Workpiece debris deposition on tool electrodes and secondary discharge phenomena in micro-EDM

During electrical discharge machining (EDM), ablated workpiece material is rapidly solidified upon ejection into the dielectric and thought not to become reattached to the electrode surfaces. This work furthers the understanding of the little understood discharge gap phenomena by investigating the attachment of machined material back onto the tool electrode surface and explains the mechanism of this attachment. After the machining of high-aspect ratio slots, SEM and EDS techniques along with single discharge and cross-sectional analysis were used to explain that debris reattachment onto the tool electrode does not occur randomly but is dependent on its remelting in the dielectric by the secondary discharge process. The subsequently bonded material is present mainly in the centre of the discharge crater, with no attachment occurring outside of discharge affected regions. The surfaces of electrodes subject to intense secondary sparking are therefore liable to transient surface properties dependent on the composition of the deposited material. It is also observed that the deposited material on the tool electrode can offer a protective effect against wear from further secondary discharges and so potentially enhancing tool life.