Dependence of Crater Formation in Dry EDM on Electrical Breakdown Mechanism

The research effort in dry electrical discharge machining (DEDM) has increased during the last years. The process has advantages in comparison with EDM in oil-based dielectric media, such as low tool electrode wear, thin recast layers on the work piece surface and much more environmentally friendly conditions. However, the plasma-material surface interactions involved in dry EDM are not properly explained yet. Therefore, a characterization of the erosion phenomena in dry EDM under different breakdown mechanisms is presented. Plasmas and craters generated by single electrical discharges are studied. The plasma expansion is recorded and measured by high-speed imaging experiments, while electrical signals, current and voltage, are monitored. The plasma species are identified by emission spectroscopy. The results show that gas discharges triggered by streamers have larger plasma expansions than sparks generated from vacuum breakdown mechanism. Several small craters can be generated from a single gas discharge, spread over of the work piece surface. The plasma formed from vacuum breakdown is restricted to smaller areas, providing larger and less spread craters. This explains the higher material removal rate of the process with vacuum breakdown in very small gaps.