Black layer characterisation and electrode wear ratio in electrical discharge machining (EDM)

This article gives a new proposal about the electrode wear ratio (EWR) in the die-sinking electrical discharge machining (EDM) process. During the machining of a work piece (BS 4695 D2), a black layer is produced on the tool surface (W/Cu). The black colour is due to the migrated carbon from the dielectric during the electrical discharge. This article shows that a bi-dimensional laminate of carbon crystals with random phases forms the black layer, which is strongly attached to the tool. Moreover, it demonstrates that there is not only carbon in the black layer composition but also other elements such as iron, chromium, vanadium and molybdenum. The most of these elements forms the equivalent carbon, which is the main cause of wear decrease in the tool according to the set of EDM input parameters used. An association between the equivalent carbon value of the black layer and EWR was done, proving that the equivalent carbon has a direct effect on tool wear. Thus, the equivalent carbon increases on the tool surface from the beginning until the end of the work piece machining leading to the decrease of the EWR. This study confirms that there is a EWR decrease by selecting in the beginning of machining a predetermined set of EDM input parameters.