Electrical discharge phenomena in fluids have been observed using suspended small steel balls placed between electrodes, in order to investigate the effect of debris not attached to tool electrodes or a workpiece on the generation of discharge in electrodischarge machining (EDM). Discharge occurs at certain voltages only when a ball is inserted between electrodes. Images taken using a high-speed camera show that, with one ball, the first dielectric breakdown takes place between one electrode and the ball, followed by the second breakdown between the other electrode and the ball, before spark discharge begins between the electrodes. With two balls, the breakdowns between an electrode and a ball precede that between the balls. These results suggest that the presence of suspended debris facilitates the generation of discharge in EDM and that the discharge between electrodes occurs after breakdowns take place between an electrode and debris, between a workpiece and debris, and among debris. Electrical fields are calculated by the finite element method, indicating that the first breakdown occurs at the gap with the highest electrical field.
