Observation of relationship between bubbles and discharge locations in EDM using transparent electrodes

•EDM gap phenomena were observed using transparent SiC single crystal electrodes.•More than 70% of the working surface was occupied by bubbles.•Electrical discharges occur not only in liquid but also in bubble and at boundary.•Probability of discharge through debris located at bubble boundary was high.•Rise of discharge current is slow in liquid and quick in bubble.

This paper describes the observation of EDM gap phenomena through a transparent electrode from the direction normal to the machining surface using a high-speed video camera. To visualize the gap, SiC single crystal was used as the electrode material since it is optically transparent and electrically conductive. It was found that more than 70% of the working surface was occupied by bubbles generated by discharge after a few hundred pulse discharges from the start of machining. Electrical discharge occurs not only in liquid but also in bubble and at the boundary between liquid and bubble. The probability of discharge occurrence per unit area of each atmosphere was highest at the boundary between liquid and bubble. Furthermore, the probability of discharge occurrence through debris particles located at this boundary generated by the last discharge was highest.