GB2171822A

GB2171822A - Method and apparatus for controlling an electric discharge machine

Info

Publication number: GB2171822A
Application number: GB8524583A
Authority: GB
Inventors: Yasunori Ohno; Eiichiro Asano
Original assignee: Amada Co Ltd
Current assignee: Amada Co Ltd
Priority date: 1984-10-05
Filing date: 1985-10-04
Publication date: 1986-09-03
Also published as: GB2171822B; GB8524583D0

Abstract

A control circuit for a wire cutting electric discharge machine comprises a power circuit (3) for applying a pulse voltage across a working gap (1) between a wire electrode and a workpiece; a discriminant circuit (13) which detects the discharge status of the working gap (1); a counter (3) which calculates the number of abnormal discharges which are detected by the discriminant circuit; a pulse control circuit (17) which extends the OFF-time interval of the pulse when the calculated value of the number of successive abnormal discharges, calculated in the counter, reaches a set value; and a drive circuit (11) which drives by means of the output of the pulse control circuit a switching element (9) which is connected into the power circuit. <IMAGE>

Description

SPECIFICATION A method of and an apparatus for controlling an electric discharge machine This invention relates to a method of and an apparatus for controlling an electric discharge machine and, in particular, to a control circuit for a wire cutting electric discharge machine, particularly a control circuit for a wire cutting electric discharge machine wherein the OFFtime interval of the pulse is controlled in accordance with the status of the electric discharge in the working gap between the wire electrode and the workpiece.

A power circuit system for an electric discharge machine generally includes an independent and a dependent impulse power system.

The independent impulse power system applies a pulse voltage through a D.C. switching transistor and a current control resistance at the working (interpolar) gap between the electrode and the workpiece. This independent impulse power system applies the pulse voltage at the working gap by utilizing a circuit which controls the ON-OFF time intervals of the transistor, and it repeatedly switches ON or OFF for a fixed time period regardless of the status of the working gap. Accordingly, such an independent impulse power system is not used as a power system in a wire cutting electric discharge machine which utilizes an exceedingly short electric discharge, but is generally used as the power system for an electric discharge shaping machine.

In the dependent impulse power system, a capacitor is connected in parallel with the working gap and the electric charge stored in this capacitor through a resistance is discharged by way of the breakdown of the insulation in the working gap. In this dependent impulse power system, the amperage pulse obtained has a narrow pulse width and a high peak current value. However, because the charging and discharging of the capacitor is controlled according to the status of the working gap, if, for example, the working gap is wide, the initial discharge voltage is high, and conversely, if the working gap is narrow, the initial discharge voltage is low.Accordingly, the voltage at which the capacitor is charged and discharged cannot be controlled, and considerable variation is produced in the discharge energy, which adversely affects the roughness or smoothness of the processed surface of the workpiece. In addition, because a shift to lumped discharging occurs readily, so that a break in the wire electrode is easily produced, it is not possible to select precise processing conditions or increase the speed of the process.

It is an object of the present invention to enable the provision of a method of and an apparatus for controlling an electric discharge machine whereby the above disadvantages may be overcome or at least mitigated.

According to a first aspect of the present invention, there is provided a method of controlling an electric discharge machine, which method comprises adjusting the time interval between successive pulses of a pulsed discharge in accordance with a detected characteristic of the discharge.

According to a second aspect of the present invention there is provided apparatus for controlling an electric discharge machine, which apparatus comprises means for adjusting the time interval between successive pulses of a pulsed discharge in accordance with a detected characteristic of the discharge.

The present invention thus enables the provision of a current control circuit which can achieve a current pulse for the high peak current value with a narrow pulse width, while eliminating the use of a capacitor.

The present invention further enables the provision of a current control circuit which detects abnormal discharges to prevent the cutting of the wire electrode.

The present invention still further enables the provision of a current control circuit which will provide an increase in processing speed and an improvement in the smoothness of the finished surface.

These objectives of the present invention are achieved in a preferred aspect by the provision of a current control circuit comprising a power circuit for applying a pulse voltage across the working gap between the wire electrode and the workpiece, a discriminant circuit which detects the discharge status of the working gap, a counter which calculates the number of abnormal discharges which are detected by the discriminant circuit, a pulse control circuit which extends the OFF-time interval of the pulse when the calculated value of the number of abnormal discharges in succession, calculated in the counter, reaches a set value, and a drive circuit which drives means of the output of the pulse control circuit switching elements which are connected into the power circuit.

For a better understanding of the present invention and to show how the same may be put into effect, reference will now be made, by way of example, to the accompanying drawing, in which: Figure 1 is a block diagram of a control circuit in accordance with the present invention, Figure 2 is a diagram illustrating discrimination between normal and abnormal discharges, and Figure 3 is a diagram illustrating extension of the OFF-time interval when abnormal discharges occur in succession.

Referring now to Fig. 1, a power circuit 3 for applying a pulse voltage across a working gap 1 between a wire electrode and workpiece comprises a DC power source 5, a re sistance 7 for limiting an electric current, and a switching element 9, which can be a device such as a transistor, all connected in series. A drive circuit 11, which drives the switching element 9, is connected to the switching element 9. Accordingly, a pulse voltage is applied to the working gap 1 by the ON-OFF action of the switching element 9 activated by the drive circuit 11.

A discriminant circuit 13 is connected in parallel with the working gap 1. A counter 15 is connected to the discriminant circuit 13. A pulse control circuit 17 is connected to the counter 15, and this pulse control circuit 17 is also connected to the drive circuit 11.

The discriminant circuit 13 differentiates between normal and abnormal electrical discharges across the working gap 1. Specifically, the discriminant circuit 13 detects the voltage at the working gap 1 when a pulse voltage is applied across the working gap 1 to cause a discharge to occur. When the discharge commences after the voltage rises above a certain standard voltage, this discharge, is classified as normal. Conversely, when the voltage does not rise above this standard voltage before the discharge commences, this discharge is considered to be abnormal.

The counter 15 calculates the number of successive abnormal discharges which are detected by the discriminant circuit 13. When a normal discharge occurs during the calculation of the abnormal discharges, the counter 15 is reset. When the counter 15 is counting the number of successive abnormal discharges and the calculated value reaches a preset value, then the counter 15 outputs a signal to a pulse control circuit 17.

This pulse control circuit 17 is used to control the discharge ON and OFF time intervals when a pulse voltage is applied across the working gap to cause a discharge to commence. The pulse control circuit 1 7 is connected to a discharge-ON time interval setting circuit 19 in which it is possible to optionally set the discharge-ON time interval, and a discharge-OFF time interval setting circuit 21 in which it is possible to optionally set the discharge-OFF time interval. The pulse control circuit 1 7 is used to extend the discharge-OFF time interval according to the input signal received from the counter 1 5 by controlling the drive circuit 11.

In a configuration such as outlined above, a pulse voltage is applied across the working gap 1 by turning the switching element 9 into a conductive condition by appropriately controlling the drive circuit 11. After the pulse voltage has been applied across the working gap 1, the occurrence of a breakdown of the interpolar insulation, as shown in (1), (2), and (5) of Fig. 2, causes the voltage to suddenly drop, and the discharge commences. When the voltage suddenly drops in this manner and goes below a predetermined standard voltage (DSC level) slightly higher than the discharge voltage, the discharge-ON time interval is set, with the points (awl, A2) below the standard voltage taken as the starting points of the discharge. This discharge-ON time interval is previously set in the discharge-ON time interval setting circuit 19.When a predetermined discharge-ON time interval has elapsed, the drive circuit 11 is controlled by the pulse control circuit 17 to turn the switching element 9 into a non-conductive condition. By the nonconductive condition of the switching element 9, the voltage is prevented from applying across the work gap 1, and the discharge-OFF time interval starts. This discharge-OFF time interval is previously set in the discharge-OFF time interval setting circuit 21. When the discharge-OFF time interval has elapsed, the switching element 9 is once again turned into a conductive condition through the drive circuit 11 under the control of the pulse control circuit 17.

When the pulse voltage is applied across the working gap in this manner, the voltage at the working gap 1 is detected by the discriminant circuit 13. The discriminant circuit 13 determines that the discharges is normal when the discharge commences after the voltage rises above the standard voltage (DSC level) as a result of the application of the pulse voltage, as shown in (1), (2), and (5) of Fig. 2. In addition, as shown in (3) and (4) of Fig. 2, when a pulse voltage is applied but the voltage does not rise above the standard voltage even after the discharge-ON time interval has elapsed, this is classified as an abnormal discharge, and the discharge-OFF period is immediately entered.

In the discriminant circuit 13, the number of successive discharges judged to be abnormal are calculated in the counter 15, but when, during calcualtion of the abnormal discharges, a normal discharge occurs, the couner 15 is reset. The number of successive abnormal discharges in the working gap 1 is calculated in the counter 15. When this calculated value reaches the set value, it is determined that a lumped discharge occurs in the working gap, and the discharge-OFF time interval is extended by the pulse control circuit 17. Specifically, as shown in Fig. 3, when, for example, several abnormal discharges occur in succession (three occurrences in Fig. 3), the discharge-OFF time interval is extended, for example, doubled.

As can be readily understood from the foregoing explanation, the present invention controls the discharge-ON time interval from the point where the discharge commences by applying a voltage across the working gap. In addition, because the discharge-OFF time interval can be optionally set, it is possible to obtain a amperage pulse with a narrow pulse width and a high current peak value, while at the same time equalizing the discharge energy, thus improving the smoothness of the finished surface. In addition, cutting of the wire electrode as the result of a lumped discharge can be prevented. Furthermore, depending on the situation, it is possible to increase the repeating frequency of the discharges in relation to the status of the working gap, providing an improvement in the efficiency of the process.

Claims

1. A method of controlling an electric discharge machine, which method comprises adjusting the time interval between successive pulses of a pulsed discharge in accordance with a detected characteristic of the discharge.

2. A method according to Claim 1, which comprises counting abnormal discharges and adjusting the said time interval in accordance with the number of abnormal discharges counted.

3. A method according to Claim 2, which comprises increasing the said time interval if the number of successive abnormal discharges exceeds a preset value.

4. Apparatus for controlling an electric discharge machine, which apparatus comprises means for adjusting the time interval between successive pulses of a pulsed discharge in accordance with a detected characteristic of the discharge.

5. Apparatus according to Claim 4, which comprises means for counting abnormal discharges and means responsive to the counting means for adjusting the said time interval.

6. Apparatus according to Claim 5, which comprises means for increasing the said time interval if the number of successive abnormal discharges exceeds a preset value.

7. A control circuit for a wire cutting electric discharge machine, comprising; a power circuit for applying a pulse voltage across a working gap between a wire electrode and a workpiece; a discriminant circuit for detecting the discharge status of the working gap; a counter for calculating the number of abnormal discharges detected by the discriminant circuit; a pulse control circuit for extending the OFFtime interval of the pulse if the number of successive abnormal discharges, calculated in the counter, reaches a set value; and a drive circuit for driving, by means of the output of the pulse control circuit, a switching element which is connected into the power circuit.

8. A method of controlling an electric discharge machine, substantially as hereinbefore described with reference to the accompanying drawing.

9. Apparatus for controlling an electric discharge machine, substantially as hereinbefore described with reference to, and as shown in the accompanying drawing.

10. Any novel feature or combination of features described herein.