Electric discharge machining process control.

Popa, Marcel Sabin ; Contiu, Glad ; Preja, Dan 等

1. INTRODUCTION

In 1942 the Lazarenko couple discovered the Electric Discharge Machining process. At the beginning it was used only in the military industry. EDM performed a long way till its present high performances. EDM was created in order to fulfill the requirement to manufacture materials with better mechanic and thermal characteristics. The major advantage of EDM as compared to other manufacturing processes is that the hardness of material is not important, and the only condition that has to be fulfilled by the processed material is to be electro conductive (Popa M, Contiu G., Precup M., 2008).

Development of unconventional technologies in the field of manufacturing processes was possible due to discovery of new materials that are harder and sometimes impossible to process with conventional technologies. The new dimensions in the electronic and automation opened also the gates for the unconventional technologies to take the lead in the research departments.

Even though the unconventional technologies have been studied for a long time, some aspects are not yet perfectlyunderstood. For the industry that is using those technologies (for example EDM) it is important to understand every detail that could influence the quality of the work piece that is manufactured.

Electric discharge machining process is complex and stochastic in nature. The process involves a combination of several disciplines such as electrodynamics, electromagnetic, thermodynamic, and hydrodynamic which makes it difficult to structurein a comprehensive model. (S.H. Yeo,2 008)

The properties of diatomic plasma were considered to be constant and the fluid dynamic equation was included in the model. Eubank reported variable mass cylindrical plasma which expands with time. For an EDM process with a current of 2.34 A, the temperature and pressure of the plasma channel were approximated to be 11,210 K and 54 bar after 6 us. Another approximation of plasma channel for micro-EDM process was reported by Dhanik and Joshi (2005) where the temperature and pressure were found to be in the range of 8100 [+ or -] 1750K and 6-8 bar, respectively (S.H. Yeo,2008, Eubank, P.T.1993, Dhanik, S., Joshi, S.S., 2005).

[FIGURE 1 OMITTED]

One of the parameters that depend on these phenomena is the roughness of the surface obtained by processing a work piece with EDM process (Popa, M. S. 2008).

2. SINGLE CRATER RESEARCHES

The phenomena of electric discharge machining was studied and mathematically modeled by some scientists such as Jilani, Di'Bitonto, Van Dijck, Beck etc. By these models, they tried to approximate the dimensions of the crater created by a single spark during the EDM process.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

To be able to obtain a single (fig. 1 and 2) crater some researches were made. The best method was to use plate electrodes, one anode of cupper and one cathode of steel. Moreover, in order to be able to obtain only a few sparks, the pulse-off-time had to be modified up to 50000 us. By extending the pulse off time obtaining a longer period, it was possible to obtain single defined craters.

The experiments were performed on different work pieces with the aim to observe the variation of the roughness of the surface and the speed of machining as related to the thickness of the material. In figure 3 measured the volume of the crater by with the help of an electronic microscope. In this case the volume above the crater is [V.sub.above]= 11513351 [micro][m.sup.3] and the volume above the surface [V.sub.below]=21322934 [micro][m.sup.3]. Two problems occur during the process. One is the accuracy of the measurements that isperformed by the microscope and the second oneis the light reflection of the processed surface. As in figure 2, some white points can be seen. Some of these errors appear because of the surface that is too dark and the light from the microscope is not reflected and some of them because the surface reflect too much light. During the measurements all these reflections generate errors of the results.

Rough surfaces or complex form geometries are challenging for tactile and conventional optical instruments. Tactile methods can produce errors caused by the tip radius of the stylus which can cause mechanical misrepresentation of the true surface. The complex reflective properties of extremely sculptured surfaces can be extremely difficult to resolve and capture with conventional optical systems. (http://www.alicona.com)

A researche was performed on a number of types of materials that were processed by wire EDM and than the surface roughness was measured. Six types of material were used. Three of them were steel alloys and the other three were Al, Cu and Graphite.

Measurements of the roughness of the surface and the material removal rate are shown in figure 4. The diagram presented in fig. 1 shows the approximated form of the crater that can be measured with an electronic microscope.

[FIGURE 4 OMITTED]

A mathematical calculation and a statistic evaluation of the dimension of the crater would be more precise than the microscopic measurement of the volume. Also an approximation of the thermal influence could provide us the structural changes in the superficial layers of the surface.

3. CONCLUSIONS

The roughness of the surface is constituted from all the craters that are melted during the process. A state of the art machining technology adapted for each specific material should provide a precise value of the surface roughness.

One of the most important problems to be solved is a mathematical model like one of DiBitonto or Jilani, which should provide a precise approximation of the one spark crater. These could solve the problem that industry still has.

Until this point, the research has focused on measuring the dimensions of the craters that are generated on the surface. Next step will be to improve or to create a mathematical model of the crater.

All the measurements that were made till now will be used in validation of the future mathemathical model.

A mathematical model could help in building a data base for every machine type.

For a better planning of the technology, it is necessary to build a data base with most common materials that are to be used in a factory. Although the machining parameters (such as the type of material, the thickness of the work piece, the roughness that must be obtained and the tool material) are optimized, the quality of surface is different for each machining because external conditions are different for each machining process. (Popa M. S., Contiu G, Preja D.., 2008).

4. REFERENCES

Ceausescu, N, Popescu, I., 1982--Tehnologii neconventionale, "Unconventional Technlogies" Ed. Scrisul Romanesc, Craiova

Dhanik, S., Joshi, S.S., 2005. Modeling of a single resistance capacitance pulse discharge in micro-electro discharge machining. Trans. ASME J. Manufact. Sci. Eng. 127 (4), 759-767

Dodun, Oana, Tehnologii neconventionale. Prelucrari cu scule materializate. "Unconventional Technologies. Machining with materialized tools". Editura Tehnica Info Chisinau 2001

Eubank, P.T., Patel, M.R., Burrufet, M.A., Bozkurt, B., 1993.Theoretical models of the electrical discharge machining process. III. The variable mass, cylindrical plasma model. J. Appl. Phys. 73 (11), 7900-7909

Popa M, Contiu G., Precup M.. : Unconventional Technologies and Competitive Engineering in the 21st Century. Proceedings of the TMCE 2008, April 21-25, 2008, Kusadasi, Turkey

Popa M. S., Contiu G, Preja D., New Trends in Non Conventional technologies and Electric Discharge Machining, The 2nd European DAAAM International Young Researchers' and Scientists' Conference, 22-25th October 2008, Slovak University of Technology, Trnava, Slovakia

S. H. Yeo, W. Kurnia, P.C. Tan Critical assessment and numerical comparison of electro-thermal models in EDM. Journal of materials processing technology 2 0 3 (2008) 241-251