Patent GB-1388301-A (GB)
Title: Process and an installation for machining by electro-erosion
IPC: B23H7/20
Filed: 1972-03-21
Abstract: 1388301 Automatic control of electro-discharge machining ATELIERS DES CHARMILLES SA 21 March 1972 [25 March 1971] 13038/72 Heading G3R In an electro-erosion process, voltage pulses are supplied to a machining gap filled with fluid. The voltage and/or current across the gap are measured, together with the rate of decrease of the voltage, and at least one of the following parameters is automatically controlled: (a) a pulse characteristic, (b) the composition of the machining fluid, (c) the size of the gap. In the arrangement shown, a pulse generator 5 supplies controlled pulses to an electrode 3 spaced from a workpiece 2. The electrode is movable by a servomechanism 4, and the machining gap is filled with fluid, in the form of a dielectric liquid, supplied through a valve 12 from a reservoir 10. The mechanism 4 and valve 12 are both controlled by a circuit 13 which receives signals representing the machining voltage and current on lines 17, 17a and 18, 18a respectively. The circuit 13 also controls the operation of the pulse generator 5 over lines 14a-14f and may energize fault indicator lamps 31-34. The generator 5 includes monostable multivibrators 5a, 5b connected to a switching device 36 which controls a series transistor T 1  in a relay circuit 7. The current may be increased by turning on a parallel transistor T 2  through an AND gate ET1 controlled by the circuit 13. A third transistor T 3  ensures the establishment of discharge current by applying high voltage from a source 6b, under the control of an AND gate ET2. The beginning of the discharge can be detected by a device 5c connected to the multivibrators, and a line 19a receives a pulse when transistor T 1  turns on. The control circuit 13 includes a number of monitoring and comparator circuits, logic circuitry, and output control signal circuits. The monitoring circuits, Figs. 4-9 (not shown) receive current, voltage and timing signals, and use comparator circuits, transistors, field effect transistors and Zener diodes to produce signals indicative of various operating conditions of the machining process. These signals are then combined and distributed to appropriate control circuits connected to the pulse generator, electrode position servo, machining fluid valve, and warning lamps. The functions of the six monitoring circuits are as follows: Circuit IV, Fig. 4 (not shown) detects the presence of abrupt voltage variations during the current pulses. An insufficient rate of such variations causes illumination of lamp 31. Circuit V, Fig. 5 (not shown) responds to voltage variations from one discharge pulse to another, indicating that sparking is occurring at different points on the workpiece. Circuit VI, Fig. 6 (not shown) monitors the pollution level of the machining fluid. High pollution allows conduction through the fluid and prevents sparking. Warning lamp 32 may be energized and a control signal is fed on line 16 to the fluid supply valve 12, Fig. 12 (not shown). Circuit VII, Fig. 7 (not shown) detects the occurrence of a short circuit, illuminating lamp 33, and circuit VIII, Fig. 8 (not shown) monitors the electrode potential between the start of a voltage pulse and the beginning of current flow. Circuit IX, Fig. 9 (not shown) responds to non-operation of the transistor relay circuit 7 by illuminating lamp 34 and producing a signal on line 14f to energize a relay 35 which switches off the power supply. The outputs from monitoring circuits V, VII and VIII are combined in a control circuit, Fig. 10 (not shown) which produces the signal on line 15 to operate the electrode position servo 4. The characteristics of the pulses produced by generator 5 are controlled by signals on lines 14c, 14d, 14e. The pulse duration is determined by the signal on line 14c, connected to the multivibrator 5a and derived from monitoring circuits IV and V through an output circuit, Fig. 15 (not shown). The duration of the discharges is reduced if the rate of arcing is too high. The intervals between successive pulses are determined by the multi-vibrator 5b under the control of the signal on line 14d in such a way that the intervals are increased if a high rate of unsatisfactory pulses occurs. The signal is produced by an output circuit, Fig. 11 (not shown) of the control circuit 13. Finally, logic circuitry, Fig. 3 (not shown) is used to produce a line 14e a signal to cause switching device 36 to interrupt the discharges momentarily.