Surface quality in the electric discharge machining process.

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

Abstract: The unconventional manufacturing methods technologies, mainly electric discharge machining has been performed for over 20 years within the Faculty for Machine Building from Technical University of Cluj-Napoca. Lately, the researches have been extended also, on the last generation of machines that were received within international research programs. Concerning the Electric Discharge Machining method, it is very important to record the variation of the parameters depending on the material and on the depth of the work piece. It must be mentioned that for a better planning of electric discharge machining technology is necessary a very good knowledge of the working parameters. The article presents some results about the roughness variation depending on the depth of the work piece, speed of cutting and materials.

Keywords: Unconventional, technology, discharge, machine-building.

1. INTRODUCTION

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 working or when they are switched on or off.

The discharge in the electric sparks holds a very short time ([10.sup.-6]/[10.sup.-8] s), producing with high energy a lot of craters on the very small surfaces.

The very fast developing of this manufacturing machines by electro erosion and of the electro technologies are owned to the usage in the machine building, equipments, instruments, integrated circuits, etc. of special materials that are very hard workable or even impossible using the classic methods.

The process of electric discharge machining avoids the appearance of the distortion in the work piece and of the internal stress in the superficial layer of the manufactured surface by avoiding the contact between work piece and tool.

The process was developed very fast so that, today can be realized any kind of manufacturing that is made with classic methods: drill, saw, turn, mill, ream, grind, hone, etc. (Dodun, 2001)

2. GENERAL ASPECTS OF THE WIRE ELECTRIC DISCHARGE MACHINING PROCESS

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 gets in contact with.

Comparing with the classic machines, the work piece flows forward and the wire rolls only. The experiments for this work were made on a Wire Electric Discharge Machine (WEDM) model MAKIO EE3.

One of the parameters that are followed to be obtained after machining is the roughness of the surface. Machining with this method can be obtained very good surfaces with roughness that can arrive till 0,2 [micro]m, but only in finishing mode. Only some special elements require such a good quality for the surface. So in the industry, is important to anticipate the roughness of the surface to be able to prepare a technology for a work piece that is economic. Obtaining a roughness that is wanted by the client, it avoids an additional machining which would require an additional time and costs.

[FIGURE 1 OMITTED]

For practical applications it considers:

h = [C.sub.H] x [E.sub.i.sup.p] (1)

where:

[C.sub.H]--material coefficient;

[C.sub.H]=190 for steel and steel alloyed with Cr;

[C.sub.H]=67 for hard alloys; p--exponent p= 0.33-0.4

Expressing the energy [E.sub.i]

[E.sub.i] = [U.sub.m] x [I.sub.m]/f (2)

it obtains:

h [C.sub.H] [([I.sub.m] x [u.sub.m]/f).sup.p] (3)

where:

[I.sub.m]--medium value of the intensity;

[U.sub.m]--medium value of the tension on the interstice;

f--frequency. (Ceausescu, 1982)

The roughness is increasing when the intensity of the current is growing and when the frequency is decreasing.

Also the results of the machining don't coincide with the results obtained by calculation. Some problems appear because the machines don't have a library with the parameters for all materials. Also the parameters of the machine are encoded so the technician can't calculate all this values.

3. EXPERIMENTAL RESULTS

In our researches for the work piece, we used hard steel.

* OLC 45

* 42MoCr11

* OSC7

For a higher hardness, the test pieces were thermal treated.

The roughness of the surface was obtained on the work pieces with thickness between 10 and 50 mm.

The surface doesn't have the same texture. For small dimensions of the thickness h, the values of the roughness are not radically modified as it can be observed in the table 2.

From the figure 2 can be observed that 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.

But after a thickness of 70 mm the roughness changes from the bottom and the upper part of the work piece to the centre as it can be seen in the fig. 4. This phenomenon takes place because the nozzles 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.

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. On a work piece with a thickness of 110 mm, the measurements were made from the bottom to the middle of the work piece and the roughness grows from 2, 7 [micro]m up to 5, 5 [micro]m as it can be seen in fig. 3

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

4. CONCLUSIONS ABOUT THE WIRE ELECTRIC DISCHARGE MACHINING PROCESS

Today is important that this technological method of machining is expanding in the industry more and more and is to be mentioned that begins to be used in advanced processes i.e. micromachining. Considering this, it must be establish a very good technological itinerary and after this an optimal price for a work piece. The work presents only one parameter that influences the quality of the machined surface. 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 actual tables of the machine-tools, the parameters are optimized but it is not specified the quality of the surface that can be obtained. Some of these aspects, including the preparing of the mentioned parameters bibliotheca, will be tackling in our future researches.

5. REFERENCES

Han, F.; Kunieda, M., Sendai, T.; Imai, Y.--High precision simulation of WEDM using parametric programming, in the Annals of the CIRP, vol. 51/1, 2002.

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

Popa, M.S., Gaciu, A.--A new method for cutting dies trough WEDM. In Proceedings of the 4th International DAAAM Symposium, Tech. Univ. Brno, Czech Rep, 1993.

Popa, M.S.,--Tehnologii si masini neconventionale, pentru mecanica fina si mecatronica. "Unconventional Technologies and Machine tools, for fine mechanic and mechatronics" Editura UT Pres, Cluj-Napoca, 2005.

Ceausescu, N.; Popescu, I.--Tehnologii Neconventionale, "Unconventional Technlogies" Vol.1. Ed. Scrisul Romanesc, Craiova 1982, "Unc onventional Technologies".

Table 1. Values of the hardness after thermal treatment.

Material OLC45 42MoCr11 OSC7

Obtained 241 HB (21,2 HRC) 263 HB (24,7 HRC) 640 HB (61,9 HRC)
hardness

Table 2. Values of the hardness after thermal treatment.

Surface roughness Ra[[micro]m] after machining OLC45, 42MoCr11
and OSC7

 Material

 OLC45 42MoCr11 OSC7

h = 10mm 2.45 2.05 2.45
h = 20mm 2.00 2.25 2.50
h = 30mm 2.30 2.45 2.00
h = 40mm 2.20 1.85 2.20
h = 50mm 2.70 2.50 2.15