Small hole electrical discharge machining apparatus and small hole electrical discharge machining method

Abstract

To provide a small-hole electrical discharge machining apparatus that enables high-precision machining while suppressing a decrease in machining efficiency. [Solution] According to the present invention, a small-hole electrical discharge machining apparatus is provided, which includes a control device that controls the movement of the tool electrode by controlling the machining speed, which is the speed at which the tool electrode moves relative to the workpiece in the machining direction, wherein when an index of discharge instability satisfies predetermined conditions, the control device controls the movement of the tool electrode so that the machining speed is lower than the machining speed for differential voltage when the discharge is stable, and the differential voltage is the value obtained by subtracting a reference voltage from the average machining voltage, which is the average value of the inter-electrode voltage between the tool electrode and the workpiece.

Description

Technical Field

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[0001] The present invention relates to a micro-hole electric discharge machining apparatus and a micro-hole electric discharge machining method.

Background Art

[0002] Conventionally, micro-hole electric discharge machining for providing holes in a workpiece has been known.

[0003] Patent Document 1 discloses a micro-hole electric discharge machining apparatus for performing micro-hole electric discharge machining. In micro-hole electric discharge machining, while rotating an elongated pipe-shaped or rod-shaped tool electrode and bringing it close to the workpiece, a voltage pulse is applied between the tool electrode and the workpiece to cause discharge, thereby forming a hole in the workpiece. <00000​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​[1] A small-hole electrical discharge machining apparatus comprising a control device that controls the movement of the tool electrode by controlling the machining speed, which is the speed at which the tool electrode moves relative to the workpiece in the machining direction, wherein the control device controls the movement of the tool electrode so that, when an index of discharge instability satisfies predetermined conditions, the machining speed is lower than the machining speed for differential voltage when the discharge is stable, and the differential voltage is the value obtained by subtracting a reference voltage from the average machining voltage, which is the average value of the inter-electrode voltage between the tool electrode and the workpiece. [2] The small-hole electrical discharge machining apparatus described in [1], wherein the index of the instability of the discharge includes the frequency of discharges occurring between the tool electrode and the workpiece, and the predetermined condition includes a state in which the discharge frequency is maintained at a value smaller than a first threshold for a predetermined time. A small-hole electrical discharge machining apparatus according to [3][1] or [2], wherein the indicator of discharge instability includes the average machining voltage, and the predetermined condition includes a state in which the ratio of the average machining voltage obtained during machining to the average machining voltage during machining when the discharge is stable is less than or equal to a second threshold. A small-hole electrical discharge machining apparatus according to any one of [4][1] to [3], wherein the control device is configured to reduce the machining speed by switching the machining speed characteristic used to control the machining speed from a first machining speed characteristic to a second machining speed characteristic when the predetermined conditions are met, the machining speed characteristic is the relationship between the machining speed and the differential voltage, and the second machining speed characteristic is such that, when the differential voltage is 0, the change in the machining speed in response to the change in the differential voltage is more gradual than that of the first machining speed characteristic. [5] A method for machining a workpiece by moving a tool electrode, comprising a machining state determination step and a machining speed adjustment step, wherein the machining state determination step determines whether an index of discharge instability satisfies predetermined conditions, and the machining speed adjustment step adjusts the machining speed so that, if the predetermined conditions are met, the machining speed is lower than the machining speed when the predetermined conditions are not met. [Effects of the Invention]

[0008] In the small-hole electrical discharge machining apparatus according to the present invention, by controlling the movement of the tool electrode so that the machining speed decreases when an index indicating the instability of the discharge satisfies a predetermined condition compared to the machining speed when the predetermined condition is not met, it is possible to perform stable machining while suppressing a decrease in machining efficiency. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic block diagram of a small-hole electrical discharge machining apparatus 1 according to one embodiment of the present invention. [Figure 2] This is a schematic block diagram of the area around the control device 70 of a small-hole electrical discharge machining apparatus 1 according to one embodiment of the present invention. [Figure 3] This is an example of the first processing speed characteristics provided by the processing speed characteristic determination unit 76 according to one embodiment of the present invention. [Figure 4] This is an example of a second processing speed characteristic provided by the processing speed characteristic determination unit 76 according to one embodiment of the present invention. [Figure 5] Figures 5A, 5B, 5C, 5D, 5E, 5F, and 5G are schematic diagrams illustrating a micro-hole electrical discharge machining method according to one embodiment of the present invention. [Figure 6] This is a schematic diagram illustrating the cutting action during a small-hole electrical discharge machining method according to one embodiment of the present invention. [Modes for carrying out the invention]

[0010] Embodiments of the present invention will be described below. The various features shown in the embodiments below can be combined with each other. Furthermore, each feature constitutes an independent invention. In addition, any elements in the embodiments below that are not defined in the claims are optional and can be omitted. Any number of zeros (for example, one or two) may be added to the end of the numerical values ​​disclosed in the following description. For example, one or two zeros may be added after "1.4" to make it "1.40" or "1.400".

[0011] 1. Small hole electrical discharge machining In small-hole electrical discharge machining, machining is difficult to perform and requires extra time, resulting in a rough surface finish, when the tool electrode 10 and the workpiece W approach each other and the first electrical discharge contributing to machining begins to occur in the machining gap formed between the tool electrode 10 and the workpiece W (hereinafter referred to as "cut-in"), and when the tool electrode 10 begins to penetrate the workpiece W until it has completely penetrated (hereinafter referred to as "exit-in"). The present invention aims to achieve high-precision machining by adjusting the machining speed control method mainly during cut-in and exit-in, and during other machining processes.

[0012] In this specification, high-precision machining refers to a machining surface with a relatively small surface roughness. Short circuits and abnormal discharges are prone to occur at the beginning and end of the machining process, making the generation of normal discharges that contribute to machining unstable. Because the discharge does not occur stably, machining is difficult to proceed, and the machined surface becomes rough. Therefore, the inventors have found that machining accuracy can be improved by adjusting the machining speed control using the instability of the discharge as an indicator. Below, an electrical discharge machining apparatus according to one embodiment of the present invention and a method using the same will be described in detail.

[0013] 2. Small hole electrical discharge machining equipment 1 2.1 Processing equipment main body This section outlines the main components of the processing apparatus body. The processing apparatus body includes a tool electrode 10, an electrode holder 20, a power supply member 30, a power supply unit 40, a moving unit 50, and a discharge characteristic detection unit 60.

[0014] The tool electrode 10 is formed from a conductive material into an elongated pipe or rod shape. During machining, the tool electrode 10 is rotated while a voltage is applied between the tool electrode 10 and the workpiece W to cause a discharge. The tool electrode 10 is inserted into the electrode holding portion 20.

[0015] The electrode holding part 20 is configured to hold and rotate the tool electrode 10. Further, the electrode holding part 20 is connected to the power supply member 30. The power supply member 30 is configured to supply power to the tool electrode 10 and the workpiece W respectively. A power supply part 40 is connected to the power supply member 30. Also, similar to a known micro-hole electric discharge machining apparatus, the machining apparatus main body includes at least one electrode guide 80 corresponding to the length of the tool electrode 10.

[0016] The moving part 50 includes a moving body (not shown) for relatively moving the tool electrode 10 with respect to the workpiece W, and a servo motor (not shown) for driving and moving the moving body. The moving part 50 is connected to the electrode holding part 20 and is configured to move the tool electrode 10 via the electrode holding part 20 by driving the moving body.

[0017] Also, the moving part 50 is configured to move the tool electrode 10 according to the machining speed output from the control device 70 and machine the workpiece W. Here, the machining speed is the speed at which the tool electrode 10 is relatively moved in the machining progress direction with respect to the workpiece W. The moving body can be configured as, for example, a moving body that reciprocates in the vertical one-axis direction (Z-axis). However, various configurations can be applied to the moving part 50 according to the specifications of the micro-hole electric discharge machining apparatus 1.

[0018] The discharge characteristic detection part 60 is configured to be able to detect the inter-electrode voltage and the discharge frequency. Specifically, the inter-electrode voltage is the voltage between the tool electrode 10 and the workpiece W during machining. The inter-electrode voltage rises to the no-load voltage when a voltage is applied between the electrodes, decreases due to discharge, and then decreases to almost zero when the voltage application is turned off, so it is constantly fluctuating. Therefore, the average machining voltage, which is the average value of the inter-electrode voltage, is calculated and can be used for control of the machining speed and the like. The average machining voltage is the average value of the inter-electrode voltage within a predetermined detection period. The predetermined detection period is not particularly limited as long as it is a time during which the average machining voltage can be stably calculated, and is, for example, 1 microsecond to 200 microseconds, preferably 10 microseconds to 150 microseconds.

[0019] Furthermore, the discharge frequency is expressed as the ratio (%) of the number of times a discharge is actually considered to have occurred to the number of discharges that should occur between the tool electrode 10 and the workpiece W per predetermined sampling period (e.g., 0.1 seconds to 1 second). The discharge characteristic detection unit 60 can have any configuration as long as it can detect the inter-electrode voltage and the discharge frequency.

[0020] 2.2 Control device 70 The control device 70 controls the operation of the micro-hole electrical discharge machining apparatus 1. The control operation of the control device 70 will be described below.

[0021] Each component of the control device 70 may be implemented by software or by hardware. When implemented by software, various functions can be realized by the CPU executing a computer program. The program may be stored in the built-in memory unit or in a computer-readable non-temporary recording medium. Alternatively, the program may be read from an external memory unit and implemented through so-called cloud computing.

[0022] When implemented in hardware, it can be implemented using various circuits such as ASICs (Application Specific Integrated Circuits), FPGAs (Field Programmable Gate Arrays), or DRPs (Dynamically Reconfigurable Processors). In this embodiment, various types of information and the concepts encompassing them are handled, which are represented by high and low signal values ​​as a set of binary bits consisting of 0s and 1s, and communication and calculations can be performed by the aforementioned software or hardware configurations.

[0023] As shown in Figure 2, the control device 70 according to this embodiment includes a processing condition holding unit 71, a voltage comparison unit 72, a processing speed determination unit 73, a movement control unit 74, a condition determination unit 75, a processing speed characteristic determination unit 76, and a power supply control unit 77.

[0024] 2.2.1 Machining condition holding section 71 The machining condition holding unit 71 temporarily stores machining information data, including machining conditions that have been previously input by operators and set in the storage unit (not shown) of the control device 70 of the small-hole electrical discharge machining apparatus 1, which is required to operate the small-hole electrical discharge machining apparatus 1 so that the desired machining is performed. The machining information data is stored, for example, in the form of a single dataset. Specifically, for example, the machining condition holding unit 71 holds a reference voltage for machining conditions, which can be output to the voltage comparison unit 72. In addition, the machining condition holding unit 71 specifically holds machining condition data for operating the electrical discharge machining circuit, such as machining current value, power supply voltage, and on-time and off-time of the discharge pulse, which can be output to the power supply control unit 77.

[0025] 2.2.2 Voltage Comparison Section 72 The voltage comparison unit 72 can derive a differential voltage by comparing the reference voltage with the average machining voltage. Specifically, for example, the differential voltage is calculated to be the value obtained by subtracting the reference voltage obtained by the machining condition holding unit 71 from the average machining voltage obtained by the discharge characteristic detection unit 60.

[0026] 2.2.3 Condition judgment section 75 The condition determination unit 75 can determine whether the indicator of discharge instability satisfies predetermined conditions. Here, if the predetermined conditions are met, it means the discharge is unstable, and if the predetermined conditions are not met, it means the discharge is stable. If it is determined that the discharge is unstable, specifically, for example, it indicates that the machining situation is at the engagement or release stage.

[0027] For example, the instability of the discharge can be expressed as the "discharge frequency," which indicates how many normal discharges contributing to machining occurred in the machining gap formed between the tool electrode 10 and the workpiece W during a predetermined sampling period. In the following explanation, the predetermined sampling period will be set to 1 second. The power supply control unit 77 repeatedly supplies voltage pulses to the machining gap with predetermined on and off times based on the machining conditions held in the machining condition holding unit 71. Since a small amount of material is removed from the workpiece W each time a normal discharge occurs, when the discharge is occurring most stably, machining is progressing smoothly at a machining speed that can be estimated from the amount of material removed. Therefore, when the discharge is occurring most stably, a number of discharges corresponding to the repetition frequency of the voltage pulse should be occurring in the machining gap. In that case, if the discharge frequency is considerably low compared to the number of applied voltage pulses, it can be considered that the discharge is unstable and machining is not progressing. Thus, the discharge frequency can be used as an indicator of the instability of the discharge.

[0028] In the micro-hole electrical discharge machining apparatus 1 of the embodiment, when the discharge frequency is used as an indicator of instability, the ratio of the number of discharges considered normal to the repetition frequency of the voltage pulse during a predetermined sampling period is used as the threshold (hereinafter referred to as the first threshold, assuming that instability is determined by two or more indicators). In this case, the predetermined condition can be a state in which the discharge frequency is maintained at a value smaller than the first threshold for a predetermined time.

[0029] The first threshold depends on the positional relationship between the tool electrode 10 and the workpiece W, but specifically it is, for example, 10% to 50%. When the workpiece W is positioned parallel to the lower end surface of the tool electrode 10 (Figures 5A to 5G), the first threshold is preferably 10 to 30%, specifically, for example, 10, 15, 20, 25, 30% / second, and may be within the range of any two of the values ​​exemplified here. When the workpiece W is positioned at an angle to the lower end surface of the tool electrode 10 (Figure 6), the first threshold is preferably 30 to 50%, specifically, for example, 30, 35, 40, 45, 50%, and may be within the range of any two of the values ​​exemplified here. The predetermined time is specifically, for example, 0.1 seconds to 1 second. Specifically, for example, these could be 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0 seconds, or they could be within the range of any two of the numbers exemplified here.

[0030] For example, the "average machining voltage" in the machining gap formed between the tool electrode 10 and the workpiece W can be used as an indicator of discharge instability. The power supply control unit 77 repeatedly supplies voltage pulses to the machining gap with predetermined on and off times based on the machining conditions held in the machining condition holding unit 71. When discharge is occurring stably, after applying voltage pulses to the machining gap, discharge occurs after an uncertain discharge waiting time, the voltage drops, and the discharge is cut off after a predetermined on time. Since a small amount of material is removed from the workpiece W each time a normal discharge occurs, when discharge is occurring most stably, machining is progressing smoothly at a machining speed that can be estimated from the amount of material removed. Therefore, the average machining voltage at this time is always close to the reference voltage of the set machining conditions.

[0031] When the machining gap is narrow and normal discharge is unlikely to occur (i.e., when there is an arcing or short-circuit tendency), current flows through the machining gap before the inter-electrode voltage reaches the no-load voltage, causing the voltage to drop to the arc voltage or short-circuit voltage. As a result, the average machining voltage in the machining gap will be lower than when discharge is occurring stably.

[0032] Therefore, if the average machining voltage during a predetermined detection period is considerably lower than when machining is performed normally, the machining can be considered unstable. Consequently, if the detected average machining voltage is significantly lower than when the machining is performed under optimal conditions, it can be determined that the discharge is not occurring normally and that the discharge is unstable.

[0033] In the small-hole electrical discharge machining apparatus 1 of this embodiment, when the indicator of discharge instability is the average machining voltage, the ratio of the average machining voltage obtained during machining to the average machining voltage during a predetermined detection period when the discharge is stable can be used as a threshold (hereinafter referred to as the second threshold when a first threshold exists). The predetermined average machining voltage when the discharge is stable can be, for example, the value obtained when test machining is performed. In this case, the predetermined condition can be a state in which the average machining voltage is less than or equal to the second threshold. Specifically, the second threshold depends on the specifications of the small-hole electrical discharge machining apparatus 1, but for example, it is between 10% and 70%, and preferably between 10% and 30%.

[0034] Furthermore, in cases of unstable discharge, the occurrence of a short circuit and the subsequent retraction of the tool electrode 10 due to the short circuit are repeated. As a result, the waveform of the average machining voltage tends to become an oscillating wave that repeatedly alternates between a nearly 0V state and a no-load voltage state over time. Therefore, the "amplitude of the oscillation of the average machining voltage waveform" can also be used as an indicator of discharge instability.

[0035] The condition determination unit 75 may determine whether the discharge instability index satisfies predetermined conditions based on either the discharge frequency or the average machining voltage, or it may determine based on both the discharge frequency and the average machining voltage. When the determination is made based on both the discharge frequency and the average machining voltage, the condition can be determined to be satisfied if both predetermined conditions are met.

[0036] The condition determination unit 75 can output the determination result to the processing speed characteristic determination unit 76.

[0037] 2.2.4 Machining speed characteristics determination section 76 The machining speed characteristic determination unit 76 can switch machining speed characteristics based on the determination result obtained from the condition determination unit 75. The machining speed characteristic determination unit 76 has a first machining speed characteristic and a second machining speed characteristic. The first and second machining speed characteristics are the relationship between the machining speed and the differential voltage, which is the value obtained by subtracting the reference voltage from the average machining voltage.

[0038] Figure 3 shows an example of the first machining speed characteristic, and Figure 4 shows an example of the second machining speed characteristic. The first machining speed characteristic is used when the condition determination unit 75 determines that the predetermined conditions are not met (discharge is stable). The second machining speed characteristic is used when the condition determination unit 75 determines that the predetermined conditions are met (discharge is unstable).

[0039] Here, a positive machining speed indicates the speed at which the machine is moving towards the workpiece W, while a negative speed indicates the speed at which the machine is moving away from the workpiece W. If the differential voltage is negative, it means the distance between the electrode and the workpiece W has become too small, making short circuits or concentrated discharges likely. Therefore, the speed is configured to be negative so that the electrode can be retracted. On the other hand, if the machining is proceeding without problems, the differential voltage will be 0 or greater.

[0040] The first machining speed characteristic can be represented by a straight line passing through the differential voltage corresponding to the upper and lower limits of the machining speed. Here, the point passing through the upper limit of the machining speed (positive side) and the corresponding differential voltage is defined as the upper limit point on the machining speed characteristic, and the point passing through the lower limit of the machining speed (negative side) and the corresponding differential voltage is defined as the lower limit point on the machining speed characteristic.

[0041] In the first machining speed characteristic, when the differential voltage exceeds the upper limit point, the machining speed remains constant at the upper limit regardless of the increase in the differential voltage. The lower limit of the machining speed has the same absolute value as the upper limit. In the first machining speed characteristic, when the differential voltage falls below the lower limit point of the machining speed, the machining speed remains constant at the lower limit regardless of the decrease in the differential voltage.

[0042] Specifically, the example in Figure 3 shows the first machining speed characteristic when the upper limit of the machining speed is 22 mm / min and the lower limit is -22 mm / min. In this case, the differential voltage corresponding to the upper limit of the machining speed is 40 V, and the machining speed is constant at 22 mm / min in the range where the differential voltage is 40 V or higher. Also, the differential voltage corresponding to the lower limit of the machining speed is -40 V, and the machining speed is constant at -22 mm / min in the range where the differential voltage is -40 V or lower. The upper limit of the machining speed is specifically determined based on the set machining conditions. Furthermore, the corresponding differential voltage can be calculated and derived from the amount of material removed per unit time, which is determined based on the machining conditions.

[0043] As mentioned above, when the differential voltage is negative (the third quadrant on the coordinate plane), it is sufficient for the electrodes to retract quickly, so a straight line can be drawn that reaches the lower limit of the machining speed with any slope (preferably a value greater than or equal to the slope when the differential voltage is positive). Therefore, in the example in Figure 3, a straight line is drawn with the same slope as on the positive side for the negative machining speed characteristics, but unlike the example in Figure 3, it may have different slopes with respect to the origin.

[0044] Next, we will explain the second machining speed characteristic. In the second machining speed characteristic, first an upper limit of the machining speed is set. Similarly, a differential voltage corresponding to the upper limit of the machining speed is set, and this becomes the upper limit point. The upper and lower limits of the machining speed and the corresponding differential voltages for the second machining speed characteristic are the same as those for the first speed characteristic. If the differential voltage exceeds the upper limit point, the machining speed will remain constant at the upper limit regardless of the increase in value. The same applies to the negative side. The average machining voltage corresponding to the positive upper limit can be derived based on the machining conditions. The example in Figure 4 shows the second machining speed characteristic when the upper limit of the machining speed is 22 mm / min and the lower limit is -22 mm / min.

[0045] Furthermore, in the second machining speed characteristic, the portion where the differential voltage is small can be set as a dead zone. When the differential voltage is 0V, in actual machining, the average machining voltage in the machining gap remains close to the reference voltage, and machining is progressing smoothly. If the feed rate is increased in such a machining state, the tool electrode 10 may pass the position where a good machining gap is maintained, and the machining gap may suddenly narrow. In such cases, it is better to keep the tool electrode 10 in the position where a good machining gap is maintained, so a dead zone is deliberately created to maintain stable machining.

[0046] In the dead zone region, the corresponding machining speed is 0 mm / min. Specifically, the dead zone region is 0 to 15V, more preferably 0 to 10V, although this depends on the machining conditions, including the machining method. The second machining speed characteristic is a speed characteristic that, by setting the dead zone region, enables behavior that keeps the machined gap at a position where the size of the machined gap can be maintained, thereby inducing the generation of continuous discharges.

[0047] In the second machining speed characteristic, the machining speed characteristic between the upper limit point and the upper limit of the dead zone can be expressed as a polynomial function. The order of this polynomial function is preferably n=2 to 5, and more preferably n=4. In the example in Figure 4, the characteristics are shown when the upper limit of the machining speed is 22 mm / min, the differential voltage in the dead zone is 0 to 10 V, and the order is n=4.

[0048] Even in the negative portion of the second processing speed characteristic (the third quadrant on the coordinate plane), the processing speed can be expressed by a polynomial function. As mentioned above, when the differential voltage is negative, it is sufficient that the electrodes can be quickly retracted in the second processing speed characteristic as well. Therefore, it is preferable that the absolute value of the differential voltage at the lower limit point is smaller than the absolute value of the differential voltage at the upper limit point. Specifically, for example, the absolute value of the differential voltage at the lower limit point relative to the absolute value of the differential voltage at the upper limit point of the second processing speed characteristic is preferably 1 / 10 to 1, and more preferably 1 / 5.

[0049] In the example shown in Figure 4, the degree of the polynomial function is n=1 on the negative side of the machining speed characteristics. Also, the differential voltage at the upper limit point of the machining speed characteristics is 40V, and the differential voltage at the lower limit point is -8V. Therefore, the example in Figure 4 shows machining speed characteristics where the upper limit of the machining speed is reached with a differential voltage that is one-fifth of that on the negative side compared to the positive side.

[0050] If the condition determination unit 75 determines that the discharge is unstable (i.e., at the time of engagement or disengagement), the machining speed characteristic determination unit 76 decides to adopt the second machining speed characteristic. When the differential voltage is 0V or a small value, the second machining speed characteristic shows that the change in machining speed in response to changes in differential voltage is more gradual than that of the first machining speed characteristic. Therefore, if the discharge is determined to be unstable, the machining speed will decrease as the average machining voltage approaches the reference voltage compared to the machining speed in response to the differential voltage when the discharge is stable.

[0051] For example, during the initial engagement phase, there is almost no machining chips in the machining gap that can induce secondary electrical discharge, making it difficult for discharge to occur. In this state, even if the tool electrode 10 is brought close to the workpiece W at the same machining speed as when there is a moderate amount of machining chips in the machining gap and the machining conditions are stable, discharge may not occur at the same machining gap position (hereinafter referred to as the required machining gap) as in the stable machining conditions. As a result, the tool electrode 10 may overshoot the required machining gap position, short-circuit, and then repeatedly try to widen the machining gap, leading to a hunting behavior. In this case, machining will not progress, and the machined surface will become rough.

[0052] In such cases, a suitable method of adjusting the machining speed is one in which the machining speed is gradually reduced as the tool electrode 10 approaches the required machining gap size, and the tool electrode 10 is brought closer to the workpiece W more slowly from well before the required machining gap position, thereby keeping the tool electrode 10 near the position where the required machining gap size can be maintained for as long as possible and inducing the generation of continuous discharges.

[0053] 2.2.5 Machining speed determination section 73 The machining speed determination unit 73 can determine the machining speed corresponding to the differential voltage obtained from the voltage comparison unit 72, based on the machining speed characteristics determined by the machining speed characteristic determination unit 76. The machining speed determination unit 73 can output the determined machining speed to the movement control unit 74.

[0054] Using Figures 3 and 4, the configuration in which the machining speed determination unit 73 determines the machining speed will be specifically explained. Here, we will explain the case where the differential voltage obtained from the voltage comparison unit 72 is 40V. In the machining speed characteristic determination unit 76, regardless of whether the set machining speed characteristic is the first machining speed characteristic or the second machining speed characteristic, the machining speed determination unit 73 determines the machining speed to be 22 mm / min (Figures 3 and 4).

[0055] 2.2.6 Movement Control Unit 74 The movement control unit 74 is configured to control the movement unit 50 according to the machining speed obtained from the machining speed determination unit 73. This makes it possible to control the movement of the tool electrode 10.

[0056] 1.2.7 Power supply control unit 77 The power control unit 77 controls the power supply unit 40 according to the machining condition data obtained from the machining condition holding unit 71. As a result, a predetermined voltage pulse is applied between the tool electrode 10 and the workpiece W, causing repeated discharges to occur in the machining gap. Then, a machining hole of a size dependent on the diameter of the tool electrode 10 is formed in the workpiece W.

[0057] 3. Small hole electrical discharge machining method This section describes a method for performing electrical discharge machining on a workpiece W using the small-hole electrical discharge machining apparatus 1 according to this embodiment. The small-hole electrical discharge machining method comprises a machining state determination step and a machining speed adjustment step.

[0058] In the machining state determination step, it is determined whether the indicator of discharge instability meets predetermined conditions. Specifically, in the machining state determination step, first, the discharge characteristic detection unit 60 acquires the discharge frequency and / or average machining voltage and outputs it to the condition determination unit 75. The condition determination unit 75 determines whether predetermined conditions are met based on the acquired discharge frequency and / or average machining voltage, and outputs the determination result to the machining speed characteristic determination unit 76.

[0059] In the machining speed adjustment process, if the machining state determination process determines that predetermined conditions are met, the machining speed is adjusted so that it is lower than the machining speed when the predetermined conditions are not met. Specifically, the machining speed characteristics are changed so that the machining speed gradually decreases as the tool electrode 10 approaches the position of the required machining gap size. Specifically, in the machining speed adjustment process, the machining speed characteristic determination unit 76 first determines the machining speed characteristics based on the determination result from the condition determination unit 75.

[0060] For example, if a first processing speed characteristic, which is beneficial during stable processing, is initially set as the processing speed characteristic to be used, and the condition determination unit 75 outputs a determination result that the predetermined conditions are not met, the processing speed characteristic to be used will not be changed from the first processing speed characteristic.

[0061] For example, if the first processing speed characteristic is initially set as the processing speed characteristic to be used, and a determination result is output indicating that predetermined conditions are met, the processing speed characteristic determination unit 76 can decide to switch the processing characteristic to the second processing speed characteristic.

[0062] Similarly, if the second processing speed characteristic is set as the processing characteristic to be used, and the predetermined conditions are no longer met, it is possible to decide to switch the processing speed characteristic to be used from the second processing speed characteristic to the first processing speed characteristic.

[0063] Furthermore, the voltage comparison unit 72 compares the average processing voltage obtained from the discharge characteristic detection unit 60 with the reference voltage to derive the difference voltage.

[0064] Next, in the machining speed adjustment step, the machining speed determination unit 73 determines the machining speed corresponding to the differential voltage obtained from the voltage comparison unit 72, using the machining speed characteristics determined by the machining speed characteristic determination unit 76. Based on the determined machining speed, the movement control unit 74 adjusts the machining speed of the tool electrode 10.

[0065] Here, using the machining speed characteristics in Figures 3 and 4, and referring to the schematic machining diagram in Figure 5, we will specifically explain the case of forming a hole in a workpiece W with a thickness of 50 mm, along with the change in the relative height (hereinafter, relative height H) of the lower end surface of the tool electrode 10 relative to the upper surface of the workpiece W. The relative height H is the height when the upper surface of the workpiece W is set to 0 mm and the positive and negative directions are set as shown in Figure 5. The discharge frequency is used as an indicator of discharge instability. The predetermined condition is when the discharge frequency is less than 10% / second (the predetermined time is 1 second).

[0066] As shown in the table below, the machining process is divided into (1) to (7) stages according to its progression. The relative height H corresponding to each machining stage is assumed as shown in the table below. For the sake of explanation, the machining stages are divided by relative height H, but in reality, the machining stage transitions depending on whether the discharge conditions meet the predetermined conditions. [Table 1]

[0067] (1) At the start of operation (1 ≤ H, Figure 5A) A schematic diagram at the start of driving is shown in Fig. 5A. At the start of driving, the first machining speed characteristic is set. Also, the tool electrode 10 can be positioned with, for example, a position where the relative height H is 1 mm as the initial position by detecting contact with the workpiece W. The gap between the workpiece W and the tool electrode 10 (hereinafter referred to as the gap) is large (Fig. 5A). Specifically, for example, when the relative height is 1 ≤ H as the initial position, discharge is not started. Therefore, specifically, the discharge frequency at the start of driving is 0%, and while the machining situation is at the start of driving, the discharge frequency is below the first threshold value. Therefore, in order to satisfy the predetermined conditions, the machining speed characteristic determination unit 76 sets the second machining speed characteristic. Also, the operator may check the occurrence situation of discharge and set the speed characteristic.

[0068] Also, since basically no discharge occurs at the start of driving, the average machining voltage increases and the differential voltage becomes large on the positive side. Therefore, in the second machining speed characteristic, the electrode moves at a relatively fast speed close to or at the upper limit value of the machining speed. For example, when the differential voltage obtained from the voltage comparison unit 72 is 50 V, the machining speed determination unit 73 sets the machining speed to 22 mm / min from the second machining speed characteristic in Fig. 4, and the movement control unit 74 controls the movement of the tool electrode 10 at the machining speed.

[0069] (2) At the time of biting (-1 < H < 1, Fig. 5B) Subsequently, as the gap narrows with the movement of the tool electrode 10, discharge starts to occur. Therefore, as the relative height H decreases and the tool electrode 10 approaches the workpiece W, the discharge frequency slightly increases. Even when the movement of the tool electrode 10 further progresses and it approaches or contacts the workpiece W, since sufficient secondary discharge does not occur in the machining gap yet, short circuit or abnormal discharge is likely to occur, and the state where the discharge frequency does not increase much continues (Fig. 5B).

[0070] Also, for example, in an arrangement where the workpiece W is inclined with respect to the lower end surface of the tool electrode 10 (Fig. 6), even if one point of the tool electrode 10 contacts the workpiece W, the distance between the lower end surface of the tool electrode 10 and the workpiece W remains large at the other point, and the discharge frequency remains low. Further, the electrode holding portion 20 or the electrode guide 80 cannot hold or support and guide the tool electrode 10 near the lower end portion of the tool electrode 10 in order to avoid interference between the electrode holding portion 20 or the electrode guide 80 and the workpiece W. For example, in Fig. 6, the position of the electrode guide 80 from the lower end surface of the tool electrode 10 is higher compared to the example of Fig. 5B. Therefore, due to the occurrence of blurring in the distance between the tool electrode 10 and the workpiece W, the discharge is not stable. That is, during biting, the discharge frequency decreases regardless of the arrangement relationship with the workpiece W.

[0071] When the relative height -1 < H < 1, the discharge frequency is specifically, for example, 10% / second or less. Therefore, the condition determination unit 75 determines that the discharge is unstable because a predetermined condition is satisfied. Therefore, the processing speed characteristic that is still used remains the second processing speed characteristic. ​​​​​​Specifically, for example, when the differential voltage is 0V, from the second machining speed characteristic in FIG. 4, the machining speed determination unit 73 sets the machining speed to 0 mm / min. Therefore, machining is performed without moving the tool electrode 10. When such a state continues for a certain period and depressions start to be formed in the workpiece W, normal discharges continuously occur, the distance from the workpiece W gradually increases, the differential voltage increases, and the machining speed also tends to increase. Specifically, for example, when the differential voltage reaches 20V, based on the second machining speed characteristic in FIG. 4, the machining speed determination unit 73 sets the machining speed to 0.27 mm / min.

[0074] As described above, during the biting-in process, the tool electrode 10 may move forward, contact, or retreat with respect to the workpiece W. Therefore, the discharge frequency is low, and the value of the differential voltage (average machining voltage) is in an unstable state. Also, if the tool electrode 10 is moved according to the first machining speed characteristic having a linear characteristic without an insensitive region set, the tool electrode 10 is more likely to move forward significantly, contact, or retreat significantly with respect to the workpiece W. Therefore, in the micro-hole electric discharge machining apparatus 1 of the embodiment, during the biting-in process, based on the second machining speed characteristic, the tool electrode 10 is moved at a very low machining speed as it approaches a position where the required machining gap size is achieved, or the tool electrode 10 is not moved, or the tool electrode 10 is slightly retracted, and machining is performed over time.

[0075] (3) Transition to the stable state (-2 < H ≤ -1, FIG. 5C) When the machining of the workpiece W starts to progress, from a state where the workpiece W and the tool electrode 10 are too close and short circuits are likely to occur, appropriate machining powder intervenes in the machining gap and spreads to an appropriate interval, and discharges occur sequentially, so that the discharge frequency starts to increase. Specifically, for example, when the relative height H is -2 < H ≦ -1 mm, the discharge frequency is, specifically, for example, about 70% / second or more. Therefore, since the discharge frequency exceeds the threshold value, it is determined by the condition determination unit 75 that a predetermined condition is no longer satisfied (that is, the discharge is stable). Subsequently, in the machining speed adjustment process, first, the machining speed characteristic determination unit 76 switches the machining speed characteristic to be used to the first machining speed characteristic.

[0076] At this time, the value of the average machining voltage also starts to gradually stabilize. Specifically, when the relative height H is -2 < H < -1 mm, the average machining voltage is about 25V to 30V. For example, when the differential voltage is 30V, by switching from the second machining speed characteristic to the first machining speed characteristic, the machining speed determination unit 73 changes the machining speed from 4.35 mm / mi to 16.5 mm / mi and machining is performed.

[0077] (4) Stable state (-50 < H ≦ -2, Fig. 5D) As the machining progresses, for example, in the state shown in Fig. 5D, the distance between the position of the tool electrode 10 and the workpiece W is stable, and the flow of discharge of machining powder, etc. is also established and a certain discharge frequency is maintained. When the relative height is -50 < H ≦ -2, the discharge frequency is, specifically, for example, about 70% / second or more. Therefore, since it is a stable state that does not satisfy the predetermined condition, the first machining speed characteristic is maintained.

[0078] Since the discharge occurs stably, the value of the average machining voltage also stabilizes. Specifically, when the relative height is -50 < H ≦ -2, the average machining voltage is about 25V to 30V. For example, when the differential voltage is 25V, from the first machining speed characteristic in Fig. 3, the machining speed determination unit 73 sets the machining speed to 13.75 mm / min. <(

[0079] (5) Penetration initial stage (-51 < H ≦ -50, Fig. 5E) While machining the hole, a jet of machining fluid is supplied to the machining gap through the inner bore of the pipe-shaped tool electrode 10. As a result, a large amount of machining dust generated in the machining gap is pushed upward and discharged together with the machining fluid from between the side of the machining hole and the side of the tool electrode 10 by the pressure of the machining fluid jet.

[0080] Towards the end of machining, when the tool electrode 10 begins to penetrate, the tip of the tool electrode 10 becomes tapered due to wear caused by electrical discharge (Figure 5E). Therefore, at the stage when penetration begins, a hole smaller than the electrode diameter is formed, and the jet of machining fluid supplied through the inner hole of the pipe-shaped tool electrode 10 is discharged from this small hole. As a result, the discharge of machining chips is hindered, and a large amount accumulates between the side of the machining hole and the side of the tool electrode 10, making it difficult for normal electrical discharges that contribute to machining to occur. In addition, the machining area between the tapered tool electrode 10 and the part of the workpiece W in the direction of machining progress becomes extremely small, so machining does not progress well, the tool electrode 10 is more likely to get caught on the edge of the hole which is smaller than the electrode diameter, and the machining is unstable.

[0081] Therefore, abnormal discharges and short circuits are more likely to occur, and the time during which the discharge frequency decreases significantly becomes longer. In the initial stages of penetration, there is variation in the discharge frequency value, but as mentioned above, the time during which the discharge frequency decreases is long, and when the discharge frequency decreases, specifically, for example, the discharge frequency becomes 10% / second. Therefore, in this case, the condition determination unit 75 determines that the predetermined conditions are met and the discharge is unstable. Next, in the machining speed adjustment process, the machining speed characteristic determination unit 76 first sets the second machining speed characteristic as the machining speed characteristic to be used.

[0082] The average machining voltage decreases as the distance between the tool electrode 10 and the workpiece W decreases, resulting in a smaller differential voltage, which may even become negative. When the differential voltage becomes negative, the average machining voltage increases again when the tool electrode 10 retracts, increasing the distance between the tool electrode 10 and the workpiece W. Therefore, the behavior is similar to that at the time of engagement, and the average machining voltage value is not stable even at the time of exit. Specifically, the average machining voltage changes between, for example, 0V and the no-load voltage.

[0083] For example, when the differential voltage obtained from the voltage comparison unit 72 is 20 V, the machining speed determination unit 73 sets the machining speed to 0.27 mm / min from the second machining speed characteristic in FIG. 4, and the movement control unit 74 controls the movement of the tool electrode 10 at the machining speed.

[0084] (6) Penetration middle stage (-60 < H ≤ -51, FIG. 5F) As the machining progresses and the tool electrode 10 begins to protrude from the lower surface of the workpiece W, the machining area formed between the tool electrode 10 and the workpiece W decreases. As a result, the discharge frequency remains in a low state. Specifically, for example, although the value of the discharge frequency still varies, the time during which the discharge frequency decreases, as in the initial stage of penetration, becomes longer. Therefore, the condition determination unit 75 determines that a predetermined condition is satisfied (that is, the discharge is unstable). The machining speed characteristic determination unit 76 determines to maintain the second machining speed characteristic as the machining speed characteristic to be used.

[0085] In addition, since the machining conditions change at any time due to, for example, the decreasing machining area formed between the tool electrode 10 and the workpiece W, the value of the average machining voltage also remains unstable. Specifically, for example, the average machining voltage changes between 0 V and the no-load voltage.

[0086] (7) Penetration completion (H ≤ -60, FIG. 5G) In the state where the machining progresses and a through hole is completely formed, almost no discharge occurs because the lower end of the electrode and the workpiece W are separated. Therefore, the discharge frequency gradually decreases and finally becomes 0 times / second. Therefore, the machining ends with the second machining speed characteristic remaining as the machining speed characteristic to be used.

[0087] Furthermore, the average machining voltage, similar to the middle stage of through-cutting, is unstable due to changes in machining conditions associated with the movement of the tool electrode 10. Specifically, for example, the average machining voltage fluctuates between 0V and the no-load voltage. For example, when the differential voltage obtained from the voltage comparison unit 72 is 30V, the machining speed determination unit 73 sets the machining speed to 4.35 mm / min based on the second machining speed characteristics in Figure 4, and the movement control unit 74 controls the movement of the tool electrode 10 at that machining speed.

[0088] 4. Effects In the small-hole electrical discharge machining apparatus 1 according to the present invention, the movement of the tool electrode 10 is controlled such that when an indicator of discharge instability satisfies predetermined conditions, the machining speed is lower than when the predetermined conditions are not met. By reducing the machining speed when the predetermined conditions are met (i.e., at the start of cutting or when the tool is about to cut off), the machined surface is less likely to become rough and stable machining can be performed.

[0089] Furthermore, since the machining speed is reduced only when the discharge is determined to be unstable, machining can be performed without reducing machining efficiency. [Explanation of symbols]

[0090] 1:Small hole electrical discharge machining equipment 10: Tool electrode 20: Electrode holding part 30: Power supply component 40: Power supply section 50: Mobile unit 60: Discharge characteristic detection unit 70: Control device 71: Machining condition holding section 72: Voltage comparison section 73: Machining speed determination section 74: Movement Control Unit 75: Condition judgment section 76: Machining speed characteristics determination section 77: Power supply control unit 80: Electrode guide W: Workpiece

Claims

1. A small-hole electrical discharge machining apparatus, The system includes a control device that controls the movement of the tool electrode by controlling the machining speed, which is the speed at which the tool electrode moves relative to the workpiece in the machining direction. The control device, when an indicator of discharge instability satisfies predetermined conditions, controls the movement of the tool electrode such that the machining speed is lower than the machining speed with respect to the differential voltage when the discharge is stable. The differential voltage is the value obtained by subtracting the reference voltage from the average machining voltage, which is the average value of the inter-electrode voltage between the tool electrode and the workpiece, in a small-hole electrical discharge machining apparatus.

2. A small-hole electrical discharge machining apparatus according to claim 1, The indicator of the instability of the discharge includes the frequency of discharges occurring between the tool electrode and the workpiece. The predetermined conditions include a state in which the discharge frequency is maintained at a value smaller than a first threshold for a predetermined time, in which case the small hole electrical discharge machining apparatus.

3. A small-hole electrical discharge machining apparatus according to claim 1, The indicator of the instability of the discharge includes the average machining voltage, The aforementioned predetermined conditions include a state in which the ratio of the average machining voltage obtained during machining to the average machining voltage during machining when the discharge is stable is less than or equal to a second threshold, in a small hole electrical discharge machining apparatus.

4. A small-hole electrical discharge machining apparatus according to claim 1, The control device is configured to reduce the processing speed by switching the processing speed characteristics used to control the processing speed from a first processing speed characteristic to a second processing speed characteristic when the predetermined conditions are met. The aforementioned processing speed characteristic is the relationship between the processing speed and the differential voltage, The second machining speed characteristic is that, when the differential voltage is 0, the change in machining speed in response to a change in the differential voltage is more gradual than that of the first machining speed characteristic, in a small-hole electrical discharge machining apparatus.

5. A small-hole electrical discharge machining method in which a workpiece is machined by moving the tool electrode, It comprises a processing state determination step and a processing speed adjustment step. In the aforementioned processing state determination step, it is determined whether the indicator of discharge instability satisfies predetermined conditions. A method for small-hole electrical discharge machining, wherein in the machining speed adjustment step, the machining speed is adjusted so that when the predetermined conditions are met, the machining speed is lower than when the predetermined conditions are not met.

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