Abstract: After pre-forming of a cavity in a workpiece by means of an electrode of male form, movements of circular translation of increasing relative eccentricity are imparted to the electrode or the workpiece in order to complete machining of the cavity by the spark-erosion process while adjusting the eccentricity in successive steps. The orbital motion is stored in memory during at least one period of travel while the electrode-workpiece potential difference is compared with a reference voltage. A control signal delivered as a result of the comparison initiates the progression of eccentricity by one step when the potential difference has remained continuously higher than the reference voltage during the period of travel.

Abstract: The electrode-holder of an electro-erosion machine is displaced at right angles to the direction of feed and to the same extent in a plurality of uniformly distributed directions by means of a reaming device placed between the machining-head spindle and the electrode-holder. A mechanism placed laterally with respect to the spindle and secured thereto imparts circular translational motion to a member comprising a plate which supports the electrode-holder.