JPH048165B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to an electrical discharge machining apparatus that performs machining by generating intermittent electrical discharge between machining gaps where a machining electrode and a workpiece are opposed to each other. It is related to.

[Background of the invention]

Although the machining speed and machining accuracy of wire electric discharge machining devices have been significantly improved in recent years, when the machining speed is increased, the machining accuracy tends to decrease. In particular, it has become clear that recently developed pulse power sources for high-speed machining have a relatively large electrolytic effect, which is one of the causes of reduced machining accuracy.

FIG. 1 is a diagram showing the overall configuration of a conventional wire electric discharge machining apparatus having such a problem. In FIG. 1, 1 indicates a wire electrode. This wire electrode 1 is guided by rollers 2 and 3 and faces a workpiece 4, and the relative position between the electrode 1 and the workpiece 4 is determined by a positioning device operated by a command from a numerical control device (not shown). Controlled by motors 5 and 6.

Ion-exchanged water, which is a machining fluid, is supplied to the machining gap Q formed by the electrode 1 and the workpiece 4 by machining fluid supply nozzles 7 and 8, and is also supplied to the electrode 1 side via current-carrying terminals 9 and 10. Power is supplied from a pulse discharge circuit described later.

The pulse discharge circuit 11 that supplies this power includes a DC power supply 12, a switching element 13, its control circuit 14, a current reducing resistor 15, and a discharging capacitor 16.
It is made up of.

During machining of this type of pulse discharge circuit 11, the voltage and current waveforms during machining Q are as shown in the upper diagram of FIG.
When the discharge capacitor 16 is charged to a voltage of 2 and a discharge occurs during the machining interval Q, an oscillating current flows as shown in the lower diagram of FIG. 2a. This oscillating current waveform is from the discharge capacitor 16 to the machining interval 〓Q
It is determined by the inductance and resistance value of the current-carrying system and the capacity of the discharge capacitor 16,
Generally, the oscillating current waveform is as shown in the lower diagram of FIG. 2a.

Further, a pulse discharge circuit (not shown) in which the discharge capacitor 16 of FIG. 1 is removed has also been used in the past, and its waveform is as shown in FIG. 2b.

In both cases of FIGS. 2a and 2b, the polarity of the applied pulse at the time of dielectric breakdown is such that the workpiece 4 is the positive polarity. This polarity was selected as the polarity that increases the speed of machining the workpiece 4, and experiments have confirmed that it is better to flow current from the workpiece 4 to the electrode 1 in order to increase the machining speed. .

However, since water is used in the machining fluid, when voltage is applied during machining Q, an electrolytic reaction occurs, making the machining surface of the workpiece 4 soft and deteriorating the quality of the machining surface. was. Since this amount ranges from several micrometers to more than ten micrometers, it has been a major obstacle to high-precision machining with dimensional accuracy on the order of several micrometers.

To solve this problem, there is a method of applying a bipolar pulse voltage so that the average voltage between the poles is OV, but this method reduces the frequency of the machining pulses that contribute to machining, resulting in a decrease in machining speed. This left the problem of decline.

Therefore, without reducing the electrical discharge machining speed,
An electric discharge machining apparatus was invented in which a soft layer on the machining surface caused by electrolytic reaction during machining was eliminated (Japanese Patent Application Laid-Open No. 56-56341).

It has two discharge circuits, high and low voltage.
After the low-voltage discharge circuit provides a threshold of discharge, the high-voltage discharge circuit performs the main discharge for machining, and at this time, the low-voltage discharge circuit sets the wire electrode side to a positive potential, The high voltage discharge circuit is connected with the workpiece side at a negative potential, and
The wire electrode side is connected to a negative potential and the workpiece side is connected to a positive potential.

However, in this conventional technology, the main discharge after the start of discharge by the cut-off voltage discharge circuit is carried out by discharging the charge previously charged in the capacitor provided in the high voltage discharge circuit to the machining interval 〓Q using a switch. It was a so-called capacitor discharge type.

In such a capacitor discharge type main discharge circuit, the current waveform is determined by the RLC of the energization path and the discharge starting voltage, but the degree of freedom is narrow. In particular, since the current rise speed is slow, the peak current value
There were problems in that it was difficult to obtain a high current density waveform with a high I _P and a narrow pulse width, and the processing efficiency was low.

[Purpose of the invention]

The object of the present invention has been made in view of the problems of the prior art described above, and it is possible to eliminate the soft layer on the machined surface caused by the electrolytic reaction during machining without reducing the electrical discharge machining speed, and to
The main discharge current waveform can be controlled relatively freely, and in particular, it is easy to obtain a high current density waveform with a high peak current value I _P and a narrow pulse width, and it is possible to improve machining efficiency. It is an object of the present invention to provide an electrical discharge machining device in which arc generation is performed stably and reliably.

[Summary of the Invention] The present invention is characterized by: a pulse discharge circuit for dielectric breakdown in which the workpiece is connected to the negative side and a machining electrode is connected to the positive side; The main pulse discharge circuit is connected to the negative side of the working electrode, and after the dielectric breakdown between the machining electrode and the workpiece is performed by the dielectric breakdown pulse discharge circuit, the In an electric discharge machining apparatus that performs main discharge for machining using a main pulse discharge circuit, (1) the main pulse discharge circuit includes: (1-1) a main discharge switching element; (1-2) this main discharge switching element; a main discharge DC power supply having a negative electrode connected to the processing electrode and a positive electrode connected to the workpiece through an element; (1-3) an anode between the processing electrode and the main discharge switching element; and (1-4) a sub-DC power source, the positive electrode of which is connected to the cathode of the diode, and the negative electrode of which is connected between the workpiece and the positive electrodes of the main discharge DC power source. (2) The pulse discharge circuit for dielectric breakdown includes (2-1) a switching element for dielectric breakdown, and (2-2) a negative electrode connected to the workpiece, and at least the switching element for dielectric breakdown. (2-3) between the negative electrode of the dielectric breakdown power source and the dielectric breakdown switching element and the machining electrode; and a dielectric breakdown discharge capacitor which is connected to the inductance of the current carrying path leading to the machining gap and set to a value that satisfies vibration conditions; (3) the discharge of the dielectric breakdown discharge capacitor causes the A control circuit that controls the main discharge switching element in order to superimpose the main discharge current of the main pulse discharge circuit on the oscillating discharge current output from the dielectric breakdown discharge capacitor after dielectric breakdown of the machining gap, The purpose is to equip and configure the system.

[Embodiments of the Invention] Hereinafter, embodiments of the present invention will be described with reference to FIGS. 3 and 4.

FIG. 3 is a circuit diagram showing the main parts of an embodiment of the electrical discharge machining apparatus according to the present invention, mainly showing the electrical discharge circuit portion. In FIG. 3, the same reference numerals as in FIG. 1 indicate the same or corresponding parts.

In FIG. 3, the dielectric breakdown pulse discharge circuit includes a dielectric breakdown DC power supply 17, a dielectric breakdown switching element, a current reducing resistor 19, a dielectric breakdown switching element control circuit 20 for controlling the switching element 18, and a discharge dielectric breakdown switching element control circuit 20. It is composed of a capacitor 30.

Here, the DC power supply 17 is, for example, 70 to 15 OV.
The negative electrode is connected to the workpiece 4, and the positive electrode is connected to the current-carrying terminal 10 through the current reducing resistor 19, the switching element 18, the diode 29, and the current detector 28 in this order. Further, the discharge capacitor 30 connects the negative electrode of the DC power supply 17, the switching element 18, and the current detector 2.
8 are connected to each other. This discharge capacitor 30 is set so that the discharge current reaches a value that satisfies the vibration conditions by the inductance 31 and resistance 32 of the current-carrying path from here to the machining interval Q.

In addition, the main pulse discharge circuit is connected to the main discharge DC power supply 2.
1. It is composed of a main discharge switching element 22, a main discharge switching element control circuit 23 that controls this switching element 22, a diode 24, and a sub-DC power supply 25 for absorbing electromagnetic energy.

Here, the DC power supply 21 has a voltage of, for example, 200 to 300V.
At an output voltage of about
It is connected to the current-carrying terminal 10 via the energizing terminal 10, and its positive electrode is connected to the workpiece 4. In addition, the diode 24
The anode is connected between the current-carrying terminal 10 and the switching element 22. Further, the auxiliary DC power supply 25 has an output voltage of, for example, about 50 to 100V,
A positive electrode is connected to the cathode of the diode 24, and a negative electrode is connected between the workpiece 4 and the positive electrodes of the DC power source 21.

Note that the current detector 28 detects the start of electrical discharge (insulation breakdown) during the machining interval Q.

Further, 27 indicates a pulse control circuit, which processes the output signal of the current detector 28 and outputs a pulse signal to the switching element control circuits 20 and 23.

Next, the operation of the above-mentioned apparatus of the present invention will be explained using FIG. 4.

Waveform a in Figure 4 is the voltage during machining = Q, and the same waveform b
represents the current during machining 〓Q. Similarly, waveform c shows on and off signals of switching element 18, and waveform d shows on and off signals of switching element 22, respectively.

First, from the pulse control circuit 27, the waveform c in FIG.
It is assumed that the signal 2 is input to the switching element control circuit 20, and the switching element 18 is turned on. At this time, as can be seen from the waveform d, the switching element 22 is off.

When the switching element 18 is turned on, the discharging capacitor 30 is charged from the DC power supply 17 via the resistor 19. At this time, since the polarity of the workpiece 4 is negative, the workpiece 4 is not electrolyzed.

When the discharge starts, the voltage drops as shown by _a1 of waveform a in FIG.
The oscillating current of the RLC circuit is caused by the discharging capacitor 30.
During machining 〓Q begins to flow like waveform b ₁ ,
The current is detected by a current detector 28, and the detection signal is input to the pulse control circuit 27.

As a result, the pulse control circuit 27 outputs a pulse-off signal to the switching element 18 via the switching element control circuit 20 to turn it off, and at the same time, as shown in waveform d in FIG. A pulse-on signal is output to the switching element 22 via the switching element control circuit 23 to turn it on so that the time it takes for the element 22 to turn on becomes t ₃ ', which will be described later. As a result, the discharge current flowing during machining Q becomes as shown in the waveform b in _FIG . The large current b from .

Note that after the dielectric breakdown of the machining interval Q, an oscillating discharge current flows, and if the machining interval Q is in a transient arc state, the large current b ₃ can be superimposed without breaking the arc (smooth and reliable arcing). ). In particular, when the large current b ₃ is superimposed on the second half-wave b ₂ of the oscillating discharge current, the superposition has the same polarity, so that the transition to an arc becomes smoother and more reliable.

In waveform b in FIG.
The reason why the current _b4 continues to flow even after the power supply 2 is turned off is because the electromagnetic energy accumulated in the inductance L (not shown) of the wiring in the current-carrying system is released through the diode 24 (absorbed by the DC power supply 25). It is.

Note that _t4 of waveform b in FIG. 4 is the time from the start of discharge to the start of the second half wave _b2 of the oscillating discharge current waveform of the RLC circuit formed by the discharging capacitor 30, inductance 31, and resistor 32. Further, t ₃ ' of waveform d in FIG. 4 is an appropriate time after the elapse of the time t ₄ from the start of discharge and before the end of the second half wave b ₂ . Therefore, this time t ₃ ' is the duration of the second half wave b ₂ and the delay time from the detection of the start of discharge by the current detector 28 until the switching element 22 is turned on (200 to 500 ns at the minimum). are taken into account and set.

t ₅ is the delay time from the detection of the start of discharge by the current detector 28 until the switching element 18 is turned off. Further, the times t ₁ and t ₂ are usually controlled to be constant.

According to the above-mentioned device of the present invention, the conventional technology
56341), it is possible to eliminate the soft layer on the machined surface caused by the electrolytic reaction during the machining interval Q without reducing the electrical discharge machining speed. In addition, the main discharge current waveform can be controlled relatively freely, and in particular, it is easy to obtain a high current density waveform with a high peak current value I _P and a narrow pulse width, which improves machining efficiency. I can do it. That is, unlike the capacitor discharge type main discharge circuit of the prior art, in the switching element (mainly switching transistor) type main discharge circuit as in the present invention, the voltage of the DC power supplies 21 and 25 and the on-time t ₁ of the switching element 22 , peak current value I _P
(Peak value of the triangular wave in waveform b in Figure 4),
The current pulse width (width of the triangular wave) can be easily changed. Therefore, it is easy to obtain a high current density waveform with a high peak current value I _P and a narrow pulse width, and it is possible to improve processing efficiency.

This will be discussed below. In the configuration shown in FIG. 3, when the switching element 22 is turned on,
A DC power supply 21 is used as a power supply, and a switching element 22
A discharge current flows through the machining interval Q. The rising current b ₃ (see Figure 4b) is the DC power supply 21
The voltage is E _M , the voltage corresponding to the back electromotive force during machining Q (= arc voltage: about 20 V) is Ea , the inductance of the current-carrying path between the DC power supply 21 and the switching element 22 is L ₁ , the DC power supply 21 The inductance of the current-carrying path leading to the switching element 22 via Q is L ₂ , and the switching element 22 is
When the time after turning on is _t1 , it is expressed by the following equation (1).

b ₃ = E _M −Ea/L ₁ +L ₂・t ₁ ...(1) Also, when the switching element 22 is turned off, the discharge current from the DC power supply 21 will pass through the diode 24 and the auxiliary DC power supply 25 side. However, since the auxiliary DC power supply 25 has a polarity opposite to that of the discharge current, the discharge current falls rapidly. The falling current b ₄ (see Fig. 4b) increases the voltage of the auxiliary DC power supply 25 by Ec,
The inductance of the current-carrying path from the connection between the switching element 22 and the diode 24 to the connection between the DC power supply 21 and the auxiliary DC power supply 25 is L ₃ , and the time after the switching element 22 is turned off is t _f (Ea and L ₂ are the same as above). Similarly), it is expressed by the following equation (2).

b ₄ = Ec + Ea / L ₂ + L ₃ · t _f ... (2) As can be seen from the above equations (1) and (2), the changes in the rising edge b ₃ and falling edge b ₄ of the discharge current are respectively expressed as It can be arbitrarily adjusted by the voltage E _M and the voltage Ec of the auxiliary DC power supply 25. In other words, the peak current I _P can be set to a desired value by adjusting the rising current b ₃ , and the pulse width can be reduced by making the changes in the rising current b ₃ and falling current b ₄ as steep as possible. Setting is possible by adjusting the voltages _EM and EC.

In the above embodiment, a current detector of a pulse discharge circuit for dielectric breakdown was used to detect the start of discharge.
A similar effect can be obtained by using a voltage detector that directly measures the voltage during machining Q and detects the voltage drop at the time of dielectric breakdown.

Furthermore, in the above-mentioned embodiments, the case where the present invention is applied to a wire electrical discharge machining device using the wire electrode 1 as a machining electrode was also explained, but a die-sinker electrical discharge machining device in which a water-based machining fluid is used. It goes without saying that the same effect can be obtained even when applied to.

〔Effect of the invention〕

As described above, according to the present invention, there is an effect that deterioration of the machined surface caused by electrolytic reaction can be prevented without reducing the electrical discharge machining speed. Further, the transition to the arc after dielectric breakdown during machining becomes smooth and reliable, and there is also the effect that arc generation is performed stably and reliably.

[Brief explanation of drawings]

Fig. 1 is an overall configuration diagram of the conventional device, Fig. 2 is a voltage and current waveform diagram in the conventional device, Fig. 3 is a circuit diagram showing the main parts of an embodiment of the device of the present invention, and Fig. 4 is the same as the above device. FIG. 3 is a pulse waveform diagram for explaining the operation. 1... Wire electrode, 4... Workpiece, 17...
DC power supply for pulse discharge circuit for insulation breakdown, 18,2
2...Switching element, 20...Switching element control circuit of pulse discharge circuit for dielectric breakdown, 21
... DC power supply of the main pulse discharge circuit, 23 ... Switching element control circuit of the main pulse discharge circuit, 25
... Sub-DC power supply of main pulse discharge circuit, 27 ... Pulse control circuit, 28 ... Current detector, 30 ... Discharge capacitor, 31 ... Inductance, 32 ...
...Resistance, Q...During machining〓.

Claims (1)

[Claims] 1. The workpiece is on the negative side, the processing electrode is on the positive side,
A pulse discharge circuit for dielectric breakdown is connected to each other, and a main pulse discharge circuit is connected to the workpiece on the positive side and the machining electrode on the negative side, and the pulse discharge circuit for dielectric breakdown In an electric discharge machining apparatus that performs a main discharge for machining by the main pulse discharge circuit after performing dielectric breakdown during machining between a machining electrode and a workpiece, (1) the main pulse discharge circuit comprises: , (1-1) a switching element for main discharge, and (1-2) a switching element for main discharge in which a negative electrode is connected to the machining electrode and a positive electrode is connected to the workpiece through this switching element for main discharge. (1-3) a diode with an anode connected between the processing electrode and the main discharge switching element; (1-4) a positive electrode connected to the cathode of the diode, (2) The pulse discharge circuit for dielectric breakdown includes: (2-1) a switching element for dielectric breakdown; (2-2) a dielectric breakdown DC power supply having a negative electrode connected to the workpiece and a positive electrode connected to at least the processing electrode via the dielectric breakdown switching element; The inductance of the current-carrying path connected between the negative electrode of the dielectric breakdown power supply and the dielectric breakdown switching element and the machining electrodes to the machining gap is set to a value that satisfies the vibration conditions. (3) After the dielectric breakdown of the machining gap is caused by the discharge of the dielectric breakdown discharge capacitor, the oscillating discharge current output from the dielectric breakdown discharge capacitor is An electric discharge machining apparatus comprising: a control circuit that controls the main discharge switching element so as to superimpose the main discharge current of the main pulse discharge circuit.