried out by suspended powders also helps improve the surface finish rapidly.

All of the above methods have been demonstrated in the laboratory to result in mirror-like finishes. However, in most of these cases, large areas (i.e., greater than 300 mm2) cannot be successfully machined by EDM for a good surface finish. A new method of achieving mirror-like finishes with EDM has recently been attempted (Ref 20). In this method, narrow spark duration and small discharge current (i.e., with the smallest of discharge energies) have been used. In order to reduce the dependence of surface roughness on the working area, a partially induced electric field is used during machining. The concept is to reduce the space in which the electric energy is stored before breakdown.

It has also been demonstrated that surface roughness achieved after EDM can be improved by the use of a radio-frequency (RF) controller (Ref 21). In the presence of bad flushing conditions, the occurrence of stable arcs results in thermal damage to the workpiece surface. Hence it is important to detect the occurrence of stable arcs. Research shows that normal spark discharges generate intense high-frequency (HF) noise signals that are a component of the discharge voltages and are emitted as RF signals. The intensity of the HF or RF signal drops as the gap conditions change from normal sparking to harmful arcing (Ref 21). The RF controller has been modified to include an isoenergetic function, and the sensing gain of the RF detecting circuit has been improved. The EDM operations carried out with the RF controller have been shown to improve the surface finish by 40%.

As mentioned earlier, a uniform surface finish on large areas is difficult to achieve by EDM. However, an electrical discharge texturing system has been developed to obtain improved machining performance and precise control of the surface roughness (Ref 22). The electrical discharge texturing system incorporates an RF monitoring and controlling unit, a unique gap voltage measuring circuitry, and MOSFET (metal oxide semiconductor field-effect transistors) for power switching.

In another attempt to achieve good surface finish by EDM, surface modification by the use of composite electrodes has been performed (Ref 23). In this new technique, the electrode used for EDM is made from fine powder or green compact and shows higher wear than a conventional solid electrode. The machined surface shows the presence of components of the electrode, and there are fewer microcracks in the machined surface layer.

EDM has recently become an important method for machining advanced ceramic materials for many applications, due to the fine finishes that it provides. It has been found that the grain size of the dispersed phase plays the dominant role in affecting the surface roughness (Ref 24).

New developments in the field of materials science have led to new engineering methods for metallic materials, composite materials, and high-tech ceramics. EDM (both rough and finishing) of ceramics turns out to be a very good alternative to traditional machining techniques such as grinding, milling, turning, and sawing. Because EDM is a thermal machining process, it provides a means of machining ceramic materials, irrespective of their hardness and strength. Recorded machining speeds when EDM is applied on those ceramics are much better than those obtained with traditional machining techniques. This, combined with the extremely good surface finishes obtained makes EDM a viable alternative to traditional finishing techniques for ceramic materials (Ref 25).

However, because ceramic materials have higher melting points compared to metals, in general the metal removal rate of ceramics is lower. The removal mechanism during EDM of metals is based on melting and vaporization phenomena. In the case of ceramics, metal removal is due to two types of mechanisms, depending on melting point. The lower-melting-point ceramics exhibit metal removal due to failure by thermal shock (spalling). Spalling occurs when high internal stresses created by steep temperature gradients cause the material to flake. Ceramics that are machined by spalling exhibit very smooth surfaces (1.11 to 1.67 ^m). It has been observed that EDM of ceramics results in higher machining speeds compared to the traditional machining techniques for the same surface roughnesses (Ref 25).

In other recent research into the machining of new materials, polycrystalline diamond (PCD) has been machined by WEDM (Ref 26). PCD is challenging to shape due to its hardness, high strength, and high toughness. Diamond grinding is one of the most commonly used techniques for shaping of PCD. However, diamond grinding results in rapid tool wear during machining of PCD, and WEDM has been found to be a much more cost-effective method. The surface quality of workpieces machined by WEDM is very good, and there is not much difference in the surface roughness with a change in the diamond grain size.

References cited in this section

1. G.E. Benedict, Nontraditional Manufacturing Processes, Marcel Dekker, 1987

14. E.J. Weller, Ed., Nontraditional Machining Processes, Society of Manufacturing Engineers, 1984

15. H.C. Konig and L. Jorres, Electrodischarge Polishing as a Surface Finishing Process, EDM Technology, Vol 1, EDM Technology Transfer, 1993, p 12-16

16. Y. Hisashi, M. Takuoi, S. Kiyoji, Y. Tsuyoshi, and K. Kazuhiko, High Quality Electrical Discharge Machining Using an Anti-Electrolysis Power Source, EDM Technology, Vol 1, EDM Technology Transfer, 1993, p 25-30

17. H. Narumiya, N. Mohr, N. Saito, H. Ootake, Y. Tsunekawa, T. Takawashi, and K. Kobayashi, EDM by Powder Suspended Working Fluid, Proc. 9th Int. Symp. Electromachining (Nagoya, Japan), Japan Society of Electromachining, 1989, p 5-8

18. O. Yoshihara, M. Furuya, N. Saito, and N. Mohri, Finish EDM on Large Surface Area, Report No. 1--Finish Machining Phenomena, Toyota Tech. Rev., Vol 41 (No. 1), Dec 1991

19. N. Mohri, N. Saito, and M. Higashi, A New Process of Finish Machining on Free Surface by EDM Methods, Ann. CIRP, Vol 40/1, 1991, p 207-210

20. Y.F. Luo, Z.Y. Zhang, and C.Y. Yu, Mirror Surface EDM by Electric Field Partially Induced, Ann. CIRP, Vol 37/1, 1988, p 179-181

21. K.P. Rajurkar and W.M. Wang, Improvement in EDM Surface Roughness and Other Performances with an R.F. Control System, Proc. 10th Int. Symp. Electromachining, Magdeburg Technical University, Germany, 1992, p 383-394

22. S.M. Ahmed, "Texturing Large Surface Areas by EDM," Increase Machining Precision and Efficiency with EDM--Conf. Proc., Society of Manufacturing Engineers, 1987

23. N. Mohri, H. Momiyama, N. Saito, and Y. Tsunekawa, Surface Modification by EDM--Composite Electrode Method, Proc. 10th Int. Symp. Electromachining, Magdeburg Technical University, Germany, 1992, p 587-593

24. A.M. Gadalla and N.F. Petrofes, Surfaces of Advanced Ceramic Composites Formed by Electrical Discharge Machining, Mater. Manuf. Process., Vol 5 (No. 2), 1990, p 253-271

25. D.F. Dauw, C.A. Brown, J.P. van Griethuysen, and J.F.L.M. Albert, Surface Topography Investigations by Fractal Analysis of Spark-Eroded, Electrically Conductive Ceramics, Ann. CIRP, Vol 39/1, 1990

26. S.Z. Wang, K.P. Rajurkar, and J. Kozak, Effect of Grain Size on Wire Electrical Discharge Machining of Polycrystalline Diamond, Proc. Int. Conf. Advanced Material Processing (Gaithersburg, MD), S. Jahanmir, Ed., Special Publication 847, National Institute of Standards, and Technology, 1993, p 535-542

Mass Finishing

Revised by Edward H. Tulinski, Harper Surface Finishing Systems, Inc.

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