The material removal rate and the surface roughness increase with increased power in the gap. In this way, rough and fine machining is distinguished. When rough machining is performed, the material removal rate should be as high as possible, while the achieved surface roughness does not play an important role.
The EDM process stability is determined by the proportion of harmful discharges in the gap between a workpiece and an electrode, i.e. arc and short-circuit discharges, which not only lower the material removal rate, but also increase the electrode wear. The process is more stable in the case of lower proportion of the harmful discharges. The main cause for unstable EDM process is the contamination of the gap with discharge products. But the surface power density in the gap also affects the process stability. To achieve the highest material removal rate, the roughing setup parameters should be tuned to the eroding surface size. The eroding surface is a projection of the engaged surface of the electrode to the plane perpendicular to the machining direction as shown in Figure. This was analyticaly prooved in the In general, the engaged surface is not plane and the eroding surface size changes with the depth of machining. To select the appropriate roughing setup parameters at any machining depth, the eroding surface size has to be determined on-line.
Figure: The eroding surface is a projection of the engaged surface of the electrode to the plane perpendicular to the machining direction.
Voltage and current in the gap define electric power in the gap (P=UI). There exists the optimal set of the setup parameters' values to obtain the certain power in the gap and the discharge voltage is nearly constant at all machining regimes, thus the power in the gap depends only on the current in the gap. In the literature, the boundary surface current density is given rather then boundary surface power density and it is stated that stable EDM process is achieved when the surface current density is less than 0.1 A. The relation between the surface current density and the material removal rate Vw is presented in Figure. At constant eroding surface size A1, the material removal rate increases with increased surface current density until the boundary surface current density is reached. Higher surface current density causes unstable machining process and the material removal rate decreases. When greater eroding surface is employed (A2), the higher current is needed to reach the boundary surface current density, thus the material removal rate is higher compared to the material removal rate at eroding surface A1.
To select the appropriate roughing setup parameters when eroding surface size varies during the machining, the eroding surface size has to be determined on-line.
For on-line detection of the eroding surface size, it is necessary to monitor the appropriate process quantities z. Proper evaluation of the process quantities is the key to gain suitable process attributes x for the determination of the eroding surface size. The process attributes are the inputs into the model for the selection of the optimal rough machining parameters(see figure)
Figure: On-line selection of the roughing setup parameters of the EDM process
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