GB800449A - Improvements in spark discharge machining apparatus - Google Patents

Abstract

800,449. Cutting by spark discharge. CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE. Jan. 7, 1955 [Jan. 12, 1954], No. 601/55. Class 83 (4). In apparatus for machining a workpiece 5 by sparks discharged between it and a tool electrode 4, the discharge current is supplied from an alternating current source 1 through a rectifying system including rectifier gas tubes, such as thyratron tubes 3a, 3b, 3c whose grids are subjected to a negative bias to thereby interrupt the discharge current when the latter exceeds a predetermined value. The discharge circuit includes a condenser 6, a self-inductance coil 7 and a resistance 8. An electromagnet relay having its coil 9 in series with an adjustable resistance 10 is connected in shunt with the resistance 8 and the grids of the thyratron tubes are each connected through a resistance 11a, 11b and 11c and a common resistance 12, which has a condenser 13 in shunt therewith, to the common point M of the cathodes. When the discharge current reaches the value that causes arcing between the electrode and workpiece, or a suitable fraction such as 0.8 of that value, the armature 9a of the coil 9 is pulled inwards and a battery 14 causes a negative bias on the tube grids which interrupts the flow of discharge current. When the current falls the armature 9a is released and normal sparking is resumed, the condenser 13 serving to prevent vibration of the armature. In a modification, Fig. 2 (not shown), the relay 9, 9a is replaced by a thyratron tube connected to an alternating network and acting both as a rectifier and a relay, the tube grid being biased by the voltage drop across resistance 8 through further resistances and a potentiometer so that it is conductive and causes a negative bias on the grids of tubes 3a, 3b and 3c when the predetermined discharge current is exceeded. In a further modification, Fig. 3 (not shown), the relay coil 9 is connected so that it is subjected to the spark gap voltage opposed by a voltage from a separate battery acting across a resistance adjusted so that the armature 9a is operated when the mean voltage across the spark gap falls below a critical value representing the change from sparking to arcing.