New trends in non-conventional technologies and electric discharge machining.

Popa, Marcel ; Contiu, Glad ; Precup, Mircea 等

1. INTRODUCTION

In 1942 the Lazarenko couple discovered the EDM(Electric Discharge Machining) process. At the beginning it was used only in the military industry. EDM performed a long way till to its present high performances. EDM has appeared as a necessity to manufacture materials with better mechanic and thermal characteristics. The major advantage of EDM in comparison with other manufacturing processes is represented by the fact that the hardness of material is not important, the only condition being that the processed material must be electro conductive.

We are participating today to the most extraordinary growth of electronic and automatic, aeronautic and spatial industry, nuclear industry or micro technologies. It is noted also, a sharp rise of unconventional techniques and technologies. In this field we find today from the unconventional "economic war" and "unconventional terrorism" to the "clasic" unconventional technologies--laser, EDM, etc.- from unconventional forms of informational or economical war and "exotic" techniques as those para psychological, satellite telepathy, to the unconventional weapons.

The most important means of development, but also the principal financial support of the unconventional technologies is represented by the military industry and techniques. We note also, that some unconventional technologies are still in different stages of investigation or improvement. All the new technological applications have an unconventional character, but after it became known and applied, there are rapidly turned into conventional(Popa et. al., 2008).

2. GENERAL ASPECTS OF EDM PROCESS

The phenomena of electro erosion of metals and their alloys were well known in the electro technology industry, because switches, contactors, micro switches etc. are destroyed by the sparks that appear during functioning or when they are switched on or off.

The discharge in the electric sparks last a very short time, producing high energy on the very small surfaces and punctiform a very high temperature instead of discharges through electric arcs which are very powerful on the more bigger surfaces and can last longer(welding, shortcutting).

Today, the machining by electro erosion can practically realize any kind of manufacturing that is made with classic methods: drill, saw, turn, mill, ream, grind, hone, etc.

This machining can be classified in two groups: EDM (machining with massive electrode), WEDM (machining with wire electrode)(Westkampfer & Warnecke, 2001).

The EDM machines can be separated in two types: universal and special machines. The physic process of material machining and the evacuation of the processed material from the spark take place in the interstice between work piece and tool. In the process of electro erosion, the electric parameters have the main importance.

In EDM, conductive workpiece materials are removed for the purpose of machining in a dielectric by electrical discharge. The material removal results from the erosive effect of subsequent, time wise separated, no stationary or quasistationary discharges between electrodes, i.e., between tool and work piece. Each discharge generates a microscopic removal on the two electrode surface. In principle, the process is based on thermal erosion. Hence, an efficient EDM process can only be realized by a purposefully uneven material removal on the two electrodes. Wire--EDM as kinematical variant of EDM allows hereby the machining or respectively, production of complex geometrical contours.

3. ELECTRICAL DISCHARGE MACHINING. EXPERIMENTAL RESULTS.

3.1 Wire Electrical Discharge Machining

The process can be compared with the classic process of sawing or cutting with diamond wire, but the particular characteristic of not stressing the work piece and the wire that don't get in contact with. Comparing with the classic machines, the work piece flows forward and the wire rolls only.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

One of the parameters that are followed to be obtained after machining is the roughness of the surface. By machining with this method can be obtained very good surfaces with roughness that can arrive till 0,2 um, but only in finishing mode. In the industry is not always necessarily such a good surface. So in the industry, it is important to know the roughness of the surface to be obtained in order to prepare a technology as economic as possible. Obtaining a roughness that is wanted by the client, it avoids an additional machining which would require additional time and cost. The autors realised some experiments to obtain results regarding the time of machining and the quality of the processed surface. The roughness will increase when the intensity of the current grows and when the frequency decreases. The process can be compared with sawing or cutting with diamond wire. As it can be observed in the fig. 1, the texture of the surface is not the same on the entire surface of the work piece. This phenomenon is due to the dielectric, that doesn't wash equable the processed work piece. As in the conventional processes, the EDM can also do rough, finishing or super finishing works. With the new machines of EDM, the roughness [R.sub.a] that is obtained on the existing machines that work in the industry is between 0, 2/6,3 [micro]m.

In other cases, due to the aspect that must be obtained on the surface, the work pieces are processed with very high Ra value, over 12, 5 [micro]m. The productivity that is obtained in this case is very high due to the very high energy that is used. For the very high precision processes, it can be obtained in special conditions a roughness Ra of 0, 05/0,1 um. (Dodun Oana, 2001).

Because usually, the process is used in industry after a thermal treatment of the work piece, we used hard steel.

* OLC 45; 42MoCr11; OSC7.

For a higher hardness, the test pieces were thermal treated. (Table 1.)

The dimensions of the test pieces are:

--length L = 80mm;

--width l = 20mm;

-height h = 10-50mm;

The roughness of the surface was measured on the middle of the height. For the same thickness of the work pieces the value of the roughness are oscillating between 1, 85 [micro]m and 2, 70 [micro]m as it can be seen in the figure 2.

For the work pieces that have thicknesses over 70 mm, it can be observed the difference between the roughness from the bottom and the middle of the work piece--figure 3.

This phenomenon takes place because the nozzles for dielectric are placed at the top and at the bottom of the work piece and the pressure is not big enough to be able to wash all the particles.

In the industry the roughness that is obtained is very important. On this parameter depends the quality and the aspect of the product.

Today the new machines with the command and control system that assist the process make the work of the operator easier.

4. CONCLUSIONS

In the industry, this technological method of machining is expanding more and more. To be mentioned is that it begins to be used in micromachining. It's important to be able to establish a technological itinerary and after this a price for a work piece or for a large series of production.

The operator is using the CAM programs in order to write the CNC program, but the parameters of the machine must be set concerning the material, the thickness of the work piece, the roughness that must be obtained and the tool material. For a better planning of the technology, it's necessary to build a bibliotheca with most common materials that are to be used in a factory. For the same kind of machining (roughing or finishing), the values of parameters can be obtained from the tables of the machine-tools. Although the parameters are optimized, the quality of surface is different for each machining because external conditions are deferent for each machining.

But for industry it is important to be prepared a bibliotheca with specified parameters for machining different materials. For example for the same kind of machining (roughing or finishing), in the tables of the machine-tools, the parameters are optimized but it is not specified the quality of the surface that can be obtained.

Our researches have shown the knowledge importance of all parameters that can influence the quality of the process and in the future we will prepare a bibliotheca that will complete the CAM programs by having a larger table of materials and machining parameters.

5. REFERENCES

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

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

Popa, M., Contiu, G. Precup, M., Preja, D., Gaina, O., Fagarasan, C.. Unconventional Technologies and competitive engineering in the 21st century, Proceeding of the TMCE 2008, Kusadasy Turkey,

Popa M., Contiu G., Precup M., Preja D., Gaina O., Mathematical model and simulation of the heat transfer in the EDM process, IEEE International Conference of Automation , Quality and Testing, Robots AQTR 2008, May 22-25, 2008.

Westkampfer, E., Warnecke, H.J. Einfuhrung in die Fertigungstechnick, "Introduction in the machininng technology", Teubner Verlag, Stuttgart 2001.