in the electrical discharge machining of the surface of a recess in a workpiece electrode to a shape corresponding to that of a tool electrode , the electrodes are relatively displaced , during a finishing phase of machining , along an axis of penetration and simultaneously in a plane perpendicular to the axis of penetration with a cyclic translational movement whose amplitude increases with the penetration of the tool in the workpiece . this provides a virtual 3 - dimensional dilatation of the tool as it advances , and enables control of the sparking in both the frontal and lateral parts of the machining zone , such that the shape of the machined .. . iadd . portion . iaddend . is an image of the tool shape . the cyclic translational movements can be obtained with an eccentric member whose eccentricity relative to a shaft is controlled by limiting axial movement of the shaft at a position corresponding to a predetermined penetration .