Patent GB-780139-A (GB)
Title: Improvements in or relating to machines for the electric machining of conductive materials
IPC: B23H7/18
Filed: 1955-08-05
Abstract: 780,139. Automatic control systems. ATELIERS DES CHARMILLES SOC. ANON. Aug. 5, 1955 [Sept. 10, 1954], No. 22564/55. Class 38 (4). [Also in Group XXIX] In electric spark erosion apparatus a condenser which discharges to produce the sparks is charged from a D.C. source and the average value of the voltage across the spark gap controls, electromagnetically, a hydraulic servomotor adjusting electrode position to maintain a predetermined size of gap. In Fig. 1 a condenser C, connected between an electrode E and the work P is periodically discharged to produce a succession of sparks through kerosene in a vat 20 in which the work is immersed. Between successive sparks condenser C is re-charged from D.C. mains A, B through a resistance R and an inductance L. The circuit constants are chosen as disclosed in Specification 780,138 so that the interval between successive sparks is not greater than one quarter of the natural period of oscillation of the circuit. The voltage between the electrode and the work is compared with an adjustable reference voltage obtained from a potentiometer 5 and any difference applied to a winding 3 of an electromagnet controlling the operating member 1 of an hydraulic servo-mechanism, shown in Fig. 3, controlling the position of the electrode E to maintain constant spacing between the electrode and the work. The current through a biasing winding 4 of the electromagnet is maintained constant by a hydrogen and iron resistance 7. Periodical withdrawal of the electrode during operation may be obtained by closing a switch 58 so that a relay 59 is operated repeatedly to periodically increase the current in winding 4. In Fig. 3, member 1, centred by springs 9, controls the opening of a valve 11 in series with a restriction 33 between highpressure and exhaust oil conduits 14 and 15. The pressure in a conduit 34 between valve 11 and restriction 33, which is normally half that in the high-pressure conduit, is applied to the upper face of a piston 18 having an area twice that of its lower face which communicates with high pressure conduit 14. Piston 18 is coupled to electrode E and adjustment of valve 11 in either sense in consequence of deviation in the distance between the electrode and the work thus causes movement of piston 18 to correct the electrode position. A restricted orifice in a spring-centred piston 36 located in an enlargement in the conduit leading to the upper side of piston 18 limits the rate of movement of the electrode when the displacement of valve 11 is sufficient to move piston 36 to an end of its travel. The hydraulic unit is contained in a frame 22 mounted on an upright support 41 so that it may be raised or lowered by operating a valve 44 to couple a cylinder 42,to the high pressure or exhaust conduit respectively. A valve 16 venting to exhaust maintains a constant high pressure in conduit 14. A manual control unit with indicator lamps for starting the apparatus is described and includes means for varying C and R and indicating unstable operation by means of a voltmeter. Electrical connections to the electrode are designed to reduce radio interference.