Some considerations concerning the gap evolution during the electrochemical machining.

Coteata, Margareta ; Slatineanu, Laurentiu

Abstract: The electrochemical machining uses a dissolution process to remove the material from the workpiece. For a profile of the active part of the electrode tool obtained as an intersection of two lines under a certain angle and for an electrochemical erosion process without work motions of the two electrodes, a theoretical model for the work gap was elaborated. The experimental researches proved the validity of principle of the theoretical model.

Key words: electrochemical erosion process, work gap

1. INTRODUCTION

The electrochemical machining is based on the electrochemical (anodic) dissolution of the workpiece material within some characteristic processes of electrical charge and mass changes among the electrolyte, the anode and the cathode. Even the electrochemical machining was industrially applied since the beginning of the XX century, there are yet some problems concerning the control of the work gap size between the workpiece and the electrode tool.

The gap size could be influenced by different factors. The researchers established that the gap size firstly depends on the shape of the surface to be obtained; the workpiece zones found at a small distance in comparison with the active part of the electrode tool are more intense affected by the dissolution process, due to concentration of the electric field lines on the tops of the workpiece prominent zones (Gavrila[degrees] & Marinescu, 1991; Rajurkar et al., 1999; Slatineanu et al., 2004). A mathematical model for numerical modeling of the electrolyte flow in the work gap during the electrochemical machining process was elaborated by L. Dabrowski and T. Paczkowski ((Dabrowski & Paczkowski, 2005). The current distribution study in electrochemical systems to elaborate an adequate model was made by Maria Georgiadou (Georgiadou, 2003). We must mention that the electrochemical erosion process develops in closed spaces (in the so-called electrolytic cells) and this situation does not permit to directly follow the evolution of the work gap during the machining time.

2. THEORETICAL MODELLING

Firstly, we will suppose an electrochemical erosion without relative motions between the electrodes and without electrolyte circulation; of course, the machining process will develop in immersion. We know [De Ba rr, 1973] the mathematical relation for defining the gap size between the two electrodes:

s = [square root of (2Ct + [s.sub.0.sup.2])], (1)

where C is a constant, t - the machining time and [s.sub.0] - the initial gap size. The constant C is given [De Barr, 1973] by the relation:

C = A(U - [delta]U)[KAPPA] / [F][[rho].sub.m], (2)

where A is the equivalent gram of the substance affected by the electrochemical dissolution process, U--the voltage applied on the electrodes, [DELTA]U- the voltage necessary for the electrodes polarization, k--the electroyte electric conductivity, F--the Faraday's constant, and [[rho].sub.m]--the substance mass density (for steel, nm= 7.8 g/[cm.sup.3]).

We will take into consideration the case of a profiled electrode tool; the active zone of the electrode tool is obtained by the intersection of two plane surfaces under an angle 2 a = 90[degrees] (fig. 1). The work gap size corresponds to the relation:

[s.sub.0] = [s.sub.min]+ [DELTA]s, (3)

where smin is the minimum gap size before the machining, respectively, in this case, the distance between the electrode tool corner and the workpiece surface, [DELTA]s--the difference between the real gap size and the minimum size of th initial gap, in a certain point of the active surface of the electrode tool. We will use a cartesian coordinate system xOy, having the origin O in the top point of the electrode tool. Geometrical consideration permit us to write:

tg[alpha] = x/[DELTA]s, (4)

where x is the size of the side AB belonging to the right triangle ABC. From the above written relation, we have:

[DELTA]s = x/tg[alpha]. (5)

Introducing the expression for the difference As in the relation (3), we obtain:

[s.sub.0] = [s.sub.min] + x/tg[alpha]. (6)

Thus, the relation (1) becomes:

S = [square root of (2Ct + [([s.sub.min] + x/tg[alpha]).sup.2]]. (7)

If we take into consideration the relation (2), we obtain:

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

s = [square root of 2t A(U - [DELTA]U)k/F[[rho].sub.m] + [([s.sub.min] +/tg[alpha]).sup.2]]. (8)

The mathematical relation (8) characterizes the profile of the machined surface by taking into consideration the erosion time t, the geometry of the active part belonging to the electrode tool

(here, by the angle of the electrode tool corner, 2 a), the work voltage U, the electric conductivity of the electrolyte k.

Specialized software in Basic medium was elaborated to determine the coordinates of the points which define the profile of the machined surface, when using the electrode tool having the active surface obtained by the intersection of the two plane surfaces under an angle of 90 degrees.

3. EXPERIMENTAL RESEARCH

Within the nontraditional technologies laboratory of the Technical University "Gh. Asachi" of Iasi, some experiments were made in order to verify the above mentioned theoretical considerations. We used an electrode tool and a workpiece made of thin plates (the thickness of the workpiece was [t.sub.w]=0.18 mm and the thickness of the electrode tool was [t.sub.et]=0.10 mm).

The electrode tool was made of thin copper sheet. The workpiece was stainless steel sheet; we preferred to use the stainless steel as workpiece material in order to avoid the chemical erosion when the electric field is not present. Initially, the electrodes were isolated by the using of a transparent insulating layer; the transparency permitted us to directly observe and measure the evolution of the machined surface.

As electrolyte, an aqueous solution containing 5% of sodium chloride was used. The verifying of the mathematical model validity was achieved by means of an electrolytic cell; we used a forced circulation of the electrolyte through the work gap and the obtained profile was in a certain measure similar to the profile defined by the relation (8). In figure 2 we can see the work zone before to start the electrochemical dissolution and minutes of developing of the erosion process (b) the same zone after 5 minutes of developing the erosion process.

[FIGURE 3 OMITTED]

The presence and the circulation of the hydrogen bubbles can be observed in figure 2, b, emphasizing yet the direction of the electrolyte circulation (from the right side to the left side). The intensity of the electric current was of about 1.4 A; thus, an electric current density of about 0.27 A/[mm.sup.2] was ensured. By the using of the mathematical relation (8), where we took into consideration A=28, U- [DELTA]U=40 V, k=0.2, F=1608 Axmin [[rho].sub.m]=7.8 g/[cm.sup.3], t=5 min and [x.sub.min]=0.44 mm (corresponding to the initial gap), we obtained the theoretical sizes of the work gap in different points belonging to the axis Ox.

On the other hand, we measured the size of the real gap by means of the graphical representation included in figure 2, b; the sizes obtained thus were used to elaborate the graphical representation from the figure 3. We can see that for small sizes of the distance x, the theoretical sizes of the work gap are bigger than the effective sizes; an explanation of this situation could be based on the less intense circulation of the electrolyte at the electrode tool corner.

4. CONCLUSIONS

Even the electrochemical machining process was discovered long time ago, there are yet problems concerning the machining accuracy and the design of the electrode tool, so that the profile of the machined surface to correspond to the desired profile.

A theoretical model concerning the work gap size evolution for a certain shape of the active part belonging to the electrode tool was proposed.

Some experimental researches destined to verify the theoretical model were made within the laboratory of non-conventional technologies from the Technical University "Gh. Asachi" of Iasi.

The experimental researches proved the validity of principle of the theoretical model.

In the next period, we have the intention to extend the theoretical researches for other types of the electrode tool corner shape.

On the other hand, the experimental researches could be directed to the study of the influence exerted by other different factors (chemical composition of the electrolyte, temperature, viscosity and pH of the electrolyte, chemical composition of the workpiece material etc.) on the work gap size evolution.

5. REFERENCES

Dabrowski, L., Paczkowski, T. (2005). Computer simulation of two dimensional electrolyte flow in electrochemical machining. Russian Journal of Electrochemistry, Vol. 41, No. 1, 102-110, ISSN 1023-1935

De Barr, A.E., Oliver, D.E. (1968). Electrochemical machining. MacDonald, London

Gavrilas, I., Marinescu, N.I. (1991). Non-conventional machining methods in machine building (in Romanian). Editura Tehnica, ISBN 973-31-0226-1, ISBN 973-31-0225-3, Bucuresti

Georgiadou, M. (2003). Modelling current density distribution in electrochemical systems. Electrochimica Acta, Vol. 48, 4089-4095, ISSN 0013-4686

Rajurkar, K.P. et al. (1999). New developments in electrochemical machining. Annals of the CIRP, Vol. 48, No. 2, 567-579, ISSN 3-905-277-22-0

Slatineanu, L., Nagit , Gh., Dodun, O., Coteata, M., Chinesta, F., Goncalves-Coelho, A., Pamies Teixeira J., San Juan, M., Santo, L., Santos, F. (2004). Non-traditional manufacturing processes. Publishing House Tehnica Info, ISBN 975-63-164-9, Kishinew (Republic of Moldova).