Patent GB-941272-A (GB)
Title: Improvements in or relating to spark erosion machines
IPC: H03K3/08
Filed: 1961-11-28
Abstract: 941,272. Removing metal by spark erosion. A. MARTIN. Nov. 28, 1961, No. 42545/61. Heading B3V. [Also in Divisions G3, H1 and H3] A spark erosion machine comprises a frame carrying spark machining heads, a casing containing a spark pulse generator and control and safety devices and a cabinet for the purification and distribution of a machining liquid. Machining heads.-In the embodiment described, there are two machining heads mounted on a common support carried on a vertical spindle, enabling each head in turn to be swung into a working position over a co-ordinate motion machining table. The first head, Fig. 3, analogous to a drilling head, comprises a casing 11 mounted about its axis 13 on the machining head support by means of flange 12. A motor 20 drives a gear train 24, 23, 22, 21, thus rotating a screw-threaded shaft 16 in a fixed nut integral with a sleeve 14 non-rotatably slidable in a fixed guide 15. The lower end of the sleeve 14 carries a vibrator unit 26 and an electrode 34 in a holder 33. The vibrator comprises a permanent magnet and a moving coil supplied with sinusoidal current and is carried on two diaphragms 30, 31. The second machining head, Fig. 4, analogous to a milling or grinding head, is mounted on the machining head support by means of a flange 40. The spark gap on this head is handset by means of a screw 42 fixed axially to the housing and a nut 43 fixed to the mounting flange. Rotation of a disc-shaped electrode 45 is effected by a motor 53 and gearing 55, 54, 52, 31, being finally transmitted to the electrode by an electromagnetic clutch 56. Spark impulses are fed to the electrode by a brush 58 and a slip ring 59. The electrode is made easily removable by use of a spring- loaded centre 48, thus facilitating profiling of the electrode on some other machine tool. The electrical equipment is housed in a separate cabinet and includes a pulse generator and control circuits for a cutting motor. The pulse generator comprises a multi-vibrator and three stages of amplification. The motor control circuits (Figs. 9 and 10) comprise a sawtooth pulse generator tapped off at the collector of a uni-junction transistor 111. The pulses are applied through a condenser 114 to an impedance matching transistor 115, are then amplified by a transistor 116 and applied through a condenser 117 to a point 118 in a potentiometer chain 119, 120, 121 connected across a D.C. power source. The resistance 120 is variable as a function of the spark conditions. The potentiometer chain is connected to a Schmidt type trigger circuit 124. The width of the output pulse from circuit 124 is dependent on the setting of resistor 120. The output is applied to the base of a transistor 126, the collector of which controls the current in transistors 127, 128 in diagonally opposed branches of a bridge circuit. It is also applied through a phase inversion transistor 130 to a transistor 131 whose collector voltage controls the current in two further transistors 132, 133 disposed in the other two diagonally opposed legs of the bridge circuit. Thus, dependent on the setting of resistance 120, current is caused to flow through the armature 134, connected across two apices of the bridge, in either direction F or direction F<SP>1</SP>. By suitable matching of the armature inertia and the pulse frequency, it is possible to induce vibrating oscillation in addition to feed, thus obviating the need for a separate vibrator. The machining liquid system is housed in a separate cabinet and comprises a tank, pump and filter. The filter unit comprises a filter of a cellulose base material of the vegetable sponge type in the form of a sleeve. The liquid is filtered by passing from the exterior to the interior of the sleeve. The Spark Generator (Fig. 5) comprises the pulse generator 60 and three amplifying stages 61, 62 and 63, the latter being an electronic switch valve inserted in a power circuit 66 a branch of which comprises a primary winding 71 of a transformer 74 and reactance windings 72, 73. Condensers 77, 78, 79 are each connected between two windings and earth, the terminal 70 being at a high D.C. potential (e.g. 6000 v.). To this circuit is connected a discharge circuit comprising a primary winding 80 and a diode 82. In operation, a pulse transmitted to the grid of the valve 63 causes it to become conductive, thus building up current flow in primary winding 71 with a consequent power pulse being produced in the secondary winding 83. The build up of current is augmented by the discharge of capacitor 58 via diode 65. When the pulse ceases and the valve 63 ceases to conduct, the excess potential in the reactance windings 71-73 establishes the conductivity of the diode 82 and is thus discharged through winding 80 which results, through a secondary winding 86, in a second power impulse to the output terminal.