Improvements in or relating to apparatus and method for machining a workpiece by spark erosion

807,897. Machining by spark discharge. FIRTH STERLING Inc. Sept. 12, 1955, No. 26031/55. Class 83 (4). In a method of and apparatus for machining a workpiece 5, particularly of tungsten carbide or hard alloy steel, by spark discharges, the discharge circuit is of low inductance. The apparatus is mounted on a pedestal 21-23 and includes means for maintaining a predetermined spark-gap between the workpiece 5 and a brass electrode 6. The latter are supported in a tank 29 of copper or steel lined with copper containing a dielectric 8 such as kerosene. Insulated posts 35 support a copper plate 34 on to which the workpiece is held by conductive clamps 36. An oil-filled metal-cased condenser 3, charged with direct current from a full-wave rectifier 11 and filter 2, Fig. 1 (not shown), is mounted below the plate 34 and its positive terminal is connected therewith by a strap 39 whilst the negative casing of the condenser is in contact with the tank which is connected with the electrode 6 by one or more flexible straps 40. The electrode may be rotated especially where used for cutting threads. The concentric arrangement of the electrode workpiece and condenser 3 within the tank, which forms the negative conductor of the discharge circuit, produces a low inductance thereof and thus increases the peak discharge current and the frequency of sparking. In a modification, Fig. 4 (not shown), the tank has a lid through which the electrode passes in sliding contact. In another modification, Fig. 5 (not shown), the condenser is disposed outside the tank and is connected thereto by a coaxial conductor. In a further embodiment, Fig. 6, a closed loop 65 is disposed adjacent the conductors 66, 67 and the electrode 6 for minimizing the discharge circuit inductance. The loop 65 is preferably insulated from the discharge conductors but may be electrically coupled thereto on one side only of the condenser 3, Figs. 7, 8 (not shown). The tank arrangement of Fig. 4 may be combined with a closed loop 70, Fig. 9 (not shown), or with a flat conducting sheet 74, Fig. 10 (not shown), serving the same function as the loop. A number of condensers 3 connected in series or parallel may be used instead of a single condenser.