JP5357298B2 - Wire electrical discharge machine to detect machining status - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wire electric discharge machine little effected by variations in a detection circuit of inter-electrode voltage. <P>SOLUTION: An electric discharge ratio and an opening ratio per unit time are mostly determined in accordance with the opening ratio per unit time from an experiment of measuring the inter-electrode voltage and determining the discharge state, furthermore, the relationship changes very little among the machines. Consequently, only with detection of a clearly distinguishable opening state, electric discharge frequency and short circuit frequency per unit time are estimated. A trapezoidal wave voltage is applied between the electrodes during one application cycle, an intermission time is provided after voltage application, and the opening state is determined during the intermission time when the wire electric discharge machine is little effected by the variations in the detection circuit. An inter-electrode average voltage is determined based on the opening ratio for the application cycle frequency of the detected opening state and on a power supply voltage and the average voltage of one application cycle. By applying the conventional feed control using the frequency of the discharge thus found and the average voltage found from the opening ratio as indexes, highly precise machining can be actualized even in a high frequency machining. <P>COPYRIGHT: (C)2012,JPO&amp;INPIT

Description

  The present invention relates to a wire electric discharge machine capable of detecting a machining state of a wire electric discharge machine.

  In a wire electrical discharge machine, a voltage is applied between the wire electrode and the workpiece to cause discharge, and at the same time, the relative position between the wire electrode and the workpiece is changed to process the workpiece into a desired shape. To do. Since the machined surface is formed by collecting the discharge traces generated by the discharge, the surface roughness is determined by the size of the individual discharge traces. For this reason, it is known that a good machined surface can be obtained by applying a high-frequency voltage between the electrodes and repeating a short duration discharge at a high frequency. For example, Patent Document 1 discloses that a processed surface having a surface roughness of 1 μmRmax or less can be obtained by processing a workpiece by applying an AC high frequency voltage of 1 MHz to 5 MHz between the electrodes.

  In finishing processing, it is required to correct the shape processed by roughing with higher accuracy. It is necessary to control so that the processing amount increases at the time of finish machining for the part that becomes larger than the target dimension after rough machining, and conversely, the part that becomes smaller than the target dimension after the rough machining needs to be controlled so that the processing amount decreases at the finishing process. There is. That is, at the time of finishing, it is necessary to detect whether it is larger or smaller than the target dimension from the machining state, and to control the relative position between the wire electrode and the workpiece or to change the machining condition accordingly.

FIG. 7 shows a general high-frequency voltage waveform. Between each application, a necessary minimum dead time for preventing a short circuit of the bridge circuit provided in the power supply circuit is provided. The waveform between the poles is sinusoidal as shown in the figure.
The most common index indicating the machining state is an average value of absolute values of the interelectrode voltage. Since the average voltage between the poles generally represents the gap distance between the poles, a highly accurate machining shape can be obtained by controlling the shaft feed so that the average voltage between the poles is constant.
However, in order to obtain good surface roughness, when finishing is performed by applying a high frequency voltage as shown in FIG. 7 between the electrodes, the response of the detection circuit for the voltage between electrodes deteriorates in the high frequency region, and accurate average voltage information is obtained. Hard to get. In addition, since the variation in the component characteristics of the detection circuit increases in the high frequency region, the detection value varies from machine to machine. When the shaft feed control is performed based on such information, a difference occurs in the machining result for each machine.

  As a countermeasure, Patent Document 2 applies a DC voltage superimposed on an AC high-frequency voltage, extracts only the low-frequency voltage component of the interelectrode voltage by a low-pass filter, and controls the feeding of the wire electrode according to the change in this voltage. The technique of doing is disclosed. In this technique, since the average voltage does not become zero, there is a risk of causing electric corrosion on the workpiece or the processing machine body. Further, since a low-pass filter is used, when the discharge state changes suddenly, the response is poor and there is a possibility that it cannot follow.

As an index other than the average voltage, there is the number of discharges per unit time. Patent Document 3 discloses a technique for controlling a shaft feed rate and a pause time in accordance with the number of discharges. In order to realize this, it is necessary to classify the interelectrode state into three states of open, discharge, and short circuit. Here, the term “open” refers to a case where no discharge occurs even after voltage application, and the term “short circuit” refers to a state where the wire electrode and workpiece are in contact with each other and no discharge occurs, and neither contributes to machining.
However, when a sinusoidal high-frequency voltage as shown in FIG. 7 is applied, the application time during one application cycle is very long. Since the voltage between the electrodes is increased, the time during which the voltage between the electrodes is actually decreased is short, and the detection circuit cannot respond to such a very fast frequency, so that discharge and open cannot be distinguished.

Japanese Patent Laid-Open No. 61-260915 International Publication No. 2004/022275 JP 2002-254250 A

As described above, in the prior art, there is only a method for detecting the inter-electrode voltage as an index of the high-frequency machining state. However, in the high-frequency region, due to the influence of the response of the detection circuit and the variation of parts, there is a difference in the detection voltage value of the inter-electrode voltage for each machine of the wire electric discharge machine, and the difference appears in the machining result. There was a problem.
Accordingly, an object of the present invention is to provide a wire electric discharge machine that is less susceptible to the influence of variations in the inter-electrode voltage detection circuit for each machine in view of the problems of the prior art.

  The invention according to claim 1 of the present application is a wire discharge for machining a workpiece by generating a discharge by applying a high-frequency voltage between the wire electrode and a workpiece disposed at a predetermined interval between the wire electrode. In the processing machine, between the wire electrode and the workpiece, a positive voltage and a negative voltage with a period of 1 microsecond or less and a pause time of at least an application time between individual voltage applications A voltage applying unit for applying a voltage, an inter-electrode voltage detecting unit for detecting an inter-electrode voltage generated between the electrodes, and counting the number of times the voltage applied by the voltage applying unit is applied per unit time. Based on the inter-electrode voltage detected by the inter-electrode voltage detection means, the open-counting means for determining an open state in which no discharge occurs after the voltage is applied, and the open-discriminating means determined by the open determination means. An open rate counting means for counting the state per unit time, and an open ratio determined by the number of applications per unit time counted by the application count counting means and the number of open times per unit time counted by the open count means And determining the discharge ratio from the relationship between the discharge ratio and the short circuit ratio with respect to the open ratio per unit time determined in advance, and using the number of times of application and the discharge ratio, the number of discharges per unit time and the unit distance per unit distance It is a wire electric discharge machine characterized by obtaining at least one of the number of discharges, the number of short circuits per unit time, and the number of short circuits per unit distance.

  In the invention according to claim 2, the number of discharges and the number of short circuits per unit time are obtained from the discharge ratio, the short circuit ratio, and the number of times of application, and at least one of a machining amount per unit time and a machining amount per unit distance is obtained. The wire electric discharge machine according to claim 1.

  In the invention according to claim 3, the open state determining means compares the inter-wave voltage waveform subjected to full-wave rectification with the reference voltage within the pause time, and determines that it is in the open state when higher than the reference voltage. 3. The wire electric discharge machine according to claim 1, wherein when the voltage is lower than the reference voltage, it is determined as a non-open state. 4.

  According to the present invention, it is possible to provide a wire electric discharge machine that is not easily affected by variations in the detection circuit of the interelectrode voltage for each machine.

It is a schematic block diagram of a wire electric discharge machine. It is a figure explaining an example of the high frequency voltage which provided the idle time. It is a figure explaining the example of the voltage waveform between electrodes in process. It is a figure explaining the open | release discrimination | determination method. It is a figure explaining an example of a determination circuit. It is a figure explaining the relationship between an open ratio and a discharge ratio. It is a figure explaining an example of a general high frequency voltage.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram of a wire electric discharge machine. A machining tank (not shown) is disposed in the traveling path of the wire electrode 1 in the electric discharge machining section in the wire electric discharge machine. The processing tank is filled with the processing liquid. The wire electrode 1 and the workpiece 2 are connected to a machining power supply 4 that is a high-frequency power supply device for wire electric discharge machining via a feeder line 5 that is a machining voltage supply cable.
The machining power supply 4 includes a DC voltage source 41, a high-speed switching element 42 constituting a bridge circuit, and a current limiting resistor 43. The machining power supply 4 sends a machining power output (see FIG. 2) between the wire electrode 1 and the workpiece 2 via the feeder line 5 and the feeder 6 in accordance with a command from the machining power supply control device 8. Applied to the wire electrode 1. The workpiece 2 is connected to a machining power source 4 via a feeder line 5.

The machining power supply control device 8 receives a command related to the application time, the pause time, and the detection time from the numerical control device 9. The machining power supply control device 8 controls the machining power supply 4 based on a command from the numerical control device 9. The machining power supply control device 8 generates a pulse every application cycle corresponding to the detection time. The application time, rest time, and detection time will be described later with reference to FIGS.
The machining state detection device 7 detects a voltage generated between the electrodes between the wire electrode 1 and the workpiece 2. From the detected voltage generated between the electrodes, the number of application cycles and the number of releases per unit time are determined by a determination circuit described with reference to FIG. Data on the number of application cycles and the number of releases per unit time are sent to the numerical controller 9.
The numerical control device 9 controls the servo drive device 10 based on the number of application cycles and the number of releases per unit time sent from the machining state detection device 7. A servo motor (not shown) is driven by the servo drive device 10 to move the wire electrode 1 and the workpiece 2 relative to each other, thereby performing electric discharge machining on the workpiece 2.

FIG. 2 shows that between the electrodes between the wire electrode 1 and the workpiece 2, a voltage of both positive and negative polarity with a period of 1 microsecond or less and at least an application time between each voltage application. It is a figure explaining the example which provides the above idle time and applies a voltage.
FIG. 2A shows an ON command for the high-speed switching element 42 shown in FIG. FIG. 2B shows a waveform of the machining power output output from the machining power supply 4. FIG. 2 (3) shows the waveform of the interelectrode voltage applied between the electrodes generated by the machining power output of (2). In FIG. 2, the positive polarity and the reverse polarity are alternately applied, but the positive polarity may be continued twice or more, or the reverse polarity may be continued twice or more. In this method, since there is a stray capacitance between the poles and the power supply line, if the discharge does not occur, the voltage drop is negligible. Is trapezoidal.

  However, even in the trapezoidal waveform of the voltage between the electrodes, it is difficult to distinguish between a short circuit and a discharge. This is because the voltage waveforms at the time of short-circuiting and when discharging immediately after reaching the arc voltage after application are similar. That is, one pair of cables for transmitting the interelectrode voltage to the machining state detection device 7 is attached between a power supply unit 6 in FIG. 1 and the other between a table (not shown) that fixes the workpiece 2. The contact impedance between the supply electrode (not shown) and the wire electrode 1, the impedance of the wire electrode 1 itself, the contact impedance between the wire electrode 1 and the workpiece 2, and the contact between the workpiece 2 and the table Impedance is a major factor in the impedance between the poles, and these increase with higher frequencies. For this reason, particularly at high frequencies, a voltage is generated between the electrodes even at the time of a short circuit, which is difficult to distinguish from the time of discharge. An example of such a waveform is shown in FIG. As an example of a method for discriminating between short circuit and discharge, a certain discharge judgment level is provided, and if the discharge judgment level is never exceeded, it is judged that it is short-circuited, and if it exceeds even once, it is judged that it is discharged. it can).

  Like the average voltage between the electrodes, at high frequencies, the influence of variations in detection circuit responsiveness and component characteristics is large. Therefore, as in the discharge determination level in FIG. However, in the case of the discharge waveform as shown in (b) and (c) of FIG. 3 (1), the machine is still affected by the response of the detection circuit and variations in component characteristics. As a result, it is determined as a short circuit or a discharge, and eventually, although not as much as the average voltage, a difference occurs in the processing result for each machine.

Therefore, in the present invention, an application method (see FIG. 2) for applying a high-frequency voltage with a pause time is used to detect only the open state ((e) in FIG. 3 (2)) from the voltage between the electrodes. Based on the above, the shaft feed control of the wire electric discharge machine is performed. In this system of the present invention, there is no difference in processing results because it is not affected by the variation of the detection circuit as described above.
Next, a method for determining an open state in machining using the method for applying an interelectrode voltage in FIG. 2 will be described with reference to FIG. Here, the period from the start of application of a positive polarity or reverse polarity voltage, the stoppage of application until the start of application of the positive polarity or reverse polarity voltage is defined as one application cycle, V1 is an open determination level, and T1 is applied. The time from the start to the start of time until the open determination is made in each application cycle is shown. The open determination level V1 is optimally selected to be about 50 to 60% of the power supply voltage. In the open state, the absolute value of the interelectrode voltage exceeds the determination level V1 at time T1, whereas in the discharge or short circuit state, the absolute value of the interelectrode voltage falls below the determination level V1 at time T1. Therefore, in order to detect an open state by applying a high-frequency voltage, a voltage application method with a pause time as shown in FIG. 2 is very effective.
In FIG. 4 (2), T1 is the end time of one application cycle. However, as shown in FIG. 4 (3), after any application time within one application cycle, any time within the rest period There is no problem even after a certain time after the voltage is applied between the electrodes.

FIG. 5 illustrates an example of a determination circuit that determines an open state between the electrodes. The machining state detection device 7 includes a determination circuit. The inter-electrode voltage is input to a differential amplifier circuit composed of operational amplifiers 71 and 72 and amplified or attenuated to a predetermined level. Here, a voltage having a polarity opposite to that of the operational amplifier 71 is input to the operational amplifier 72.
The interpolar voltage amplified or attenuated by the differential amplifier circuit composed of the operational amplifiers 71 and 72 and the open determination level V1 are input to the comparators 73 and 74, and the output signal level of the comparator at time T1 is read. If the output of at least one of the comparators 73 and 74 is high level, it is open, and if it is low level, it is discharge or short circuit. The open determination level V1 is given by the reference voltage VR in the drawing. The outputs of the comparators 73 and 74 are input to the OR gate 75, and their logical sum is output from the OR gate 75 to the AND gate 76. The AND gate 76 also receives a pulse generated at any timing within each application cycle and within a rest period after the application time. The output from the AND gate 76 outputs a high signal when the comparator 73 or the comparator 74 determines an open state. The counter 78 is a counter that counts the number of times the AND gate 76 is opened. The counter 77 obtains the number of application cycles by counting the number of pulses generated during each application cycle.

By counting the number of times of opening per unit time by the counter 78 and transferring it to the numerical control device 9 (see FIG. 1), the numerical control device 9 can acquire the number of times of opening per unit time. At the same time, the counter 77 counts the number of application cycles per unit time and transfers it to the numerical control device 9 so that the numerical control device 9 can calculate the release ratio (= the number of releases relative to the number of application cycles).
In addition, by using the number of times of opening per unit time, it is possible to approximately obtain the average voltage between the poles from Formula 1 and Formula 2.

  Here, the average voltage of one application cycle needs to be experimentally obtained in advance as a value corresponding to the processing conditions. The average voltage calculated by Equation (1) or Equation (2) is very little affected by variations in the detection circuit for each machine, which is a problem in the prior art. For this reason, by carrying out axial feed so that the average voltage becomes constant, it is possible to obtain uniform machining accuracy with no variation for each machine.

Next, a method for obtaining the number of discharges and the number of short circuits from the open ratio will be described. From the experiment, it was found that the discharge rate per unit time and the short circuit rate are almost determined according to the open rate per unit time (see FIG. 6). Moreover, the relationship hardly changes depending on the machine.
In the experiment, the interelectrode voltage was measured using a voltage waveform measuring instrument capable of measuring the interelectrode voltage with high accuracy, and the discharge state was determined. From this fact, the number of discharges and the number of short circuits per unit time can be estimated by detecting only the open state that can be clearly distinguished.

  Note that (discharge ratio) + (short-circuit ratio) = 1− (open ratio), and the discharge ratio and the short-circuit ratio are obtained from FIG. 6 and the measured open ratio.

  As shown in the inter-electrode voltage waveform in FIG. 3 (1), various energy sources are mixed in the discharge, and the amount of machining by one discharge is not constant. In view of such circumstances, it is also conceivable to use the quantities of Equation 6 and Equation 7 as indices representing the machining state. This approximately represents the machining amount per unit time.

  Here, α and β represent the degree to which one discharge and one short circuit contribute to processing, respectively, and are constants obtained experimentally. In general, the value of β is 0 (zero). These machining amounts are very little affected by variations in the detection circuit for each machine, which is a problem in the prior art.For example, by performing axial feed so that the machining amount per unit distance is constant, It is possible to obtain uniform processing accuracy without any variation.

  Since the number of discharges per unit time and the average voltage obtained in this way are used for axial feed control for machining with high accuracy except for high frequencies, wire electric discharge machines accumulated in the past. The control algorithm (software) may be diverted as it is.

DESCRIPTION OF SYMBOLS 1 Wire electrode 2 Workpiece 3a, 3b Servo motor 4 Processing power supply 41 DC voltage source 42 High-speed switching element 43 Current limiting resistor 5 Feeding line 6 Feeding part 7 Processing state detection device 8 Processing power supply control device 81, 82 Operational amplifier 83 Comparator (Positive side)
84 Comparator (reverse polarity side)
85 OR gate 86 AND gate 87,88 Counter 9 Numerical control device 10 Servo drive device

Claims (3)

In a wire electric discharge machine that processes a work by generating a discharge by applying a high-frequency voltage between poles between a wire electrode and a work arranged at a predetermined interval in the wire electrode,
Between the wire electrode and the workpiece, a voltage having a positive polarity and a negative polarity with a period of 1 microsecond or less and a pause time of at least an application time is provided between each voltage application. Voltage applying means for applying
An inter-electrode voltage detecting means for detecting an inter-electrode voltage generated between the electrodes;
Application number counting means for counting the number of times of application of the voltage applied by the voltage application means per unit time; and
An open discriminating means for discriminating an open state in which no discharge occurs after the voltage is applied, based on the inter-electrode voltage detected by the inter-electrode voltage detection device;
An opening number counting means for counting the opening state determined by the opening determining means per unit time;
With
The open ratio determined by the number of applications per unit time counted by the application count counting means and the number of releases per unit time counted by the open count means, and the discharge ratio and short circuit with respect to the pre-determined open ratio per unit time Determine the discharge ratio from the relationship of the ratio, using the number of times of application and the discharge ratio, the number of discharges per unit time, the number of discharges per unit distance, the number of short circuits per unit time, and the short circuit per unit distance A wire electric discharge machine characterized by obtaining at least one of the number of times.   The number of discharges per unit time and the number of short circuits are obtained from the discharge rate, the short-circuit rate, and the number of applications, and at least one of a machining amount per unit time and a machining amount per unit distance is obtained. The wire electric discharge machine described.   The open state discriminating means compares the full-wave rectified inter-electrode voltage waveform with the reference voltage within the pause time, determines that the voltage is higher than the reference voltage, and determines that the voltage is lower than the reference voltage. The wire electric discharge machine according to claim 1, wherein the wire electric discharge machine is determined as a state.

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