Electrical discharge machine and method for producing a machined object using the same

Abstract

An electrical discharge machine for applying a voltage between an electrode (23) and a workpiece (24) to generate an electrical discharge so that the workpiece (24) is melted and machined by the electrical discharge, the electrical discharge machine comprising: an electrode holder (22) which carries the electrode (23); an ultrasonic motor (21) having a fingertip (21a) that comes into contact with the electrode holder (22) and moving the electrode holder (22) by moving the fingertip (21a) in an annular manner at an ultrasonic range frequency in a predetermined driving direction; a rolling bearing (28) which guides the movement of the electrode holder (22) in the drive direction, the rolling bearing (28) comprising: Movement blocks (28b and 28c) attached to the electrode holder (22); a rail (28a) which supports the movement blocks (28b and 28c) and guides the movement of the movement blocks (28b and 28c); and a plurality of rolling elements (28d) interposed between the moving blocks and the rail (28a); and a control circuit (7) which controls a position of the electrode holder (22), wherein the control circuit (7) moves the electrode holder (22) by a distance which corresponds at least to the circumference of the largest rolling element (28d) when the mechanical resistance of the rolling bearing (28) has changed.

Description

Background[Technical Field]The present invention relates to an electric discharge machine and a method for manufacturing a machined object using the electric discharge machine.[Prior Art]US 2011 0186 551 A1 discloses an electric discharge machine for machining a workpiece, which includes a plurality of electrodes and a plurality of discharge power supply units. The plurality of electrodes each generate a plurality of discharges with the workpiece. The plurality of discharge power supply units respectively and independently apply voltages to the plurality of electrodes.An ultrasonic motor is well known (for example, refer to JP 2011-104735 A). The ultrasonic motor is used in an electric discharge machine to move an electrode holder which holds an electrode. The ultrasonic motor has a fingertip which comes into contact with the electrode holder. The ultrasonic motor drives the electrode holder by moving the fingertip in an annular manner at an ultrasonic range frequency.When an ultrasonic motor such as this is used, the movement of the electrode holder can be controlled to be at a very short distance unit (for example, 1 μm or less). However, according to experiments conducted by the inventors, problems may occur when a rolling bearing is used as a member for guiding the movement of the electrode holder in a driving direction. In the rolling bearing, a plurality of rolling elements are interposed between a moving block and a rail.In particular, when the electrode holder is moved over a long phase at a very short line unit, the sliding resistance of the rolling bearing increases. The moving speed of the electrode holder decreases. Consequently, the time required for machining is highly likely to increase.The reason for such a phenomenon occurring is that dust is generated between the rolling elements and the surrounding path. When the movement of the electrode holder by a very short moving unit is frequently repeatedly performed, the plurality of rolling elements inside the rolling bearing move by minute vibrations without rotating. Consequently, dust is generated by the rolling elements and the surrounding path rubbing against each other.Summary of the InventionIt is therefore an object of the present invention to provide an electric discharge machine which drives an electrode holder using an ultrasonic motor and guides the electrode holder using a rolling bearing, in which an increase in the processing time required due to an increase in the sliding resistance in the rolling bearing is less likely.The object is achieved by the discharge machine having the features of claim 1 and by the method having the features of claim 8. Further advantageous developments and embodiments of the invention are the subject matter of the claims that follow.An exemplary embodiment provides an electric discharge machine that applies a voltage between an electrode and a workpiece, thereby generating electric discharge. The workpiece is melted and machined by the electric discharge. The electric discharge machine includes: an electrode holder that supports the electrode; an ultrasonic motor that has a fingertip that comes into contact with the electrode holder and moves the electrode holder by moving the fingertip in an annular manner at an ultrasonic range frequency in a driving direction; a rolling bearing that guides the movement of the electrode holder in the driving direction; and a control circuit that controls the position of the electrode in the driving direction by driving the ultrasonic motor. The rolling bearing includes: moving blocks fixed to the electrode holder; a rail that supports the moving blocks and guides the movement of the moving blocks; and a plurality of rolling elements interposed between the moving blocks and the rail. The control circuit moves the electrode holder based on an abnormality that occurs in resistance to the movement of the electrode holder in the driving direction. The electrode holder is moved by a moving distance equivalent to when the largest rolling element among the plurality of rolling elements rolls and rotates once without sliding or longer.When the sliding resistance between the rail and the moving blocks becomes larger than this in a normal state, the resistance against the movement of the electrode holder in the driving direction is no longer normal. Therefore, based on an abnormality occurring in the resistance against the movement of the electrode holder in the driving direction, a process for canceling the increase in the sliding resistance is performed. Consequently, the increase in the sliding resistance can be suppressed appropriately. In addition, due to the movement of the electrode holder through a moving distance, when the largest rolling element among the plurality of rolling elements rolls and rotates once without sliding, or longer, even when abrasion is generated, the generated abrasion can be dispersed. Therefore, the increase in the sliding resistance can be suppressed.Reference numerals in the parentheses in the claims give corresponding relationships between the expressions mentioned in the claims and specific constituent elements that exemplify the expressions described according to the embodiments described below and the like.BRIEF DESCRIPTION OF THE DRAWINGSIn the accompanying drawings, FIG. 1 is a configuration diagram of an electric discharge machine 1 according to an embodiment of the present disclosure; FIG. 2 is a cross-sectional view of a mechanical unit; FIG. 3 is a diagram showing a relationship between elliptical movement of a fingertip and ascending movement of a holder main body; FIG. 4 is a diagram showing a relationship between an elliptical movement of the fingertip and a descending movement of the holder main body; FIG. 5 is a configuration diagram of a linear guide; FIG. 6 is a cross-sectional view of an unlimited circulation path in the linear guide along an extending direction of the unlimited circulation path; FIG. 7 is a flowchart of a position control process; FIG. 8 is a time chart showing changes with time in an electrode driving signal, an electrode position, a discharge state and the like when a single injection hole is processed by the electric discharge; FIG. 9 is a graph showing an example of the transition with time of a discharge state; FIG. 10 is a graph showing an example of changes in the position of an electrode holder when a plurality of injection holes are machined; FIG. 11 is a flowchart of a guidance abnormality determination process according to a first embodiment; FIG. 12 is a diagram showing a state in which balls are arranged unequally on the unlimited circulation path; and FIG. 13 is a flowchart of the guidance abnormality determination process according to a second embodiment.DESCRIPTION OF EMBODIMENTS(First Embodiment)Next, a first embodiment of the present disclosure will be described. An electric discharge machine 1 (see FIG. 1 ) according to the present embodiment corresponds to a device that generates an electric discharge by applying a voltage between an electrode and a workpiece (an object to be machined). The workpiece is melted by the electric discharge and thereby machined.As shown in FIG. 1, the electric discharge machine 1 includes a mechanical unit 2, a discharge power supply 3, a discharge state detection circuit 4, a position detection circuit 5, a motor amplifier 6, and a control circuit 7. The mechanical unit 2 includes an ultrasonic motor 21, an electrode holder 22, an electrode 23, and the like. A workpiece 24 is set on the mechanical unit 2.The ultrasonic motor 21 moves the electrode holder 22 in a driving direction (specifically, an up / down direction and a feeding direction of the electrode 23). The electrode holder 22 holds the electrode 23, and the electrode 23 is a wire-formed electrode having a narrow diameter (such as a diameter of 0.2 mm or less). The electrode 23 is configured by, for example, a thin, hollow (or solid) rounded rod made of copper, tungsten, or the like. When the electrode holder 22 is moved in the driving direction by the ultrasonic motor 21, the electrode 23 similarly moves in the driving direction.The discharge power supply 3 is a device that repeatedly applies a predetermined voltage between the electrode 23 and the workpiece 24 in a pulsating manner. The repetition cycle corresponds, for example, to several tens of thousandths of a second up to several tens of mils of a second. When the electrode 23 is separated from the workpiece 24 by an appropriate distance and the voltage is applied between the electrode 23 and the workpiece 24, an electric discharge is generated between the electrode 23 and the workpiece 24. Processing is performed by a portion of the workpiece 24 being melted.The discharge state detection circuit 4 detects a discharge state of the discharge power supply 3 (in other words, the discharge state between the electrode 23 and the workpiece 24) at all times. The discharge state detection circuit 4 then outputs the obtained discharge state as the detection result as a discharge state signal to the control circuit 7. The detected discharge state may correspond to the discharge voltage applied between the electrode 23 and the workpiece 24, the discharge current flowing between the electrode 23 and the workpiece 24, or the like.The position detection circuit 5 is a known circuit that detects the position of the electrode holder 22 in the driving direction. The position detection circuit 5 then outputs the detected position to the control circuit 7.The motor amplifier 6 corresponds to a circuit that drives the ultrasonic motor 21 based on an electrode drive signal received from the control circuit 7. Specifically, the motor amplifier 6 operates the ultrasonic motor 21 only during a phase in which the following electrode driving signal is received from the control circuit 7. The electrode driving signal during this phase has a voltage level at which the absolute value exceeds a predetermined value (specifically, zero). The motor amplifier 6 drives the ultrasonic motor 21 at a higher speed while increasing the absolute value of the voltage level of the received electrode driving signal, thereby moving the electrode holder 22 at a high speed. Hereinafter, reception of an electrode drive signal having a voltage level whose absolute value exceeds the predetermined value (specifically, zero) is simply referred to as reception of an electrode drive signal.The control circuit 7 is a known microcomputer including a central processing unit (CPU), a random access memory (RAM), a read only memory (ROM), a flash memory, and the like. The CPU executes programs stored in the ROM. Thus, the control unit 7 outputs the electrode drive signal to the motor amplifier 6 based on the discharge state signal received from the discharge state detection circuit 4. In general, the control circuit 7 controls the position of the electrode 23 in the driving direction by outputting the electrode driving signal to the motor amplifier 6 so that the distance between the electrode 23 and the workpiece 24 corresponds to an appropriate distance.FIG. 2 shows a detailed configuration of the mechanical unit 2. the mechanical unit 2 includes the ultrasonic motor 21 described above, the electrode holder 22, and the electrode 23. in addition, the mechanical unit 2 includes an XY stage 25, a back plate 26, a fixed plate 27, a linear guide 28, a side plate 29, an electrode guide holder 31, and an electrode guide 32.The XY stage 25 corresponds to a stage on which the workpiece 24 is placed. The XY stage 25 is capable of changing the position of the workpiece 24 with respect to the electrode 23 by moving the workpiece 24 in a direction of an X movement axis 25 xand a direction of a Y movement axis 25 ywithin a horizontal plane. In addition, the XY stage 25 is electrically connected to a ground-side terminal of the discharge power supply 3 via a feeder line (not shown).The back plate 26 corresponds to a standing wall fixed to the XY platform 25. The fixed plate 27 corresponds to a plate screwed and fixed to the back plate 26 so as to be parallel to the back plate 26. The ultrasonic motor 21 and the electrode holder 22 are fixed to a surface of the fixed plate 27 which is opposite to the surface on the back plate 26 side.Here, the ultrasonic motor 21 is described. The ultrasonic motor 21 drives the electrode 23. The ultrasonic motor 21 corresponds to a motor that moves a fingertip 21 aain an annular manner (specifically, elliptical) at an ultrasonic range frequency. The operation speed of the ultrasonic motor 21 corresponds to the rotational frequency of the elliptical motion of the fingertip 21 aaper unit time. The elliptical motion of the fingertip 21aa corresponds to a clockwise or counter-clockwise elliptical motion on the paper surface of FIG. 2, A micromotor described in JP H07-184 382A can be used as the ultrasonic motor 21 like this. The micromotor is widely available under the name HR1 motors from Nanomotion, Ltd.In the ultrasonic motor 21, electrodes are formed at four positions in a checkerboard net shape on a rectangular piezoelectric ceramic element. In addition, the ultrasonic motor 21 includes the fingertip 21 aaat a central portion of one side of the rectangle. A high frequency voltage of substantially 50 kHz is applied to the electrodes at two positions on a diagonal line of the electrodes at the four positions of the ultrasonic motor 21. Consequently, the ceramic member is expanded, contracted, and bent. In the fingertip 21 aa, an elliptical motion is generated at a rotation frequency of substantially 50 kHz.The electrode holder 22 includes a holder main body 22 aa, a first holding member 22 b, a first bearing 22 c, a rotation transmission member 22 d, a rotation conductive member 22 e, a conductive brush 22 f, a second bearing 22 g, and a second holding member 22 h.The holder main body 22 aasupports the other members 22 bto 22 hof the electrode holder 22. A ceramic plate is embedded in the portion of the holder main body 22 athat comes into contact with the fingertip 21 aa. When the ultrasonic motor 21 is operated and the fingertip 21 aais elliptically moved, the elliptical movement of the fingertip 21 aais transmitted to the holder main body 22 aaby a spring fixed to the piezoelectric ceramic element. The holder main body 22 aalinearly moves in the driving direction. In addition, the holder main body 22 aaand the other members 22 bto 22 hof the electrode holder 22 integrally move in the driving direction.Here, with reference to FIGS. 3 and 4, the relationship between the elliptical motion of the fingertip 21 aaand the motion of the holder main body 22 ais described. As shown in FIG. 3, when the holder main body 22 aais moved upward (in other words, in a direction away from the workpiece 24 in the driving direction; corresponding to an example of a second direction D 2), the fingertip 21 aais elliptically moved in a counterclockwise direction on the paper surface. In addition, as shown in FIG. 4, when the holder main body 22 aais moved downward (in other words, in a direction close to the workpiece 24 in the driving direction; corresponding to an example of a first direction D 1), the fingertip 21 aais elliptically moved in a clockwise direction on the paper surface.The fingertip 21 aaand the holder main body 22 aaare in a contact state at all times during the counterclockwise elliptical motion. However, the fingertip 21 aapresses more strongly against the holder main body 22 aawhen the fingertip 21 aatravels as compared with when the fingertip 21 aafalls. Thereby, the frictional force of the fingertip 21 aaon the holder main body 22 aais larger when the fingertip 21 aatravels up. Consequently, the holder main body 22 aatravels up when viewed in the entirety.In addition, the fingertip 21 aand the holder main body 22 aare also in a contact state at all times during the clockwise elliptical motion. However, in this case, the fingertip 21 apresses more strongly against the holder main body 22 awhen the fingertip 21 afalls as compared with when the fingertip 21 atravels up. Thereby, the frictional force of the fingertip 21a on the holder main body 22a is larger as the fingertip 21a descends. Consequently, the holder main body 22 adecommits when viewed in the entirety.In addition, when the fingertip 21a stops the elliptical movement, the holder main body 22a also stops due to the frictional force between the fingertip 21a and the holder main body 22a caused by the contact between the fingertip 21a and the holder main body 22a.The electrode 23 is inserted through the central portions of the first holding member 22 b, the rotation transmission member 22 d, the rotation guide member 22 e, and the second holding member 22 h. The first holding member 22 b, the rotation transmission member 22 d, the rotation guide member 22 e, and the second holding member 22 eholder the electrode 23 inserted through the central portions.The rotation transmission member 22 dis rotated by a rotation mechanism (not shown). A contact type rotation transmission mechanism such as a gear mechanism may be used as the rotation mechanism. Alternatively, a non-contact type rotation transmission mechanism such as an air spindle mechanism (refer to, for example, JP-A-2011-104735) may be used. For example, when the gear mechanism is used, the rotation transmission member 22 dis a gear that rotates about an axis (the portion through which the electrode 23 is inserted). The rotation transmission member 22d meshes with another gear (not shown), thereby receiving and rotating the transmission of the rotational force from the other gear.The rotation transmission member 22 dis configured to rotate integrally with the first holding member 22 b, the rotation guide member 22 e, and the second holding member 22 h.In addition, the first holding member 22 bis held by the holder main body 22 awith the first bearing 22 ctherebetween. The first holding member 22 bis capable of rotating with respect to the holder main body 22 a. In addition, the second holding member 22 his held by the holder main body 22 awith the second bearing 22 fbetween. The second holding member 22 his capable of rotating with respect to the holder main body 22 a.As a result of this configuration, when the rotation transmission member 22 dis rotated with respect to the holder main body 22 a, the first holding member 22 b, the rotation guide member 22 e, and the second holding member 22 hrotate together with the rotation transmission member 22 d. Along with the rotation, the electrode 23 also rotates with respect to the holder main body 22 a.In addition, the rotation guide member 22 eis electrically conductive with a non-ground side terminal of the discharge power supply 3 with the conductive brush 22 ftherebetween. Consequently, the discharge power supply 3 can apply a voltage to the electrode 23 via the conductive brush 22 f.The electrode guide holder 31 is fixed to the back plate 26 below the electrode holder 22. The electrode guide 32 is supported by the electrode guide holder 31 so as to be capable of rotating with respect to the electrode guide holder 31. In addition, the electrode guide 32 has a through hole through which the electrode 23 is inserted. The electrode 23 extends downward from the second holding member 22 h. Since the electrode 23 passes through the through hole in the electrode guide 32, the electrode 2 is positioned.The side plate 29 corresponds to a standing wall fixed to the XY stage 25. The linear guide 28 is fixed to the side plate 29.The holder main body 22a is fixed to linear moving blocks 28b and 28c of the linear guide 28. The linear moving blocks 28b and 28c are engaged with a linear rail 28a so as not to be disengaged from the linear rail 28a. The linear moving blocks 28 band 28 care capable of moving freely on the linear rail 28 aalong the driving direction.As shown in FIG. 2, the linear guide 28 includes the linear rail 28 aand the linear motion blocks 28 band 28 c. The linear rail 28a is fixed to the side plate 29. The linear moving blocks 28 band 28 care engaged with the linear rail 28 ato be able to move with respect to the linear rail 28 a.FIG. 5 is a partial perspective view of the linear guide 28. However, the linear motion block 28c also has the same configuration as that of the linear motion block 28b.The linear moving blocks 28 band 28 care fixed to the linear rail 28 ato be able to slip in the driving direction with numerous spherical balls 28 dtherebetween (corresponding to an example of a rolling element). The linear rail 28a corresponds to a track member. The linear rail 28a corresponds to a long member whose cross-sectional shape is perpendicular to the longitudinal direction (corresponding to the driving direction). In the corner portions of the linear rail 28a on the side of the linear moving blocks 28b and 28c, four ball rolling grooves 28e are formed over the entire length of the linear rail 28a. The four ball rolling grooves 28e serve as paths when the balls 28d roll. The ball rolling grooves 28 eextend linearly in the longitudinal direction.In each of the linear motion blocks 28 band 28 c, four ball rolling grooves 28 jare also formed. The ball rolling grooves 28j correspond to the ball rolling grooves 28e, respectively. The plurality of balls 28 dare accommodated between the ball rolling grooves 28 ein the linear rail 28 aand the ball rolling grooves 28 jin the linear moving blocks 28 and 28. Moreover, four rows of return paths 28m are formed in the linear motion blocks 28b and 28c. The return paths 28 mextend parallel to the ball rolling grooves 28 j, respectively. The linear motion blocks 28b and 28c are also provided with turning paths. Each turning path connects a return path 28m and a ball rolling groove 28j.FIG. 6 is a cross-sectional view of an unlimited circulation path. The unlimited circulation path is configured by a combination of a single ball rolling groove 28 e, a single ball rolling groove 28 j, a single return path 28 m, and two turning paths 28 p. The two turning paths 28p connect the ball rolling grooves 28e and 28j and the return path 28m.Each unlimited circulation path is provided with numerous balls 28d. The balls 28 dare in a single row to be able to contact and separate from each other. In addition, the unlimited circulation path is filled with a lubricating oil. In general, each ball 28 drolls through the unlimited circulation path, thereby moving and circulating while receiving a load as the linear moving blocks 28 band 28 cmoves relative to the linear rail 28 a. Fig. 5 shows the arrangement of the unlimited circulation path and the balls 28d. A portion of the linear motion block 28 bis omitted in FIG. 5.According to the present embodiment, the diameters of the balls 28 din the linear guide 28 are all equal to 1 mm. However, there may be individual differences in the diameters of the balls 28d.Next, an operation of the electric discharge machine 1 configured as described above and a processing method using the electric discharge machine 1 will be described.First, the workpiece 24 is placed on the XY stage 25. The workpiece 24 according to the present embodiment corresponds to a member serving as a base for a nozzle body of a fuel injection nozzle. The fuel injector injects fuel (such as gasoline or diesel fuel) into a cylinder of an engine. The workpiece 24 corresponds to a member having an outer appearance of a nozzle body. However, the workpiece 24 may be a member that serves as a base for another machined object.In the electric discharge performed using the electric discharge machine 1, a step of generating an electric discharge between the electrode 23 and the workpiece 24 and forming a single spray hole in the workpiece 24 is performed. The step is repeatedly performed while changing, as necessary, the X-axis direction position and the Y-axis direction position of the XY stage 25 at each time a single spray hole is formed. Consequently, a plurality of injection holes for injecting fuel are formed at a plurality of positions in the workpiece 24. As a result of these formed injection holes, the workpiece 24 becomes a finished nozzle body. Therefore, the method of machining the workpiece 24 using the electric discharge also constitutes a method of machining a nozzle body (corresponding to an example of a workpiece).A worker places the workpiece 24 on the XY platform 25, then the worker operates the discharge power supply 3. The discharge power supply 3 then starts applying a pulse voltage between the electrode 23 and the workpiece 24 at a predetermined cycle (such as a cycle of several tens of mils of a second).In addition, the worker operates the discharge state detection circuit 3, the position detection circuit 5, the motor amplifier 6, and the control circuit 7. The discharge state signal indicates the discharge state (such as the discharge current or the discharge voltage) between the electrode 23 and the workpiece 24. In addition, the position detecting circuit 5 starts to output the position signal to the control circuit 7. The position signal indicates the position of the electrode holder 22 in the driving direction.In addition, the control circuit 7 is started. The control circuit 7 starts to perform a position control operation based on the received discharge state signal. The control circuit 7 performs the position control operation to output the electrode drive signal to the motor amplifier 6 so that the distance between the electrode 23 and the workpiece 24 becomes an appropriate electric discharge distance. Subsequently, the motor amplifier 6 drives the ultrasonic motor 21 based on the electrode drive signal received from the control circuit 7.Here, with reference to Figs. 7 to 11, the details of the position control operation performed by the control circuit 7 will be described. FIG. 7 is a flowchart of the position control process performed by the control circuit 7. FIG. 8 is a time chart showing the change with time of the electrode driving signal, the electrode position, the discharge state, and the discharge state detection time during electric discharge.In the position control process, the control circuit 7 first determines whether machining of a new single injection hole is started at step S 110. The control circuit 7 performs the determination based on, for example, the worker performing a predetermined starting operation on the electric discharge machine 1. The control circuit 7 repeatedly performs the determination until the machining is started.When it is determined that the machining of a new single injection hole is started, the control circuit 7 subsequently performs a single hole machining preparation process (steps S 111 to S 114). Specifically, the control circuit 7 first proceeds to step S 111. The control circuit 7 resets a flag S to OFF. The flag S relates to a short circuit between the electrode 23 and the workpiece 24, and the flag S corresponds to data held in a memory (such as the RAM) of the control circuit 7.Subsequently, at step S 112, the control circuit 7 starts lowering control to lower the electrode holder 22 downward (corresponding to the first direction) from a predetermined original position (home position) in the driving direction toward a predetermined machining start position. Specifically, the control circuit 7 starts to output, to the motor amplifier 6, the electrode driving signal for lowering the electrode holder 22. The machining start position corresponds to a position set in advance as a position at which electric discharge between the electrode 23 and the workpiece 24 can be started (specifically, a position at which the lower end of the electrode 23 approaches the upper surface of the workpiece 24 to enable electric discharge).The motor amplifier 6, which has received the electrode driving signal, elliptically moves the fingertip 21a of the ultrasonic motor 21. The motor amplifier 6 moves the fingertip 21a at a speed corresponding to the absolute value of the level of the electrode driving signal. The direction of the elliptical motion corresponds to the direction for raising the electrode holder 22 when the level of the electrode driving signal is positive (in other words, counterclockwise in FIG. 3 ). The direction of the elliptical motion corresponds to the direction for lowering the electrode holder 22 when the level of the electrode driving signal is negative (in other words, clockwise in FIG. 4 ). The level of the electrode driving signal, the output of which is started at step S 112, corresponds to a negative value. Therefore, the electrode holder 22 and the electrode 23 start to lower along the driving direction to approach the workpiece 24.Subsequently, at step S 113, the control circuit 7 measures the lowering speed Vd (corresponding to an example of a first moving speed) of the electrode holder 22 and the electrode 23 based on the position signal from the position detection circuit 5. During this lowering phase, the control circuit 7 also continues to output the electrode driving signal to the motor amplifier 6, and continues to lower the electrode holder 22 and the electrode 23. The lowering speed Vd may correspond to an average value, a maximum value, a minimum value, or a mode value of the moving speed of the electrode holder 22 during the predetermined lowering phase. However, the lowering speed Vd is calculated as a positive value. In other words, the lowering speed Vd corresponds to a statistical representative value of the lowering speed of the electrode holder 22 during the predetermined lowering phase.The predetermined lowering phase corresponds to a phase whose length is set in advance as a phase during which the electrode holder 22 starts to move from the original position and has not reached the machining start position yet. The predetermined lowering phase may correspond to, for example, a phase corresponding to half the amount of time required until the electrode holder 22 reaches the machining start position after the start of the movement from the original position.Subsequently, the control circuit 7 waits until the electrode holder 22 reaches the machining start position based on the position signal outputted from the position detection circuit 5 at step S114. When it is detected that the electrode holder 22 has reached the machining start position, the control circuit 7 stops the output of the electrode drive signal (the electrode drive signal to lower the electrode holder 22), thereby stopping the lowering control. Consequently, the electrode holder 22 and the electrode 23 temporarily stop the descent.During the phase from the start of the lowering control at step S 112 to the end of the lowering control at step S 114, the level of the electrode driving signal can be kept constant. By keeping the level constant, the frequency of the elliptical motion of the fingertip 21a is controlled to be fixed. Consequently, the lowering speed of the electrode holder 22 and the electrode 23 is also controlled to be fixed. As a result, during this phase there is little acceleration or deceleration of the electrode holder 22 and the electrode 23. Therefore, the lowering speed Vd more largely reflects the effects of the sliding resistance in the linear guide 28. However, the level of the electrode driving signal may be changed during the phase from the start of the lowering control at step S 112 to the end of the lowering control at step S 114.Subsequently, at step S 115, the control circuit 7 proceeds to output the electrode drive signal 51 (see FIG. 8 ) to the motor amplifier 6 via a predetermined phase T 1. In the first step S115 after the start of the position control operation, the level Y of the output electrode drive signal corresponds to a default value Y0 (Y0 corresponds to a negative value) which is stored in the ROM. In the present example, since step S 115 corresponds to the first after the start of the position control operation, the electrode drive signal whose level corresponds to the set value Y 0 is output to the motor amplifier 6 via the set phase T 1.The motor amplifier 6 receives the electrode driving signal of the level Y0 as described above over the fixed phase T1. The motor amplifier 6 elliptically moves the fingertip 21a of the ultrasonic motor 21 at a speed corresponding to the absolute value of the level Y0. The direction of the elliptical motion varies depending on whether the level of the electrode driving signal is positive or negative as described above. The level Y0 in this case corresponds to a negative value. Therefore, the position 52 in the driving direction of the electrode holder 22 and the electrode 23 decreases by a distance substantially corresponding to the level Y0 to approach the workpiece 24.The discharge power supply 3 continues to apply the pulse voltage between the electrode 23 and the workpiece 24 repeatedly even during the output of the electrode drive signal 51. The interval T0 corresponds to, for example, 0.5 ms. During the interval T0 of the output time of the electrode drive signal 51, the pulse voltage is applied approximately ten thousand times. At each time the pulse voltage is applied, an electric discharge is generated based on the distance from the workpiece 24 to the electrode 23 or the like in the discharge state 53 (an electric discharge cannot be generated depending on the distance or the like).Subsequently, when the output of the electrode drive signal 51 is completed over the fixed phase T 1, the control circuit 7 resets a timer value t to zero at step S 120. The timer value t varies based on the lapse of time. Subsequently, the control circuit 7 waits at step S 125 until the timer value t reaches T 2 (in other words, from the time 54 ato the time 54 bduring which the phase T 2 passes, after completion of the continuous output of the electrode drive signal 51). When the phase T2 is reached, the control circuit 7 then proceeds to step S130. Also, while the control circuit 7 waits until the timer value t reaches T2, the discharge power supply 3 continues to apply the pulse voltage between the electrode 23 and the workpiece 24 repeatedly, as during the output of the electrode drive signal 51.In addition, while the control circuit 7 waits until the timer value t reaches T2, the electrode drive signal 51 is not output to the motor amplifier 6. Therefore, the motor amplifier 6 does not drive the ultrasonic motor 21. However, up to this point, the moving speed of the electrode holder 22 and the electrode 23 does not suddenly become zero due to inertia, and then the electrode holder 22 and the electrode 23 continue to move forward in the same moving direction while being decelerated due to friction with the fingertip 21 a. At this time, even if the phase T 2 passes, if the moving speed becomes zero before the phase T 2 passes, the moving speed remains zero.At step S 130, the control circuit 7 detects the discharge state 53 between the electrode 23 and the workpiece 24 at this current time, in other words, at the time when the moving speed of the electrode 23 has decreased to a certain extent. The control circuit 7 detects the discharge state 53 based on the discharge state signal received from the discharge state detection circuit 4. The discharge state 53 between the electrode 23 and the workpiece 24 at the present time may correspond to an amount based on one or both of the discharge voltage Vg and the discharge current received from the discharge state detection circuit 4.Alternatively, the discharge state 53 between the electrode 23 and the workpiece 24 at the current time may correspond to an amount based on one or both of the average value Vg of the discharge voltage and the average value of the discharge current during a phase from the current point to a point that is past by a predetermined amount of time. For the predetermined amount of time, it is only required that the time does not return to the time at which the last output of the electrode drive signal 51 ended (in other words, the time at which the electrode 23 starts decreasing). For example, if the predetermined amount of time is half of the phase T 2 or less, the predetermined amount of time is more preferable because the moving speed of the electrode 23 is considered to be sufficiently decreased.Subsequently, at step S 140, the control circuit 7 determines whether the electrode 23 and the workpiece 24 are short-circuited and the continuous short-circuit phase exceeds a reference time Ts, based on the discharge state 53 acquired at the last step S 130 and the previously performed step S 130.Here, the reference time Ts is, for example, 50 ms. When the reference time Ts is 50 ms and the detection interval T 0 of the discharge state signal is 0.5 ms, the phase of the continuous short circuit is determined to exceed the reference time Ts when the number of times a short-circuit discharge state signal is continuously obtained exceeds a number of 100.As shown in FIG. 9, the discharge state 53 corresponds to a voltage value based on the discharge voltage Vg received from the discharge state detection circuit 4 at each time. The discharge state 53 may indicate a greater distance between the electrode 23 and the workpiece 24 as the voltage value increases.In this case, no current flows between the electrode 23 and the workpiece 24 when the voltage value becomes a maximum value (8.0 V). In other words, no electric discharge is generated between the electrode 23 and the workpiece 24. When the voltage value becomes a minimum value (0.0 V), the electrode 23 and the workpiece 24 are in a short-circuit state.When it is determined that the electrode 23 and the workpiece 24 are short-circuited and the continuous short-circuit phase exceeds the reference time Ts, the control circuit 7 proceeds to step S 145. If otherwise determined, the control circuit 7 bypasses step S 145 and proceeds to step S 150. At step S 145, the control circuit 7 sets the flag S to ON and then proceeds to step S 150.At step S 150, the control circuit 7 determines the level Y of the electrode drive signal 51 based on the discharge state detected at step S 130. the level Y is determined using, for example, an equation Y=K(Vg-Vc) as described above. The level Y of the electrode drive signal 51 determined in this manner corresponds to the level of the electrode drive signal 51 which is subsequently continuously output. A control coefficient K corresponds to a negative value. The control coefficient K prescribes the proportional relationship between the deviation of the discharge state detected at step S 130 with respect to an ideal discharge state and the level Y of the electrode drive signal 51 based on the deviation. The control coefficient K is determined in advance as a fixed value. In addition, Vc corresponds to a positive value which is previously determined as the voltage level of the discharge state signal corresponding to the ideal discharge state.The level Y of the electrode driving signal to be subsequently output to the motor amplifier 6 is determined using a method such as this. Therefore, for example, when the distance between the electrode 23 and the workpiece 24 to the current point is longer than the distance for updating the ideal electric discharge, the level Vg of the discharge state becomes higher than the reference level Vc. Therefore, the value of the level Y of the electrode driving signal is calculated as a negative value. Consequently, in the next step S 115, the control circuit 7 performs control to lower the electrode holder 22 and the electrode 23.In addition, for example, when the distance between the electrode 23 and the workpiece 24 at the current point is shorter than that of the distance for updating the ideal discharge, the level Vg of the discharge state becomes lower than the reference level Vc. Therefore, the value of the level Y of the electrode driving signal is calculated as a positive value. Consequently, in the subsequent step S 115, the control circuit 7 performs control to lift the electrode holder 22 and the electrode 23.Specifically, when the electrode 23 and the workpiece 24 are short-circuited at the present point, the level Vg of the discharge state becomes zero. Therefore, the value of the level Y of the electrode driving signal is calculated as the maximum value. Consequently, in the next step S 115, the control circuit 7 performs control to raise the electrode holder 22 and the electrode 23 to the maximum height.The distance over which the electrode holder 22 is moved in the next step S 115, based on the level Y determined in step S 150, corresponds to a maximum of several μm. The distance is substantially shorter than the diameter of the ball 28d.Subsequently, in step S 160, the control circuit 7 determines whether the machining of the injection hole, that is, the current machining object, is completed. For example, when it is determined that the electrode 23 has penetrated the workpiece 24 based on the discharge state signal from the discharge state detection circuit 4, the control circuit 7 may determine that the machining of the injection hole, that is, the current machining object, is completed. Alternatively, when it is determined that the electrode holder 22 has reached a predetermined machining completion position based on the position signal from the position detection circuit 5, the control circuit 7 may determine that the machining of the injection hole, that is, the current machining object, is completed.When it is determined that the electrode holder 22 has not reached the predetermined machining completion position, the control circuit 7 then performs step S 165. When it is determined that the electrode holder 22 has reached the predetermined machining completion position, the control circuit 7 proceeds to step S 170 to return the electrode 23 to the original position.At step S165, the control unit 7 waits until the timer value t reaches the time (T0-T1). The time T 1 corresponds to the phase over which a single output of the electrode drive signal 51 is maintained. Therefore, the elapse of time (T 0-T 1) from the end of the previous output of the electrode drive signal 51 indicates the elapse of the output cycle T 0 of the electrode drive signal 51 from the start time of the previous output of the electrode drive signal 51. When the timer value t reaches the time (T0-T1), the control circuit 7 returns to step S115 again. The control circuit 7 continues to output the electrode drive signal 51 having the level Y determined for the fixed phase T1 at the last step S150 to the motor amplifier 6.As a result of repeatedly performing the loop from steps S 115 to S 165 thereafter as described above, the control circuit 7 performs the operations (A), (B), and (C) thereafter in this order as shown in FIG. 8.(A) The control circuit 7 outputs the electrode drive signal 51 at the set cycle T 0 and drives the electrode 23 (step S 115). After completion of the output of the electrode drive signal 51, when the predetermined time T 2 has elapsed (step S 125), the control circuit 7 detects the discharge state (step S 130).(B) Only when the short circuit continues for the reference time Ts based on the detected discharge state (step S 140), the control circuit 7 changes the state of the flag S to ON (step S 145).(C) The control circuit 7 determines the level Y of the electrode drive signal 51 based on the detected discharge state (step S 150), and outputs the next electrode drive signal 51 (step S 165) as the electrode drive signal 51 having the determined level Y.The cycle of the detection time of the discharge state at step S 130 is also T 0.The time for detecting the discharge state as described above corresponds to the time at which the ultrasonic motor 21 is not in operation, the electrode 23 is not driven, and the moving speed of the electrode 23 is attenuated due to inertia. However, as another example, the time for detecting the discharge state may correspond to the time at which the ultrasonic motor 21 operates and the electrode 23 is driven.In addition, when it is determined at step S 160 that the electrode holder 22 has reached the predetermined machining completion position as described above, or in other words, when the machining of a single injection hole is completed, the control circuit 7 proceeds to step S 170. At steps 170 to 195, the control circuit 7 performs a single hole post-processing operation. Specifically, the control circuit 7 first performs start-up control to lift the electrode holder 22 in the upward driving direction (corresponding to an example of the second direction) from the current position (such as the machining completion position) to the original position at step S 170. Specifically, the control circuit 7 starts to output the electrode driving signal to the motor amplifier 6 to lift the electrode holder 22.The motor amplifier 6, which has received the electrode driving signal, elliptically moves the fingertip 21a of the ultrasonic motor 21. The motor amplifier 6 moves the fingertip 21a at a speed corresponding to the absolute value of the level of the electrode driving signal. Since the level of the electrode driving signal is positive, the direction of the elliptical motion corresponds to the direction for raising the electrode holder 22 (in other words, counterclockwise in FIG. 3 ). Therefore, the electrode holder 22 and the electrode 23 start to ascend along the driving direction to separate from the workpiece 24.Subsequently, at step S 175, the control circuit 7 measures the ramp-up speed Vu (corresponding to an example of a second moving speed) of the electrode holder 22 and the electrode 23 based on the position signal from the position detection circuit 5. During this startup phase, the control circuit 7 also continues to output the electrode drive signal to the motor amplifier 6, and continues to lift the electrode holder 22 and the electrode 23. The ramp-up speed Vu may correspond to an average value, a maximum value, a minimum value, or a mode value of the moving speed of the electrode holder 22 during the predetermined ramp-up phase. However, the ramp-up speed Vu is calculated as a positive value. In other words, the ramp-up speed Vu corresponds to a statistical representative value of the ramp-up speed of the electrode holder 22 during the predetermined ramp-up phase.The predetermined start-up phase corresponds to a phase whose length is set in advance as a phase during which the electrode holder 22 starts moving from the original position and has not reached the machining start position yet. The predetermined start-up phase may correspond to, for example, a phase corresponding to half the amount of time required for the electrode holder 22 to reach the machining start position after the start of the movement from the original position.Subsequently, at step S 180, the control circuit 7 waits until the electrode holder 22 reaches the original position based on the position signal output from the position detection circuit 5. When it is detected that the electrode holder 22 has reached the original position, the control circuit 7 stops the output of the electrode drive signal (the electrode drive signal to lift the electrode holder 22), thereby stopping the start-up control. Consequently, the electrode holder 22 and the electrode 23 temporarily stop the start-up.During the phase from the start of the start-up control at step S 170 to the end of the start-up control at step S 180, the level of the electrode drive signal may be kept constant. By keeping the level constant, the frequency of the elliptical motion of the fingertip 21a is controlled to be fixed. Consequently, the ascending speed of the electrode holder 22 and the electrode 23 is also controlled to be fixed. Therefore, during this phase, there is little acceleration or deceleration of the electrode holder 22 and the electrode 23. Therefore, the ramp-up speed Vu more clearly reflects the effects of the sliding resistance in the linear guide 28. However, the level of the electrode driving signal may be changed during the phase from the start of the start-up control at step S 170 to the end of the start-up control at step S 180.Subsequently, at step S 185, the control circuit 7 performs a guidance abnormality determination process. In the guidance abnormality determination process, the control circuit 7 determines whether a guidance abnormality has occurred. The guide irregularity is present when the sliding resistance in the linear guide 28 becomes excessively larger than that during an ordinary state.At step S 190, the control circuit 7 determines whether a guidance abnormality has occurred based on the result of the guidance abnormality determination process at step S 185. When it is determined that a guidance abnormality has occurred, the control circuit 7 performs upper / lower limit stroke control at step S 195. The control circuit 7 then returns to step S 110 to process the next injection hole. When it is determined that no guidance abnormality has occurred, the control circuit 7 bypasses step S 195 and returns to step S 110 to process the next injection hole. Details of the upper / lower limit stroke control at step S 195 will be described below.As described above, the control circuit 7 performs the position control operation shown in FIG. 7 when a plurality of injection holes in a single workpiece 24 are machined or when a plurality of injection holes in a plurality of workpieces 24 are machined. In the position control process, the control circuit 7 performs the single-hole machining preparation process (steps 111 to 114), then performs the actual machining process (steps 115 to 165), and then performs the single-hole machining process (steps 170 to 195) when each hole is machined (step S 110).In the single hole machining preparation process, the control circuit 7 lowers the electrode holder 22 from the original position to the machining start position. In addition, the control circuit 7 detects the moving speed Vd of the electrode holder 22 during the above-described predetermined lowering phase.In addition, the control circuit 7 generates an electric discharge between the workpiece 24 and the electrode 23 in the actual machining operation. the control circuit 7 advances the boring of the injection hole in the workpiece 24 while the workpiece 24 is melted by the electric discharge. Subsequently, in the actual machining operation, the control circuit 7 sets the flag S to ON (step S 145) when the electrode 23 and the electrode holder 22 and the workpiece 24 are short-circuited for a longer phase than the reference time Ts (step S 140).As described above, when a short circuit occurs only once during the actual machining operation, the control circuit 7 performs control to move the electrode holder 22 in the direction to remove the electrode 23 from the workpiece 24 at step S 150 at this time. Therefore, a short-circuit state is usually not continuously detected a plurality of times during repetitions of step S 130. Frequent continuous detection of the short-circuit condition indicates that the sliding resistance in the linear guide 28 has increased. The probability is high that the linear motion blocks 28b and 28c have difficulty in moving over the line of the rail 28a.In addition, in the single hole post-processing operation, the control circuit 7 lifts the electrode holder 22 from the processing completion position toward the original position. In addition, the control circuit 7 detects the moving speed Vu of the electrode holder 22 during the above-described predetermined startup phase. The control circuit 7 then performs the upper / lower limit stroke if an abnormality has occurred based on the result of the guidance abnormality determination process.When the electrode holder 22 moves in the driving direction in any one of the above-described single-hole machining preparation process, the actual machining process, and the single-hole machining process, the linear moving blocks 28 band 28 cmoves integrally with the electrode holder 22 along the linear rail 28 aat all times. Due to the movement of the linear moving blocks 28 band 28 calong the linear rail 28 a, the linear rail 28 aguides the movement of the electrode holder 22 in the driving direction.Here, with reference to FIG. 10, an example in which the worker processes a plurality of spray holes in a single workpiece 24, or an example in which the worker processes a plurality of spray holes in a plurality of workpieces 24, using the electric discharge machine 1, will be described.As shown in FIG. 10, at time t 10, the worker first sets the workpiece 24 on the XY stage 25 as shown in FIG. 1 when machining a first hole. The worker then operates the discharge power supply 3, the discharge state detection circuit 4, the position detection circuit 5, the motor amplifier 6, and the control circuit 7.Subsequently, at time t 11, the worker performs a predetermined starting operation on the electric discharge machine 1. The control circuit 7 then proceeds from step 110 to 111 and performs the single hole machining preparation process. Subsequently, the position of the electrode holder 22 falls at a fixed speed higher than the speed during actual machining from the original position toward the machining start position as shown by a solid line 60 in FIG. 10.Subsequently, the control circuit 7 terminates the single hole machining preparation operation and starts the actual machining operation when the electrode holder 22 reaches the machining start position at the time point t 12. The amount of time required from time t 11 to time t 12 is, for example, 0.5 seconds.After time t12, the injection hole in the workpiece 24 is slowly drilled by the electrode 23. At time t 13, the electrode 23 penetrates the workpiece 24. at subsequent time t 14, the control circuit 7 determines that machining of the current injection hole is completed at step S 160. The control circuit 7 terminates the actual machining operation. The amount of time required from t 12 to t 14 is, for example, 10 to 30 seconds.Here, it is assumed that during the machining of the first injection hole, no short circuit has occurred over a phase exceeding the reference time Ts. Therefore, during the phase from the time t 12 to the time t 14 at each time, the control circuit 7 bypasses steps 140 to 145 and proceeds to step S 150. Therefore, the flag S remains OFF at time t14.After time t 14, the control circuit 7 proceeds to step S 170 and performs the single hole post processing operation. Subsequently, as described above, the position of the electrode holder 22 rises at a fixed speed higher than the speed during actual machining from the machining completion position toward the original position, as indicated by a solid line 60 in FIG. 10.When the electrode holder 22 reaches the original position at time t 15, the control circuit 7 performs the guidance abnormality determination at step S 185. As shown in FIG. 11, in the guidance abnormality determination at step S 205, the control circuit 7 first determines whether the lowering speed Vd (positive value) measured at the last step S 113 is lower than a predetermined lowering reference speed Vd 0 (corresponding to an example of a first reference speed). Subsequently, when it is determined that the lowering speed Vd is lower than the lowering reference speed Vd 0 (positive value), the control circuit 7 proceeds to step S 225. The control circuit 7 determines that a guidance abnormality has occurred and ends the guidance abnormality determination. In addition, the control circuit 7 proceeds to step S 210 when it is determined that the lowering speed Vd is equal to or higher than the lowering reference speed Vd 0.Usually, the balls 28d rotate within the linear guide 28, causing them to roll through the space as the linear moving blocks 28b and 28c move. Consequently, the sliding resistance between the linear rail 28 aand the linear moving blocks 28 band 28 cis reduced. The reduction of the sliding resistance is achieved by the resistance reducing effect of rolling the balls 28d. In addition, the reduction of the sliding resistance is also achieved by the effect of the movement of the lubricating oil within the unlimited circulation path in association with the rolling of the balls 28 d(rolling movement), thereby enabling the lubricating oil to be uniformly distributed over the unlimited circulation path.However, as described above, the distance over which the electrode holder 22 moves at step S 115 during the actual machining operation (in other words, the moving distance of the linear moving blocks 28 band 28 c) is several μm at maximum, whereas the diameter of the balls 28 dinside the linear guide 28 is large is about 1 mm. Therefore, the balls 28d move while vibrating to a very small extent at a high frequency without rolling. In addition, the variations in the amount of movement of the individual balls 28d increase.When this state continues, as shown in FIG. 12, uneven positioning occurs in the distribution of the balls 28d within the unlimited circulation path. In a portion where the balls 28 dare concentrated, mutual contact between the balls 28 dis increased. Consequently, the sliding resistance between the linear rail 28a and the linear moving blocks 28b and 28c increases due to the frictional force between the balls 28d.In addition, it becomes difficult for the lubricating oil to be uniformly distributed over the unlimited circulation path because the moving amount of the balls 28 dis small. Consequently, the balls 28 are engaged with each other. In addition, when the balls 28 dare moved while vibrating to a very small extent at a high frequency without rolling, abrasion is generated in a state where the lubricating oil is not uniformly distributed, because the balls 28 dand the unlimited circulation path rub each other. The thus generated output interacts with the balls 28d, thereby increasing the sliding resistance between the linear rail 28a and the linear moving blocks 28b and 28c. Such a state corresponds to a state of the guide abnormality in which the sliding resistance becomes excessively larger than that in an ordinary state.When the sliding resistance between the linear rail 28 aand the linear moving blocks 28 band 28 cis increased in this manner, if the driving force of the electrode holder 22 is equal, the moving speed of the electrode holder 22 decreases. According to the present embodiment, this decrease in the moving speed is used as follows. When the lowering speed Vd of the electrode holder 22 is lower than the predetermined lowering reference speed Vd 0 during the predetermined lowering phase (step S 205), the control circuit 7 determines that a guidance abnormality has occurred (step S 225). The lowering reference speed Vd0 may be determined in advance as half of the prescribed speed at which the electrode holder 22 ascends during a conventional single hole machining preparation operation.Subsequently, at step S 205, the control circuit 7 determines whether the ramp-up speed Vu (positive value) measured at the last step S 175 is lower than a predetermined reference ramp-up speed Vu 0 (corresponding to an example of a second reference speed). When it is determined that the ramp-up speed Vu is lower than the ramp-up reference speed Vu 0 (positive value), the control circuit 7 proceeds to step S 225 and determines that a guidance abnormality has occurred. When it is determined that the ramp-up speed Vu is equal to or higher than the reference ramp-up speed Vu 0, the control circuit 7 proceeds to step S 215.In other words, as described above, the moving speed of the electrode holder 22 decreases as the sliding resistance increases. The control circuit 7 uses this circumstance as follows. When it is determined that the ramp-up speed Vu of the electrode holder 22 during the predetermined ramp-up phase is lower than the predetermined ramp-up reference speed Vu 0 (step S 210), the control circuit 7 determines that a guidance abnormality has occurred (step S 225). The startup reference speed Vu 0 may be determined in advance as half of the assumed speed at which the electrode holder 22 starts up during an ordinary single-hole machining preparation operation.In consideration of the gravitational force applied to the electrode holder 22 and the linear moving blocks 28 band 28 c, when the absolute value of the level of the electrode driving signal output by the control circuit 7 is equal during both the predetermined lowering phase and the predetermined ascent phase, the lowering reference speed Vd 0 is set higher than the ascent reference speed Vu.In this manner, the control circuit 7 compares the moving speeds Vd and Vu in the driving direction of the electrode holder 22 with the reference speeds Vd 0 and Vu 0, thereby determining whether a resistance against the movement of the electrode holder 22 is abnormal.The moving speed Vd, which is compared with the reference speed Vd 0, corresponds to the moving speed of the electrode holder 22 when the electrode holder 22 is moved from the original position in the direction of the workpiece 24 to start machining of a single injection hole. In addition, the moving speed Vu compared with the reference speed Vu 0 corresponds to the moving speed of the electrode holder 22 when the electrode holder 22 is returned to the original position after the machining of the same injection hole is completed.The steps for moving before and after the start of machining a single injection hole correspond to steps that are usually present in the steps for machining. Thereby, by using the moving speed at such steps for comparison, it is not necessary to provide a special phase for detecting the moving speed. Therefore, this results in a faster processing. In addition, the steps of moving before and after the start of machining a single spray hole correspond to a phase during which the electrode holder can be moved in one direction over a long distance from the machining steps for a single hole. Therefore, the moving speed during this phase can be detected with high accuracy.In addition, the control circuit 7 determines that the guidance abnormality has occurred when at least one of the condition that the lowering speed Vd is lower than the lowering reference speed Vd 0 and the condition that the ramp-up speed Vu is lower than the ramp-up reference speed Vu 0 is satisfied, even if the other condition is not satisfied.One reason for performing the determination in this manner is that, as the sliding resistance increases due to uneven positioning of the plurality of balls 28 d, the movement of the electrode holder 22 in both the upward and downward directions does not necessarily become difficult. In some circumstances, movement in only one direction becomes difficult. As described above, as a result of the determination that the guidance abnormality has occurred, when an abnormality is detected in even one of the lowering speed Vd and the ascent speed Vu, situations in which the movement of the electrode holder 22 only in one direction becomes difficult can be addressed.At step S 215, the control circuit 7 determines whether the flag S is set to ON. When it is determined that the flag S is set to ON, the control circuit 7 proceeds to step S225. The control circuit 7 determines that a guidance abnormality has occurred and ends the guidance abnormality determination. When it is determined that the flag S is set to OFF, the control circuit 7 proceeds to step S220. The control circuit 7 determines that no guidance abnormality has occurred and ends the guidance abnormality determination.In this manner, the control circuit 7 determines that the resistance against the movement of the electrode holder 22 is not normal when the flag S is set to ON, in other words, when the phase during which the electrode 23 and the workpiece 24 are short-circuited exceeds the reference time Ts in the last machining operation proper.When the sliding resistance between the linear rail 28 aand the linear moving blocks 28 band 28 cis abnormal, even if control is performed to separate the electrode 23 from the workpiece 24 during the short circuit of the electrode 23 and the workpiece 24, the electrode 23 cannot be separated from the workpiece 24. Therefore, as described above, it can be determined whether the resistance to the movement of the electrode holder 22 is abnormal, in other words, whether a guide abnormality has occurred, based on the phase during which the electrode 23 and the workpiece 24 are short-circuited.For example, in the example in FIG. 10, it is assumed that no guidance abnormality has occurred during the single hole machining preparation process for machining the first hole (t 11 to t 12), the actual machining process (t 12 to t 14), and the single hole post-machining process t 14 to t 15. In this case, the electrode holder 22 is moved at the normal speed. Even if a short circuit occurs, the short circuit is quickly cleared. Therefore, the control circuit 7 determines at step S 205 in FIG. 11 that the lowering speed Vd is equal to or higher than the lowering reference speed Vd 0, and proceeds to step S 210.Subsequently, at step S 210, the control circuit 7 determines that the start-up speed Vu is higher than the start-up reference speed Vu 0, and proceeds to step S 215. The control circuit 7 determines that the flag S is set to OFF at step S 215 and proceeds to step S 220. The control circuit 7 then determines that no abnormality has occurred. Thereafter, the control circuit 7 determines that no abnormality has occurred in step S 190. The control circuit 7 bypasses step S 195 and returns to step S 110.After the machining of the first injection hole is completed, during the phase from the time t 15 to the time t 16, the worker temporarily removes the workpiece 24 from the XY stage 25.Thereafter, the worker or the control circuit 7 moves the workpiece 24 so that the position at which the second injection hole is to be formed in the workpiece 24 is positioned directly below the electrode 23. Subsequently, at time t 16, the worker performs the predetermined starting operation on the electric discharge machine 1.The control circuit 7 then fails from step 110 to 111. The control by the control circuit 7 and the operation of the electric discharge machine 1 during the subsequent phase from the time t 16 to the time t 17 are the same as the control and the operation from the time t 11 to the time t 15. As a result of repeatedly performing such operations by the worker, the plurality of spray holes are formed in the workpiece 24.Subsequently, the worker moves the workpiece 24 so that the position at which the last injection hole is to be formed in the workpiece 24 is positioned directly below the electrode 23. At time t 18, the worker performs the predetermined starting operation of the electric discharge machine 1. The control circuit 7 then proceeds from step S 110 to S 111.The control by the control circuit 7 and the operation of the electric discharge machine 1 during the subsequent phase from the time t 18 to the time t 22 are the same as the control and the operation from the time t 11 to the time t 15. However, it is assumed that a guidance abnormality has already occurred at time t 18.Subsequently, it is assumed that only one, only any two, or all of the following items (p), (q), and (r) have occurred.(p) The lowering speed Vd of the electrode holder 22, which is measured during the single hole machining preparation process at the time t 18 to the time t 19, is lower than the lowering reference speed Vd 0.(q) A short circuit occurs for a phase exceeding the reference time Ts as in phase T 3 in FIG. 9 during the actual machining phase at time t 19 to t 21.(r) The ramp-up speed Vu of the electrode body 22, which is measured during the single hole post-processing operation at the time t 21 to the time t 22, is lower than the reference ramp-up speed Vu 0.Subsequently, in the guidance abnormality determination process performed at time t 22, when the above-described item (p) has occurred, the control circuit 7 proceeds from step 205 to 225, and determines that guidance abnormality has occurred even when (q) has not occurred and (r) has not occurred, in the guidance abnormality determination process performed at time t 22. In addition, when (p) has not occurred and (r) has occurred, the control circuit 7 proceeds from step 210 to 225 and determines that a guidance abnormality has occurred even if (q) has not occurred.Moreover, if neither (p) nor (q) has occurred and (q) has occurred, the control circuit 7 proceeds from step 215 to 225 and determines that a guidance abnormality has occurred. Therefore, the control circuit 7 determines at step S 190 in FIG. 7 that a guidance abnormality has occurred, and proceeds to step S 195.At step S 195, the control circuit 7 performs the upper / lower limit stroke control. Specifically, the control circuit 7 first waits for a predetermined waiting period. During the waiting period, the worker first temporarily removes the workpiece 24 from the XY stage 25, or alternatively, the worker moves the XY stage 25 and moves the workpiece 24 toward a position that does not interfere with the movement of the electrode 23. Subsequently, at time t 23 when the waiting period has elapsed, the electrode holder 22 is moved by a method as shown in FIG. 10.Specifically, the control circuit 7 first proceeds to output the maximum positive electrode drive signal to the motor amplifier 6 from the time t 23 to the time t 24. Consequently, the motor amplifier 6 uses the ultrasonic motor 21 and continues to lift the electrode holder 22 at the maximum possible ramp-up speed.Subsequently, at time t 24, the control circuit 7 detects that the electrode holder 22 has reached the upper limit position corresponding to the highest possible performance based on the position signal from the position detection circuit 5. subsequently, starting from time t 24 to time t 25, the control circuit 7 continues to output the lowest negative electrode drive signal to the motor amplifier 6. Consequently, the motor amplifier 6 uses the ultrasonic motor 21 and continues to lower the electrode holder 22 at the maximum lowering speed in terms of performance.Subsequently, at time t 25, the control circuit 7 detects that the electrode holder 22 has arrived at the lower limit position corresponding to the lowest possible in terms of performance based on the position signal from the position detection circuit 5. subsequently, starting from time t 25 to time t 26, the control circuit 7 continues to output the maximum positive electrode drive signal to the motor amplifier 6. Consequently, the motor amplifier 6 uses the ultrasonic motor 21 and continues to lift the electrode holder 22 at the maximum performance ramp-up speed.Subsequently, at time t 26, the control circuit 7 detects that the electrode holder 22 has reached the original position based on the position signal from the position detection circuit 5. the control circuit 7 sets the level of the electrode drive signal to zero. Consequently, the motor amplifier 6 stops the operation of the ultrasonic motor 21 and stops the movement of the electrode holder 22. At step S 195, the control circuit 7 returns to step S 110.In the present example, the upper / lower limit stroke control is performed after the machining of the last injection hole. However, the above-described items (p), (r), and (q) may occur during the processing of shot holes other than the last shot hole (such as the first shot hole or the second shot hole). In this case, during the single hole post-processing operation, for the hole other than the last injection hole, the control circuit 7 determines that a guidance abnormality has occurred at step S 190, and performs the upper / lower limit stroke at step S 195 in a similar manner to that described above.The distance from the original position to the upper limit and the distance from the original position to the lower limit are set to a moving distance when the largest ball 28 dhaving the largest diameter rolls out of all the balls 28 dincluded in the linear guide 28 and rotates once without sliding (in other words, the length of the large circle of the largest ball 28 d), or longer.Consequently, the balls 28 drotate once or more frequently from the time t 23 to the time t 24. The balls 28 dsubsequently rotate twice or more from the time t 24 to the time t 25. The balls 28 dsubsequently rotate once or more from the time t 25 to the time t 26. Thereby, the uneven positioning in the arrangement of the balls 28 dmay be suppressed. In addition, the lubricating oil is more easily evenly distributed because the balls 28d move over a long distance simultaneously. In addition, even when abrasion is generated, the generated abrasion can be dispersed. Consequently, the increase in the sliding resistance between the linear rail 28 aand the linear moving blocks 28 band 28 cis suppressed.The amount of time from time t23 to time t26 is substantially shorter (such as 3 seconds) than the amount of time required for the actual machining operation of a single injection hole.In this manner, the control circuit 7 moves the electrode holder 22 at a high speed through a distance having an amount of the moving distance when the largest ball rolls and rotates once without slipping. Consequently, the increase in the sliding resistance between the linear rail 28 aand the linear moving blocks 28 band 28 cis suppressed. Therefore, the movement of the electrode holder 22 becomes smooth. The amount of time required for the processing of the injection hole can be reduced.In addition, in a single upper / lower limit stroke control operation, the electrode holder 22 continuously moves upward by a distance corresponding to the length of the large circle of the largest ball 28 dor longer. In addition, the electrode holder 22 continuously moves downward by a distance corresponding to the length of the large circle of the largest ball 28d or longer.Depending on the unequal positioning of the balls 28 d, the unequal positioning cannot be eliminated only by the electrode holder 22 moved by a distance corresponding to the length of the great circle of the largest ball 28 dor longer, in only one direction. In such circumstances, moreover, the likelihood of the mispositioning released becomes higher when the electrode holder 22 is moved in both directions by a distance corresponding to or longer than the length of the large circle of the largest ball 28d.(Second Embodiment)Next, a second embodiment of the present disclosure will be described. According to the present embodiment, the content of the guidance abnormality determination process performed by the control circuit 7 at step S 180 in FIG. 7 is changed from that according to the first embodiment. The guidance abnormality determination process is changed from the process in FIG. 11 to the process in FIG. 13.In the process in FIG. 13, steps S 205 and S 210 have been replaced with step S 208 from the process in FIG. 11. At step S 208, the control circuit 7 determines whether the absolute value of the difference between the lowering speed Vd (positive value) measured at the last step S 113 and the ramp-up speed Vu (positive value) measured at the last step S 175 is greater than a predetermined reference speed difference VT.When it is determined that the absolute value is larger than the reference speed difference VT, the control circuit 7 proceeds to step S 225 and then determines that a guidance abnormality has occurred. When it is determined that the absolute value is equal to or less than the reference speed difference VT, the control circuit 7 proceeds to step S 220 and determines that no guidance abnormality has occurred.In addition, as a result of determining whether guidance abnormality has occurred based on comparison of the lowering speed Vd and the raising speed Vu in this manner, situations in which movement of the electrode holder 22 in only one direction becomes difficult can be addressed.In addition, when the sliding resistance increases due to uneven positioning of the balls 28d, the resistance to downward movement and the resistance to upward movement do not become equal. Rather, the resistance to downward movement and the resistance to upward movement are likely to vary substantially. Therefore, situations can also be addressed in which the movement of the electrode holder 22 in both directions becomes difficult.(Other Embodiments)The present disclosure is not limited to the above-described embodiments. Variations are possible in a suitable manner within the scope mentioned within the scope of the claims. In addition, the above-described embodiments are not unrelated to each other. Except for circumstances in which combinations are clearly not possible, the embodiments may be combined as appropriate. In addition, it is understood that elements configuring each of the above-described embodiments do not necessarily represent required elements, except for circumstances in which the element is clearly and clearly indicated as being necessary, circumstances in which the element is clearly required based on the principle, and the like.Moreover, in the above-described embodiments in which numerical values such as an amount, a numerical value, an amount, or range of a constituent element of the embodiment are mentioned, the numerical value is not limited to the specific number except for circumstances in which the numerical value is clearly and clearly indicated as being required, circumstances in which the numerical value is clearly limited to the specified number based on the principle, and the like.Moreover, in the above-described embodiments in which the shape, positional relationship, and the like of constituent elements and the like are mentioned, the constituent elements are not limited to the shape, positional relationship, and the like, except for circumstances in which the constituent element is clearly indicated as being limited to the shape, positional relationship, and the like, circumstances in which the constituent element is limited to a specific shape, positional relationship, and the like based on the principle, and the like. For example, the following variation examples are allowed. The following variation examples may or may not be individually selectively applied to the above-described embodiments. In other words, any combination of the following variation examples can also be applied to the above-described embodiments.(First Variation Example)In the above-described embodiments, the balls 28d in the unlimited circulation path of the linear guide 28 are not connected by a ball holder. The ball retainer maintains the distance between the balls 28d at a fixed distance and connects the balls 28d to each other. However, the balls 28 din the unlimited circulation path of the linear guide 28 may be balls 28 dconnected by a ball holder. In this case, the problem of generated abrasion also occurs in a similar manner to the balls 28 dwhich are not connected by a ball holder in the present disclosure. This problem is solved by the above-described upper / lower limit stroke control.(Second Variation Example)In the above-described embodiments, the linear guide 28 is configured such that spherical balls 28 dare interposed between the linear rail 28 aand the linear moving blocks 28 band 28 c. However, instead of the spherical balls 28d, circular columnar rollers may be inserted. In other words, it is only required that the members interposed between the linear rail 28 aand the linear moving blocks 28 band 28 crepresent a plurality of rolling elements. In addition, the linear guide 28 that guides the linear motion may be replaced with a guide that guides a curved motion. In other words, it is only necessary that the guide 28 is a rolling bearing.

Claims

An electric discharge machine for applying a voltage between an electrode (23) and a workpiece (24) to generate an electric discharge so that the workpiece (24) is melted and processed by the electric discharge, the electric discharge machine comprising: an electrode holder (22) supporting the electrode (23); an ultrasonic motor (21) having a fingertip (21a) contacting the electrode holder (22) and moving the electrode holder (22) by moving the fingertip (21a) in an annular manner at an ultrasonic range frequency in a predetermined driving direction; a rolling bearing (28) guiding the movement of the electrode holder (22) in the driving direction, the rolling bearing (28) comprising: moving blocks (28b and 28c) fixed to the electrode holder (22); a rail (28a) that supports the moving blocks (28b and 28c) and guides the movement of the moving blocks (28b and 28c); and a plurality of rolling elements (28d) interposed between the moving blocks and the rail (28a); and a control circuit (7) that controls a position of the electrode holder (22), wherein the control circuit (7) moves the electrode holder (22) by a distance corresponding to at least the circumference of the largest rolling element (28d) when the mechanical resistance of the rolling bearing (28) has changed.The electric discharge machine according to claim 1, wherein the control circuit (7) is configured to determine whether an abnormality occurs in resistance to the movement of the electrode holder (22) by comparing a moving speed (Vd, Vu) in the movement of the electrode holder (22) in the driving direction with a predetermined reference speed.The electric discharge machine according to claim 2, wherein the moving speed compared with the reference speed corresponds to: a moving speed of the electrode holder (22) when the electrode holder (22) moves from a predetermined original position in a predetermined direction toward the workpiece (24) so that single hole machining is started; or a moving speed of the electrode holder (22) when the electrode holder is returned to the original position in the predetermined direction after the single hole machining is completed.The electric discharge machine according to claim 1, wherein the control circuit (7) is configured to determine that irregularity occurs in resistance to the movement of the electrode holder (22) when a first condition and / or a second condition is satisfied, the first condition corresponding to a condition that a first moving speed (Vd) in a first direction in the driving direction of the electrode holder is lower than a first reference speed (Vd0), the second condition corresponding to a condition that a second moving speed (Vu) in a second direction opposite to the first direction in the driving direction of the electrode holder (22) is lower than a second reference speed (Vu0).The electric discharge machine according to any one of claims 1 to 4, wherein the control circuit is configured to determine whether an abnormality occurs in resistance to the movement of the electrode holder (22), by comparing a first moving speed (Vd) in a first direction along the driving direction of the electrode holder (22) with a second moving speed (Vu) in a second direction opposite to the first direction along the driving direction of the electrode holder (22).The electric discharge machine according to any one of claims 1 to 5, further comprising: a discharge state detection circuit that detects a discharge state between the electrode (23) and the workpiece (24) to generate a discharge state signal, and outputs the discharge state signal, wherein the control circuit (7) is configured to: move the electrode holder (22) in the driving direction based on the discharge state signal output by the discharge state detection circuit, such that when the electrode (23) and the workpiece (24) are short-circuited, the electrode (23) moves in a direction away from the workpiece (24); and further based on when a time period during which the electrode ( 23) and the workpiece ( 24) are short-circuited exceeds a reference time (Ts), it is determined that an abnormality occurs in resistance to the movement of the electrode holder.The electric discharge machine according to any one of claims 1 to 6, wherein the control circuit (7) is configured to move the electrode holder (22) based on an abnormality occurring in resistance to the movement of the electrode holder in both a first direction and a second direction opposite to the first direction along the driving direction of the electrode holder, so that the electrode holder (22) is moved by a moving distance when the largest rolling element among the plurality of rolling elements rolls and rotates once without sliding.A method for manufacturing an object, comprising: a step of providing a workpiece (24) for the electric discharge machine according to any one of claims 1 to 7; and a step of machining the workpiece (24) using the electric discharge machine.

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