GB1076171A - Method of and apparatus for electrically machining conductive workpieces - Google Patents

Abstract

A metal workpiece 19 is machined to roughly the required shape by electrolytic erosion and subsequently is finished by spark erosion. The workpiece is clamped within a vessel 25 containing the working liquid 53 and which is adjustible in mutually perpendicular directions by lead screws 28, 31. The hollow electrode 10 has a leading portion 12 used for electrolytic machining separated by an insulating sheath 15 of melamine resin from a second portion 11 used for spark erosion finishing. Portion 12 may be of copper, brass or a silvertungsten alloy; portion 11 comprises a metal plus an electronegative substance such as: copper oxide, tin oxide, iron oxide, manganese dioxide, aluminium oxide, iron carbide, copper sulphide, iron sulphide, molybdenum carbide, chromium oxide, tungsten oxide. Preferably the portion 12 is of copper having uniformly dispersed therein 3.2% of copper oxide. In other embodiments the leading end of the electrode may comprise a separate collar, Fig. 7 (not shown) or the electrode may have a hemispherical leading end, Fig. 8 (not shown). During electrolytic machining the electrode 10 is fed downwardly through the workpiece to form a cavity 17, by a servomechanism 56, controlled through feedback amplifier 61 in dependence upon the electrical conditions in the machining gap. A change-over switch 60 selects the control conditions for electrolytic machining or for spark discharge. This horizontal movement may be of square or circular form, Figs. 3,4 (not shown). Working current is supplied from a three-phase source 62 through a three-phase saturable-core reactor 63 having a blasing winding 64 fed by a battery 67 through rheostat 68, and a feedback control winding 65. The output of reactor 63 feeds the primary winding of transformer 69. Low voltage secondary windings 70 supply through rectifiers 75 direct current to the electrode 10 and <PICT:1076171/C6-C7/1> workpiece 19 during electrolytic machining. Impedances 76, 77, 78 may be switched in selectively and periodically for reversing the current. High frequency impulses from source 85 and transformer 83 may also be fed to the machining gap upon closing switch 82. During electrolytic machining sodium chloride in water is supplied from reservoir 42 by pump 48 through conduits 47, 50 to the hollow bore 14 of the electrode. A gas such as nitrogen may also be admitted through pipe 49. During both the electrolytic and spark-erosion processes, an emulsion of oil and water in different proportions may be circulated to the electrode by a pair of pumps 46, 45 drawing from the reservoirs 41, 42. The proportion of the emulsion is determined by the output of the pumps. An emulsifying agent, such as nonylphenyl ether of polyethylene glycol, is added. The discharge tube 32 from the vessel 25 has conductivity sensing electrodes 34 which control a two-way valve 38 for discharging dielectric liquid through filter 43 into reservoir 41, and electrolyte through filter 43 into reservoir 42. In another embodiment, Fig. 9, changes in conductivity in the outlet pipe 156 are detected by an electromagnetic coil 157 which controls the two-way valve 159. An electromagnetic sensing coil 140 is also provided in the feed pipe 128 leading to the electrode. This coil 140 serves to maintain the correct proportion of electrolyte from reservoir 136 and dielectric liquid from reservoir 137 by controlling the variable-speed motors 132, 133 of respective feed pumps 130, 131. The control is effected by a spring-biased potentiometer 154 actuated by a solenoid 150 fed through a saturable reactor 147 whose bias is set by potentiometer 149. In another embodiment, Fig. 10 (not shown) both the electrolyte and dielectric liquid are fed into a single reservoir. An electrol control system is described for this embodiment. Examples are given for the machining of high carbon steel and a multi-layer plate of tungsten carbide on high carbon steel. Reference has been directed by the Comptroller to Specification 1, 032, 799.