Patent GB-950943-A (GB)
Title: Improvements in or relating to spark machining apparatus
IPC: B23H7/18
Filed: 1960-06-28
Abstract: 950,943. Cutting by spark discharge. PHILIPS ELECTRICAL INDUSTRIES Ltd. June 28, 1960 [July 1, 1959], No. 22618/60. Heading B3V. [Also in Division G3] In spark machining apparatus, an electrode holder is adjusted in position relative to a workpiece by means of an electromagnetic means controlled in dependence on the magnitude of a current flowing through the spark, and further means is provided to adjust the position of the electrode holder if, in operation, the current through the spark falls below a predetermined value. As shown, coarse adjustment is effected by a motor 15 which moves a guide member 4 carrying an electrode holder 5<SP>1</SP> towards the workpiece 8. A projection 7 of the holder 5<SP>1</SP> is attached to the member 4 by springs 6. An electromagnet 14 which is also mounted on the member 4 provides the fine adjustment of the electrode by controlling the holder 5<SP>1</SP>. Assuming the electrode is clear of the workpiece when a D.C. source is connected to terminals 9, capacitors 10, 11 are charged via resistors 12, 13 and the support member 4 is moved towards the workpiece by the motor 15. After a delay due to a capacitor 18 the contacts 16 of a relay 17 break and the motor stops. If the electrode is insufficiently close to the workpiece, the relay armature drops off after further delay and the cycle is repeated until discharge of the capacitor 11 across the electrode and workpiece is able to take place. Charging current then begins to flow through the resistance 12, the resulting voltage drop across it causing the relay 17 to remain energized through contacts 20 and the electromagnet 14 to be energized. The gap between the electrode and the workpiece thus increases slightly, and discharges of the capacitor 11 at a frequency which may be from 1-10 kc./s. take place across the gap. Erosion of the workpiece increases the gap and the capacitor 11 is charged to a higher voltage before discharge occurs. The mean current through resistor 12 thus decreases, and the voltage drop across it decreases so that current through the electromagnet 14 is reduced. The electrode holder 5<SP>1</SP> is thus allowed to move downwards slowly and the rate of increase of the discharge gap is reduced. When the limit of downward movement of the electrode has been reached, the discharge gap continues to increase and the current through the resistor 12 further decreases so that the armature of the relay 17 drops off and the motor is re-energized. The support member 4 is thus moved downwards until the mean electrode current through the resistor 12 is sufficient for the relay 17 to be energized. The contacts 16, 19 are then opened and the contacts 20 closed. Machining continues until the gap between the electrode and the workpiece is such that the relay 17 falls off again when the sequence occurs again. In a modification, Fig. 3 (not shown), the voltage across the electrode and the workpiece operates the motor. As a further modification, the motor may be energized by constant D.C. in one direction in conjunction with an energization in the opposite direction derived from the current passing through the resistor 12. If this energization in the opposite direction becomes too small, the D.C. energization will cause the motor to rotate. Hydraulic drive may be used for the member 4, and a second electromagnet may be used to control a valve such that movement of the member 4 is accelerated by decrease in the electrode current. Specification 800,061 is referred to.