Electrochemical discharge, or arc, machining (ECAM) is a process combining features of electrochemical (ECM) and electrodischarge (EDM) machining. With the combined process ECM action is assisted by the thermal erosive effect of electrical discharges in the machining gap filled with electrolyte. Machining rates are thereby obtained which can be as much as five and forty times faster than respectively ECM and EDM. The complexity of the process renders difficult any full theoretical treatment. Indeed most work to date has dealt with experimental investigations of this industrially interesting process, particularly for drilling applications. In this paper, some effects of the pulsed voltage and vibrating tool-electrode waveforms are subjected to theoretical analysis. From this treatment, conditions which are favourable to either ECM or sparking can be identified. The effect of the phase-angle between the voltage and vibration waveforms on metal machining rates is also analysed. These results are compared with experimental results, and the influence of the electrical discharges in the electrolyte is shown to become the major factor promoting enhanced rates of metal removal, as both phase-angle and amplitude of vibration increase. Further theoretical and experimental studies deal with the effect of machining voltage and feed-rate, with the sparking element playing an increasing role in raising the metal removal rate, the higher the voltage and the feed-rate used.