Patent GB-1091571-A (GB)
Title: Improvements relating to spark machining apparatus
Inventor: CARVER BRYAN GEORGE
IPC: B23H1/02
Filed: 1963-05-24
Abstract: 1,091,571. Spark erosion. G.K.N. GROUP SERVICES Ltd. May 25, 1964 [May 24, 1963], No. 20860/63. Heading B3V. [Also in Divisions G3 and H3] In a spark erosion apparatus having a tool head Th carrying the electrode and a worktable Wt disposed in a tank of liquid dielectric such as paraffin, the spark pulses are suppressed in the event of a short circuit or similar condition in the spark gap. The spark pulses are supplied by a rotary A.C. generator through a rectifying circuit including an electronic switch V 1  which is triggered at a frequency less than that of the generator. The latter is preferably of the homopolar type connected across t 1 , t 2  and also across the primary t 3 , t 4 , of transformer T 1  for supplying the triggering circuit. Valve V 1  is an ignitron having its trigger electrode Te connected to a thyratron V 7 . The secondary of transformer T 1  is connected to a phase shift network R 3 , R 4 , R 5 , C 1  which produces a phase lag of about 30 degrees and its output passes through a square wave forming circuit V 2  and amplifier V 3  to a multi-vibrator circuit V 4 , V 5 . The output signal, whose period is variable by resistance R 9 , is applied to valve V 6  having a cathode follower output feeding the grid of the thyratron V 7 . Bias is applied by a bias circuit fed from transformer T 3  and the anode of the thyratron is supplied through rectifier W 1  from the secondary of transformer T 2 . A D.C. supply of 200 volts is connected to t 5 . The tool electrode may be positive or negative with respect to the workpiece. Work-table Wt may be mounted on a compound slide for mutually perpendicular adjustment. A pump circulates the paraffin dielectric to the spark gap and a filter is provided for removing sediment and eroded particles. In another embodiment, Fig. 2, the electronic switch comprises parallel connected silicon controlled rectifiers W 3 , W 4  supplied through variable resistance banks R13, R14 from A.C. input t 9 , t 10  supplied with 210 volts at 800 e.p.s. from the rotary generator. Rectifiers W 6 -W 9  provide a high resistance leak path to protect against high voltage. The control electrodes T e 3 and T e 4 of the silicon rectifiers are fed with trigger signals from a transistorized trigger circuit supplied with an A.C. input of 240 volts 50 c.p.s. across primary of transformer T 5 . The secondary is connected through bridge rectifier W 10 -W 13  to a phase shift network C 8 , R 16  whose output is applied to a Schmidt trigger circuit TR 1 , TR 2  to give square wave pulses. These are fed through a monostable multivibrator circuit TR 3 , TR 4  whose output appearing across resistor 25 is connected through resistors R 26 , R 27  with the control electrodes T e 3, T e 4. A Zener diode W 14  limits the positive voltage on the electrodes. The electrode is fed automatically as described in Specification 843,548 by a servomotor (M), Fig. 5 (not shown), controlled by the difference between the voltage (V 1 ) across the gap and a reference voltage (V 2 ). This difference is fed to the servomotor rotor (M) and also to the terminals, t 13 , t 14  of a protective circuit, Fig. 2. If a short circuit occurs across the gap, a relay A/1 is energized to open contacts A1 and stop the spark pulses. A second relay B/1 with contacts B1 causes the pulses to be re-applied a t  intervals of about one second thereby to check for removal of the short circuit whereupon normal spark pulses begin. This protective circuit is also incorporated in the circuit of Fig. 1.