Influence of Plasma Extinction on Machining Stability in EDM Process

This paper describes experimental observation and numerical analysis of the arc plasma extinction in the electrical discharge machining (EDM) process. The arc extinguishing time must be shorter than the pulse interval, because it is closely related to the stability of the machining process and hence the machining rate of the process. The spark locations and the waveforms of discharge current and gap voltage were observed in the continuous process in order to distinguish a discharge whose location was influenced by the insufficiently extinguished plasma which was made by the previous discharge. The experimental results show that the plasma extinguishes within 10 micro seconds under the condition that the distribution of discharge location is uniform, so that the machining process is stable. However, in the case that the discharge localization occurs, it takes longer to deionize the arc plasma. The time variation of the plasma temperature after a single pulse discharge was calculated with the finite difference method to estimate the arc extinguishing time. The calculated results show that the arc extinguishing time depends on the temperature of the electrode surface, and so that the local temperature rise causes such a instability as the discharge localization.