Electric heating – Metal heating – Cutting or disintegrating
Patent
1984-01-30
1987-09-22
Pellinen, A. D.
Electric heating
Metal heating
Cutting or disintegrating
219 69C, 219 69W, B23H 102, B23H 704
Patent
active
046956964
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to electric discharge machines, and more particularly to an electric discharge machine which comprises a main switching circuit for supplying a current high in peak value between an electrode and a workpiece, and an auxiliary switching circuit for supplying a current low in peak value between the electrodes, to intermittently cause electric discharge between the electrodes.
BACKGROUND ART
Systems for supplying machining energy from a machining electric source to the electrodes of a wire cutting electric discharge machine can be roughly classified into a first group of systems in which a capacitor is connected between an electrode and a workpiece and the capacitor thus connected is charged through a switching transistor which is turned on and off, so that electric discharge machining is carried out by utilizing the voltage of the capacitor thus charged, and a second group of systems in which the current between the electrodes is directly controlled on and off by a switching circuit connected between the power source and the electrodes.
In the first group of systems, as shown in FIG. 1, a capacitor 14 is connected between a wire electrode 10 and a workpiece 12, and machining energy from a power source 16 is supplied through a current limiting resistor 18 and a switching transistor 20 to the capacitor 14 thus connected. The capacitor 14 is charged through the switching transistor 20 which is turned on and off by an on-off pulse signal outputted by an oscillator 22. Thus, pulse voltage and current are applied between the wire electrode 10 and the workpiece 12 by the capacitor 14 thus charged.
The parts (a) and (b) of FIG. 2 are waveform diagrams respectively showing an interelectrode voltage V and a machining current I in the electric discharge machine in FIG. 1. The waveform of the interelectrode voltage V is defined by the time constant CR which is determined from the capacitance C of the capacitor 14 and the resistance R of the resistor 18. In the machine shown in FIG. 1, the timing of occurrence of electric discharge between the electrodes depends on the gap between the electrodes, the specific resistance of the machining solution between the electrodes and the presence of powder which is created during electric discharge machining. Therefore, for instance, electric discharge starts before the charge voltage of the capacitor 14 reaches the supply voltage Vcc, or with a delay time after it reaches the interelectrode voltage. In this case, the peak value Ip and the pulse width .tau.p of the current between the electrodes are as follows: ##EQU1## In these equations, E is the discharge start voltage, and L is the inductance of the current path. Accordingly, in the machine in FIG. 1, the machining current is determined by the voltage E which is provided at the start of electric discharge. Therefore, the machining current value is not constant for every electric discharge. On the other hand, as the amount of a part of the workpiece, which is removed by one discharge machining operation depends on the value of current which is provided during electric discharge, the machined surface roughness is determined by the maximum value of current during electric discharge. In general, as a discharge current is increased, the discharge machining speed is increased. However, the wire cutting electric discharge machine in FIG. 1 suffers from a difficulty that, since the machining current is not constant for every electric discharge, the machining speed for a given surface roughness is decreased.
One example of the second group of systems has been disclosed by Japanese Patent Application Publication No. 13195/1969. It will be briefly described. The system has a main switching circuit for supplying a current large in peak value between the electrodes, and an auxiliary switching circuit which is small in current peak value and is used only to supply a voltage across the electrodes. First the auxiliary switching circuit applies the voltage across the electrodes, and after the occurrence
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Ozaki Yoshio
Tsurumoto Kazuo
Yamamoto Masahiro
Yatomi Takeshi
Evans Geoffrey S.
Mitsubishi Denki & Kabushiki Kaisha
Pellinen A. D.
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