JPH07108485B2 - Electric discharge machine - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electric discharge machining apparatus, in which the resistance value of a machining power supply circuit is made as small as possible, and a discharge current is cut off from the middle within a limited capacity of a switching element when a short circuit occurs. The present invention relates to an electric discharge machine equipped with the electric discharge circuit.

2. Description of the Related Art Generally, an electric discharge machine is composed of a capacitor charging circuit and a discharging circuit.In its basic circuit, the charging circuit includes a DC power supply, a series circuit of a resistor and a capacitor, and the discharging circuit of the capacitor is A series circuit including a capacitor, a switching element, a safety resistor, a machining gap between an electrode and a workpiece is included, and a DC power supply is turned on intermittently by a switching element such as a transistor without using a capacitor. There is a power supply circuit that controls off and forms a voltage pulse, and applies this voltage pulse between an electrode and a workpiece.

Problems to be Solved by the Invention In either case, if a large safety resistance is included when the machining gap is short-circuited, a slightly larger current will flow, but without such a safety resistance, the current value will be much higher. As a result, a current exceeding the rated current value flows through the switching element transistor or FET, which causes a problem. However, if the resistance value of the safety resistor is too large, a large amount of machining energy is consumed here, which is not desirable as the power supply circuit of the electric discharge machine. Further, the short circuit may be caused by the machining waste accumulated in the machining gap, and may be eliminated by removing the machining waste.

Means for Solving the Problems In view of such problems, the present invention turns on the DC power supply. An object of the present invention is to reduce the safety resistance of a switching element that is controlled to be off to save energy, prevent damage to the switching element due to overcurrent when a short circuit occurs, and remove machining waste to eliminate the short circuit.

In order to achieve this object, the electric discharge machining apparatus of the present invention intermittently applies a voltage pulse while leaving a pause between the electrode and the object to be machined opposite each other with a machining gap, and repeatedly generates a discharge pulse. In the electric discharge machining device that performs machining by turning on, the machining DC power supply and the power supply are turned on. It is composed of a series circuit of a switching element that controls off and forms a voltage pulse, its safety resistance and a machining gap, and the value of the safety resistance is a current of a rated current or more in the voltage pulse forming switching element when the machining gap is short-circuited. A voltage pulse applying circuit that is set to a small value, a control pulse generating device that outputs an open / close control pulse signal of the voltage pulse forming switching element, a short-circuit discriminator that detects a short-circuit state of a machining gap, and the voltage. A discharge current value discriminator for detecting that the discharge current flowing through the pulse forming switching element has reached a predetermined value less than or equal to the rated current value of the voltage pulse forming switching element, the control pulse generator, the short circuit discriminator, and the discharge current value. A logical product circuit using each output signal of the discriminator as an input signal, and the voltage pulse forming pulse depending on the output signal of the logical product circuit. And characterized in that it comprises a control circuit for the switching element in the open state.

Action Therefore, in this proposal, the resistance value of the safety resistance is made as small as possible so that it is equal to or less than the predetermined resistance value at which the rated current of the switching element flows. Less. Also, when a short circuit occurs due to the inclusion of machining chips, the switching element is cut off from the middle of the voltage pulse (discharge current) below the rated current value of the transistor or FET of the switching element. It is possible to prevent damage to the element and maintain safety. Moreover, since a voltage pulse cut off from the middle is applied to the machining gap at a predetermined interval and a slight discharge current flows, that is, a discharge current having a narrow pulse width and a high current peak flows with a sufficient pause time. The gap is quickly released from the arc discharge environment, and the machining waste existing in the machining gap is removed by the shocking discharge current to eliminate the short circuit and restart the electric discharge machining.

EXAMPLE The present invention will be described with reference to an example. FIG. 1 shows a basic circuit according to the present invention. 1 is a DC power source for machining, 2 is an electrode, 3 is a workpiece, 4 is a power source 1, an electrode 2 and a workpiece. 3 is a safety resistor that also functions as a current detector in a circuit provided in series, 5 turns on the machining DC power supply 1. It is a switching element that is off-controlled to form a voltage pulse, and a voltage pulse application circuit is formed by the above respective configurations. 6 is a control pulse generator that outputs an opening / closing control pulse signal for the switching element 5, 7 is a detection resistor that detects the voltage of the machining gap between the electrode 2 and the workpiece 3, and 8 is a machining gap based on the voltage detected by the detection resistor 7. The short-circuit discriminator for detecting the short-circuit state of 11 detects that the discharge current flowing through the switching element 5 has reached a predetermined value below the rated current value of the switching element 5 from the current value detected by the detection resistance (safety resistance) 4. A discharge current value discriminator, 9 is a logical product circuit using the output signals of the control pulse generator 6, the short circuit discriminator 8 and the discharge current value discriminator 11 as input signals, and 10 is an output signal of the logical product circuit 9. The control circuit includes a switching element or the like that opens the switching element 5 in accordance with the above. As shown in FIG. 2, when a pulse signal having a predetermined rest interval is sent from the control pulse generator 6 to the switching element 5, the switching element 5 is actuated so that the power source 1
DC voltage is turned on. By being controlled to be off, a second gap is formed in the machining gap G between the electrode 2 and the workpiece 3 via the safety resistor 4.
Applying a voltage pulse with a rest interval as shown in the figure,
When the machining gap G between the electrode 2 and the workpiece 3 is normal, the electric discharge machining is performed while showing the change in voltage as shown in FIG. 3 and the change in current as shown in FIG. A pulse signal having a predetermined interval of rest shown in FIG. 2 is sent from the controller 6 to the logic circuit 9. The resistance value of the safety resistor 4 inserted in the discharge circuit is a resistance value that is extremely low enough to cause a current exceeding the rated current value to flow through the switching element 5 when the machining gap is short-circuited. The value is chosen as small as possible so that it is less than 1/2.
When the detected current value of the safety resistor 4 also serving as the current detection reaches a predetermined value equal to or less than the rated current value of the switching element 5, an output signal is sent from the discharge current value discriminator 11 to the AND circuit 9. The detection signal of the detection resistor 7 is sent to the short-circuit discriminator 8, and when the machining gap G is short-circuited, it is output from the short-circuit discriminator 8, and the output signal is sent to the AND circuit 9. Therefore, when the pulse discharge is performed under the normal machining gap G, no signal is output from the short-circuit discriminator 8 and the discharge current value discriminator 11, so that the output of the AND circuit 9 is 0. Is.

Therefore, the machining pulse is continuously applied from the power source 1 to the machining gap G, and the electric discharge machining is continued. At that time, the electric power consumed by this discharge circuit is smaller than that of the conventional one because the resistance value of the safety resistor 4 is selected to be as small as possible. In the unlikely event that a short circuit accident occurs in the machining gap G, the current of the switching element 5 rises according to the current and time characteristics of FIG. 5, and when it reaches the point T below the rated current value P, the discharge current value discriminator. An output signal from 11 is sent to the AND circuit 9, and at this time, the AND circuit 9
Since a short-circuit signal is input from the short-circuit discriminator 8 and a pulse-on signal is input from the control pulse generator 6, the output of the AND circuit 9 becomes 1 and the output signal of the AND circuit 9 becomes Accordingly, the switching element of the control circuit 10 is turned on to bypass the output pulse signal of the control pulse generator 6, thereby opening the voltage pulse forming switching element 5 and protecting it from damage due to overcurrent. When the switching element 5 is opened and the supply of the voltage pulse is cut off to reduce the discharge current, the output of the discharge current value discriminator 11 becomes 0, the switching element of the control circuit 10 turns off, and the switching element 5 returns to the control pulse. Opening and closing is controlled by the generator 6. If the short circuit continues when the next opening / closing control pulse signal is output from the control pulse generator 6, the detected current value of the safety resistor 4 reaches the predetermined value T again, and the output of the discharge current value discriminator 11 becomes 1 Next to
The voltage pulse is cut off in the same manner as described above. In this way, even in the short-circuited state, as shown in FIG.
By repeatedly generating a discharge pulse having a narrow pulse width and a relatively large discharge current amplitude in the machining gap, it is possible to give a discharge impact to machining scraps, which is one of the causes of a short circuit, and remove them. Further, since this discharge pulse is repeatedly generated with a pause time larger than the pulse width, it does not cause an arc state.

EFFECTS OF THE INVENTION According to the present invention, the safety resistor 4 can be selected to be a value as small as possible than a predetermined resistance value for flowing a rated current to the switching element, and the loss of processing energy consumed by the safety resistor 4 is minimized. be able to. When a short circuit accident occurs in the machining gap G, when the rise of the discharge current reaches a certain value T which is equal to or less than the rated current value P of the switching element 5, the switching element 5 is turned off to ensure the safety of circuit elements such as the switching element. You can measure. Moreover, by repeating this action, the slight discharge current is caused to flow through the machining gap G at a predetermined pulse interval, so that the pulse current is used to remove the machining chips that are one of the causes of the short circuit. This has the effect that the electric discharge machining can be automatically restarted.

[Brief description of drawings]

FIG. 1 is a basic circuit diagram of the present invention, FIG. 2 is a diagram of a machining voltage having a predetermined rest interval, FIG. 3 is a diagram showing a voltage change during machining by the voltage pulse of FIG. 2, and FIG. The figure shows a change in current corresponding to the voltage change in FIG. 3, and FIG. 5 is a current vs. time characteristic diagram of the switching element. 1 is a DC power supply for processing, 2 is an electrode, 3 is a workpiece, 4 is a safety resistor, 5 is a voltage pulse forming switching element, 6 is a controller, 7
Is a detection resistor, 8 is a short-circuit discriminator, 9 is a logical product circuit, 10 is a control circuit, and 11 is a discharge current value discriminator.

Claims (1)

[Claims] 1. An electric discharge machining apparatus which performs machining by intermittently applying a voltage pulse with a pause between an electrode and a body to be machined, which are opposed to each other via a machining gap, and repeatedly generating a discharge pulse. Then, turn on the DC power supply for processing and the power supply. It is composed of a series circuit of a switching element that controls off to form a voltage pulse, its safety resistance and a machining gap, and the value of the safety resistance is a current more than the rated current in the voltage pulse forming switching element when the machining gap is short-circuited. A voltage pulse applying circuit that is set to a small value, a control pulse generating device that outputs an open / close control pulse signal of the voltage pulse forming switching element, a short-circuit discriminator that detects a short-circuit state of a machining gap, and the voltage. A discharge current value discriminator for detecting that the discharge current flowing through the pulse forming switching element has reached a predetermined value less than or equal to the rated current value of the voltage pulse forming switching element, the control pulse generator, the short circuit discriminator, and the discharge current value. A logical product circuit using each output signal of the discriminator as an input signal, and the voltage pulse forming pulse depending on the output signal of the logical product circuit. Discharge machining apparatus characterized by comprising a control circuit for the switching element in the open state.