Patent Description:
In the wire electrical discharge machine, the wire electrode can break due to various factors such as when the discharge time becomes relatively long, when the voltage application time becomes relatively short, or when the distance between the workpiece and the wire electrode becomes relatively short, etc..

In order to reduce breakage of the wire electrode, a wire electrical discharge machine is disclosed in <CIT>, for example. In the wire electrical discharge machine disclosed in <CIT>, by taking into consideration the fact that the machining energy at a wire breakage under the abnormal discharge condition is smaller than that under the normal discharge condition, weight coefficients are pre-defined for the normal discharge pulse and the abnormal discharge pulse, which are generated during electrical discharge machining. The numbers of normal discharge pulses and abnormal discharge pulses, generated at the electrode gap, are multiplied by respective coefficients so as to calculate energy evaluation data, which is compared with a threshold. When the calculated energy evaluation data is greater than the threshold, the off time of voltage pulses is increased to thereby reduce wire breakage.

<CIT> relates to a method for continuously controlling, controlling and optimizing EDM machining with a machine. Therein, it is proposed to take into account all types of pulses emitted: normal discharges, open circuits, short circuits and arcs. Specifically, the method uses the power P dissipated during the unit of time in the wire electrode to be a quantity characteristic of the risk of wire electrode breakage.

A convention method of EDM process analysis and on-line control is disclosed by <NPL>. Therein, an ignition delay time is used to detect arcing pulses.

A convention method of improving the efficiency of machining a workpiece with varying thickness in the wire electrical discharge machining is provided by<NPL>. Therein, a period of the high voltage sustained before sparking, i.e., the ignition delay time, is used to detect arcing pulses.

However, in <CIT>, the definitions of the normal discharge state and the abnormal discharge state are unclear. Therefore, an improvement measure for reducing breakage of the wire electrode is needed.

It is therefore an object of the present invention to provide a wire electrical discharge machine and an electrical discharge machining method which can reduce wire electrode breakage.

A first aspect of the present invention resides in a wire electrical discharge machine defined in claim <NUM>.

A second aspect of the present invention provides an electrical discharge machining method defined in claim <NUM>.

According to the present invention, it is possible to reduce breakage of the wire electrode by changing the machining conditions based on the degree of instability calculated using the number of non-discharge pulses.

The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.

The wire electrical discharge machine and the electrical discharge machining method according to the present invention will be detailed below by describing preferred embodiments with reference to the accompanying drawings.

<FIG> is a diagram showing an overall schematic configuration of a wire electrical discharge machine <NUM>. The wire electrical discharge machine <NUM> is a machine tool that applies a voltage between a workpiece to be machined and the wire electrode <NUM> in a dielectric working fluid to generate electric discharge at the electrode gap between the workpiece and the wire electrode <NUM>, thereby performing electrical discharge machining on the workpiece. The wire electrical discharge machine <NUM> includes a main machining body <NUM>, a dielectric fluid unit <NUM> and a control device <NUM>.

The wire electrode <NUM> is formed of, for example, metal material such as tungsten-based, copper-alloy based, brass-based material and the like. On the other hand, the material of the workpiece is a metal material such as, for example, iron-based material, superhard (tungsten carbide) material and the like.

The main machining body <NUM> includes a supply system 20a for supplying the wire electrode <NUM> toward a target (workpiece, object to be machined), and a collection system 20b for collecting the wire electrode <NUM> that has passed through the workpiece.

The supply system 20a includes a wire bobbin <NUM> on which unused wire electrode <NUM> is wound, a torque motor <NUM> for applying torque to the wire bobbin <NUM>, a brake shoe <NUM> for applying a braking force by friction to the wire electrode <NUM>, a brake motor <NUM> for applying a brake torque to the brake shoe <NUM>, a tension detection unit <NUM> for detecting the magnitude of tension of the wire electrode <NUM>, and a wire guide (upper wire guide) <NUM> located above the workpiece to guide the wire electrode <NUM>. The torque motor <NUM> and the brake motor <NUM> are provided with encoders EC1 and EC2 for detecting rotational positions or rotational speeds. The control device <NUM>, based on the detection signals detected by the encoders EC1 and EC2, performs feedback control of the torque motor <NUM> and the brake motor <NUM> so as to keep the rotational speeds of the torque motor <NUM> and the brake motor <NUM> at predetermined values for rotational speed.

The collection system 20b includes a wire guide (lower wire guide) <NUM> disposed under the workpiece to guide the wire electrode <NUM>, a pinch roller <NUM> and a feed roller <NUM> capable of holding the wire electrode <NUM> therebetween, a torque motor <NUM> for applying a torque to the feed roller <NUM>, and a collection box <NUM> for collecting the used wire electrode <NUM> conveyed by the pinch roller <NUM> and the feed roller <NUM>. The torque motor <NUM> is provided with an encoder EC3 for detecting a rotational position or a rotational speed. The control device <NUM>, based on the detection signal detected by the encoder EC3, performs feedback control of the torque motor <NUM> so as to keep the rotational speed of the torque motor <NUM> at a predetermined value for rotational speed.

The main machining body <NUM> includes a work pan <NUM> capable of storing a dielectric working fluid such as deionized water or oil used in the electrical discharge machining. The work pan <NUM> is disposed on a base <NUM>. The wire guides <NUM> and <NUM> are disposed inside the work pan <NUM>, and a workpiece is set between the wire guides <NUM> and <NUM>. The wire guides <NUM> and <NUM> have respective die guides 32a and 34a that support the wire electrode <NUM>. The wire guide <NUM> further includes a guide roller 34b that deflects and guides the wire electrode <NUM> toward the pinch roller <NUM> and the feed roller <NUM>.

The wire guide <NUM> ejects clean working fluid free from sludge (machined waste), toward the electrode gap between the wire electrode <NUM> and the workpiece. This makes it possible to fill the gap with the clean working fluid suitable for electrical discharge machining, and hence prevent accuracy deterioration of electrical discharge machining due to sludge generated during electrical discharge machining. The wire guide <NUM> may also eject clean fluid free from sludge, toward the gap.

The workpiece is supported on a table (not shown) movable in the X- and Y-directions. The wire guides <NUM>, <NUM>, the workpiece and the table are immersed in the working fluid stored in the work pan <NUM>.

Here, the workpiece has a start hole or a kerf or machined groove formed therein, which is a start point for electrical discharge machining at which the machining is started. The wire electrode <NUM> is inserted through the start hole or the kerf to thereby feed the wire electrode <NUM>. The gap between the start hole or the kerf of the workpiece and the wire electrode <NUM> forms the electrode gap. After the wire electrode <NUM> is passed through the start hole or the kerf of the workpiece and fed, the wire electrical discharge machine <NUM>, while feeding the wire electrode <NUM> downward (negative Z-direction) to the workpiece, moves the table (workpiece) on a plane parallel to the XY plane, to thereby machine the workpiece. The wire feeding of the wire electrode <NUM> means that the wire electrode <NUM> wound on the wire bobbin <NUM> is passed through the wire guide <NUM>, the workpiece, and the wire guide <NUM>, and pinched between the pinch roller <NUM> and the feed roller <NUM>. When the wire electrode <NUM> is fed, a predetermined tension is applied to the wire electrode <NUM>. The X-direction and the Y-direction are orthogonal to each other, and the direction orthogonal to the XY-plane (horizontal plane) is defined as the Z-direction.

The dielectric fluid unit <NUM> is a device that removes chips (sludge) arising in the work pan <NUM> and adjusts the electrical resistivity, temperature and the like to control the liquid quality of the working fluid. The working fluid, whose fluid quality is controlled by the dielectric fluid unit <NUM>, is returned to the work pan <NUM> again, and this working fluid is ejected from at least the wire guide <NUM>. The control device <NUM> controls the main machining body <NUM> and the dielectric fluid unit <NUM>.

<FIG> is a diagram showing a configuration of main components of the wire electrical discharge machine <NUM>. Specifically, in <FIG>, the main parts of the main machining body <NUM> and the control device <NUM> of the wire electrical discharge machine <NUM> are shown.

The main machining body <NUM> has a power supply control unit <NUM> and a power supply unit <NUM>. The control device <NUM> includes a pulse detection unit <NUM>, a pulse information acquisition unit <NUM>, a storage medium <NUM>, an instability calculation unit <NUM>, and a machining condition changing unit <NUM>.

The power supply control unit <NUM> controls the power supply unit <NUM> so that a voltage is repeatedly applied between the workpiece W and the wire electrode <NUM> at predetermined intervals. That is, the power supply control unit <NUM>, based on information on the workpiece W supplied from the outside, determines the application conditions such as the voltage value, the pulse width, the pulse interval and the like of the pulse of voltage to be applied (hereinafter referred to as the voltage pulse). The pulse interval is a time between the voltage pulses, and is an off time during which no voltage is applied between the workpiece W and the wire electrode <NUM>.

The power supply control unit <NUM> determines the application conditions, then generates drive pulse signals for driving the power supply unit <NUM> to apply voltage according to the determined application conditions, and outputs the generated drive pulse signals to the power supply unit <NUM>.

The power supply unit <NUM>, based on the drive pulse signals, repeatedly applies a voltage pulse between the workpiece W and the wire electrode <NUM> at predetermined intervals. <FIG> illustrates a state in which a start hole Wh is formed in the workpiece W and the wire electrode <NUM> is passed therethrough.

The pulse detection unit <NUM> detects voltage pulses that are repeatedly applied between the workpiece W and the wire electrode <NUM>. The shapes of the voltage pulses differ depending on the presence or absence of discharge and the timing of occurrence of the discharge.

Specifically, as shown in <FIG> the voltage pulses include three types, i.e., a normal discharge pulse P1, an abnormal discharge pulse P2 and a non-discharge pulse P3. The normal discharge pulse P1 is a voltage pulse in which a discharge delay time Ta from starting of voltage application until voltage drop due to occurrence of discharge is equal to or greater than a predetermined time. The abnormal discharge pulse P2 is a voltage pulse in which the discharge delay time Ta is less than the predetermined time. The non-discharge pulse P3 is a voltage pulse without voltage drop due to discharge. That is, the pulse time Tb of the non-discharge pulse P3 is a duration time during which the voltage is applied between the workpiece W and the wire electrode <NUM> (i.e., pulse width of the voltage pulse).

When detecting a voltage pulse, the pulse detection unit <NUM> performs predetermined signal processing such as shaping processing on the detected voltage pulse, and outputs the voltage pulse obtained as a result of the signal processing to the pulse information acquisition unit <NUM>.

The pulse information acquisition unit <NUM> acquires pulse information on the voltage pulses detected by the pulse detection unit <NUM> per unit time. That is, the pulse information acquisition unit <NUM>, based on the pulse width of each voltage pulse supplied from the pulse detection unit <NUM>, determines which type of pulse the voltage pulse belongs to, i.e., normal discharge pulse P1, abnormal discharge pulse P2 or non-discharge pulse P3. In addition, the pulse information acquisition unit <NUM> counts the number of voltage pulses supplied from the pulse detection unit <NUM> and measures the time (discharge delay time Ta or pulse time Tb) for each unit time, to obtain the measurement result per unit time as the pulse information.

This pulse information contains the number of normal discharge pulses P1 and the total time of the discharge delay time Ta for the number of pulses P1, the number of abnormal discharge pulses P2 and the total time of the discharge delay time Ta for the number of pulses P2, and the number of non-discharge pulses P3 and the total time of the pulse time Tb for the number of pulses P3. Upon obtaining the pulse information, the pulse information acquisition unit <NUM> stores the obtained pulse information into the storage medium <NUM>.

The instability calculation unit <NUM>, based on the pulse information stored in the storage medium <NUM>, calculates the degree of instability that indicates how unstable the discharge state is (i.e., the extent of instability of the discharge state), per unit time. The higher the degree of instability, the more unstable the discharge state. That is, the probability that the wire electrode <NUM> is broken increases as the degree of instability increases.

Now, specific examples of calculation of the degree of instability will be described. For example, when the numbers of normal discharge pulses P1, abnormal discharge pulses P2 and non-discharge pulses P3 are used, the degree of instability can be calculated according to the following formula (<NUM>) or (<NUM>), for example. <MAT> <MAT>.

In the above formulae (<NUM>) and (<NUM>), IT is the degree of instability, and NA is the number of normal discharge pulses P1 detected per unit time. Further, in the above formulae (<NUM>) and (<NUM>), NB is the number of abnormal discharge pulses P2 detected per unit time, and NC is the number of non-discharge pulses P3 detected per unit time. The number of non-discharge pulses P3 may be replaced by a value obtained by multiplying the number of pulses P3 by a coefficient.

As other examples, when the times of the normal discharge pulse P1, the abnormal discharge pulse P2 and the non-discharge pulse P3 are used, the degree of instability can be calculated according to the following formula (<NUM>) or (<NUM>), for example. <MAT> <MAT>.

According to the invention the degree of instability IT is calculated according to formula (<NUM>).

In the above formulae (<NUM>) and (<NUM>), TA is the total time obtained by summing all the discharge delay time Ta of the normal discharge pulses P1 detected per unit time, and TB is the total time obtained by summing all the discharge delay time Ta of the abnormal discharge pulses P2 detected per unit time. TC in the above formulae (<NUM>) and (<NUM>) is the total time obtained by summing all the pulse time Tb of the non-discharge pulses P3 detected per unit time.

The instability calculation unit <NUM> calculates the degree of instability based on the above formula (<NUM>) using not only the discharge delay time Ta of normal discharge pulses P1 and abnormal discharge pulses P2 but also the pulse time Tb of non-discharge pulses P3.

The pulse time Tb of non-discharge pulses P3 is the duration time during which the voltage is applied between the workpiece W and the wire electrode <NUM> (i.e., the pulse width of the voltage pulse) as described above, and is thus substantially constant. Therefore, TC is a value obtained by multiplying the pulse width by the number of non-discharge pulses P3. That is, the number of non-discharge pulses P3 is also used in the above formula (<NUM>) or (<NUM>).

After calculating the degree of instability, the instability calculation unit <NUM> outputs the calculated degree of instability to the machining condition changing unit <NUM>.

Based on the degree of instability calculated by the instability calculation unit <NUM>, the machining condition changing unit <NUM> changes the machining conditions for the workpiece. That is, as shown in <FIG>, the machining condition changing unit <NUM> compares the degree of instability calculated for each unit time by the instability calculation unit <NUM> with a predetermined first threshold (upper limit threshold).

Here, when the degree of instability becomes equal to or greater than the first threshold, the machining condition changing unit <NUM> determines that the discharge state has become unstable, and starts changing the machining conditions. That is, the machining condition changing unit <NUM> starts changing the machining conditions at the time when the degree of instability reaches the first threshold or greater.

Once the machining condition changing unit <NUM> starts changing the machining conditions, it changes the off time (the pulse interval between the drive pulse signals), which is one of the application conditions in the power supply control unit <NUM>, every unit time until the degree of instability becomes less than a predetermined second threshold (lower limit threshold) smaller than the first threshold.

Specifically, the machining condition changing unit <NUM>, using a preset proportional constant (change ratio), changes the off time to be longer (i.e., lengthens the off time) in such a manner that the change ratio of the off time to the normal value (initial value) increases as the degree of instability is greater. In this connection, the machining condition changing unit <NUM> can prevent excessive delay of the electrical discharge machining, by setting a maximum length (maximum limit) for the off time to be changed, in advance.

When the off time (pulse interval between the drive pulse signals) is changed longer (i.e., lengthened), the machining condition changing unit <NUM> causes the power supply control unit <NUM> to output drive pulse signals corresponding to the changed off time, to the power supply unit <NUM>. Thereby, breakage of the wire electrode <NUM> can be diminished.

When the degree of instability becomes less than the second threshold, the machining condition changing unit <NUM> shortens the off time every unit time until the off time returns to a normal value (initial value). Specifically, the machining condition changing unit <NUM> shortens the off time (pulse interval) in such a manner that the change ratio of the off time to the normal value (initial value) increases as the degree of instability is greater, and causes the power supply control unit <NUM> to output drive pulse signals corresponding to the changed off time, to the power supply unit <NUM>. When the off time returns to the normal value (initial value), the machining condition changing unit <NUM> stops changing the machining conditions until the degree of instability becomes equal to or greater than the first threshold again.

Next, the electrical discharge machining method of the wire electrical discharge machine <NUM> will be described. It is assumed herein that voltage pulses are repeatedly applied between the workpiece W and the wire electrode <NUM> at predetermined intervals. It is also assumed that changing the machining conditions has been started. <FIG> is a flowchart showing a control processing sequence of the control device <NUM>.

At step S1, the pulse detection unit <NUM> monitors the voltage applied between the workpiece W and the wire electrode <NUM>. When the voltage pulse applied from the power supply unit <NUM> to the workpiece W and the wire electrode <NUM> is detected, the control proceeds to step S2.

At step S2, the pulse information acquisition unit <NUM> determines the type of voltage pulse detected at step S1 and measures the number of the voltage pulses and the time concerning the voltage pulses (discharge delay time Ta or pulse time Tb). In addition, upon measuring the number of voltage pulses detected per unit time and the time (discharge delay time Ta or pulse time Tb), the pulse information acquisition unit <NUM> acquires the measurement result as pulse information, and the control proceeds to step S3.

At step S3, the instability calculation unit <NUM> calculates the degree of instability based on the pulse information obtained at step S2, and the control proceeds to step S4. The machining condition changing unit <NUM> compares the degree of instability calculated at step S4 with a predetermined threshold (upper limit threshold).

Here, when the degree of instability is equal to or higher than the predetermined threshold (upper limit threshold), the control proceeds to step S5 where the machining condition changing unit <NUM> lengthens the off time (pulse interval) in which no voltage is applied between the workpiece W and the wire electrode <NUM>. That is, the machining condition changing unit <NUM> changes the off time to be longer in such a manner that the change ratio of the off time to the normal value (initial value) increases as the degree of instability is greater, and outputs drive pulse signals corresponding to the changed off time (pulse interval), to the power supply unit <NUM>, and then the control returns to step S1.

On the other hand, when the degree of instability is less than the predetermined threshold (upper limit threshold), the machining condition changing unit <NUM> proceeds to step S6 and shortens the off time (pulse interval). That is, the machining condition changing unit <NUM> changes the off time (pulse interval) to be shorter in such a manner that the change ratio of the off time to the normal value (initial value) increases as the degree of instability is greater, and outputs drive pulse signals corresponding to the changed off time (pulse interval), to the power supply unit <NUM>, and then the control returns to step S1. When the shortened off time (pulse interval) returns to the normal value (initial value), the machining condition changing unit <NUM> stops changing of the off time without returning to step S1.

As described heretofore, the control device <NUM> causes the main machining body <NUM> to change the off time based on the degree of instability calculated from the pulse information containing the number of non-discharge pulses P3 and the pulse time Tb to thereby diminish breakage of the wire electrode <NUM>.

Though the above embodiment has been described as one example of the present invention, the technical scope of the invention should not be limited to the above embodiment. It goes without saying that various modifications and improvements can be added to the above embodiment. It is also apparent from the scope of claims that the embodiment added with such modifications and improvements should be incorporated in the technical scope of the invention.

Though in the above embodiment, the change of the machining condition (off time) is started at the time when the first threshold (the upper limit threshold) is exceeded, the change of the machining condition (off time) may be started at the time when the second threshold (the lower limit threshold) is reached. That is, only one threshold may be used to determine whether or not to change the machining condition (off time).

That is, as shown in <FIG>, the machining condition changing unit <NUM> compares the degree of instability calculated for each unit time by the instability calculation unit <NUM> with a threshold. Here, when the degree of instability is equal to or greater than the threshold, the machining condition changing unit <NUM>, using a prescribed proportional constant (change ratio), lengthens the off time in such a manner that the change ratio of the off time to the normal value (initial value) increases as the degree of instability is greater.

On the other hand, when the degree of instability is less than the threshold, the machining condition changing unit <NUM> using a prescribed proportional constant (change ratio), shortens the off time in such a manner that the change ratio of the off time to the normal value (initial value) increases as the degree of instability is greater.

In this way, the machining condition changing unit <NUM> can determine whether or not to change the machining condition (off time), by using one threshold.

In this connection, the machining condition changing unit <NUM> can prevent excessive delay of the electrical discharge machining, by setting a maximum length (maximum limit) for the off time to be changed, in advance. In addition, the machining condition changing unit <NUM> can prevent excessive increase of discharge energy, by setting a minimum length (minimum limit) for the off time to be changed, in advance.

Though in the above embodiment, the off time is adopted as the machining condition, the flow rate of the working fluid may be adopted.

That is, as shown in <FIG>, at the time when the degree of instability becomes equal to or greater than a first threshold (the upper limit threshold), the machining condition changing unit <NUM> starts changing the flow rate to be smaller (i.e., decreasing the flow rate) every unit time until the degree of instability becomes less than a second threshold (the lower limit threshold). Specifically, the machining condition changing unit <NUM> changes the flow rate to be smaller in such a manner that the change ratio of the flow rate to a normal value (initial value) increases as the degree of instability is greater, and controls the pump in the dielectric fluid unit <NUM> so as to jet working fluid from the wire guide <NUM> at the changed flow rate.

In this connection, the machining condition changing unit <NUM> can prevent degradation of accuracy in electrical discharge machining due to increased generation of sludge at the electrode gap during electrical discharge machining, by setting a minimum value (minimum limit) for the flow rate to be changed, in advance.

On the other hand, when the degree of instability is less than the second threshold (lower limit threshold), the machining condition changing unit <NUM> changes the flow rate greater (i.e., increases the flow rate) every unit time until the flow rate of the working fluid returns to the normal value (initial value). Specifically, the machining condition changing unit <NUM> changes the flow rate greater in such a manner that the change ratio of the flow rate to the normal value (initial value) increases as the degree of instability is greater, and controls the pump in the dielectric fluid unit <NUM> so as to jet working fluid from the wire guide <NUM> at the changed flow rate.

Thus, when the flow rate of working fluid is adopted as the machining condition, it is possible to diminish breakage of wire electrode <NUM>, similarly to the above embodiment.

As shown in <FIG>, whether or not to change the flow rate of working fluid may be determined by using one threshold, as in Modification <NUM>.

Though in the above embodiment, the off time is adopted as the machining condition, the tension of the wire electrode <NUM> may be adopted.

Specifically, the machining condition changing unit <NUM> can change the tension of the wire electrode as in the case where the flow rate of working fluid is changed. That is, at the time when the degree of instability becomes equal to or greater than a first threshold (the upper limit threshold), the machining condition changing unit <NUM> starts changing the tension of the wire electrode <NUM> smaller (i.e., decreasing the tension) every unit time until the degree of instability becomes less than a second threshold (the lower limit threshold). More specifically, the machining condition changing unit <NUM> changes the tension smaller in such a manner that the change ratio of the tension to a normal value (initial value) increases as the degree of instability is greater, and controls the torque motors <NUM>, <NUM> and the brake motor <NUM> so as to produce the changed tension.

On the other hand, when the degree of instability is less than the second threshold (lower limit threshold), the machining condition changing unit <NUM> changes the tension greater (i.e., increasing the tension) every unit time until the tension of the wire electrode <NUM> returns to the normal value (initial value). Specifically, the machining condition changing unit <NUM> changes the tension greater in such a manner that the change ratio of the tension to the normal value (initial value) increases as the degree of instability is greater, and controls the torque motors <NUM>, <NUM> and the brake motor <NUM> so as to produce the changed tension.

Thus, when the tension of the wire electrode <NUM> is adopted as the machining condition, it is possible to diminish breakage of wire electrode <NUM>, similarly to the above embodiment.

As shown in <FIG>, as in Modification <NUM>, whether or not to change the tension of the wire electrode <NUM> may be determined by using one threshold.

Though in the above embodiment, the off time is adopted as the machining condition, the feed rate of the wire electrode <NUM> may be adopted.

Specifically, the machining condition changing unit <NUM> can change the feed rate as in the case where the flow rate of working fluid is changed. That is, when the degree of instability becomes equal to or greater than a first threshold (the upper limit threshold), the machining condition changing unit <NUM> starts changing the feed rate of the wire electrode <NUM> smaller (i.e., lowering the feed rate) every unit time until the degree of instability becomes less than a second threshold (the lower limit threshold). More specifically, the machining condition changing unit <NUM> changes the feed rate smaller (lower) in such a manner that the change ratio of the feed rate to a normal value (initial value) increases as the degree of instability is greater, and controls the torque motors <NUM>, <NUM> and the brake motor <NUM> so as to produce the changed feed rate.

On the other hand, when the degree of instability is less than the second threshold (lower limit threshold), the machining condition changing unit <NUM> changes the feed rate greater (i.e., increasing the feed rate) every unit time until the feed rate of the wire electrode <NUM> returns to the normal value (initial value). Specifically, the machining condition changing unit <NUM> changes the feed rate greater (higher) in such a manner that the change ratio of the feed rate to the normal value (initial value) increases as the degree of instability is greater, and controls the torque motors <NUM>, <NUM> and the brake motor <NUM> so as to produce the changed feed rate.

Thus, when the feed rate of the wire electrode <NUM> is adopted as the machining condition, it is possible to diminish breakage of wire electrode <NUM>, similarly to the above embodiment.

As shown in <FIG>, as in Modification <NUM>, whether or not to change the feed rate of the wire electrode <NUM> may be determined by using one threshold.

Claim 1:
A wire electrical discharge machine (<NUM>) for performing electrical discharge machining on a workpiece (W) by generating electrical discharge at an electrode gap formed between the workpiece and a wire electrode (<NUM>), comprising:
a pulse detection unit (<NUM>) configured to detect voltage pulses repeatedly applied between the workpiece and the wire electrode;
a pulse information acquisition unit (<NUM>) configured to determine which type of pulse the voltage pulses detected by the pulse detection unit (<NUM>) belong to;
an instability calculation unit (<NUM>) configured to calculate a degree of instability indicating how unstable a discharge state is; and
a machining condition changing unit (<NUM>) configured to change a machining condition for the workpiece, based on the calculated degree of instability,
characterised in that
the pulse information acquisition unit (<NUM>) is configured to:
determine that the voltage pulses are normal discharge pulses (P1), in a case where a discharge delay time (Ta), which is a time from start of application of the voltage until occurrence of voltage drop due to the electrical discharge, is equal to or longer than a predetermined time,
determine that the voltage pulses are abnormal discharge pulses (P2), in a case where the discharge delay time is less than the predetermined time, and
determine that the voltage pulses are non-discharge pulses (P3), in a case where the voltage pulses have no voltage drop due to the electrical discharge and the voltage is applied between the workpiece and the wire electrode,
wherein the instability calculation unit is configured to calculate the degree of instability, by using a ratio of total time of the discharge delay time of the abnormal discharge pulses and pulse time of the non-discharge pulses, detected per unit time, to total time obtained by summing the discharge delay time of the normal discharge pulses, the discharge delay time of the abnormal discharge pulses and pulse time of the non-discharge pulses, detected per unit time.