GB774091A - Improvements in machining methods and apparatus making use of intermittent electric discharges - Google Patents

Abstract

774,091. Cutting by spark discharge. CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE. May 4, 1954 [May 7, 1953], No. 13013/54. Classes 83 (2) and 83 (4). In a method of cutting, piercing or otherwise shaping a workpiece 9 by electric spark discharge between the workpiece and an electrode 8 which are fed with rapid pulsations of unidirectional current through a circuit including an electric energy storing system 4, the potential between the electrode and the workpiece reaches its maximum when the potential at the terminals X, Y, of the unidirectional current source is at its minimum. Alternating current supplied by a generator 1 is fed through a transformer 2, full-wave rectifier 3, which may be an electronic valve or a dry rectifier, and a self-inductance coil 5 to one side of the storing system 4, the other side being fed through a variable resistance 7. A rectifier 6 is used to prevent oscillation of potential across the electrode and workpiece and a thermal ammeter 10 measures the current in the discharge circuit. The storing system may be a condenser, but is preferably a delay network of the kind described in Specification 774,090. Three-phase current with a three-phase rectifier may alternatively be used. The workpiece and electrode are preferably immersed in a liquid such as transformer oil or water and the electrode is adjusted by a screw 16, Fig. 3 (not shown). In an embodiment for cutting a threaded hole in a workpiece 20 the latter is clamped against a plate 17 by jaws 21, 22 and a screw 27 which forms the electrode is mounted in a threaded support 28 and is rotated during the operation so as to gradually move through the workpiece. The electrode may be fed automatically by means responsive to the mean value of the potential difference between the electrode and workpiece. Reference is also made to the cutting of toothed gears. The workpiece may be of tempered steel, titanium, metallic carbides of tungsten, molybdenum cobalt or ferrite materials. The electrode may be of copper or copper alloy.