The dependence of interspace discharge transitivity upon the gap debris in precision electrodischarge machining

Abstract

The present study confirms the indispensable stabilizing role of gap debris in precision electrodischarge machining (EDM). It is well known that failure to evacuate surfeited debris in a spark gap results in arcing that damages the tool electrode as well as the work. A machining process in pure kerosene is very unstable due to arcing. The latter cause for arcing is frequently observed, but is poorly understood. It is a well-known fact that the presence of minute particles in insulating liquids drastically lowers breakdown strength. A stable process without arcing depends on discharge transitivity rather than on the ease of breakdown. As a result, an easy breakdown process with the help of the foreign particles does not show the entire stabilizing contribution of debris. Our experimental and analytical investigations on general precision EDM reveal that discharge transitivity in gap space relies on the presence of a sedimentary debris layer on the work's surface. Actually, the inherent transitivity due to surface irregularity will disappear because of the very low surface roughness (R_a < 2 μm). Spark movement should be attributed to effects from the mechanical impact of discharge pulses, the distribution of breakdown strength, and the distribution of electrical field. We can derive a process model regarding the tendency of spark movement to expound the functions of debris content and gap size. Pertinent new understanding will eventually give an explanation of the complicated role of gap debris. A process model will lead to a more relevant conception of gap size and its control than most currently accepted understanding.