Patent GB-872921-A (GB)
Title: Improvements in or relating to spark machining
IPC: H01J37/34
Filed: 1959-06-08
Abstract: 872,921. Removing metal by spark erosion. FIRTH STERLING Inc. June 8, 1959 [June 9, 1958], No. 19486/59. Class 83 (4). In the removal of metal by spark erosion means are provided for detecting ionization of the spark gap before the metal removing spark is released thereto. The metal-removing spark is of high voltage and is not released if a shortcircuit occurs in the gap or if the gap is too wide. In one embodiment, Fig. 2, an alternating supply 10 of 400 c.p.s. acts through transformer 11 and inductor 14 to charge a working condenser 15 to 1200 volts. The discharge circuit to the gap G between workpiece W and electrode T includes a thyratron or ignitron 16 acting as a switch, and a thyratron, mercury-pool, mercury vapour or other gaseous discharge tube 18 acting as a rectifier, whose grid is biased by a D.C. source 19 and resistor 20. A pilot pulse passes through current-limiting resistor 17 and rectifier 18 and applies a potential of about 90 volts to the workpiece W which charges gap condenser 22. If the gap G is within working limits a spark is produced which ionizes the gap causing rapid discharge of gap condenser 22 through resistor 23, the voltage drop across which produces a triggering pulse, Fig. 4C (not shown), which is passed through amplifier 24 and transformer 25 to the grid of tube 16 which closes to permit discharge of condenser 15 through rectifier 18 and the gap G. If a shortcircuit occurs condenser 22 is not charged or if the gap is too large condenser 22 discharges slowly through shunt resistor 21; in both cases no triggering pulse is produced for closing tube 16. Condenser 15 may be replaced by an inductance-capacitance pulse forming network. The gap voltage, averaged over several pulses, is compared with a reference voltage to produce a signal which is passed through amplifier 28 to a gap-adjusting motor 29. In a modification, Fig. 6 (not shown), the rectifier 18, preferably a triode, is in series with resistor 17 in the by-pass of tube 16 whereby the high current spark does not pass through the rectifier. In another embodiment, Fig. 8, a direct current main power supply 60 charges condenser 61 through rectifier 63 and saturable-core magnetic switch 62 which prevents direct connection of the supply to the gap G. The discharge circuit is controlled by a mercury pool ignition 64 whose control electrode 64a is fed from a control thyratron 68 supplied with control current from a condenser 66 charged through an impedance or a magnetic switch 67 from a source 65. Pilot pulses of rectangular waveform produced by source 81 pass to the control grid or grids of one or more beam tetrodes 77 of a pulse generator circuit 76 whose output is fed to the gap G through rectifier 74 and resistor 73. A rectifier 75 is used to prevent oscillation in the spark gap. Ionization of the gap produces a signal across resistor 72 which is fed through amplifier 82 and transformer 83 to grid of tube 68 which thus conducts and closes ignition 64. In a modification, Fig. 7 (not shown), the thyratron 68 is omitted and the tetrode 77 is replaced by a thyratron, mercury-pool or mercury vapour tube. A pulse generating circuit including gating amplifiers to prevent parasitic triggering is described, Figs. 12, 13 (not shown).