Observation of EDM plasma behavior influenced by parasitic working gap capacitance

Abstract

Machining large surfaces to good surface qualities is currently difficult, as optimisation of parameters does not always lead to a satisfactory outcome. This appears to be an issue with traditional EDM, and the cause is not understood very well. Observing this effect by looking at current and voltage is very difficult because of the parasitic capacitance. As the capacitance is a function of surface area and gap width, small gaps, as they occur when using small energies, result in a large capacitance. The charge of the capacitance can have the same order of magnitude as the discharge itself. This makes it hard to understand what actually happens in the gap while measuring current and voltage on the outside. In order to understand this phenomenon large, optically transparent SiC electrodes were used to observe the gap with a high speed camera. By doing so it is possible to locate consecutive discharges and to determine if the capacitance has a large effect on the behaviour of the plasma. The goal of this work is to identify the effect that is responsible for the difficulties with the machining of large high-quality surfaces in order to work out possibilities to enable those machining operations.