PROCESS AND APPARATUS FOR MAINTAINING A GAP IN ELECTROCHEMICAL MACHINING

In a method of manufacturing an electrolytically machined workpiece in which the workpiece 10 and a cathode 13 are mounted for relative movement towards or away from each other with a gap therebetween, an electrolyte is supplied to said gap and the workpiece is maintained at a positive potential with respect to the cathode, a first signal is derived which is representative of the conductivity of the electrolyte and a second signal is derived which is independent of the electrolytic current and is representative of the rate of said relative movement between the workpiece and the cathode, the first and second signals are combined to produce a control signal which controls the relative movement between the workpiece and the cathode so as to maintain the gap there-between at a predetermined size. The first signal is derived by passing A.C. current through a conductivity cell 19 through which the electrolyte flows. The second signal is produced by a servomotor generator 25, driven by servomotor 14 which effects the movement of the cathode 13. By means of controls 29, 30 the signals are arranged to be equal when the gap is of the required value. The two signals are rectified at 23, 26 respectively and fed to the servo-control device 17. The resultant current is converted to a pulsing signal by means of chopper 27, and passed through amplifier 28 to the servo-motor 14. An arrangement of relay 32, cathode follower 34 and associated circuitory is provided which enables the cathode to be advanced or retracted independently of the servo-control device.