JP5056907B2 - Electric discharge machining apparatus and electric discharge machining method - Google Patents

Description

  The present invention relates to an electric discharge machining apparatus and an electric discharge machining method, and more particularly to a technique for recognizing an electric discharge machining state and controlling feed of a machining axis based on the recognition result.

The electric discharge machining apparatus is configured to generate an electric discharge between a tool electrode provided in a machining fluid and a workpiece to melt and remove the workpiece in the machining fluid.
In electric discharge machining, machining scrap generated by melting and removing the workpiece is generated between the tool electrode where the electric discharge occurs and the workpiece (hereinafter referred to as the machining gap), and if the machining scrap is not excluded from the machining gap by any means, It is well known that the insulation recovery of the machining gap and the repetition of electric discharge cannot be maintained in a normal state, and there are adverse effects such as a reduction in machining efficiency and deterioration of the machined surface properties.

In order to eliminate the machining waste and maintain the machining gap, the electric discharge machining apparatus detects the discharge voltage and controls the machining axis with respect to the change in the discharge voltage every moment. For example, in the method of Japanese Patent Publication No. 44-13195, the average voltage (Vg) within a specific sampling time is treated as a discharge state and compared with a servo reference voltage (SV) which is a preset target average voltage. In addition, the stability of electric discharge during machining is maintained by performing machining axis feed control, that is, servo control by an electric discharge machine.
Specifically, a detection line is provided in the machining gap formed by the tool electrode and the workpiece, the voltage of the machining gap is obtained with a detector, and the discharge voltage at that time is passed through a filter circuit and averaged The average voltage detected as a result of comparison with the servo reference voltage (SV) determined in advance on the axis control device, which is smoothed and extracted within a specific sampling time as an average voltage (Vg) Is lower than the target average voltage, the machining axis is returned to the direction opposite to the machining direction, and when it is higher, the machining axis is sent in the machining axis direction.

In order to perform machining axis control, in the method of detecting the gap state from the voltage fluctuation of the machining gap through the filter, the sampling time and the time constant of the filter circuit are close, and if the time constant is sufficiently smaller than the sampling time, the circuit If at least the time constant of the filter circuit is set to 2 to 3 times the sampling time, the charge / discharge characteristics of the configured filter are affected, resulting in a difference from the target value (FIG. 8). Designing a filter with the natural vibration characteristics of the machine is a very difficult problem.
In addition, a detection line is required to detect the voltage, or even if a dedicated detection line is not required, the detection line may be used together with the supply line from the power supply. When the length becomes longer, the L component increases on the electric circuit, and the state of the machining gap and the detected voltage component become the voltage through the L component, which is different from the actual machining state. .

Japanese Patent Application Laid-Open No. 6-262435 discloses an electric discharge machining apparatus having means for counting no-load time (Td), pulse width (Ton), and rest time (Toff) using a clock pulse.
In this method, it seems as if the above problem has been solved by eliminating the filter circuit that detects discharge, but the controlled object is the servo reference voltage (SV) itself, and the servo reference voltage (SV) is set depending on the machining state. The change can be improved in terms of stability, but as a result, there is a problem that the servo reference voltage is high, that is, the machining is performed in a state where the machining efficiency is lowered, and the machining speed is greatly reduced.

  Japanese Patent Laid-Open No. 7-246518 discloses a method of counting the discharge frequency and the number of short circuits, and estimating and controlling the discharge gap length from the result and a separately determined no-load time (Td). The rest time (Toff) and the no-load time (Td) are long with respect to the width (Ton), and the discharge energy is only intended for small finishing. When this technology is applied to normal machining, the no-load time As a result, there remains a problem that the processing speed decreases.

In Japanese Patent Laid-Open No. Hei 6-170645, similarly, the discharge frequency is counted, the variation in the discharge frequency and the determination of the quality of the discharge are corrected by fuzzy inference, and the membership function related to the state change is performed so that proper control is performed. A means for preparing and controlling is disclosed.
Although this method mentions how to avoid an exceptionally unstable situation that was a problem of Japanese Patent Laid-Open No. 7-246518, the definition of the membership function defines its design. A lot of know-how is required for itself, and the stability and results of machining are strongly influenced by the membership function itself.

Japanese Patent Publication No. 44-13195 JP-A-6-262435 Japanese Patent Laid-Open No. 7-246518 JP-A-6-170645

  However, the conventional problem is that the discharge state in the discharge gap cannot be accurately detected. Servo control can be performed between the electrodes regardless of whether a filter circuit is used or the discharge frequency is detected by a counter. If the state is detected accurately, the basic control itself is not significantly different.

The present invention has been made paying attention to the problems as described above, and correctly detects the state of the machining gap formed by the tool electrode and the workpiece even in a relatively simple apparatus configuration, The machining axis feed control, so-called servo control, is performed so that it is reflected in the discharge state and can be adapted to the momentary change according to the state.

In order to achieve this object, in an electric discharge machining apparatus that performs machining axis control so that the machining average voltage Vg within the predetermined sampling time Ts becomes the servo reference voltage SV, the gap between the tool electrode and the workpiece is determined. Power supply means for supplying power to the battery, discharge detection means for detecting a discharge waveform between the electrodes based on the power supplied by the power supply means, and generation of discharge within a predetermined sampling time in the discharge waveform A discharge occurrence counter that counts the number of times Nd, and a short-circuit occurrence that counts the number N1 of short-circuit discharges in which the discharge due to voltage application from the power supply means falls below a preset short-circuit voltage threshold Vsh in the discharge waveform Number counter means, discharge occurrence number Nd, short circuit number N1, preset no-load voltage V0, pulse width Ton, rest time Toff, discharge voltage eg, sample The calculation means for calculating the estimated average voltage Vgs between the poles using the measurement time Ts, and the estimated average voltage Vgs calculated by the calculation means so as to become the servo reference voltage SV within the sampling time Ts. Electrode position control means for performing machining axis control.

  According to the present invention, even with a relatively simple apparatus configuration, the state of the machining gap formed by the tool electrode and the workpiece is correctly detected, reflected in the discharge state, and momentarily according to the state. It is possible to perform machining axis feed control, so-called servo control, which can cope with changes in the angle.

It is a block diagram which shows schematic structure of the electric discharge machining apparatus in Embodiment 1. FIG. It is a figure for demonstrating the detection of the frequency | count of discharge generation in a certain sampling time. It is a figure which shows a certain discharge phenomenon. It is the figure which showed the relationship between the average voltage of a process gap | interval, and the frequency | count of discharge generation. It is the figure which showed the relationship between the average voltage of an actual process space | interval, and the frequency | count of discharge generation. It is the figure which showed the relationship between the average voltage of an actual process space | interval, and the frequency | count of discharge generation. It is the flowchart which showed the control flow in this invention. It is a figure which shows the relationship between a process gap voltage waveform and a filter circuit voltage waveform.

Embodiment 1 FIG.
FIG. 1 shows an embodiment of an electric discharge machining apparatus according to the present invention. In this embodiment, the X axis and the Y axis will be described as an example in which the work table is movable. However, the X axis and the Y axis may be an electric discharge machining apparatus in which the main shaft side is movable. The shaft mechanism and the machine configuration itself do not affect the embodiment.

The electric discharge machining apparatus includes a spindle 4 driven in the Z-axis direction by the motor 1, a work table 5 driven in the X-axis direction by the motor 2, a spindle work table 6 driven in the Y-axis direction by the motor 3, A machining tank 7 installed on the work tables 5 and 6, a tool electrode 8 is attached to the spindle 4, a machining fluid is injected into the machining tank 7, and a workpiece W is Be placed.
The tool electrode 8 and the workpiece W are opposed to each other in the machining fluid with a machining gap, and electric power is generated between the tool electrode 8 and the workpiece W by power supplied from the power supply device 9. The workpiece W is melted and removed.
When machining conditions such as a machining program are set by the machining condition setting unit 11, the electrode position control device 10 controls the motor 1, the motor 2, and the motor 3 according to the contents of the program, and controls the position of each axis. Servo control is performed. The electrode position control device 10 also performs jump control of the spindle 4 and swing control for performing processing while giving a specific locus to the workpiece W on the tool electrode 8.

In the machining condition setting unit 11, the basic machining conditions set for performing the electric discharge machining are the discharge current (IP), the pulse width (Ton), the pause time (Toff), the applied voltage (V0), and the servo reference voltage. (SV), jump control setting (JUMP), swing control setting (Orb), target machining position (Zref), etc. are registered and recorded using the input device.
In addition to this, for example, in order to determine the machining state, the discharge voltage (eg) when normal discharge is occurring, the abnormal discharge voltage threshold (Vng), the short-circuit voltage threshold (Vsh), and the minimum no-load time (Tdo) It is also possible to set a pause time (Toffs) when control is performed to extend the pause time when an abnormal discharge occurs, and a machining area (which is already an electric discharge machining portion of the tool electrode 8 to be machined ( If S) is known, it is also possible to input the processing area (S). These pieces of information can be individually set and stored for each condition to be used. When the power supply device 9 calls a predetermined basic processing condition, the information is called and stored in each control device. Is read.

The discharge detection circuit 13 records the total number of discharge occurrences (Nd) generated between the tool electrode 8 and the workpiece W at every sampling time (Ts), and the detection result is transferred to the main arithmetic unit 12.
After the transfer, each value detected by the discharge detection circuit 13 is reset, and the next sampling is started.
Moreover, in the discharge detection circuit 13, when the short-circuit voltage threshold value (Vsh) is set in the machining condition setting unit 11, the number of times that the discharge below the threshold is short-circuited based on the short-circuit voltage threshold value (Vsh). Record (N1).
Similarly, when the minimum no-load time (Tdo) is set, the discharge of the no-load time below that is set to the number of small no-load discharges (N2) and the abnormal discharge voltage threshold (Vng) is set , The discharge below that is individually recorded as the number of abnormal discharges (N3).

Here, the normal discharge has a no-load time (Td) longer than the minimum no-load time (Tdo), and the discharge voltage (eg) is higher than the abnormal discharge voltage threshold (Vng).
The short circuit is a state in which the tool electrode 8 and the workpiece W are in contact with each other. At this time, no discharge is generated, but a short circuit current is generated when the tool electrode 8 and the workpiece W are conducted.
Since a short-circuit voltage of 0 to several tens V is generated at the time of a short circuit, a voltage lower than the short-circuit voltage threshold (Vsh) is recognized as a short circuit.
Although a short circuit may be considered when the tool electrode 8 and the workpiece W are conducted even through the machining waste, it is difficult to recognize as a state of the machining gap, but when a short circuit occurs, Since it is in contact, it leads to deformation of the tool electrode in severe cases, and even if it is minor, it becomes a factor such as a stain and the quality of the machined surface is impaired.
The minimum no-load time (Tdo) is provided because the short no-load time is continuously performed in the vicinity of the occurrence of discharge, and in this case, the discharge is concentrated. Become.
Concentration of electric discharge results in local consumption and processing, and causes undulation on the processed surface and deterioration of shape transfer.

  Abnormal discharge is neither short-circuit nor short no-load time, but it is not normal discharge. For example, there is no-load time, but the applied voltage (V0) is the set value during no-load time. In this case, the current is generated in the machining gap, so that the insulation recovery is insufficient and the next discharge Is considered to be a concentrated discharge or short circuit, and when insulation recovery is not performed, it shifts to an arc and remarkably deteriorates the machined surface quality.

  Processing progresses while short-circuiting, concentration, and abnormal discharge enter and mix in the normal discharge, and it is still unclear qualitatively and quantitatively what causes each of them. When control is performed to suppress the problem from continuing by weighting each problem according to the material to be processed, etc., and prolonging the pause, etc., for each event You are using the configured pause settings.

Next, a specific operation of the discharge detection circuit 13 will be described with reference to FIG.
FIG. 2A shows the discharge state of the machining gap between the tool electrode 8 and the workpiece W at a certain sampling time (Ts) by voltage and current.
FIG. 2B shows a voltage signal indicating the time during which the voltage is applied between the electrodes, and is generated for the no-load voltage time (Td) and the pulse width (Ton) time.
The reverse of this signal is the pause time (Toff).

FIG. 2C shows a discharge time signal corresponding to a time corresponding to the pulse width (Ton) when a dielectric breakdown occurs between the electrodes and a current is generated.
FIG. 2D shows the difference between FIG. 2B and FIG. 2C, and shows the no-load voltage time (Td).
FIG. 2E shows the voltage after the pause time (Toff) is generated in order to make a comparison with the no-load voltage time (Td) when the machining condition setting unit 11 sets the minimum no-load time (Tdo). Is a signal for comparison that is generated at the timing when is applied.
FIG. 2 (F) shows one shot when the no-load voltage time (Td) is less than the minimum no-load time (Tdo) as a result of comparing the no-load voltage time (Td) and the minimum no-load time (Tdo). Is generated as
FIG. 2 (G) shows a comparison between the short-circuit voltage threshold (Vsh) and the discharge voltage (eg) during the pulse width (Ton) time when the short-circuit voltage threshold (Vsh) is set by the processing condition setting unit 11. This is a one-shot signal that is generated when it is determined that the short-circuit voltage threshold (Vsh) has been exceeded.
Here, since no applied voltage is generated at the time of a short circuit, the no-load time is also short, so it is recognized as a small no-load discharge. It is necessary to pull it out.

  FIG. 2 (H) shows a case where the abnormal discharge voltage threshold (ng) is set by the machining condition setting unit 11 and is compared with the applied voltage (V0), for example, and compared with the signal of FIG. 2 (D). This is a one-shot signal that is generated when it is determined that the voltage falls below the abnormal discharge voltage threshold value (Vng) during the no-load time.

The discharge detection circuit 13 recognizes it as the total number of discharge occurrences (Nd) by taking the signal of FIG. 2C by a counter, and the number of short circuits (N1) is the signal of FIG. The number of times (N2) is obtained by subtracting the signal of FIG. 2 (G) from FIG. 2 (F), and the number of abnormal discharges (N3) is obtained by taking the signal of FIG.
Here, the normal discharge (Nn) is obtained by subtracting the number of short circuits (N1), the number of small no-load discharges (N2), and the number of abnormal discharges (N3) from the total number of discharges (Nd).

Thus, in the present invention, what has been evaluated by taking the state of the machining gap as a voltage fluctuation so far is recognized as a more accurate discharge state by more quantitatively grasping the event of each state. This is to be reflected in the machining axis feed control.
Specifically, each state quantity acquired from the discharge detection circuit 13 is converted into an amount corresponding to the average voltage handled so far, and machining axis feed control is performed based on the signal.

The concept regarding the machining axis feed control according to the present embodiment will be described.
First, as a basic concept, a case will be described in which the machining axis feed control is performed on the assumption that the total number of discharge occurrences (Nd) obtained by the discharge detection circuit 13 is all normal discharge. Assume that the number of discharge occurrences (Nd) in a certain sampling time (Ts) is N times.

で表すことができる。 Each discharge is composed of a no-load time (Td), a pulse width (Ton), and a pause time (Toff). The pulse width (Ton) and the pause time (Toff) are values set by the machining condition setting unit 11. is there.
The no-load time (Td) is not settable and is an amount that varies depending on the machining state. In machining axis feed control using the average voltage (Vg), the machining gap average voltage (Vg) is maintained at the servo reference voltage (SV). As shown in FIG. 3, the average voltage (Vg) for one discharge is as follows:

Can be expressed as

と表すことができる。 Here, adjusting the average voltage (Vg) to the servo reference voltage (SV) means that the pulse width (Ton), pause time (Toff), and applied voltage (V0) are all set by the machining condition setting unit 11. Since the discharge voltage (eg) is a value of 20 to 30 V determined by the combination and polarity of the tool electrode 8 and the workpiece W, an attempt is made to keep the no-load time (Td), which is an unknown, constant. It turns out that it is the same as controlling.
From this, if the no-load time (Td) is the same in an ideal case where the machining state is to be controlled to be constant, the number of occurrences of discharge (Nd) in a certain sampling time (Ts) is obtained.

It can be expressed as.

となる。 That is, if the number of discharge occurrences (Nd) in a certain sampling time (Ts) is known, the no-load time (Td) at that time is

It becomes.

と表すことができる。 Although the expression (1) is an average voltage for a certain discharge, the average voltage (Vg) during a certain sampling time (Ts) can be considered that there are Nd times of this discharge collection. ) Uses equation (3),

It can be expressed as.

  This makes it possible to obtain an average voltage (Vg) for a certain sampling time (Ts), which is a state quantity of discharge only by detecting the number of occurrences of discharge (Nd) without detecting the voltage of the machining gap, By using this average voltage (Vgs) for machining axis feed control instead of the conventional detected average voltage (Vg), machining axis feed control that reflects accurate state quantities that are not affected by electrical disturbance is performed. It will be.

であることが分かる。 From the equation (4), the average voltage of the machining gap was expressed by a linear equation of the number of discharge occurrences (Nd).
This means that when the average voltage (Vg) of the sampling time (Ts) is the same value as the applied voltage (V0), the number of occurrences of discharge (Nd) is 0, that is, no discharge has occurred. When the average voltage (Vg) of time (Ts) is 0, that is, short-circuited, from the formula (4) or the formula (3),

It turns out that it is.

であり、この平均電圧（Vg）のときにも放電発生回数は最大の放電発生回数（Ndｍａｘ）になるので、式（４）は印加電圧（Ｖ0）から式（６）の範囲までが比例関係にあり、それ以上は式（５）で表される放電発生回数（Nd）を超えることがない。
つまり図４に示される関係にある。 However, it cannot be said that the number of occurrences of discharge (Nd) in Equation (5) is the maximum number of occurrences of discharge (Ndmax) that can occur.
This is because, in practice, when the no-load time (Td) is 0 under the fixed pulse width (Ton) and the pause time (Toff), the pulse width (Ton) and the pause time (Toff) are repeated. When the maximum number of discharges to be generated is determined and the no-load time (Td) is 0 in the equation (1),

Even when this average voltage (Vg) is used, the number of discharge occurrences is the maximum number of discharge occurrences (Ndmax). Therefore, equation (4) is proportional to the range of applied voltage (V0) to equation (6). No more than the number of occurrences of discharge (Nd) represented by the formula (5).
That is, there is a relationship shown in FIG.

  That is, when the average voltage (Vg) of a certain sampling time (Ts) is in the range from 0 to Equation (6), the number of discharge occurrences (Nd) becomes the same at the maximum number of discharge occurrences (Ndmax), and all discharges are generated. When all the times (Nd) are handled as normal discharge, there is a limit that an accurate average voltage (Vgs) cannot be calculated in this region.

  The problem with the method of the present invention is that when the total number of discharge occurrences (Nd) is handled as normal discharge, the average voltage (Vg) for a certain sampling time (Ts) is in the range from 0 to Equation (6). This means that the average voltage (Vg) cannot be recognized accurately, but in this range there is a state where either a small no-load discharge with a short no-load time (Td) or a short circuit or both are mixed. Since it can be seen that there are frequent occurrences, it is only necessary to recognize and reflect these two states, and since the state where the no-load time (Td) is 0 from this equation (6), In practice, it is sufficient to recognize how much a short circuit has occurred.

と表せる。 Therefore, the discharge detection circuit 13 measures the discharge that is below the short-circuit voltage threshold (Vsh) determined by the machining condition setting unit 11 as the number of short-circuits (N1). It is only necessary to know the dependency of the number of short circuits (N1) on the total number of discharges (Nd).

It can be expressed.

と表すことができる。 Further, when there is no no-load time (Td) at the time of a short circuit and it is considered that a short circuit voltage (Vsh) has occurred, Equation (4) can be expressed by Equation (7) as follows:

It can be expressed as.

とすることができる。 When a short circuit occurs, the short circuit voltage is almost 0V, and assuming that the short circuit voltage threshold (Vsh) is 0V, equation (8) is

It can be.

  Thus, when obtaining the average voltage (Vgs) for a certain sampling time (Ts), the average voltage can be correctly converted even when the number of short-circuits (N1) is mixed in the total number of discharges (Nd).

  The tool electrode 8 is made of copper having a diameter of 10 mm, and the workpiece W is made of steel, and the machining axial feed control is performed by the conventional method under the test conditions shown in Table 1 and the average voltage (Vg) of the machining gap and the total voltage. The relationship with the number of discharge occurrences (Nd) is shown in FIG.

Table 1

In FIG. 5, the straight line is obtained by applying the equation (9) to this graph. If the average voltage (Vgs) used in the machining axis feed control according to the present invention is correct, the average voltage (Vg) for each sampling time (Ts). The total number of occurrences of discharge (Nd) plotted as is on a straight line, but the test results showed that both were almost equal.
That is, it can be seen that the average voltage (Vgs) newly created in the present invention can be used instead of the average voltage (Vg) of the conventional machining axis feed control.

Next, when a discharge other than normal discharge is recognized, control has been conventionally performed to stabilize the processing by extending the pause time (Toff) to (Toffs). The correction of the equation (9) when performing the above will be described.
Since the number of short circuits (N1), the number of small no-load discharges (N2), and the number of abnormal discharges (N3) acquired by the discharge detection circuit 13 are taken into account, it is possible to grasp the discharge state other than normal discharge. .

と表され、これにより式（９）は、

と表せる。 Basically, it is only necessary to know how many times the pause control to extend the pause time is performed. The pause in the pause control by short circuit is Toffs1, the pause in the pause control by small no-load discharge is Toffs2, and the pause by abnormal discharge. Assuming that the pause in the control is Toffs3, it is sufficient to know how much the pause component at a certain sampling time (Ts) contributed.

As a result, equation (9) becomes

It can be expressed.

と表せる。 For generalization, if there are n types of suspension control and the suspension time at the suspension control is Toffn,

It can be expressed.

と表すことができる。 Reflecting this, equation (9) becomes

It can be expressed as.

That is, it has been shown that it is possible to cope with a case where pause control is performed in addition to short circuit, small no-load discharge, and abnormal discharge.
The tool electrode 8 is made of copper having a diameter of 10 mm, and the work piece W is made of steel, and machining is performed under the test conditions shown in Table 2 using machining axis feed control as a conventional method. In which the average voltage (Vgs) is recognized (a), the control in which the abnormal discharge is recognized (b) in which the average voltage (Vgs) is recognized by the equation (11), and the total number of discharges (Nd 6 is shown in FIG.

Table 2

In FIG. 6, the straight line is obtained by applying Equation (11) to this graph. If the average voltage (Vgs) used in the machining axis feed control according to the present invention is correct, the average voltage (Vg) for each sampling time (Ts) is shown. The total number of discharge occurrences (Nd) plotted as) is on a straight line.
In the machining results shown in the figure, in the former machining, not only the correct average voltage (Vgs) is not recognized, but also the average voltage (Nd) is zero when the total discharge occurrence number (Nd) is zero. Vgs) is 0 V, and when the total number of discharge occurrences (Nd) is 0, the applied gap (V0) is applied to the machining gap, that is, the so-called open state should be, but there are cases where this is not the case. .
Although there is a big difference between the short circuit and the open state, it is now possible to correctly recognize the average voltage as in the latter if the pause control is taken into account in Equation (11).

As an example of the pause control, when the applied voltage (V0) drops during the no-load time (Td) as shown in FIG. 2, if the discharge detection circuit recognizes the abnormal discharge, the number of abnormal discharges (N3) is calculated. increase. Accordingly, the power supply device 9 performs stop control, and performs control to change the stop time (Toff) to the stop time for abnormal discharge (Toff3). The same applies to the case where the pause control is performed for the short circuit and the small no-load discharge in parallel, and when such a pause control is performed, as shown in Expression (11), Recognize an accurate average voltage (Vgs) that takes into account extended pause times.
In addition, there are various definitions of abnormal discharge, and even in the current electric discharge machine, the detection means and the recognition method are different, but when abnormal discharge is recognized, the rest control is mostly performed as described above, Even if the detection means and the recognition method are different, if the pause control is performed after abnormal discharge, the average voltage of the machining gap can be correctly recognized even if the pause control is performed.

Next, FIG. 7 shows a control flowchart of the first embodiment of the present invention.
FIG. 7A is a flowchart for detecting the discharge voltage of the conventional direct machining gap and generating the average voltage (Vg) from the filter circuit to perform machining axis feed control. FIG. FIG. 7B shows a flowchart when the machining axis feed control is performed by generating (Vgs).
There is no difference in the basic control flow, and a signal used as a reference when machining axis feed control is performed by the electrode position control device 10 is generated from a filter circuit (conventional: a), or discharge generation recognized by the discharge detection circuit 13 It is the difference of generating from the number of times (the present invention: b).
As the control, the control flow is divided depending on whether or not the processing for performing the pause control is performed. If the pause control is being performed, the average voltage (Vgs) is calculated based on the equation (11), and the pause control is not performed. Then, the average voltage (Vg) is obtained based on the equation (9).

According to the present embodiment, assuming that the conventional problem lies in the characteristics of the detection line and noise, the method of the present invention does not directly detect the average voltage but directly controls the machining axis feed control. Since the average voltage (Vgs) calculated from the number of occurrences (Nd) is used, not only the filter circuit, which was a problem of the conventional technology, can be eliminated, but also a dedicated voltage detection line is eliminated, noise components, etc. Therefore, machining axis feed control can be realized with the correct average voltage (Vg).
As a result, it greatly contributes to improving the accuracy of the processed surface.
In addition, when the average voltage (Vgs) becomes small, the average voltage of the machining gap can be correctly detected by subtracting from the total number of discharge occurrences (Nd) in consideration of the number of occurrences of short circuit (N1).

  Although the embodiment of the present invention is an example using a sculpting electric discharge machine, there is a difference in the feed mechanism as long as the machining axis feed control is performed from the average voltage (Vg) by judging the discharge phenomenon. It can be said that it can be controlled by the same concept.

Embodiment 2. FIG.
Next, as Embodiment 2 of the present invention, setting of a small no-load time (Tdo) in an electric discharge machining apparatus that performs machining axis feed control according to the present invention will be described.
The machining condition setting unit 11 can set a small no-load time (Tdo) in consideration of the fact that a small no-load discharge generated during machining shifts to a concentrated discharge. As described in FIG. 1, the small no-load time (Tdo) is compared with the no-load time (Td) of each electric discharge machining.

In general, since machining with a large amount of small no-load discharge tends to cause concentrated discharge and easily shift to an arc, the no-load time (Td) needs to be set with a certain margin.
On the other hand, since this no-load time (Td) itself does not generate electric discharge, if it is too long, the processing efficiency is lowered.
For this reason, when trying to improve the machining speed, the servo reference voltage (SV) can be reduced in addition to reducing the pause time (Toff), resulting in a reduction in the no-load time (Td). Done.
For this reason, an ideal machining speed can be obtained if the no-load time (Td) can be set small enough so that concentrated discharge does not occur.

として表され、単位面積あたりのエネルギ投入量が計算される。 In addition, one of the factors required for improving the processing speed is the average current density (Id) during processing.
This is because the area of the processed portion, that is, the amount of energy that can be input per area of the tool electrode 8 is almost determined by the combination of the tool electrode 8 and the workpiece W, and almost does not exceed this average current density (Id). In this case, it is known that stable processing is maintained.
When performing machining, among the area (S) of the tool electrode 8 and the machining conditions set by the machining condition setting unit 11, the discharge current (IP), the pulse width (Ton), the pause time (Toff), the servo reference voltage ( SV) and applied voltage (V0), the target no-load time (Td) during processing is calculated from the equation (1), and the average current density (Id) during processing is

The energy input per unit area is calculated.

From various experimental results, when using copper for the tool electrode 8 and a steel material for the workpiece W and machining with the tool electrode 8 side having a positive polarity, the average current density (depending on the shape of the tool electrode 8). It is known that if Id) does not exceed 5-15 A / cm ² , the processing is stable.
Similarly, when processing is performed using graphite for the tool electrode 8 and a steel material for the workpiece W, with the tool electrode 8 side having a positive polarity, the average current density (Id) depends on the shape of the tool electrode 8. It is known that the processing is stable unless it exceeds ² to 5 A / cm ² .
Similarly, when machining is performed using a copper tungsten alloy for the tool electrode 8 and a cemented carbide for the workpiece W and the tool electrode 8 side having a negative polarity, the average current density (depending on the shape of the tool electrode 8). It is known that if Id) does not exceed ³ to 10 A / cm ² , the processing is stable.

In addition to the basic machining condition setting in the machining condition setting unit 11 of the electric discharge machining apparatus according to the present invention, when the area (S) of the workpiece W to be machined is input, the discharge current set from the equation (14) If (IP), pulse width (Ton), and pause time (Toff) are determined, the target no-load time (Td) is determined, and the result should be applied to equation (1) to be set according to the processing conditions. A servo reference voltage (SV) is determined.
If the no-load time (Td) calculated at this time is the limit no-load time (Tds), if this value is handled as a small no-load time (Tdo), a dangerous state at the time of concentrated discharge can be sensed.
In order to actually obtain an appropriate small no-load time (Tdo), processing was performed under the conditions shown in Table 3.

Table 3

The conditions shown in Table 2 (No. 1) for roughing conditions in machining with 10 mm square copper tungsten for the tool electrode 8 and cemented carbide for the workpiece W, with the polarity on the electrode side negative. In the case of processing at, if the average current density (Id) is 10 A / cm ² , the servo reference voltage (SV) is 40 V and the limit no-load time (Tds) is 60 μsec.
In this test, if a small no-load time (Tdo) is not set, and no discharge control is performed even if a discharge of a no-load time (Td) below the limit no-load time (Tds) occurs, Although there was no transfer to the arc, black spots were left on the machined surface, and there were spots where the electrodes were heavily consumed locally at the corners of the electrodes.
Therefore, in order to observe the change of the machining state by changing the small no-load time (Tdo), the small no-load time (Tdo) is the same as the limit no-load time (Tds) 60 μsec (No. 2), 10 μsec (No .3) As a setting of 20 μsec (No. 4), when a small no-load discharge (Tdo) occurred twice in succession, the test was performed under a pause control in which one pause time (Toff) was added. .

As shown in Table 2, no. Under condition 2, there was no problem with the machined surface and electrode consumption, but the machining time was 10% slower, and under condition No.4, there was no problem with the machined surface and electrode consumption and the speed could be improved. It was.
From this result, the small no-load time (Tdo) is preferably set to a value of about 0 to 1.0 times the limit no-load time (Tds), preferably about 0.3 to 0.5 times. It is thought that processing can be realized.
In other words, even when the same discharge as the limit no-load time (Tds) continues, the current density limit is not exceeded in that state, and the pause control is performed for the discharge in the no-load time (Td) in this state. However, the processing speed is rather reduced.
If small no-load discharge is considered to shift to concentrated discharge by being continuous, it is considered dangerous that discharge of no-load time (Td) smaller than the limit no-load time (Tds) continues.
For this reason, it is considered that about 1/3 of the limit no-load time (Tds) was good in this experiment.

In this experiment, the machining axis feed control according to the present invention was performed. When the test No. 4 was machined by the conventional machining axis feed control (No. 5), the machining axis feed control according to the present invention showed better results although the machining results were almost the same.
This is probably because the average voltage during machining was correctly recognized and reflected in the machining axis feed control.

Similarly, finishing machining conditions are shown in Table 2 (No. 6) in machining in which 10 mm square copper is used for the tool electrode 8 and ferrous steel material is used for the workpiece W and the polarity on the electrode side is negative. When processing is performed under such conditions, if the average current density (Id) is 10 A / cm ² , the limit no-load time (Tds) becomes negative, and abnormalities occur in processing when the current density is exceeded. You can see that there is nothing.
For this reason, it was decided not to perform pause control with small no-load discharge.
In the case of such small machining energy, the machining gap is small and there is the greatest concern about short circuit. Therefore, the servo reference voltage (SV) is set to a value somewhat larger than 1/2 of the applied voltage (V0). The experiment was conducted by giving a margin to the machining gap and performing the pause control when a short circuit occurred once.

In the machining axis feed control according to the present invention, good results could be obtained even in finish machining. In the conventional method (No. 7), there were some shorts during machining, resulting in increased wear and spots on the machined surface.
Since this is an average voltage (Vg) using a filter circuit in the conventional method, if a short-circuit occurs suddenly, it will be recognized as a voltage fluctuation due to a delay due to the time constant of the filter circuit until it reaches 0V. This is also because there was a delay, and because the new method does not depend on the time constant of the filter circuit, it was recognized immediately after the occurrence of the short circuit and was reflected in the machining axis feed control. It has been found that when the limit no-load time (Tds) is 0 or less, it is not necessary to perform pause control with a small no-load discharge.
When the area (S) becomes small, the discharge current (IP) or the pulse width (Ton) becomes large, and the limit no-load time (Tds) becomes large, the limit no-load time ( A small no-load time (Tdo) that is about 0.3 to 0.5 times (Tds) may be set.

Embodiment 3 FIG.
On the other hand, based on the pause control method in abnormal discharge, it is possible to reduce the pause time (Toff) when normal discharge continues and the current density (Id) is not exceeded.
For example, if a recognition signal is generated at the timing when normal discharge occurs five times continuously, and if the pause time is narrowed at that time, the number of times normal discharge occurs five times is set as the pause reduction number (N4). By setting the pause time as shortened pause (Toff4) in advance, the number of pause reduction times (N4) is detected by the discharge detection circuit 13 at every sampling time (Ts), and the average voltage using equation (13) By calculating (Vgs), the present invention can be applied not only to short-circuiting, small no-load discharge, and abnormal discharge, but also to control that shortens the pause in a stable state.

As described above, it is confirmed that the same control as before can be performed by using the average voltage method using the average voltage (Vgs) calculated from the total number of discharge occurrences (Nd) for the machining axis feed control. By using the counter of the number of occurrences, it is possible not only to eliminate the filter circuit, which was a problem of the prior art, but also to eliminate the dedicated voltage detection line and eliminate adverse effects such as noise components. It became possible.
It was also found that when the average voltage (Vgs) is small, the average voltage of the machining gap can be detected correctly by subtracting from the total number of discharges (Nd) in consideration of the number of short circuits (N1).

Further, in the machining axis feed control, a small unloaded time is preferably 0 to 1.0 times the limit unloaded time (Tds) calculated from the current density (Id), preferably 0.3 to 0.5 times. By performing pause control as the time (Tdo), a good machining result can be obtained.
Furthermore, even when the rest control is performed by recognizing other than the normal discharge, not only the correct average voltage is calculated and the machining is performed, but also when the control that shortens the rest in a stable state is performed. An average voltage can be calculated and processed.

  1, 2, 3 Motor, 4 Spindle 5, 6 Worktable, 7 Machining tank, 8 Tool electrode, 9 Power supply device 10 Electrode position control device, 11 Machining condition setting unit, 12 Main arithmetic unit, 13 Discharge detection circuit, 14 Discharge occurrence counter, 15 Short circuit occurrence counter, 16 Small no load time discharge occurrence counter, 17 Voltage drop no load discharge occurrence counter.

Claims (8)

In an electric discharge machining apparatus that performs machining axis control so that the machining average voltage Vg within a predetermined sampling time Ts becomes the servo reference voltage SV,
Power supply means for supplying power between the electrode between the tool electrode and the workpiece;
A discharge detection means for detecting a discharge waveform between the electrodes generated based on the power supplied by the power supply means;
In this discharge waveform, a discharge occurrence number counter means for counting the number of discharge occurrences Nd within a predetermined sampling time,
In the discharge waveform, a short-circuit occurrence number counter unit that counts the number N1 of short-circuit discharges in which the discharge accompanying the voltage application supplied from the power supply unit falls below a preset short-circuit voltage threshold Vsh;
Using the above discharge occurrence number Nd, short circuit number N1, preset no-load voltage V0, pulse width Ton, pause time Toff, discharge voltage eg, sampling time Ts,

Based on the calculation means for calculating the assumed average voltage Vgs between the poles,
Electrode position control means for performing machining axis control so that the assumed average voltage Vgs calculated by the calculation means becomes the servo reference voltage SV within the sampling time Ts;
EDM machine equipped with. The electric discharge machining apparatus according to claim 1, wherein an assumed average voltage Vgs is obtained in consideration of rest time control based on occurrence of electric discharge other than normal electric discharge or normal electric discharge. The calculation means sets Nn as the number of types of pause time control, and Toffn as the pause time during pause control.

The electric discharge machining apparatus according to claim 2, wherein an assumed average voltage Vgs is calculated based on the calculation. The discharge other than the normal discharge is a short-circuit discharge in which the discharge accompanying the voltage application supplied from the power supply means falls below a preset short-circuit voltage threshold Vsh, or within a preset small no-load time Tdo from the voltage application supplied from the power supply means. The discharge according to claim 2 or 3, wherein the discharge is a small no-load time discharge that shifts to a discharge, or an abnormal discharge in which a voltage applied from a power supply means is a discharge voltage that falls below a preset abnormal discharge threshold Vng. Processing equipment. The electric discharge machining apparatus according to claim 4, wherein the small no-load discharge Tdo is set to 0.3 to 0.5 times the limit no-load time Tds calculated based on the average current density Id. In an electric discharge machining method that performs machining axis control so that an average machining voltage Vg within a predetermined sampling time Ts becomes a servo reference voltage SV,
Detecting a discharge waveform generated based on the power supplied between the electrode between the tool electrode and the workpiece;
In this discharge waveform, a step of counting the number Nd of discharge occurrences within a predetermined sampling time Ts;
In the above discharge waveform, a step of counting the number N1 of short-circuits of the short-circuit discharge in which the discharge accompanying the voltage application supplied from the power supply means falls below a preset short-circuit voltage threshold Vsh;
Using the above discharge occurrence number Nd, short circuit number N1, preset no-load voltage V0, pulse width Ton, pause time Toff, discharge voltage eg, sampling time Ts,

A step of calculating an assumed average voltage Vgs between the electrodes based on
A step of controlling the machining axis so that the calculated average voltage Vgs thus calculated becomes the servo reference voltage SV within the sampling time Ts;
An electric discharge machining method comprising: The electric discharge machining method according to claim 6, wherein the assumed average voltage Vgs is obtained in consideration of rest time control based on occurrence of electric discharge other than normal electric discharge or normal electric discharge. In the calculation process, the number of types of pause time control is Nn, the pause time during pause control is Toffn,

The electric discharge machining method according to claim 6, wherein an assumed average voltage Vgs is calculated based on the calculation.

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