Abstract
Samples of copper, iron and graphite, commonly used as tool and workpiece electrodes in the electro-discharge machining process are subjected to single electrical discharges by breakdown of the paraffin liquid in which they are immersed. The discharge currents and durations used are typical of the repetitive discharges used in EDM. The electrode morphologies, resulting from the erosive action of the discharges, are examined using both optical and electron microscopy. The results of these examinations are related to single discharge erosion data derived from performance figures of commercial EDM equipment. It is concluded that the anode is heated by a plasma discharge which expands with time. During the discharge cathode material is evaporated by cathode spots which differ in character for thermionic and nonthermionic materials. Molten anode and cathode material is evacuated after the discharge by a fall in pressure within a bubble of vapour which surrounds the electrodes during the discharge. Graphite electrodes are eroded by sublimation during the discharge. A novel proposal for the relation between micro-cratering of iron cathodes and the plasma diameter is proposed and tested.