An evaluation of spark mobility in electrical discharge machining

Process instability and arcing failure in electrical discharge machining (EDM) are still serious problems in practice especially when high erosion rates are attempted. Most known solutions exhibit significant limit on material removal rate and do not always effectively eliminate the causes. This analysis addresses some basic mechanisms of instability and arcing which have been constantly overlooked. Spark mobility, an essential feature of any normal spark process, is proposed to characterize the inherent process resistance against spark concentration. The physical meaning of spark mobility is defined as the driving force that moves spark around in the electrode gap. It has been known, but taken for granted that the pulse sparks will keep moving in the electrode gap. The present investigation will that manifest spark mobility is highly process dependent, and not always available. From this point of view, the spark mobility is evaluated with the help of two proposed physical quantities: electrical field stress and dielectric breakdown strength. In this analysis, the difference in field stress is assessed by the stress factor, while the difference in breakdown strength is assessed by strength factor. The distributions of field stresses in a surface crater and around a surface hill are given by the solutions of a Laplace's equation in cylindrical coordinates. Both stress factor and strength factor are useful technical indexes with certain physical meaning. The differences in electrical field stress and dielectric breakdown strength are correlated with the tendency of spark movement, that is the spark mobility from a discharged spot to a new spot. Accordingly, the spark mobility is equal to the product of the stress factor and the strength factor. This systematic mechanism analysis will give exact answers to many "fuzzy" phenomena in EDM practice and resolve the complex contributions to process stability of some important parameters such as gap size and debris concentration.