Study on improvement of unstable discharge at start of electrical discharge machining using external high-voltage superimposition method: Comparison of external and internal superimposition methods and influence of externally superimposed voltage on machining characteristics

Abstract

In this study, attempts were made to resolve the problem of unstable discharge at the start of electrical discharge machining using the external superimposition method. In general, electrical discharge machining (EDM), the frontal gap distance is controlled by detecting the working voltage. Electrical discharge machining experiments were thus conducted to compare conventional power supplies for electrical discharge machining machines, which superimpose high voltage on the detected working voltage (internal superimposition) with a power supply that superimposes high voltage externally so that no superimposed voltage is added to the detected working voltage (external superimposition). Differences in abnormal discharge rate, discharge frequency, frontal gap distance, and machining characteristics between the two superimposition methods were also investigated. The results showed that external superimposition is better than internal superimposition in that it widens the frontal gap distance immediately after the start of electrical discharge machining, which decreases the abnormal discharge rate to more or less 0 and increases the discharge frequency in a very short time, and that machining characteristics are better with the external superimposition method. Consequently, the effects of the superimposed voltage level on the machining characteristics of electrical discharge machining applying external superimposition were also investigated. It was found that (1) the discharge frequency increases with increasing superimposition voltage, thereby improving the machining rate; (2) the frontal gap distance increases and the surface roughness of the machined surface improves with increasing superimposition voltage; and (3) the tool electrode wear ratio is the smallest when superimposition voltage is around 200 V.