GB937681A - Method and apparatus for the electrolytic removal of metal - Google Patents

Abstract

937,681. Electrolytic removal of metal. ROLLS-ROYCE Ltd. Jan. 31, 1962 [Feb. 7, 1961], No. 4615/61. Class 41. [Also in Group XXXV] A method for the electrolytic removal of metal from a workpiece, e.g. for forming a concave surface, hole or slot, includes directing a flow of electrolyte between the workpiece and a cathode, making the workpiece electrically positive relative to the cathode, deriving a first signal which varies with D.C. flow between the workpiece and cathode, deriving a second signal which varies with the conductivity of the electrolyte, modifying the first signal in accordance with the second signal to compensate for changes in the first signal due to variations in the conductivity of the electrolyte, and maintaining the spacing between the workpiece and cathode constant in accordance with the modified first signal. Fig. 1 shows an anodic rough forged turbine blade workpiece 12, a cathode 15 which can be advanced towards it by an electric motor 22, and a pump 24 for circulating electrolyte which may be an aqueous solution of sodium chloride through pipes 27, 23, shoot 16, anode cathode gap, tank 10 and filter 26. The electrolyte is pumped at a pressure of 100/150 1b./sq. inch, the voltage is constant at 12 volts and the electrolysing current is 100/200 amps. The first signal is supplied from a power supply unit 72 to a servo control unit 73 and the second signal from a Wheatstone bridge circuit including resistances 36, 37, 38 and electrodes 30, 31 in the electrolyte. A modified first signal is then fed from the unit 73 to the motor 22. Fig. 3 shows an anodic workpiece 12a and cathode 15a. The first signal is fed from resistance 77 to a valve 78 and the second signal; derived from conductivity measurements between electrodes 88, 89 is fed from potentiometer 85 to valve 80. 83 and 84 are split field windings of a servomotor and the circuit is such that any unbalance, i.e. if there is any current change not caused by change in conductivity, causes the motor to operate. Fig. 4 shows a D.C. power supply at 106, 107 to a workpiece 100 and cathode 101 circuit, and the first signal is fed via leads 115, 116 and a chopper unit 114 to a modifying unit 118. The second signal, which is pulsating, is fed to the modifying unit from a conductivity measuring unit 120. The resultant signal is fed, to a furtner modifying unit 122 from whence the first modified signal is fed, when required, to a servomotor 103. Circuitry including relays is also included. This effects, at the start of an electrolytic removal process, rapid approach of the cathode towards the workpiece followed by a slow approach until the workpiece cathode gap is at or near the desired constant value. There is then switching for control of the cathode advance by the modified first signal. At the end of the process, a rapid retract of the cathode is effected. Different gears in a gear-box 104 are used during these movements. The controlling of the operation of the various. relays may be by a timing mechanism, microswitches, two of which are shown at 160, or by relays responsive to current flowing between the workpiece and cathode. Relays may be provided to operate so that the cathode will be retracted if there is an undesired drop in voltage between the anode and cathode and will be moved forward again when the full voltage is restored. In a further alternative, the second, signal is derived by electromagnetically inducing an A.C. voltage in a coil, the electromagnetic coupling with the coil depending on the conductivity of the electrolyte.