Patent GB-1289612-A (GB)
IPC: B23H7/18
Filed: 1970-02-04
Abstract: 1289612 Electro-erosion MITSUBISHI DENKI KK 4 Feb 1970 [4 Feb 1969] 5452/70 Heading B3V [Also in Division G3] The working gap 18 of electric discharge machining apparatus is supplied with voltage pulses of constant width and peak value, each followed by a pause time which is varied in accordance with the state of the gap to provide optimum machining conditions, namely automatic selection of the optimum working current to obtain maximum efficiency. The pulses are provided by a smoothed D.C. supply 24, 26, 32 which is switched on and off by transistors 34a- 34m controlled via amplifier 42 by a pulse generator 40, each of whose output pulses is followed by a pause time determined by a sensor 38 connected across the gap 18. The sensor 38 is arranged to detect normal and abnormal states of the gap, which states are respectively defined by the occurrence of a predetermined number of successive pulses each including a no-load portion (40) and an arc voltage portion (42), Fig. 3 (not shown), and by the occurrence of a predetermined number of successive pulses each including only an arc voltage portion. If the abnormal state is detected the pause time is increased by a predetermined amount to decrease the mean working current, further predetermined increments being added to the pause time as necessary until the normal state of the gap is restored. The occurrence of the normal state causes the pause time to be decreased by a predetermined amount to increase the mean working current, further predetermined decrements being made to the pause time as necessary until the abnormal state of the gap is detected, whereupon the incremental cycle is recommenced. The time period corresponding to the normal state is long compared to that for the abnormal state. The predetermined increments and decrements of pause time may be chosen and varied in any desired manner. Fig. 4 shows details of the members 38, 40, the latter being a conventional astable multivibrator 70 whose pulse width is determined by members 74, 76 and whose pause time is determined by resistor 78 and that one of the capacitors 68a-68m selected by the movable contact of a switch SW1 which is controlled to move in forward or reverse directions by respective solenoids SWF 1  and SWR 1 . These are respectively controlled by a relay R 1  and a clock pulse generator C 1 , the former being energized when the abnormal state is detected and the latter providing a pulse train whose time interval defines the period corresponding to the normal state. The gap voltage is compared with that of a Zener diode 54 to control the relay R 1  through transistors 56, 58, 66 and RC circuit 60, 64. In a modification, Fig. 5 (not shown), further relays, clock pulse generators and selector switches are added to 'the circuit of Fig. 4 to provide an arrangement which immediately removes the abnormal state by a single large increment in pause time and then progressively reduces the pause time by small decrements until the optimum condition has been attained. A typical operation of this nature is described, Fig. 6 (not shown). The machining apparatus of Fig. 1 is provided with a conventional electrically-controlled hydraulic servo system 20, 22 for maintaining the gap 18 substantially constant.