Supply source for electrochemical discharge drilling.

Coteata, Margareta ; Slatineanu, Laurentiu ; Munteanu, Adriana 等

1. INTRODUCTION

The electrochemical discharge machining (ECDM) is one of the hybrid machining methods based on the anodic dissolution of the workpiece material and on the erosive effects of the electrical discharges that occur between the electrode tool and the workpiece surface (Strabalak et al., 2004). The purpose of applying the two processes in one is to have a synergetic action for the material removal.

Generally, the ECDM process implies the existence of the electroconductive working liquid and the connection of the electrode tool to the cathode of the direct current source, while the workpiece is connected to the anode. When a nonconductive material has to be machined by using the electrochemical micro discharges, an auxiliary anode electrode is used (Sanjay et al., 2007).

The properties of the working liquid affect the character of the ECDM process. Thus, if the working liquid is an electrolyte as aqueous solution of potassium chloride or natrium nitrate, the material removal process is preponderant represented by the electrochemical dissolution.

When the working liquid has a partially dielectric behavior, due to the products of the chemical reaction between the liquid and the workpiece material, the material is removed mainly by electrical erosion. This last category of working liquids include the so called semi-dielectrics, the most known being the aqueous of natrium silicate.

The electrochemical discharge drilling of the hard-to-cut materials can be a good alternative instead of the drilling by means of the high technology equipments, which are generally very expensive (laser, water jet cutting etc.), since the required equipment for ECDM process is mainly based on a direct current source and a mechanical setup of low complexity. The electrical supply basically should include a capacitor, a rectifier, a voltage regulator and the required switches.

B.R. Sarkar et al. used a pulse direct current generator for electrochemical discharge machining of silicone nitride ceramics (Sarkar et al., 2008); there are not offered details concerning the structure of the used pulse generator.

I. Basak and A. Gosh studied the electrochemical discharge process by the using of a direct current source with a separating transformer; two diodes, were included to obtain the direct current applied to the electrodes (Basak & Gosh, 1996).

Coils and electrical resistors were included in the discharge circuit to easier modify the machining parameters within electrochemical discharge grinding process (Kuromatsu, 1984).

2. PROBLEM STATEMENT

The ECDM process by means of the semidielectric liquid was used mainly for cutting (process known as anodic-mechanical cutting) and less for drilling. In small mechanical workshops, the drilling of the hard--to--cut materials may be achieved by ECDM. Thus, a mechanical setup adaptable for classical milling machine is proposed within this paper. A schematic representation of the proposed device is shown in figure 1.

[FIGURE 1 OMITTED]

The use of a semidielectric liquid (in this case, aqueous solution of natrium silicate) makes possible a better control of the electrochemical dissolution of the material by the forming of the passivanting film. In order to remove this film, a relative pressure between the electrode-tool and the workpiece material is requisite. When the electrode of small diameter (d <1 mm) is used, its buckling may occur. For this reason, the workpiece holder is sustained by a spring that assures the necessary pressure between the electrodes, avoiding the electrode-tools buckling.

In order to assure the electrical discharges occurring by electrodes contact breaking, the slotter ram of the milling machine was used for fixing the electrode-tool holder device. Thus, the frequency of the electrodes contacts may be established and, in this way, some parameters of the electrical discharges and of the electrochemical dissolution could be also controlled. The alternative linear motion and the rotation of the electrode tool allow also the expulsion of the removed particles (detached by electrical discharges) and also of the rests of the broken passivating film from the working gap, by stirring up the semidielectric liquid in the working zone.

The proposed direct current source must contain capacitors with different capacitances C that make possible the varying of the electrical discharge energy; the energy Wd of a single electrical discharge is given by the expression:

[W.sub.d] = [CU.sup.2]/2, [J] (1)

where U is the voltage applied to the work gap.

The used values of the working voltage are around 30 till 50 volts, so that a transformer was included in the electrical circuit, in order to assure a diminishing of the voltage from 220 volts to 35 volts and respectively 45 volts; these values were considered after preliminary experiments. The electric circuit schema of the supply source is shown in figure 2.

To switch the current from alternative to continuous, the circuit contains also a rectifier. The proposed device was meant for drilling of thin plates made of hard--to--cut material, such as tool steel. The preliminary experiments shown that the micro drilling of the hard--to--cut material may be achieved by ECDM with simple experimental setup. Besides the electrical parameters (as capacitance and voltage), the liquid density S and the electrode tool diameter were considered as inputs of the process. The variation of the output parameter (the hole depth) was studied.

[FIGURE 2 OMITTED]

One can formulate the hypothesis that by the increasing of the voltage and capacitance, the hole depth will increase too.

3. EXPERIMENTS RESULTS

For studying the process of the electrochemical discharge drilling, an experiments planning was done. Both the electrode-tool and the test pieces were made of cutting steel; the duration of each experiment was of 6 minutes.

The sizes of the working parameters and the obtained results are presented in table 1.

In order to mathematically process the experimental results, specialized software based on the smallest squares method was used. As result, the following empirical mathematical model was obtained:

[H.sub.d] = 2.137 x [10.sup.-4] [d.sub.ET].sup.0.2338] x [U.sup.1.8254] x [C.sup.0,1287] x [[delta].sup.4.20133] (2)

By the analysing of the mathematical relation, one can notice that the biggest influence on the depth [H.sub.d] of the obtained hole is exerted by liquid density [delta]. One can specify also that the order of the influences exerted by the considered work parameters on the hole depth is the following: the work liquid density [delta], the voltage U, the electrode tool diameter d, the capacitance C (the order of the absolute sizes of the exponents included in the relation (2) being 4.2013> 0.2338>1.8254> 0.1287).

[FIGURE 3 OMITTED]

The image concerning the influence exerted by the work parameters on the material removal rate can be completed by the graphical representations included in figures 3 and 4. In the future, one can try to establish the optimal sizes for the working parameters, so that the maximum material removal rate to be obtained in the most convenient work conditions.

[FIGURE 4 OMITTED]

4. CONCLUSION

A work schema for the electrochemical discharge drilling of the metallic workpieces was proposed. The identified supply source allows the variation of the voltage and capacitance of the discharge circuit; thus the optimisation of the electrochemical discharge process could be possible. An empirical mathematical model emphasizing the influence exerted by some of the work parameters on the depth of the hole was established.

5. REFERENCES

Basak, I. & Ghosh, A. (1996). Mechanism of spark generation during electrochemical discharge machining: a theoretical model and experimental verification. Journal of Materials Processing Technology, Vol. 62, 46-53, ISSN 0924-0136

Kuromatsu, A. (1986). Electrolytic discharging grindstone and electrolytic discharge machining. Japan Patent, no. 61050718

Sanjay K.; Chak, P. & Venkateswara, R. (2007). Trepanning of Al2O3 by electro-chemical discharge machining (ECDM) process using abrasive electrode with pulsed DC supply. International Journal of Machine Tools & Manufacture, Vol. 47, 2061-2070, ISSN 0890-6955

Sarkar, B.R.; Doloi, B. & Bhattacharyya, B. (2008) Experimental investigation into electrochemical discharge microdrilling on advanced ceramics. International Journal of Manufacturing Technology and Management, Vol., 13, 214-225, ISSN 1741-5195

Skrabalak, G.; Zybura-Skrabalak, M. & Ruszaj, A. (2004). Building of rules base for fuzzy-logic control of the ECDM process. Journal of Materials Processing Technology, vol. 149, (June, 2004), 530-535, ISSN 0924-0136

Tab. 1. Experimental results

Exp. Electrode Voltage Capacitance Liquid Hole
no. tool U, V C, [micro]F density S, depth
 diameter [delta] g/ Hd, mm
 dET, mm [cm.sup.3]

 1 0.5 35 33 1.05 0,29
 2 0.9 35 33 1.05 0,19
 3 0.5 45 33 1.05 0,68
 4 0.9 45 33 1.05 0,42
 5 0.5 35 840 1.05 0,60
 6 0.9 35 840 1.05 0,51
 7 0.5 45 840 1.05 0,16
 8 0.9 45 840 1.05 0,82
 9 0.5 35 33 1.20 0,463
 10 0.9 35 33 1.20 0,293
 11 0.5 45 33 1.20 0,85
 12 0.9 45 33 1.20 0,62
 13 0.5 35 840 1.20 0,42
 14 0.9 35 840 1.20 0,99
 15 0.5 45 840 1.20 1,39
 16 0.9 45 840 1.20 1,36