JPH0242612B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a pulse generator for a wire-cut electrical discharge machine.

SUMMARY OF THE INVENTION An object of the present invention is to provide a pulse generator for a wire-cut electrical discharge machine that can generate machining current pulses with a steep leading edge and a narrow width and large amplitude to make the cut of a workpiece smooth and clean. That's true. The width of the processing current pulse must be short enough to avoid building up enough heat in the wire to cause it to thermally break. To compensate for this short width and maximize machining speed, the amplitude of the machining current pulse must be increased. Furthermore, as is well known, self-induced inductance inevitably exists in the wiring between the pulse generator and the processing gap. In order to obtain a machining current pulse with a steep rise, it is necessary to use a power supply with a voltage sufficient to overcome the adverse effects of this inductance. This is the second reason why high amplitude pulses are advantageous. However, if the amplitude of the machining current pulse is increased while ignoring the constantly changing state of the machining gap, problems such as arcing will occur.
Therefore, in order to supply the maximum possible machining energy that the state of the machining gap allows, the discharge start time is used as a guideline for indicating the state of the machining gap. In other words, a long delay before the start of discharge indicates that a machining current pulse with a large amplitude may be applied to the machining gap, and a short delay before the start of discharge indicates that a machining current pulse with a large amplitude should not be applied to the machining gap. It is shown that. It is known that the peak of the pulse supplied by the relaxation pulse generating circuit is modulated by the discharge start time. Therefore, the condition of the machining gap is monitored using a pulse generation circuit of this type, that is, a pulse generation circuit that supplies the machining gap with a voltage pulse whose peak level changes as an increasing function of the discharge start time. This classical pulse generating circuit is not used in the present invention to generate a machining current. Relaxation pulse generation circuits have many drawbacks, but one of the most problematic is that the pulse width cannot be determined in advance. Also note that when the voltage is zero or very low (short circuit), the current level is below the threshold required to break this short circuit (e.g., 50 amps), and the slope of the current pulse changes with the starting voltage and cannot be set. There are other problems. In the present invention, these problems of the classical relaxation type pulse generation circuit are avoided, and a pulse generation device is constructed that supplies a machining current pulse whose peak level changes with the discharge start time.

A low voltage is supplied between the work piece and the wire until the electric discharge starts, thereby avoiding the wire from fusing due to energy concentration. Since the peak of the machining current pulse must be large, the pulse generator of the present invention separates the function of starting electric discharge and the function of generating the machining current pulse.

It also allows for a minimum level of current pulses to be set in order to always maintain the current level above a threshold value that rapidly breaks short circuits that may occur between the workpiece and the wire.

The pulse generator of the present invention, which achieves the above objectives and functions, applies a low-level voltage pulse between the wire and the workpiece until the discharge starts in the machining gap, and has a short predetermined width and a steep rise. Then, a machining current pulse is generated whose amplitude increases as a function of the discharge start time and whose amplitude is set above a limit value that quickly destroys the short circuit. These low-level voltage pulse generation sources and machining current pulse generation sources are separate power supplies.

The prior art pulse generator for a wire-cut electric discharge machine related to the present invention described above is disclosed in the US Patent No.
No. 2979639, it comprises a low current source for starting the discharge, a high current source for supplying the machining current, and in the first phase starts the operation of the low current source and in the second phase detects the start of the discharge. The operation of the large current source is started in the third phase, and the operation of the large current source is ended in the third phase. However, this prior art is related to a thyratron system, and a transistor-based generator as in the present invention is used. It is not related to.

Reducing the self-induced inductance of the wiring is disclosed in German Patent Publication No. 2810437, in which a capacitor is provided in the indicator arm of the wire guide close to the processing area. The disadvantages of this prior device are that it requires mounting a group of capacitors on the support arm of the wire, providing a brush for contacting the workpiece, and immersing the movable contacts in the machining fluid.

Advantages of the invention other than those mentioned above include:
The level of current may be varied as a function of discharge onset time to obtain the best effect. For example, when the discharge start time is zero, the amplitude of the current pulse is set to a predetermined minimum level. The width of the current pulse and its rise can also be selected to optimal values.

Embodiments will be described with reference to the accompanying drawings.

Figures 1a to 1c illustrate the operation of the pulse generator of the present invention. FIG. 1a shows the change over time of the voltage U between the wire and the workpiece, and FIG. 1b shows the change over time of the discharge current I. Each of the repeating periods T1, T2, T3 and T4 with different time widths includes the following three phases:

a The voltage is increased according to a predetermined time curve, such as the exponential charging curve of a capacitor, by means of an auxiliary low current source supplying an incremental voltage.

b. The high current source starts in response to the detection of the start of discharge. However, since the switching time of semiconductor devices (0.1-0.5 microseconds) is relatively slow, there is a slight delay until the discharge current reaches _IP , as seen in pulses 1, 2, and 3. This first
It is an essential feature of the invention that the magnitude of the peak current varies as a function of increasing discharge onset time T _D measured between phases. Figure 1b shows four current pulses that decrease in magnitude as the discharge onset time T _D decreases. pulse width
T _A is preset to a predetermined value of several microseconds. Let's focus on pulse 4. In the first phase, the voltage increases with a certain slope. It waits for a certain time delay _TR , observes the voltage or current, and decides whether to start discharging or not. This is what happens when the discharge start time T _D is shorter than a certain time T _R . The time from the start of the first phase to the start of the current pulse of width T _A is thus always longer than the time T _R .

c No current is supplied from the current source during the predetermined time T _B.

FIG. 1c shows the variation of the terminal voltage of the pulse generator. Initially, from the first phase to the second phase, the voltage is the voltage between the wire and the workpiece. Following the start of the second phase discharge, a small current flows, and the voltage E suddenly increases to E _P1 (the voltage of the power source E2, which is a large current source). This E _P1 is much higher than the starting voltage and is large enough to overcome the self-inductance of the wiring and generate a steeply rising pulse current (E _P1 disappears when the pulse current reaches its peak value). . In order to make the fall of the pulse similarly steep, a reverse voltage E _P2 is generated at the end of the pulse current.
It should be understood that the pulse width remains a predetermined magnitude regardless of changes in the level of the pulse current.

FIG. 2 shows an embodiment of the pulse generator of the present invention.

At the beginning of the first phase, the synchronization supervisory circuit 40 starts the auxiliary low current source 10 by a signal sent over line 100TB. This signal turns off the transistor 13. A capacitor 12 connected in parallel to this transistor is rapidly charged by the power source E1. The charging time constant is adjusted by a variable resistor 11, for example. Exponentially increasing voltages are applied to field effect transistor 14, applying an equally increasing voltage to wire 1 via line 101. Work piece 2 and all ground points are connected to a common line 102. Since the saturation value of this voltage affects the processing speed, it is adjusted to the optimum value using a group of Zener diodes 16. After the indefinite discharge start time T _D , the discharge suddenly starts, and the current limited by the variable resistor 15 becomes the first current peak value I ₀ .

In the first phase, a group of comparators 22A, 22B...2 adjust the reference value by means of potentiometers 21A, 21B...21N of the adjustment device 20 for the peak current I _P
2N monitors and compares voltage increases. As the voltage increases, comparators 22A, 22B...
The output from 22N changes from high to low, and these signals pass through the NAND gates 23A, 23B...2
3N, 24A, 24B... is sent to the RS flip-flop consisting of 24N. Synchronous monitoring circuit 40
These flip-flops are connected to wire 1 in phase 1, to which signal TB from
It memorizes the voltage level experienced by and uses that voltage in the second phase, where a discharge voltage of about 25 volts is established. The stored voltage level is the AND gate 25
A, 25B...25N are sent to the first input terminals. The second input terminals of these gates are kept low by a signal TA sent over line 103 under the control of synchronization supervisory circuit 40.

When the voltage increases sharply, the time for the voltages to reach the reference values of the potentiometers 21A, 21B, . . . , 21N one after another is too short, making it difficult to monitor changes in the voltage. In this case, the voltage is integrated and the steep voltage is converted into a gradually increasing voltage for monitoring. For this purpose, an integrator 26 and a switch are provided. Of course, integrator 26 must be reset to zero at the end of each pulse. This reset is effected by signal TB sent over line 104.

Synchronous monitoring circuit 40 monitors the voltage on the wire via line 101 and changes the potential on line 103 from a low level to a high level in response to the start of discharge.

Due to this level change, the gates 25A and 25B
. . . a predetermined number of 25N are enabled and the second phase (second operating mode) begins. The output terminals of the AND gates 25A, 25B...25N are the self-control switching transistors 32 of the large current source 30.
Preamplifier 31 for A, 32B...32N
A, 31B... are connected to 31N. Power supply E
2 and the resistors 33A, 33B, . . . , 33N. The number of transistors switched on corresponds to the number of comparators 22A, 22B . . . 22N that are at a high level in the regulating device 20 at the beginning of the discharge. This number represents the voltage that increases according to a predetermined time curve and reaches the discharge start voltage, that is, the discharge start time T _D. As a result, the second phase is characterized by generating a current pulse with a peak value I _P that is a function of the discharge onset time T _D measured in the first phase.

The current pulse continues for a time T _A set by synchronization supervisory circuit 40, and at the end of time T _A , synchronization supervisory circuit 40 switches the potential on line 103 from high to low, shutting off the transistor. At the same time, a command is given via line 100 to the low current source to short circuit capacitor 12, which acts as a voltage pilot. This marks the beginning of the third phase period TB in which the low current source and the second current source are turned off.

At the end of this time TB, the three-phase cycle is restarted.

Attention is now drawn to the important features described above with reference to FIG. Due to the limitations of semiconductor devices, the transistor of high current source 30 supplies current with a delay of 0.1-0.5 microseconds. A low current source 10, maintaining a discharge at a voltage of about 25 volts, circulates a small current of a few amperes. Due to the above-mentioned delay, the power source E2 of the high current source 30 is unable to apply its full voltage to the machining gap between the wire and the workpiece. Therefore, a moderate voltage of 200 volts or less is selected for the low current source 10, and the voltage of the power source E2 of the large current source 30 is, for example, 400 volts. This means that the current is rapidly ramped up by combining a low voltage and a high voltage to start the discharge to create short, strong current pulses that are advantageous for wire-cut electrical discharge machines. This means that the inductance that attempts to prevent a sudden rise in current is smaller than in the prior art, and even if the wiring from the pulse generator to the machining gap is long, it will not be too inconvenient and will reduce efficiency. This enables high-speed machining.

FIG. 2 also shows the operation of the synchronization monitoring circuit 40. A synchronization monitoring circuit 40 includes a monostable multivibrator 41 for setting time TB,
A monostable multivibrator 42 that sets TR, a comparator 43 that monitors the voltage on line 101 to detect the start of discharge, a potentiometer 44 that determines the level at which to detect the start of discharge, and a timer.
Includes monostable multivibrator to set TA. . These three monostable
The input/output of the multivibrator is based on German patent no.
1,565,225 and US Pat. No. 3,916,138 to create a TD-TA-TB cycle. Discharge start time
The voltage may not be established at the beginning of T _D , so it is necessary to provide a delay T _R before monitoring the voltage to determine if discharge has started. Even if the discharge start time is zero, TR-TA-
TB cycle is established. That is, the time TR between the start of the first phase and the start of the second phase is always maintained.

High current source 30 may be configured to minimize dissipated energy in accordance with the configuration described in US Pat. No. 3,832,510. In that case, the resistor 33 in FIG.
Replace it with a circuit that keeps the current level circulating in the coil within predetermined limits.

FIG. 3 shows two examples of the variation of the discharge current I as a function of the discharge start time _TD .

Curve I shows a continuous function from some minimum value I ₀ ,
The curve shows a two-stage discontinuous function, and each stage corresponds to a predetermined time up to the discharge start time.

[Brief explanation of the drawing]

Figures 1a and 1b are waveforms showing the voltage and current pulses in the machining gap, and Figure 1c is the waveform of the voltage pulses applied by the pulse generator. FIG. 2 is a circuit diagram of the pulse generator of the present invention. FIG. 3 shows two examples of the variation of the discharge current I as a function of the discharge start time _TD . In the figure, 10 is a low current source, 20 is a regulator, 30
is a large current source, and 40 is a synchronous monitoring circuit.

Claims (1)

[Claims] 1. Supplying a gradually increasing voltage whose peak level changes as _an increasing function of the discharge start time _T
In a pulse generator of a wire cut electrical discharge machine that cuts a workpiece by applying a machining current pulse having a machining current pulse to a machining gap between a wire and a workpiece, an auxiliary low current source for supplying, a high current source including a power source for supplying said machining current pulses into the machining gap, a regulating device connected to said auxiliary low current source and said high current source, and said auxiliary low current source. a synchronization monitoring circuit connected to a source, a high current source, and a regulating device, said regulating device having means for determining the value of a discharge start time T _D and following a signal from said synchronization monitoring circuit. and means for initiating the operation of said high current source and varying the current level of each machining current as an increasing function of discharge initiation time T _D , and said synchronization supervisory circuit initiates operation of said auxiliary low current source. means for initiating, detecting an electrical discharge across the machining gap, and immediately after this detection sending a signal to the regulating device to initiate the operation of the high current source; and after a time of the predetermined width T _A , A pulse generator for a wire-cut electrical discharge machine, comprising means for terminating the operation of an auxiliary low current source and a high current source. 2. The means for determining the value of the discharge start time T _D is to compare the incremental voltage supplied by the low current source with a separate reference voltage and determine the discharge start time T _D at the level of the incremental voltage up to the final sharp drop. including a comparator that determines
The high current source then includes a plurality of parallel switching devices, each connected in series with the power supply, which switch devices close one after another as the level of the increasing voltage reaches a respective reference voltage, thereby The pulse generator for a wire-cut electrical discharge machine according to claim 1, wherein each machining current pulse is increased as the discharge start time T _D increases. 3. The wire-cut electrical discharge machine according to claim 1, wherein the synchronization monitoring circuit includes means for preventing the start of operation of the high current source within a predetermined time _TR after applying the voltage of the low current source to the machining gap. pulse generator. 4. The pulse generator for a wire-cut electrical discharge machine according to claim 1, wherein the adjustment device includes a circuit that time-integrates the voltage supplied by the low current source.