Patent GB-1311511-A (GB)
Title: Electroerosion machining circuits
IPC: B23H1/02
Filed: 1970-04-01
Abstract: 1311511 Electro-erosion EX-CELL-O CORP 1 April 1970 [1 April 1969] 15488/70 Heading B3V [Also in Division H3] An electro-erosion circuit includes machining power source 12, a silicon controlled rectifier switch 24, Fig. 1, connected in series with an inductor 22 and the erosion gap 18, a capacitor 48 and a device for switching the rectifier 24 so that when it is turned off, magnetic energy stored in the inductor 22 is converted to electrical potential energy stored in the capacitor 48. When a signal is applied from a multivibrator (64) (Fig. 3, not shown) to the control electrode 46 of a second silicon controlled rectifier 40, the voltage at point 54 is made negative and the first rectifier 24 is switched off. After the rectifier 24 has been switched off, the machining pulse is continued for a short period since the voltage at point 58 is clamped to the negative side of the power source 12 by a diode 34 so that current flows through an inductor 22, capacitor 48, rectifier 40 and diode 34. The voltage at point 54 thereby rises sinusoidally (Fig. 2B, not shown) and when the voltages at points 54 and 58 are equal the machining pulse ceases. Subsequently, a pulse is applied to the control electrode 30 of the rectifier 24 from the multivibrator (64) via a diode, Fig. 4, condenser 72 and Schmitt trigger (74) to switch on the rectifier 24 which causes the gap 18 to be ionized and a machining pulse to be initiated. Preventing short circuits, Fig. 4.-The gap voltage is monitored at the end of a machining pulse and if it is below a predetermined minimum the rectifier 24 is maintained cut off for a predetermined number of pulses from the multivibrator (64). A trial pulse is then applied to the rectifier 24 and if the short persists, the rectifier 24 is subsequently maintained cut off for a greater number of pulses, the process being repeated until the time interval between trial pulses reaches a preselected maximum. During normal operation, when rectifier 24 is cut off, a condenser 98 is charged and a condenser 102 step charged through capacitor 104 to maximum voltage by the cyclic voltage variations at point 54. The negative voltage at point 54 is also transferred via diode 134 to point 138 which allows the condenser 72 to be recharged and the voltage at point 84 to fall. The next pulse from the multivibrator (64) can then operate the Schmitt trigger (74). A Zener diode 112 is set at 20 V so that when the gap voltage is below 20 V, e.g. 15 V current flows from point 52 and through a diode 120 to switch on a transistor 124. The voltage at point 126 is clamped by the diode 120 to the voltage at 52 so that the voltage at 126 rises substantially from its normal value of 20 V, e.g. to 45 V. The voltage at 142 is thereby raised which prevents a decrease in voltage at 84 so that the next pulse from the multivibrator (64) can not fire the Schmitt trigger (74). With the cessation of a pulsating voltage at 54, the voltages at 108, 110 decay. A positive voltage at 110 is produced so that current flows through a diode 146 to switch on a transistor 148. Current thereby flows from 84 through the transistor 148 to allow one trial pulse. If the short persists the condenser 102 is not fully charged, the voltage at 108 gradually falls and a progressively longer time interval is required to discharge the condenser 98.