JPH0223286B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrical discharge machining apparatus that performs machining by repeatedly supplying machining pulses generated by ON/OFF control of a switching element to a machining gap between an electrode and a workpiece.

It is known to connect a plurality of series circuits of switching elements and resistors in parallel to make the discharge current peak value a predetermined value by superimposed current, and to control the peak value by the number of parallel switching elements. Conventional switch element control involves supplying synchronizing pulses to a set number of switch elements to control them on and off in order to obtain a desired peak value. As a result, it is possible to control the pulse width (on), the rest width (off), and the peak value Ip, but it is not possible to control the rise characteristic di/dt of the discharge current.

When a pulse discharge occurs, a discharge point is created on the surface of both poles where current flows in a concentrated manner, and the two discharge points are connected by a discharge column. Immediately after the start of this discharge, the current density becomes extremely high, and at that time the temperature near the discharge point becomes high. As time passes, the area around the discharge point gradually melts and vaporizes, and the thickness of the discharge column also increases with time. At this time, discharge pressure is generated based on the metal gas released from the discharge point and the machining fluid gas heated by the discharge column, which causes the molten vaporized part to scatter and form discharge marks. . In this way, when the discharge point and discharge column are small immediately after the start of discharge, the current density A/cm ² becomes extremely large due to a discharge with a large di/dt due to a rectangular clean pulse, and as time passes, the discharge column becomes small. It is understood that the current density also decreases due to the spread of .

Therefore, with the conventional method of controlling only the peak value Ip, it is not possible to control the current density immediately after the start of discharge.
Usually the current density increases significantly instantaneously. In general, high-speed machining is performed at a current density of about 10 ^{6 to 8} A/cm ² , and for super-finishing the current density is about 10 ^{1 to 4} A/cm ² , but even at the start of electrical discharge, the machining target conditions are It is important to control the current density to correspond to the current density for proper processing.

Therefore, the present invention enables discharge by controlling the current rise characteristics so that the current density can be controlled to a predetermined value at the start of discharge. A control circuit that outputs a plurality of control pulses that are sequentially turned on with a time difference corresponding to the number of counted pulses by counting the output clock pulses and turned off simultaneously after the counting time of the set counting cycle, and an off signal of the control circuit. a time circuit which is activated thereby to set the rest time width of the control circuit independently of the clock pulses of the oscillator; and a gate circuit which supplies clock pulses from the oscillator to the control circuit by means of an output signal of the time circuit; A plurality of parallel switching in which each unit current is superimposed by a set number of switches by switching on and off a DC power supply connected in parallel to the machining gap between the electrode and the workpiece by each output control pulse of the control circuit. The device is characterized by being provided with an element.

This will be explained below with reference to an embodiment of the drawings. In FIG. 1, 1 is a machining gap formed by opposing a machining electrode (including a wire) and a workpiece, 2 is a DC power supply for power supply, and has a plurality of intermediate taps.
Apply each voltage to the machining gap. The connection circuit includes each switching element 31, 32, 33,...3n and inrush current control resistor (minimal value) 41, 42, 43,...4
A series circuit of n and 51, 52, 53, . . . 5n is inserted, and unit currents controlled by each circuit are superimposed and supplied to the machining gap. Reference numeral 6 denotes an n-ary counter whose counting cycle can be arbitrarily set, and outputs a step pulse to n output terminals every time it counts clock pulses, such as a ring counter. , 32, 33, . . . 3n time difference control is performed. 71, 72, 73, . . . 7n are circuits that self-hold the output pulses of each output terminal of the n-ary counter 6, and are all cleared by the n-th output of the n-ary counter. The n-ary counter 6 and the self-holding circuit 7
1, 72, 73...7n constitute a control circuit that outputs control pulses. 8 is a clock pulse oscillator that generates a counting signal, and the oscillation frequency can be changed in multiple stages. 9 is a monostable multivibrator that is activated by the n-th output of the n-ary counter 6 and returns to its original state after a predetermined time.
A time setting circuit is configured to set the pause time width of the control circuit, and its inversion time can be arbitrarily set, thereby arbitrarily setting the pause width τoff of the processing pulse. 10 is an AND gate which supplies a clock pulse to the n-ary counter 6 only when the output of the monostable multivibrator 9 is "1".

As described above, in the initial state at the start of machining, the output of the monostable multivibrator 9 is "1", and the output of the oscillator 8 is "1".
The clock pulse of n is passed through an AND gate 10.
Input to decimal counter 6. The counter 6 first applies a signal to the switch element 31 to turn it on. At this time, a voltage is applied from the highest voltage terminal of the power source 2 to the machining gap of the electrode workpiece to start electric discharge. Limiting resistor 4 to prevent rush current from becoming a large current
1 and 51 so that a predetermined unit current flows. Since the output signal of the counter 6 is held by the self-holding circuit 71, the switch element 31 remains on and the current continues to flow. Next, a signal is applied from the counter 6 to the switch element 32, which turns it on and causes current to flow from the intermediate tap of the power source 2 to the limiting resistor 4.
2, 52 to superimpose a predetermined unit current. Furthermore, as the count of the counter 6 progresses, the switch elements 33...3n are sequentially turned on to allow a superimposed current to flow. The current in each switch element circuit is set by a current limiting resistor so that a unit current flows. Note that the switch element circuits 33...3n are successively connected to intermediate taps where the voltage of the power supply 2 is gradually lower, but discharge has already occurred in the machining gap and the discharge point and discharge column have increased sufficiently. A predetermined unit current can be flowed at a sufficiently low voltage. Each switch element circuit 31, 32, 33,...3
The current superimposed by n becomes as shown in Figure 2,
At the start of discharge, unit currents i ₁ , i ₂ ,
By superimposing i ₃ and i _o , the rise characteristic di/dt of the flowing current can be controlled. The wave height value Ip=i ₁ +i ₂ +i ₃ + _io . When the n-ary counter 6 issues the final count output, the self-holding circuits 71-7n are cleared, and each switch element 31-3n is turned off to complete one pulse discharge. The pulse width 〓on is determined corresponding to the counting cycle of the n-ary counter. Therefore, by selecting and setting the counting cycle of the n-ary counter 6 in consideration of the set frequency of the clock pulse of the oscillator 8, the pulse width .tau.on of the machining pulse can be arbitrarily set. Also,
The signal is applied to the monostable multivibrator 9, which outputs a signal "0" during operation. After a predetermined time, the monostable multivibrator 9 returns to its original state and outputs a signal "1", so the gate 10 opens and the clock pulse is applied to the n-ary counter 6 again to start counting, controlling the switch elements 31 to 3n. Apply the following pulse current.

Therefore, the reversal time of the monostable multivibrator 9 becomes the pause width τoff.

FIG. 3 explains an embodiment in which the current rise characteristic di/dt is controlled to a desired value. If the slope is increased by shortening the time difference between when the clock pulse turns on, B is the opposite of A, and the slope is decreased by lowering the frequency of the clock pulse. Note that the unit current value and the number of superimpositions thereof by each switch element circuit can be arbitrarily switched and set, and the peak value Ip can be controlled. Furthermore, by switching and selecting the connection position between the output terminal of the n-ary counter and the switch element, the time difference between the sequentially superimposed currents can be controlled, and as a result, the current rising slope can be controlled. Switching and changing control of the rising slope of the current, the pulse width τon, and the pause width τoff can be controlled not only based on the target machining conditions but also by detecting the discharge state of the machining gap.

As described above, the rise characteristic di/dt of the discharge current can be controlled, and thereby the current density A/cm ² at the start of discharge can be controlled. Normally, in order to perform machining with increased machining speed, the current density must be increased.
10 ^{6 to 8} A/ ^cm2 , and for super finishing processing
10 Processing is performed at approximately ^{1 to 4} A/ ^cm2 , but the desired current density can be controlled by controlling the current rise characteristics at the start of each pulse discharge, and during steady state, the current density can be controlled to the desired current density by controlling the peak value Ip. As a result, the current density can be controlled, making it possible to stabilize processing under the desired conditions. Particularly in finishing machining, it is possible to perform machining with high precision without making the machined surface rough, and stable machining can be achieved without the occurrence of arcs or the like.
Processing can be performed with less electrode consumption, and in wire cutting using thin wire electrodes, rapid heating is suppressed and wire breakage is reduced, allowing stable and efficient processing. Furthermore, the pulse width τon can be set arbitrarily by setting the counting cycle of the control circuit that generates the control pulses in consideration of the set frequency of the clock pulse oscillator, and the pause width τoff can be set by changing the counting cycle of the control circuit that generates the control pulses, taking into account the set frequency of the clock pulse oscillator. , peak value Ip of discharge current
can be switched independently and arbitrarily depending on the number of superimposed unit currents flowing by each switch element connected in parallel, making it extremely easy to set processing conditions and achieving the desired Machining can be performed easily and efficiently.

In the embodiment shown in FIG. 1, a power supply with an intermediate tap is used as the DC power supply, and the superimposed currents are made to flow sequentially by step-down voltage, so the current limiting resistance can be made small, and the power efficiency can be improved. However, it is also possible to use a plurality of separate power supplies, and any number of switch element circuits can be connected in parallel. Inductance can be used for current control. Also, the clock pulse oscillator is the resistor 51, 52, 53,...5 of the switch element circuit.
When a discharge current is detected from n, the control circuit is configured to generate the next pulse to turn on the switch element, and further detect the discharge current and sequentially superimpose the discharge current so as to generate the next pulse. Can be done. The circuits used to generate control pulses for each switch element include those that convert the binary output of a register or counter into a step signal using a decoder, etc., and those that use the output selected by a switching selection circuit for the counter output. .

[Brief explanation of drawings]

FIG. 1 is a circuit configuration diagram of an embodiment of the present invention, FIG. 2 is a waveform explanatory diagram of an output processing pulse, and FIGS. 3A and 3B are other waveform explanatory diagrams. 1... Processing gap, 2... DC power supply, 31, 3
2, 33,...3n... switching element, 41,
42, 43,...4n and 51,52,53,...5
n...Resistance, 6...N-ary counter, 71, 72,
73...7n...Self-holding circuit, 8...Clock oscillator, 9...Monostable multivibrator, 10...
…andgate.

Claims (1)

[Claims] 1. An oscillator that generates clock pulses whose oscillation frequency can be switched, and by counting the output clock pulses of the oscillator, the clock pulses are turned on sequentially with a time difference corresponding to the number of counted pulses and turned off after the counting time of the set counting cycle. a control circuit that outputs a plurality of control pulses; a time circuit that is activated by an off signal of the control circuit and sets a pause time width of the control circuit independently of a clock pulse of the oscillator; and an output signal of the time circuit. A gate circuit supplies clock pulses from an oscillator to the control circuit, and a DC power supply connected in parallel between the electrode and the workpiece is turned on and off by each output control pulse of the control circuit. 1. An electrical discharge machining device characterized by being provided with a plurality of parallel switching elements that superimpose each unit current by a predetermined number of switches by switching.