JPH0211370B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in an electric discharge machining apparatus, particularly an electric discharge machining apparatus that performs machining by supplying a pulsed current to a machining gap between opposing machining electrodes and a workpiece electrode.

In this type of electric discharge machining equipment, the voltage waveform applied to the machining gap is shown in Figure 1A, on-time (T ₁ + T ₃ + T ₃ ) = T ₀ when the required voltage is applied and off-time when no voltage is applied. A method has been proposed in which the processing characteristics are improved by repeating T ₄ and changing the voltage waveform during on-time T ₀ .

That is, for example, as shown by the dotted line in FIG. 1A, voltage _E2 is applied in the initial section _T1 , voltage _E3 in the next section _T2 , and voltage _E1 in the final section _T3 . In this case, the interval T ₁ is the target time period for dielectric breakdown of the machining gap, and increasing the applied voltage E ₂ increases the machining gap length, making it easier to remove generated chips and stabilizing the machining. can get.

Further, in each section _T2 , if the discharge has not started during the section _T1 , the highest voltage _E3 is applied to serve as an ignition pulse to encourage the start of the discharge.

Furthermore, in section T ₃ , if discharge is not started in sections T ₁ or T ₂ , the machining gap is returned to low voltage E ₁ again, making it difficult to start discharge in this section and reducing the electrode wear ratio due to the short current pulse width. Preventing deterioration.

Also, during discharge in any of the sections T ₁ , T ₂ and T ₃ , an arc voltage (usually 20 to 30 V) lower than the low voltage E ₁ is maintained as shown by the solid line in FIG. 1A.

By controlling these waveforms, the current pulse width can be made uniform, the number of discharges per unit time can be increased, and a power source for electric discharge machining with good machining characteristics can be obtained.

In order to perform the above voltage waveform control at
The device shown in the figure has been proposed.

In FIG. 2, 10 is a machining electrode, 12 is a workpiece electrode facing the machining electrode 10 while maintaining a machining gap, 14 is a DC power source that outputs voltage _E1 , 16 is a DC power source that outputs voltage _E2 , 18 is a DC power supply that outputs voltage _E3 . Further, 20, 22 and 24 are switching transistors, the bases of which are supplied with control pulses S ₁ , S ₂ and S ₃ shown in FIG. 1B, C and D from a control pulse forming circuit 26, respectively. , transistors 20, 22 and 24
are connected to the positive sides of DC power supplies 14, 16 and 18, respectively. The emitter of the transistor 20 is connected to the processing electrode 10 through parallel resistors 28 and 30, a selection switch 32, and a diode 34. In this case resistor 2
By selecting the selector switch 32 between 8 and 30, the combined resistance can be variably set.
It is clear that the variable number of stages can be obtained by increasing the number of transistors and resistors as necessary to obtain the required peak current value and the desired number of stages.

Further, the emitter of the transistor 22 is connected to the processing electrode 1 through a resistor 36 and a diode 38, and the emitter of the transistor 24 is connected to the processing electrode 1 through a resistor 40.
Connected to 0. Note that the resistance values of the resistors 36 and 40 are selected to be one order of magnitude higher than those of the resistors 28 and 30.

In order to perform the voltage waveform control shown in FIG. 1A in the above configuration, first, the control pulse shown in FIG. 1B is
Total on-time T ₀ of transistor 20 by S ₁
By turning it on during the middle, a peak current is supplied to the machining gap during discharge (this current value is equal to the output voltage E ₁
- the value obtained by dividing the value of the arc voltage by the combined resistance of the resistors 28 and 30), and if the discharge has not started, the open circuit voltage _E1 is applied in the interval _T3 .

The control pulse S ₂ is controlled by the transistor 22 only in the section T ₁ .
is turned on and an open circuit voltage E ₂ is applied to the processing electrode 10. At this time, the influence of reverse voltage on transistor 20 is blocked by diode 34. This section
When discharge is started at T ₁ , a current is added to the machining gap through the resistor 36, but since the resistance value of the resistor 36 is selected to be large, its influence can be ignored.

The control pulse _S3 is controlled by the transistor 24 only in the section _T2 .
is turned on and an open circuit voltage E ₃ is applied to the processing electrode 10. At this time, the influence of the reverse voltage on the transistors 20 and 22 is blocked by the diodes 34 and 38, and the influence of the addition of current to the machining gap when discharging in the section _T2 is also prevented by the resistance value of the resistor 40. It is large and can be ignored.

Although the voltage waveform control shown in FIG. 1A is carried out through the above-described operation, this case has the following drawbacks.

That is, first, when performing fine voltage control, it is necessary to add DC power supplies with different output voltage values, and a large number of isolation transformers and rectifiers are required to constitute the DC power supply, which increases cost.

Second, since the emitter potentials of the transistors 20, 22, and 24 are independent, control power supplies for driving each transistor are required for the same number of transistors, which increases costs.

Thirdly, since the heat loss generated by the resistors 28 and 30 is large, the power efficiency is low, and the temperature inside the power supply panel housing each component is likely to rise, which may reduce reliability.

Fourth, in order to reduce the heat loss, the DC power supply 1
Although it is possible to lower both the output voltage _E1 of No. 4 and the resistance values of resistors 28 and 30, the peak current value may be lowered by changing the aforementioned arc voltage depending on the combination of materials of the processing electrode and the electrode to be processed. The range of fluctuation increases. Furthermore, since the short circuit current increases when a short circuit occurs in the processing gap, a problem arises in that the transistor 20 requires a design margin.

The present invention has been made in view of the above-mentioned conventional problems, and its purpose is to provide a novel electric discharge machining apparatus that can eliminate the drawbacks of the conventional apparatus.

In order to achieve the above object, the present invention provides an electric discharge machining apparatus that performs machining by supplying a pulsed current between opposing machining electrodes and workpiece electrodes. a first series circuit including a first output device that outputs a pulse having a variably settable peak current value, and a unidirectional conductive element; and a constant current value connected in parallel with the first series circuit. a second output device that outputs a second output device, a second series circuit including a unidirectional conductive element, a variable impedance device connected in parallel with the second output device, and a processing interval between the first output device and the first output device. A control pulse forming device that generates an impedance switching signal for switching the non-application time of the voltage pulse and the multi-stage open circuit voltage after the voltage pulse is applied to the impedance variable device after the pulse output is completed; Features.

Hereinafter, preferred embodiments of the present invention will be described based on the drawings.

FIG. 3 shows an embodiment of the present invention, in which the processing electrode 1
A series circuit including a first constant current device 42 and a diode 44, which can vary the output current value, is connected between the electrode 0 and the electrode 12 to be processed. In this case constant current device 4
2, the output current value thereof is arbitrarily variably set by a current value setting circuit 46.

Reference numeral 48 has a collector connected to the connection point between the constant current device 42 and the diode, an emitter connected to the connection point between the constant current device 42 and the electrode to be processed 12, and a base connected to the control pulse forming device 50 as an open voltage control device. This is a switching transistor to which pulse _S7 is supplied.

Further, a second circuit with a fixed output current value is connected in parallel with the series circuit.
constant current device 52, resistor 54 and diode 5
6 series circuits are connected.

Further, a variable impedance circuit 58 is connected in parallel with the constant current device 52. This circuit 58 includes a switching transistor 60, a series circuit of a resistor 62 and a switching transistor 64, a series circuit of a resistor 66 and a switching transistor 68, and a resistor 70 connected in parallel. Transistors 60, 64, 68
Control pulses S ₄ , S ₅ , and S ₆ of the control pulse forming circuit 50 are supplied to the bases of the control pulse forming circuit 50, respectively.

A voltage comparator 72 is connected between both ends of the resistor 54, detects the voltage between the terminals, compares this with an internal set voltage (for example, 50V), and when the voltage is lower than the set voltage, outputs a discharge start detection signal S. ₈ is supplied to the control pulse forming device 50.

Reference numeral 74 is a clipper circuit inserted in parallel with the transistor 48, and when the transistor 48 is turned off, the extremely high voltage generated in the processing electrode 10, the processing electrode 12, and the electrical circuit connected thereto may damage the used parts. Normally, a device with a diode and a DC power source connected in series or an element with Zener characteristics is used to prevent this, but in this embodiment, as will be described later, a constant current device Since an output current of 42 is supplied to the machining gap, an element having a small capacity Zener characteristic can be applied.

The above is an example of the configuration of the present invention, and its operation will be explained next.

In the off time before on time T ₀ ,
Control pulses S ₄ , S ₅ , S ₆ of control pulse forming device 50
and S ₇ are on as shown in FIG.
and 48 are turned on, so that the constant current device 4
The output currents of transistors 48 and 6
Since it is bypassed by 0, it is not supplied to the machining gap and the off time T ₄ is maintained.

When transitioning from this state to the on-time T ₀ , the control pulses S ₄ and S ₅ are turned off at the start time t ₀ of the section T ₁ , and the transistors 60 and 64 are turned off.
For this reason, for example, the output current value of the constant current device 52
1A, the resistance values of resistors 62, 66 and 70, respectively.
If 200Ω, 600Ω and 300Ω are selected, the combined resistance of variable impedance circuit 58 will be 200Ω, which is the parallel resistance value of 600Ω and 300Ω, and the open circuit voltage will be 200Ω×1A=
A voltage E ₂ occurs, expressed as 200V.

Also, in section _T2 , control pulse _S6 is also turned off, so the resistance value of variable impedance circuit 58 is
300Ω, and the open voltage is 300Ω×1A=300V, which generates a voltage _E3 .

Furthermore, in section _T3 , control pulses _S5 and _S6 are turned on, so that the combined resistance of variable impedance circuit 58 becomes 100Ω, the parallel resistance value of 600Ω, 300Ω, and 200Ω, and the open circuit voltage is expressed as 100Ω×1A=100V. A voltage E ₁ is generated.

However, when a discharge occurs in the machining gap at time _td in section _T1 and the voltage drops to the arc voltage, a discharge start detection signal _S8 is sent from the voltage comparator 72 as shown in FIG. 1I.
Therefore, the control pulse _S7 is turned off, the transistor 48 is turned off, and the output current of the constant voltage device 42 is supplied to the machining gap as a peak current value. At this time, the output current of the constant current device 52 is divided into the machining gap and the variable impedance circuit 58. The current value shunted to the variable impedance circuit 58 is about 0.1A to 0.3A when the arc voltage is 25V, which is very small when viewed from the whole, but the constant current device 4
When the set output current value of No. 2 is small, the influence increases relatively. In order to eliminate this drawback, as an improvement of this embodiment, the discharge start point t _d in the machining gap is
At the same time, the control pulse S ₄ is applied as shown by the chain line in FIG. 1E.
is turned on and the transistor 60 is turned on to bypass the entire current of the constant current device 52 to the transistor 60, or the control pulse S ₅ is simultaneously turned on at the discharge start point t _d .
And by turning off S ₆ and turning off transistors 64 and 68, the current flowing to the impedance circuit can be made constant.

Further, the control pulse _S7 that controls the transistor 48 is controlled to be off from the discharge start time _td to the end of the on-time _T0 , that is, the end of the section _T3 , as shown in FIG. 1H. The reason for this is that if the transistor 48 is turned off before the discharge starts in the machining gap, the output current of the constant current device 42 will reduce the minute stray capacitance formed in the machining electrode 10, the workpiece electrode 12, and the electrical circuits connected thereto. This is because it charges immediately and generates a very high voltage, which may damage the parts used, and it is necessary to select a large capacity for the clipper circuit 74 to prevent this.

The diode 44 also prevents the open circuit voltages E ₁ , E ₂ , and E ₃ applied to the machining gap from flowing into the transistor 48 or the clipper circuit 74 and damaging the normal voltages.

Furthermore, if the discharge start detection signal S ₈ obtained from the voltage comparator 72 is ANDed with the signal representing the on-time T ₀ to convert it into a true discharge detection signal, each pulse of the control pulse forming device 50 _S4 , _S5 ,
It can be used to control S ₆ and S ₇ , and by controlling the end point of on-time T ₀ as a function from the discharge start point t _d , the time of the current pulse width flowing through the machining gap can be made uniform. It becomes possible to do so.

In this embodiment, the discharge start detection signal _S8 obtained from the voltage comparator 72 is used to turn off the control pulse _S7 from the control pulse forming device 50, and turn off the transistor 48 from the discharge start time _td . I showed an example in which the on-time
The control pulse S ₇ from the control pulse forming device 50 may be turned off after a predetermined period of time has elapsed from the start of T ₀ .

As described above, according to the present invention, firstly, the voltage applied to the processing gap can be varied by simply selecting and combining the resistors and transistors of the variable impedance circuit connected in parallel with the second constant current device. It can be varied, and there is no need to provide a large number of expensive DC power supplies. Second, since the emitters of the transistors in the variable impedance circuit are all shared, it is possible to use a single control power source for driving the transistors. Thirdly, heat loss can be reduced because there is no need to insert a resistor into the closed loop formed by the variably settable first constant current device and the machining gap. Fourth, the first current device always maintains a constant current even in the event of fluctuations in arc voltage due to the combination of machining electrode material and workpiece electrode material, when a short circuit occurs between both electrodes, and fluctuations in the input voltage of the electrical discharge machining device. It has excellent effects such as being able to supply a current pulse with a peak value to the machining gap.

In the above example, a variable setting constant current device 4 is used.
2 and the transistor 48 constitute a pulse generator in which the peak current value can be variably set.
4. It is also possible to replace the peak current value consisting of the transistor 20, resistors 28, 30, and changeover switch 32 with a pulse generator that can variably set the value, and in this case, except for the third and fourth effects. It is clear that the same effect as in the case of FIG. 3 can be obtained.

Further, each switching transistor is not limited to a bipolar type, but may be an FET type, and is not limited to a transistor, but may be any controllable switching element. Furthermore, the circuit system of the constant current device can be configured as desired.

Further, the method for detecting the start of discharge is not limited to the above example, and it goes without saying that the presence or absence of a discharge current in the machining gap may be detected.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of a method of controlling the output voltage waveform of an electrical discharge machine, and is a time chart showing the inter-machining voltage waveform and control pulses in the conventional and embodiments of the present invention. FIG. 3 is a circuit diagram showing an example of the electric discharge machining apparatus according to the present invention. The same members in each figure are given the same reference numerals, 10 is the processing electrode, 12 is the processed electrode, 4
2 is a first constant current device, 44 is a diode, 50
5 is a control pulse forming device, 52 is a second constant current device, 56 is a diode, and 58 is a variable impedance circuit.

Claims (1)

[Scope of Claims] 1. In an electric discharge machining device that performs machining by supplying a pulsed current to a machining gap between opposing machining electrodes and a workpiece electrode, a variable setting device connected between the machining electrode and the workpiece electrode; a first series circuit including a first output device that outputs a pulse having a peak current value that can be adjusted, a unidirectional conductive element, and a first series circuit that outputs a constant current value that is connected in parallel with the first series circuit; a second series circuit including a second output device and a unidirectional conductive element; a variable impedance device connected in parallel with the second output device; and pulse output to the machining gap by the first output device. and a control pulse forming device that generates an impedance switching signal for switching the non-application time of the voltage pulse and the multi-stage open circuit voltage after the application of the voltage pulse to the variable impedance device after the voltage pulse is applied. electrical discharge machining equipment.