Retraction control for 3D-path controlled generic electrode electroerosion

An apparatus for controlling a numerically 3D-path controlled generic-electrode electroerosion process wherein an axial electrode tool having an end portion of a generic electrode form is positioned to face an electrically conductive workpiece and arranged to establish an electroerosive machining relationship therewith across a machining gap defined between said end portion and the workpiece, and the electrode tool and the workpiece are relatively displaced generally translationally to advance said end portion effectively and relative to the workpiece sequentially along a succession of programmed two dimensional feed paths in respective sequential parallel planes which are in mutually parallel relationship in one of the group of a third dimensional direction and the direction of an axis of the electrode tool, while maintaining the electroerosive machining relationship between the advancing electrode end portion and the workpiece to progressively develop in the latter into a three-dimensional machined form corresponding to said successive two-dimensional feed paths as a whole, said apparatus comprising:

• means (a) for monitoring the machining gap between said workpiece and said end portion of the electrode tool as it advances along a said programmed two-dimensional feed path in the corresponding plane to detect a defective condition in said gap and producing a control signal in response to the detected condition;
• and
• means (b) responsive to said control signal for retracting, from a position of said defective condition, said electrode tool and said workpiece relative to each other by a prescribed distance in a prescribed direction which is inclined to the axis of the electrode tool and substantially normal to a plane tangent to the developing three-dimensional form at said position and oblique to a respective three-dimensional path in progress and for returning the retracted electrode tool relative to the workpiece to restore said end portion to said position.

A method of controlling a three dimensional numerically controlled electroerosion process, comprising the steps of:

• using an elongate electrode to electroerosively shape an electrically conductive workpiece according to a predetermined program;
• controlling the electrode to move progressively in discrete steps axially along a Z-axis lying coaxial with the longitudinal axis of the electrode, and after each step to cause the end portion of the electrode to move stepwise in an X-Y plane extending at the right angles to the Z-axis to follow the desired contour for the workpiece in that plane;
• storing the X-Y coordinates in the said X-Y plane defining the last section of path in the said plane just traversed by the end portion of the electrode;
• detecting a fault condition in the gap between the electrode and the workpiece and in response to the said detection;
• retracting said electrode simultaneously along the Z-axis in a direction opposite to that in which the electrode is being stepped and in the X-Y plane in a direction generally at right angles to said last section of path and away from the workpiece.