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Analysis of the wear characteristics of an EDM electrode made by selective laser sintering
Journal of Materials Processing Technology 138 (2003) 475–478
Analysis of the wear characteristics of an EDM electrode made by selective laser sintering Jianfeng Zhaoa,b,*, Yue Lic, Jianhua Zhanga, Chengye Yua, Youliang Zhangb a
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, China b College of Manufacturing Engineering, Nanjing University of Science and Technique, Nanjing, China c Research Institute of Pilotless Aircraft, Nanjing University of Aeronautics and Astronautics, Nanjing, China
Abstract Selective laser sintering (SLS) is a suitable process to manufacture an EDM metal prototype directly. Metal Infiltration, as a kind of posttreatment technique, is used to improve the density and other aspects of the mechanical performance of an SLS metal prototype. A parametric experiment has been applied, achieving substantial improvements in electrode quality and machining quality. At present these electrodes are suitable for finishing cuts in EDM, and show potential for the manufacture of electrodes with comparable performance to that of solid copper. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Selective laser sintering; Rapid prototyping manufacturing; Electrode; Wear; Post-treatment
1. Introduction Electrical discharge machining (EDM), a process widely used in the machining of hardened material or even ultra hardened material to fabricate tools or moulds, is regarded as one of the main technologies in today’s tooling field. As is known complex geometries or profiles of an electrode can be achieved more easily than that of a cavity, which is why with EDM, the lead-time of machining can be greatly shortened, and at the same time, the quality of cutting can be improved. Typically, in mould production, more than 25%, or even 40% of the machining lead-time is EDM. The major cost and time spent in EDM is the manufacturing of electrodes, which can take more than 50% of the total machining costs [1]. Generally, for the fabrication of a mould which has several different cavities, more than one electrode is used sequentially to avoid the manufacturing of large integral complex electrodes. During the machining, each cavity needs a separate electrode of specific geometry. Accurate, rapid manufacturing technology of integral electrodes with minimum manual intervention would reduce lead-time and costs spent in tooling greatly. * Corresponding author. Present address: College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, China. E-mail address: [email protected] (J. Zhao).
Rapid prototyping manufacturing (RPM) technology can manufacture complex three-dimensional prototype from a CAD model directly and rapidly. This technology has the potential to reduce lead-time and development costs through the rapid manufacturing of an electrode [2]. The technology, based on stereo lithography (SL), parts has been achieved by many institutions and companies already [3–5]. At the same time, research on the technology based on selective laser sintering (SLS) has been started and preliminary research results are available. Presently, research about these technologies is mainly concentrated on the fabrication of EDM electrodes, but the electrode characteristics, such as machining quality, wear of the electrode, are neglected. Directly manufacturing an electrode which can be used in EDM to secure a fine surface finish and low wear is the final destination of this technology, which is what SLS can achieve.
2. EDM electrodes manufacturing with SLS The possible production techniques using RP models may be classified as ‘‘direct’’ and ‘‘indirect’’. The principle of ‘‘indirect’’ method is similar to the technology based on SL. This paper mainly describes the direct method of electrode fabrication via SLS. The SLS system used in this paper is RAP-I, developed by RP Center, Nanjing University of Aeronautics and
0924-0136/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00122-5
476
J. Zhao et al. / Journal of Materials Processing Technology 138 (2003) 475–478
Astronautics (NUAA), China, and the EDM system, ROBOFORM 35, by Charmilles Technology LTD, Switzerland.
Table 2 Adjustable processing parameters for EDM Parameters
2.1. Prototype manufacturing with SLS
2
For direct production, a desired electrode geometry was modeled with CAD. The numerical control (NC) codes were programmed automatically with special software. A laser beam, as a energy source, is moved upon a powder system with an NC code track. The material used in direct sintering applications is a multi-component powder system. It consists of steel, polyester and phosphate which an mixed mechanically. In the powder system, steel powder is the base material, the polyester plays as binder, and the phosphate reacts with steel to produce a kind of inorganic which is highly rigid. Under the radiation of laser, the steel powder are bounded with sintered polyester and phosphate to form continuous individual layers with sufficient strength to withstand handling. Repeating above processing layer by layer, a metal prototype is produced. 2.2. Post-treatment Post-treatment is used to improve the strength and density of the SLS metal prototype. It consists of three steps. The first step is low temperature sintering (260– 300 8C). Above 260 8C, the melting point of polyester, the polyester is decomposed and turns into dissociative carbon. At this point, the volatilized polyester is replaced with pores, which are distributed on the whole part. The second step is high temperature sintering (760–1040 8C). At the temperature of 740 8C, the phosphate reacts with steel to produce a rigid inorganic, which acts as an underprop in the model. Increasing the temperature to 1040 8C, solid phase sintering of steel commences. Steel powder melts at the granule surface partially, the granules binding to each other. After this step, an indurative metal prototype is formed and it was porous, which is necessary for metal infiltration. The last step is infiltration of the porous metal prototype. This is performed by dipping the metal prototype into a liquid infiltrant (copper used usually) that rises into the open pores through capillary action. The treatment temperature of this step is 1120 8C, the liquidus temperature of copper. This step leads to rapid shrinkage which increases the density and strength. A compact solid prototype is formed. Hereafter, the prototype can be named an ‘‘electrode’’.
Ip (A/cm ) Ton (ms) Toff (ms)
Value 10 66 195
6 250 380
4 500
2.3. Test sample of an electrode for EDM A series of test samples of EDM electrodes of simple geometry (Ø20 mm C10 mm column) have been prepared as described and used to EDM. The component proportions of the electrode material for SLS are listed in Table 1.
3. Parametric experimentation of EDM To achieve the effective application of a metal prototype as electrode, it is necessary to develop a detailed knowledge of the influence of the parameters of the EDM process. The EDM process offers a number of adjustable parameters, which are well understood for solid electrodes of specific material. In this paper, several main parameters were researched: the applied current (Ip), pulse duration (Ton) and pulse interval (Toff). Through varying these parameters, the performance of the EDM process, which is related to sacrificial wear of the electrode and surface finish of the eroded cavity, is analyzed. The material for machining is 45# steel. The EDM process parameters used in the experiment are listed in Table 2.
4. Result and discussion 4.1. Basic performance of electrode The electrode is obtained with the above process. Plate 1 shows its microstructure. After post-treatment, the steel is present as a light phase surrounded by copper. Holes are also evident as marked. These were formed after infiltration
Table 1 Component proportions of the sintering materials
No. 1 No. 2 No. 3
Steel (V/V) (%)
Polyester (V/V) (%)
Inorganic compound (V/V) (%)
59 56 67
35 33 16
6 11 17
750
Plate 1. Microstructure of electrode after post-treatment.
J. Zhao et al. / Journal of Materials Processing Technology 138 (2003) 475–478
Fig. 1. The effect of material components on electrode wear.
because of the non-continuous mesh at the first step of posttreatment. Obviously, the electrode is not full-dense, but its density is close to that of solid copper.
477
more than two components join to discharge, so the discharge is anomalous in the gap. This causes the electrode surface to be scathed, and flaws are produced. The result is that the wear rate is increased. The composition proportions of the sintering material is the other reason that results in this trend. Table 1 shows the detailed data about the three samples. The density or compact degree of the EDM electrode is determined by steel, the base material of SLS. The greater is the steel, the more uniform is the pore distributing, and the less is the quantity of copper infiltrated. On the other hand, the additions are indispensable, but the quantity should be controlled. Especially, the polyester should be as little as possible, as it will become carbon in post-treatment. A proper quantity of carbon is beneficial to EDM, but it will influence the conductivity of the electrode and the electrode wear rate will increase when the quantity goes beyond a definite range.
4.2. The effect of material components on electrode wear 4.3. The effect of processing parameters on electrode wear Fig. 1 shows the effect of material components on the electrode wear rate. The machining parameters are Ip ¼ 10 A/cm2 and Ton ¼ 250 ms. The electrode wear rate of sample no. 3 is far less than samples no. 1 and no. 2. With increase of Toff, the wear rate of no. 1 and no. 2 is increased, but that of no. 3 is stable. The reason is that the density of no. 3 is greater than that of the other two. In other words, many more holes are in no. 1 and no. 2 than in no. 3. The hole in the prototype is the primary reason of the greater wear rate. Plate 2 is the SEM photos of wear surface of these three electrodes. The dilapidation of the electrode surface, has occurred along the white line in which the hole lay. The dilapidation degree of no. 1 and no. 2 is greater than that of no. 3. At the infiltration stage, the liquid phase copper cannot flow freely to fill into all the meshes and pores in the prototype because of their unsymmetrical distributing. As a result, holes are coming into being. these holes being spread anomalously in the electrode. The lower is the density of the electrode, the greater are the holes. Around these holes, the steel particles are not surrounded by liquid phase copper. In the EDM process, holes emerge and steel particles emerge also because of the wear. On the electrode surface,
As a result of the analysis above, no. 3 represents good wear characteristics. It is used to do the machining experimentation. Fig. 2 shows that the change of wear rate with Toff at different Ip. Curves 1, 2 and 3 denote that the Ton is 66 ms, and those of the others is 250 ms. The number in bracket is the applied peak current value. The wear rate for long Ton is less than those for on short Ton, at the same time, the wear rate is reduced with increased Ip. The wear rate for different Ip will tend to be stable when the Toff rises to a specific value. These phenomena are different from general EDM, which indicates the particularity of the electrode manufactured by SLS. 4.4. The surface roughness of eroded cavity Fig. 3 shows the surface roughness of the eroded cavity in the finish machining parameters with Ip ¼ 4 A/cm2 and Ton ¼ 250 ms with these three electrodes. It shows that the surface finish of eroded cavity of no. 3 is better than that of the others. The best surface quality did not occurs when the finish machining parameters were adopted, but it appeared with the semi-finish machining parameters in
Plate 2. Microstructural of different electrodes after EDM processing.
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J. Zhao et al. / Journal of Materials Processing Technology 138 (2003) 475–478
to deposit carbon on the electrode surface. Under the same Ton, the adsorbing of carbon and the wear of the electrode come to a dynamic balance when Toff reaches a proper value. As a result, the wear tends to be constant.
5. Conclusions
Fig. 2. The change of wear with parameters.
The electrode made by SLS can be used as an EDM electrode. A parametric experiment has proven that the wear rate of the electrode approaches to that of a general electrode, and the surface roughness of the cavity is acceptable at the same machining conditions. The preferable surface finish of cavity can be obtained using rough or semi-finish machining parameters with this kind of electrode. This is significant, and shows the potential of machining using an electrode made by SLS.
References
Fig. 3. Surface roughness of eroded cavity.
this experiment. The reason is not clear at present. Further research is needed. With the increase of current density, the discharge becomes tempestuous. In the region of discharge, the temperature of the treatment fluid increases. At the high temperature, partial treatment fluid is decomposed to carbon [6]. This carbon was adsorbed on the electrode surface during the machining period. With the original carbon in the electrode formed at the first stage of post-treatment, a protection film is formed [7]. This causes the weight of the electrode to increase. As a result, the electrode was protected in discharge process. For the same reason, long Ton provides a large time
[1] G.A. Semon, Practical Guide to Electro Discharge Machining, 2nd ed., Ateliers des Charmilles, Geneva, 1975, pp. 63–76. [2] T. Lu¨ ck, Comparison of downstream techniques for functional and technical prototypes—fast tooling with RP, in: Proceedings of the Fourth European Conference on Rapid Prototyping and Manufacturing, Belgirate, Italy, July 13–16, 1995, pp. 247–60. [3] T. Altan, B.W. Lilly, Kruth, J.P. Ko¨ nig, W. To¨ nshoff, H.K. Van Luttervelt, A.B. Khairy, Advanced techniques for die and mould manufacturing, Ann. CIRP 42 (2) (1992) 706–716. [4] O. Bjo¨ rke, Layer Manufacturing—A Challenge of the Future, Trondheim, 1992. [5] K. Jensen, R. Hovtun, Making electrodes for EDM with rapid prototyping, in: Proceedings of the Second European Conference on Rapid Prototyping and Manufacturing, University of Nottingham, July 15–16, 1993, pp. 157–65. [6] B.L. McKeown, Manufacture of spark erosion electrodes, Patent 2203360A GB (11 May 1987). [7] Y. Chengye, New Technique of Non-traditional Machining, Defense Industry Publishing Company, China, January 1995, pp. 19–24.
Analysis of the wear characteristics of an EDM electrode made by selective laser sintering Jianfeng Zhaoa,b,*, Yue Lic, Jianhua Zhanga, Chengye Yua, Youliang Zhangb a
College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, China b College of Manufacturing Engineering, Nanjing University of Science and Technique, Nanjing, China c Research Institute of Pilotless Aircraft, Nanjing University of Aeronautics and Astronautics, Nanjing, China
Abstract Selective laser sintering (SLS) is a suitable process to manufacture an EDM metal prototype directly. Metal Infiltration, as a kind of posttreatment technique, is used to improve the density and other aspects of the mechanical performance of an SLS metal prototype. A parametric experiment has been applied, achieving substantial improvements in electrode quality and machining quality. At present these electrodes are suitable for finishing cuts in EDM, and show potential for the manufacture of electrodes with comparable performance to that of solid copper. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Selective laser sintering; Rapid prototyping manufacturing; Electrode; Wear; Post-treatment
1. Introduction Electrical discharge machining (EDM), a process widely used in the machining of hardened material or even ultra hardened material to fabricate tools or moulds, is regarded as one of the main technologies in today’s tooling field. As is known complex geometries or profiles of an electrode can be achieved more easily than that of a cavity, which is why with EDM, the lead-time of machining can be greatly shortened, and at the same time, the quality of cutting can be improved. Typically, in mould production, more than 25%, or even 40% of the machining lead-time is EDM. The major cost and time spent in EDM is the manufacturing of electrodes, which can take more than 50% of the total machining costs [1]. Generally, for the fabrication of a mould which has several different cavities, more than one electrode is used sequentially to avoid the manufacturing of large integral complex electrodes. During the machining, each cavity needs a separate electrode of specific geometry. Accurate, rapid manufacturing technology of integral electrodes with minimum manual intervention would reduce lead-time and costs spent in tooling greatly. * Corresponding author. Present address: College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, China. E-mail address: [email protected] (J. Zhao).
Rapid prototyping manufacturing (RPM) technology can manufacture complex three-dimensional prototype from a CAD model directly and rapidly. This technology has the potential to reduce lead-time and development costs through the rapid manufacturing of an electrode [2]. The technology, based on stereo lithography (SL), parts has been achieved by many institutions and companies already [3–5]. At the same time, research on the technology based on selective laser sintering (SLS) has been started and preliminary research results are available. Presently, research about these technologies is mainly concentrated on the fabrication of EDM electrodes, but the electrode characteristics, such as machining quality, wear of the electrode, are neglected. Directly manufacturing an electrode which can be used in EDM to secure a fine surface finish and low wear is the final destination of this technology, which is what SLS can achieve.
2. EDM electrodes manufacturing with SLS The possible production techniques using RP models may be classified as ‘‘direct’’ and ‘‘indirect’’. The principle of ‘‘indirect’’ method is similar to the technology based on SL. This paper mainly describes the direct method of electrode fabrication via SLS. The SLS system used in this paper is RAP-I, developed by RP Center, Nanjing University of Aeronautics and
0924-0136/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00122-5
476
J. Zhao et al. / Journal of Materials Processing Technology 138 (2003) 475–478
Astronautics (NUAA), China, and the EDM system, ROBOFORM 35, by Charmilles Technology LTD, Switzerland.
Table 2 Adjustable processing parameters for EDM Parameters
2.1. Prototype manufacturing with SLS
2
For direct production, a desired electrode geometry was modeled with CAD. The numerical control (NC) codes were programmed automatically with special software. A laser beam, as a energy source, is moved upon a powder system with an NC code track. The material used in direct sintering applications is a multi-component powder system. It consists of steel, polyester and phosphate which an mixed mechanically. In the powder system, steel powder is the base material, the polyester plays as binder, and the phosphate reacts with steel to produce a kind of inorganic which is highly rigid. Under the radiation of laser, the steel powder are bounded with sintered polyester and phosphate to form continuous individual layers with sufficient strength to withstand handling. Repeating above processing layer by layer, a metal prototype is produced. 2.2. Post-treatment Post-treatment is used to improve the strength and density of the SLS metal prototype. It consists of three steps. The first step is low temperature sintering (260– 300 8C). Above 260 8C, the melting point of polyester, the polyester is decomposed and turns into dissociative carbon. At this point, the volatilized polyester is replaced with pores, which are distributed on the whole part. The second step is high temperature sintering (760–1040 8C). At the temperature of 740 8C, the phosphate reacts with steel to produce a rigid inorganic, which acts as an underprop in the model. Increasing the temperature to 1040 8C, solid phase sintering of steel commences. Steel powder melts at the granule surface partially, the granules binding to each other. After this step, an indurative metal prototype is formed and it was porous, which is necessary for metal infiltration. The last step is infiltration of the porous metal prototype. This is performed by dipping the metal prototype into a liquid infiltrant (copper used usually) that rises into the open pores through capillary action. The treatment temperature of this step is 1120 8C, the liquidus temperature of copper. This step leads to rapid shrinkage which increases the density and strength. A compact solid prototype is formed. Hereafter, the prototype can be named an ‘‘electrode’’.
Ip (A/cm ) Ton (ms) Toff (ms)
Value 10 66 195
6 250 380
4 500
2.3. Test sample of an electrode for EDM A series of test samples of EDM electrodes of simple geometry (Ø20 mm C10 mm column) have been prepared as described and used to EDM. The component proportions of the electrode material for SLS are listed in Table 1.
3. Parametric experimentation of EDM To achieve the effective application of a metal prototype as electrode, it is necessary to develop a detailed knowledge of the influence of the parameters of the EDM process. The EDM process offers a number of adjustable parameters, which are well understood for solid electrodes of specific material. In this paper, several main parameters were researched: the applied current (Ip), pulse duration (Ton) and pulse interval (Toff). Through varying these parameters, the performance of the EDM process, which is related to sacrificial wear of the electrode and surface finish of the eroded cavity, is analyzed. The material for machining is 45# steel. The EDM process parameters used in the experiment are listed in Table 2.
4. Result and discussion 4.1. Basic performance of electrode The electrode is obtained with the above process. Plate 1 shows its microstructure. After post-treatment, the steel is present as a light phase surrounded by copper. Holes are also evident as marked. These were formed after infiltration
Table 1 Component proportions of the sintering materials
No. 1 No. 2 No. 3
Steel (V/V) (%)
Polyester (V/V) (%)
Inorganic compound (V/V) (%)
59 56 67
35 33 16
6 11 17
750
Plate 1. Microstructure of electrode after post-treatment.
J. Zhao et al. / Journal of Materials Processing Technology 138 (2003) 475–478
Fig. 1. The effect of material components on electrode wear.
because of the non-continuous mesh at the first step of posttreatment. Obviously, the electrode is not full-dense, but its density is close to that of solid copper.
477
more than two components join to discharge, so the discharge is anomalous in the gap. This causes the electrode surface to be scathed, and flaws are produced. The result is that the wear rate is increased. The composition proportions of the sintering material is the other reason that results in this trend. Table 1 shows the detailed data about the three samples. The density or compact degree of the EDM electrode is determined by steel, the base material of SLS. The greater is the steel, the more uniform is the pore distributing, and the less is the quantity of copper infiltrated. On the other hand, the additions are indispensable, but the quantity should be controlled. Especially, the polyester should be as little as possible, as it will become carbon in post-treatment. A proper quantity of carbon is beneficial to EDM, but it will influence the conductivity of the electrode and the electrode wear rate will increase when the quantity goes beyond a definite range.
4.2. The effect of material components on electrode wear 4.3. The effect of processing parameters on electrode wear Fig. 1 shows the effect of material components on the electrode wear rate. The machining parameters are Ip ¼ 10 A/cm2 and Ton ¼ 250 ms. The electrode wear rate of sample no. 3 is far less than samples no. 1 and no. 2. With increase of Toff, the wear rate of no. 1 and no. 2 is increased, but that of no. 3 is stable. The reason is that the density of no. 3 is greater than that of the other two. In other words, many more holes are in no. 1 and no. 2 than in no. 3. The hole in the prototype is the primary reason of the greater wear rate. Plate 2 is the SEM photos of wear surface of these three electrodes. The dilapidation of the electrode surface, has occurred along the white line in which the hole lay. The dilapidation degree of no. 1 and no. 2 is greater than that of no. 3. At the infiltration stage, the liquid phase copper cannot flow freely to fill into all the meshes and pores in the prototype because of their unsymmetrical distributing. As a result, holes are coming into being. these holes being spread anomalously in the electrode. The lower is the density of the electrode, the greater are the holes. Around these holes, the steel particles are not surrounded by liquid phase copper. In the EDM process, holes emerge and steel particles emerge also because of the wear. On the electrode surface,
As a result of the analysis above, no. 3 represents good wear characteristics. It is used to do the machining experimentation. Fig. 2 shows that the change of wear rate with Toff at different Ip. Curves 1, 2 and 3 denote that the Ton is 66 ms, and those of the others is 250 ms. The number in bracket is the applied peak current value. The wear rate for long Ton is less than those for on short Ton, at the same time, the wear rate is reduced with increased Ip. The wear rate for different Ip will tend to be stable when the Toff rises to a specific value. These phenomena are different from general EDM, which indicates the particularity of the electrode manufactured by SLS. 4.4. The surface roughness of eroded cavity Fig. 3 shows the surface roughness of the eroded cavity in the finish machining parameters with Ip ¼ 4 A/cm2 and Ton ¼ 250 ms with these three electrodes. It shows that the surface finish of eroded cavity of no. 3 is better than that of the others. The best surface quality did not occurs when the finish machining parameters were adopted, but it appeared with the semi-finish machining parameters in
Plate 2. Microstructural of different electrodes after EDM processing.
478
J. Zhao et al. / Journal of Materials Processing Technology 138 (2003) 475–478
to deposit carbon on the electrode surface. Under the same Ton, the adsorbing of carbon and the wear of the electrode come to a dynamic balance when Toff reaches a proper value. As a result, the wear tends to be constant.
5. Conclusions
Fig. 2. The change of wear with parameters.
The electrode made by SLS can be used as an EDM electrode. A parametric experiment has proven that the wear rate of the electrode approaches to that of a general electrode, and the surface roughness of the cavity is acceptable at the same machining conditions. The preferable surface finish of cavity can be obtained using rough or semi-finish machining parameters with this kind of electrode. This is significant, and shows the potential of machining using an electrode made by SLS.
References
Fig. 3. Surface roughness of eroded cavity.
this experiment. The reason is not clear at present. Further research is needed. With the increase of current density, the discharge becomes tempestuous. In the region of discharge, the temperature of the treatment fluid increases. At the high temperature, partial treatment fluid is decomposed to carbon [6]. This carbon was adsorbed on the electrode surface during the machining period. With the original carbon in the electrode formed at the first stage of post-treatment, a protection film is formed [7]. This causes the weight of the electrode to increase. As a result, the electrode was protected in discharge process. For the same reason, long Ton provides a large time
[1] G.A. Semon, Practical Guide to Electro Discharge Machining, 2nd ed., Ateliers des Charmilles, Geneva, 1975, pp. 63–76. [2] T. Lu¨ ck, Comparison of downstream techniques for functional and technical prototypes—fast tooling with RP, in: Proceedings of the Fourth European Conference on Rapid Prototyping and Manufacturing, Belgirate, Italy, July 13–16, 1995, pp. 247–60. [3] T. Altan, B.W. Lilly, Kruth, J.P. Ko¨ nig, W. To¨ nshoff, H.K. Van Luttervelt, A.B. Khairy, Advanced techniques for die and mould manufacturing, Ann. CIRP 42 (2) (1992) 706–716. [4] O. Bjo¨ rke, Layer Manufacturing—A Challenge of the Future, Trondheim, 1992. [5] K. Jensen, R. Hovtun, Making electrodes for EDM with rapid prototyping, in: Proceedings of the Second European Conference on Rapid Prototyping and Manufacturing, University of Nottingham, July 15–16, 1993, pp. 157–65. [6] B.L. McKeown, Manufacture of spark erosion electrodes, Patent 2203360A GB (11 May 1987). [7] Y. Chengye, New Technique of Non-traditional Machining, Defense Industry Publishing Company, China, January 1995, pp. 19–24.