GB843546A - Improvements relating to spark machining apparatus - Google Patents

Abstract

843,546. Cutting by spark erosion. G.K.N. GROUP SERVICES Ltd. Aug. 16, 1957 [Aug. 16, 1956], No. 25026/56. Class 83 (4). In a spark-machining apparatus wherein the spark gap between the electrode t and workpiece w is bathed in a cooling fluid, preferably a dielectric fluid, and the sparks are produced by the discharge of a condenser C<SP>1</SP>, the charging of the condenser is modified by imposition of an oscillating voltage generated in an oscillatory circuit L1, C<SP>2</SP> by shock excitation from the preceding spark whereby the voltage across the spark-gap after being initially carried below an arc extinguishing value by the condenser discharge, remains below or is carried below the arc extinguishing voltage for sufficient time to reduce the possibility of the arc persisting. Rectified but unsmoothed alternating current from a transformer T 1 and bridge rectifier w1- w4 is supplied through an inductance L3 and resistor R1. The length of the spark gap is controlled automatically by a motor m connected to the electrode t as described in Specification 843,548 and coupled in a control circuit as described in Specification 843,547; alternatively, the motor m may have a permanentmagnet field system. Following discharge an oscillating voltage produced by the circuit L1, C2 is superimposed on the normal charging voltage of the condenser to create a voltage wave 15, Fig. 3 (not shown), whose negative peaks fall below the arc extinguishing voltage during the deionization time of the spark-gap. In another embodiment, Fig. 2, oscillations are produced by a tunable condenser C<SP>3</SP> in " parallel " with a coil L2 wound on a ring M of magnetic material magnetically coupled in the discharge circuit. Both arrangements may be combined in a single circuit. A current limiting valve circuit may be used instead of resistor R1 and inductance L3. Specific examples are given.