After 60 years of EDM the discharge process remains still disputed

The ignition of electrical discharges in a dirty, liquid filled gap, when applying EDM, is mostly interpreted as ion action identical as found by physical research of discharges in air (Lichtenberg figures) or in vacuum (radio tubes) as well as with investigations on the breakthrough strength of insulating hydrocarbon liquids. The state of the servo-controlled gap in real ED-machining, however, differs very much from such condition. The author stipulates ignition of electrical discharges by the evaporation of particle bridges in the gap through excessive current. The follow-up spot of a discharge is conditioned by the remaining particles, removed from the electrodes, as well as gas bubbles from earlier discharges. The material removal reaction is grouped in an evaporation phase at start of ignition and later in the ejection of fused material by instantaneous boiling at the discharge spots. The gap width derives from the gap contamination average, depending from process settings.