Electric discharge machining method and device

Details Electric discharge machining method and device Electric discharge machining method and device

1988-08-25

1990-07-31

Pellinen, A. D.

Electric heating

Metal heating

Cutting or disintegrating

B23H 102

Patent

active

049451995

DESCRIPTION:

BRIEF SUMMARY
DESCRIPTION

1. TECHNICAL FIELD
This invention relates to an electric discharge machining method and an electric discharge machining device for use in an electric discharge machine using an electrically conductive solution as its machining solution.
2. TECHNICAL BACKGROUND
FIG. 9 is a circuit diagram showing a machining power source according to a conventional electric discharge machining method disclosed, for instance, by Japanese Patent Application (OPI) No. 85826/1985 (the term "OPI" as used herein means an "unexamined published application"). In FIG. 9, reference numeral 1 designates a machining electrode; 2, a workpiece; 3, a second DC power source; 4 and 26, power transistors; 5 and 25, current limiting resistors connected to the emitters of the power transistors 4 and 26, respectively; 9, discharge detecting means for detecting when electric discharge takes place between the electrodes, namely, the electrode 1 and the workpiece 2; 12, switching means; 13, a second drive circuit for driving the power transistor 4; 6 and 15, diodes for preventing reverse current; 18, a first drive circuit for driving the power transistor 26; and 19, a first DC power source. The switching means 12 control the first and second drive circuits 13 and 18.
The electrical characteristics of the circuit in the case where an electrically conductive solution is used will be described.
If, in this case, the machining electrode 1 and the workpiece 2 are flat plates arranged in parallel to each other, then the interelectrode impedance Rgap can be represented by the following equation (1) as indicated in FIG. 10: ##EQU1## where .rho. is the specific resistance (.OMEGA. cm) of the machining solution, l is the distance (cm) between the electrodes, and S is the confronting area (cm.sup.2) between the electrodes.
When the power transistor 4 is turned on by the second drive circuit 13, a voltage Vgopen as shown in the part (a) of FIG. 6 is developed between the electrodes, namely, the machining electrode 1 and the workpiece 2 before electric discharge takes place therebetween. In this case, according to Ohm's law, the voltage Vgopen is: ##EQU2## where R.sub.M is the current limiting resistance, and E is the DC supply voltage.
Hereinafter, the voltage Vgopen will be referred to as "a no-load voltage", and the interelectrode voltage provided after electric discharge will be referred to as "an arc voltage Vgarc", when applicable.
The current flowing between the electrodes is as follows: That is, if the total current supplied by the power source is represented by I, with respect to the interelectrode impedance Rgap an electrolytic current flowing according to Ohm's law at the application of a no-load voltage is represented by I.sub.Eopen, and that during electric discharge is represented by I.sub.Earc, and a discharge current in the electric discharge is represented by Id, then
Before electric discharge
During electric discharge ##EQU3##
As is clear from equation (1), the interelectrode impedance Rgap is decreased as the resistivity .rho. of the machining solution or the interelectrode distance l is decreased, and as the confronting area S between the electrodes is increased. Furthermore, as is apparent from equation (2), the no-load voltage Vgopen decreases as the interelectrode impedance Rgap decreases. When the no-load voltage becomes lower than the arc voltage Vgarc, then no electric discharge will take place between the electrodes; that is, the workpiece cannot be machined. Therefore, in the discharge-machining of a large area, the resistivity .rho. of the machining solution should be maintained high to some extent. For this purpose, the resistivity .rho. is controlled by using ion exchange resin for instance.
On the other hand, when it is required to decrease the discharge current Id, the resistance R.sub.M of the current limiting resistor 5 should be set to a large value; however, in this case, the no-load voltage Vgopen is decreased, thus making it difficult for electric discharge to take place; that is, the machining efficiency is

REFERENCES:
patent: 3604885 (1971-09-01), Inoue
patent: 3655937 (1972-04-01), Ullmann et al.
patent: 3987269 (1976-10-01), Inoue et al.
patent: 4288675 (1981-09-01), Inoue

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