Abstract:
A replacement determination method: detects the remaining length of the electrode; detects the electrical discharge commencement position, being the position of the electrode when electrical discharge starts; detects the throughole position, being the position of the electrode when the workpiece is pierced; sets the required length for the electrode as required for machining the next throughhole, on the basis of the difference between the detected electrical discharge commencement position and the detected throughole position; compares the detected remaining length and the set required length; and determines whether or not electrode replacement is required.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Phase patent application of PCT/JP2012/069498, filed on Jul. 31, 2012, which is hereby incorporated by reference in the present disclosure in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a replacement judgment apparatus and replacement judgment method for an electrode for electrodischarge machining which judges a need for replacement of an electrode which is used for an electrodischarge machine. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the past, when using an electrodischarge machine for forming a fine hole in a workpiece, there has been known an apparatus designed to judge a need for replacement of an electrode while considering the ratio of consumption of an electrode for electrodischarge machining and a machining depth (plate thickness of workpiece) (for example, see PLT 1). In the apparatus which is described in this PLT 1, the required electrode length which is required for machining is calculated from a preset electrode consumption ratio and machining depth, an electrode length detecting means is used to detect a current electrode length, and, when the required electrode length is longer than the electrode length, it is judged that electrode replacement is necessary and advance to the electrodischarge machining process is stopped. 
         [0004]    However, when using a turbine blade etc. as a workpiece, the plate thickness of the workpiece at the machining location is not necessarily constant. Therefore, if, like in the apparatus which is described in the above PLT 1, using a preset machining depth as the basis to calculate the required electrode length, the need for electrode replacement cannot be precisely judged. Further, there is waste in setting the electrode consumption ratio larger in view of safety and ending up replacing an electrode which can still be used. 
       PATENT LITERATURE 
       [0005]    PLT 1: Japanese Patent No. 3007911 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a replacement judgment apparatus for an electrode for electrodischarge machining which judges a need for replacement of an electrode which successively forms through holes in a workpiece by electrodischarge machining, comprising an electrode length detecting means for detecting a residual length of the electrode, an electrodischarge start position detecting means for detecting a position of the electrode at the time of start of electrodischarge as an electrodischarge start position, a penetration position detecting means for detecting a position of the electrode at the time of penetration through the workpiece as a penetration position, a required length setting means for using a difference between the electrodischarge start position which is detected by the electrodischarge start position detecting means and the penetration position which is detected by the penetration position detecting means as the basis to set a length of the electrode which is required for forming the next through hole, and a judging means for comparing the residual length which is detected by the electrode length detecting means and the required length which is set by the required length setting means to judge the need for electrode replacement. 
         [0007]    Further, the present invention provides a replacement judgment method for an electrode for electrodischarge machining which judges a need for replacement of an electrode which successively forms through holes in a workpiece by electrodischarge machining, the replacement judgment method comprising detecting a residual length of the electrode, detecting a position of the electrode at the time of start of electrodischarge as an electrodischarge start position, detecting a position of the electrode at the time of penetration through the workpiece as a penetration position, using a difference between the electrodischarge start position which is detected and the penetration position which is detected as the basis to set a required length of the electrode which is required for forming the next through hole, and comparing the residual length which is detected and the required length which is set to judge the need for electrode replacement. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a front view which schematically shows the configuration of principal parts of an electrodischarge machine which has an electrode replacement judgment apparatus according to a first embodiment of the present invention. 
           [0009]      FIG. 2  is a perspective view of one example of a workpiece to which the present invention is applied as constituted by a turbine blade. 
           [0010]      FIG. 3  is a cross-sectional view along the line III-III of  FIG. 2 . 
           [0011]      FIG. 4  is a view which shows a machining operation of a workpiece according to an electrodischarge machine of  FIG. 1 . 
           [0012]      FIG. 5  is an enlarged view of principal parts of  FIG. 4 . 
           [0013]      FIG. 6  is a block diagram which shows the configuration of an electrode replacement judgment apparatus according to a first embodiment of the present invention. 
           [0014]      FIG. 7  is a flow chart which shows an example of the processing which is performed by a control part which forms part of the electrode replacement judgment apparatus according to the first embodiment of the present invention. 
           [0015]      FIG. 8  is a view which shows a relationship between elapsed time from when a pipe electrode starts descending and an average machining voltage between the pipe electrode and a workpiece. 
           [0016]      FIG. 9  is a view which explains an advantageous effect which is achieved by the electrode replacement judgment apparatus according to the first embodiment of the present invention. 
           [0017]      FIG. 10  is a flow chart which shows an example of main processing which is performed by a control part which forms part of an electrode replacement judgment apparatus according to a second embodiment of the present invention 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Below, referring to  FIG. 1  to  FIG. 9 , a first embodiment of a replacement judgment apparatus of an electrode for electrodischarge machining according to the present invention will be explained.  FIG. 1  is a front view which schematically shows the configuration of the main parts of the electrodischarge machine  100  which has an electrode replacement judgment apparatus according to a first embodiment of the present invention. Note that, below, for convenience, as illustrated, the orthogonal  3 -axial directions (X-axis direction, Y-axis direction, and Z-axis direction) are respectively defined as the left-right direction, front-rear direction, and top-bottom direction and the configurations of the parts are explained in accordance with these definitions. 
         [0019]    In  FIG. 1 , at the rear of a foundation formed by a bed  1 , a column  2  is provided standing up. At the top surface of the column  2 , an X-slider  3  is supported to be able to slide in the X-axis direction (left-right direction). On the top surface of the X-slider  3 , a ram  4  is supported to be able to move in the Y-axis direction (front-rear direction). At the front surface of the ram  4 , a spindle head  5  is supported to be able to move in the Z-axis direction (top-bottom direction). At the bottom surface of the spindle head  5 , a front end part of the rotary spindle  6  sticks out. At the bottom part of the rotary spindle  6 , an electrode holder  7  is attached. Below the electrode holder  7  in the vertical direction, an electrode guide  8  is arranged. The electrode guide  8  is supported at a bottom end part of a holding arm  9 . The holding arm  9  is supported by a bracket  4   a  which is provided at a right side surface of the ram  4  to be able to move in the top-bottom direction. The top-bottom movement axis of this holding arm  9  is defined as the W-axis. 
         [0020]    Between the electrode holder  7  and the electrode guide  8 , an electrode  10  extends along an axis CL 0  in the top-bottom direction passing through the centers of the electrode holder  7  and the electrode guide  8 . The electrode  10  is a cylindrical shaped pipe electrode and a top end part of the electrode  10  is supported by electrode holder  7 . The bottom end part of the pipe electrode  10  runs through the electrode guide  8  in the top-bottom direction. The pipe electrode  10  is supported at the outer circumference by the electrode guide  8 . The movement (swing) of the pipe electrode  10  is retrained in the front-rear and left-right directions while the pipe electrode  10  is able to slide in the electrode guide  8  in the top-bottom direction. Inside the pipe electrode  10 , for example, water or other machining fluid is supplied. The machining fluid is ejected from the front end part (bottom end part) of the pipe electrode  10 . Note that, for the machining fluid, oil may also be used. 
         [0021]    At the top surface of the bed  1 , a table  11  is arranged at the front of the column  2 . At the top surface of the table  11 , a slanted rotary table device  12  is carried. The slanted rotary table device  12  has a front-rear pair of support members  13  which are provided sticking upward from the top surface of the table  11 , a slanted member  14  which is supported between the front-rear support members  13  to be able to pivot in the B-axis direction about a pivot axis CLb which extends in the Y-axis direction, and a rotary table  15  which is supported at a left end surface of the slanted member  14  to be able to rotate in the A-axis direction about a rotation axis CLa which is vertical to the pivot axis Lb. The rotary table  15  is provided with a chuck  16 . The chuck  16  supports a workpiece  20 . Around the table  11 , a machining tank  17  is provided so as to be able to rise to cover the entire table  11  and slanted rotary table device  12 . Note that, the one-dot chain line of the figure shows the state where the machining tank  17  is raised. 
         [0022]    While not illustrated, the electrodischarge machine  100  of  FIG. 1  has an X-axis use drive part which makes the X-slider  3  move in the left-right direction, a Y-axis use drive part which moves the ram  4  in the front-rear direction, a Z-axis use drive part which moves the spindle head  5  in the top-bottom direction, a spindle drive part which rotates the rotary spindle  6  about the axis CL 0 , an arm drive part which moves the holding arm  9  in the top-bottom direction, a B-axis use drive part which makes the pivot member  14  slant via the pivot axis CLb, and an A-axis use drive part which makes the rotary table  15  rotate via a rotation axis CLa. The X-axis use drive part, the Y-axis use drive part, the Z-axis use drive part, and the arm drive part are, for example, comprised of ball screws and servo motors which drive rotation of the ball screws, the spindle drive part is, for example, comprised of a spindle motor, and the B-axis use drive part and A-axis use drive part are, for example, comprised of DD (direct drive) servo motors. The above X-axis use drive part, Y-axis use drive part, Z-axis use drive part, arm drive part, spindle drive part, B-axis use drive part, and A-axis use drive part will sometimes be referred to all together as the drive parts  35  ( FIG. 6 ). The drive parts  35  are controlled by the control part  30  ( FIG. 6 ). 
         [0023]    By the above configuration, the electrode holder  7  and the electrode guide  8  can move relative to the workpiece  20  in the X-axis direction, Y-axis direction, and Z-axis direction and can move relative to the workpiece  20  in the B-axis direction and A-axis direction. Therefore, the workpiece  20  can be machined to a desired three-dimensional shape. Further, by the arm drive part raising and lowering the holding arm  9 , the distance between the electrode holder  7  and the electrode guide  8  can be adjusted. Despite consumption of the pipe electrode  10  and resultant change in length of the pipe electrode  10 , during machining, the electrode holder  7  and the electrode guide  8  can always support the top-bottom ends of the pipe electrode  10 . 
         [0024]    At the front surface of the ram  4 , a position detector  31   a  such as a linear scale is provided for detecting a Z-axis position in the top-bottom direction of the spindle head  5 . The signal from the position detector  31  can be used to detect the position of the electrode holder  7 , that is, the position of the top end part of the pipe electrode  10 . At the bracket  4   a  of the holding arm  9 , a position detector  32  is provided for detecting a W-axis position in the top-bottom direction of the holding arm  9  with respect to the ram  4 . The signal from the position detector  32  can be used to detect the position of the electrode guide  8  with respect to the ram  4 . Between the Z-axis position and the W-axis position, there is a certain relationship inherent to the machine (known value), so the signals of the position detectors  31  and  32  can be used to detect the distance D between the bottom end part of the electrode holder  7  and the top end part of the electrode guide  8 . Note that, while not shown in the figures, at the side of the arm  9 , an electrode magazine is provided. The electrode magazine holds a plurality of pipe electrodes  10  for replacement use which have initial lengths L 0  (known). Between the spindle  6  and the tool magazine, a not shown changing means can be used to change the pipe electrode  10 . 
         [0025]    The workpiece  20  is, for example, a turbine blade or vane which is used for a gas turbine or jet engine etc. The turbine blade is exposed to a 1000° C. to 1500° C. or so high temperature gas, so a high heat resistant nickel alloy is used as the constituent material. At the surface of this turbine blade, for cooling the surface of the turbine blade, cooling holes are formed for passing cooling air. 
         [0026]      FIG. 2  is a perspective view of a workpiece  20  (turbine blade), while  FIG. 3  is a cross-sectional view along the line III-III of  FIG. 2 . At one end part of the turbine blade  20 , for example, a Christmas tree shaped support part  20   a  is provided. The support part  20   a  is attached to the circumference of a rotatable rotor. 
         [0027]    As shown in  FIGS. 2 and 3 , the turbine blade  20  is, for example, formed by the lost wax casting method. Inside the blade part  21 , a hollow part  30  is formed. The blade part  21  has an inside surface  21   a  which faces the hollow part  30  and an outer surface  21   b  which is exposed to high temperature gas. The blade part  21  is formed with a large number of cooling holes  22  which pass through the blade part  21  at a plurality of locations in the circumferential direction of the blade part  21  and along the height direction A of the blade part  21  (arrow mark A direction of  FIG. 2 ). The plate thickness t of the blade part  21  along the center axis CL 1  of the cooling hole  22  is not constant and differs by location as shown in  FIG. 3 . At the hollow part  25 , cooling air is supplied from the rotor side. Cooling air flows out from the cooling holes  22 . Due to this, film-shaped cooling air flows out along the outer surface  21   b  whereby the blade part  21  is cooled. 
         [0028]    The nickel alloy which forms the turbine blade is hard to machine, so it is difficult to use a drill etc. to form cooling holes  22 . Therefore, in the present embodiment, an electrodischarge machine  100  is used to form the plurality of cooling holes  22  in the turbine blade. The cooling holes  22  are formed one location at a time. After one cooling hole  22   a  in  FIG. 2  finishes being formed, another cooling hole  22   b  which adjoins this cooling hole  22   a  or another cooling hole  22   c  which is closest to the cooling hole  22   a  is formed. 
         [0029]      FIG. 4  is a view which shows a machining operation of a cooling hole  22 , while  FIG. 5  is an enlarged view of principal parts of  FIG. 4 . As shown in  FIGS. 4 and 5 , when forming a cooling hole  22 , the slanted rotary table device  12  is used to hold the workpiece  20  so that the center axis CL 1  of the cooling hole  22  extends in the top-bottom direction. Furthermore, the machining program is used to instruct the W-axis so that the bottom end face  8   a  of the electrode guide  8  is positioned at an electrode support position A exactly a predetermined distance D 1  above the machining start point P. Further, the electrode holder  7  is moved while fastening the relative positions of the electrode holder  7  and the electrode guide  8 . At this time, the amount of projection of the bottom end face  10   a  of the pipe electrode  10  which sticks out from the bottom end face of the electrode guide  8  is set to a predetermined value D 2  (&lt;D 1 ). 
         [0030]    Note that, the predetermined value D 2  may be 0 or may be smaller than 0. If the predetermined value D 2  is smaller than 0, the bottom end face  10   a  of the pipe electrode  10  is positioned above the bottom end face  8   a  of the electrode guide  8 , but in this case, it is sufficient that the distance between the bottom end faces  8   a  and  10   a  be smaller than the length D 3  of the electrode guide  8  so that the pipe electrode  10  not detach from the electrode guide  8 . The above states will be referred to as the “machining preparation state”. 
         [0031]    Next, the electrode holder  7  is moved downward from the machining preparation state so as to move the pipe electrode  10  downward and the front end part of the pipe electrode  10  is used to machine the workpiece  20  by electrodischarge machining (broken lines of  FIGS. 4 and 5 ). During the machining, the holding arm  9  is fastened with respect to the ram  4  so that regardless of downward movement of the electrode holder  7 , the electrode guide  8  is supported at the electrode support position A. Due to this, the top-bottom end parts of the pipe electrode  10  are supported above the workpiece  20  and shaking of the pipe electrode  10  during machining can be suppressed. At the time of electrodischarge machining, the pipe electrode  10  is consumed along with formation of the cooling holes  22 , so the pipe electrode  10  has to be replaced at a suitable timing. To judge this replacement timing, in the present embodiment, the electrode replacement judgment apparatus is configured as follows. 
         [0032]      FIG. 6  is a block diagram which shows the configuration of an electrode replacement judgment apparatus according to a first embodiment. The control part  30  of  FIG. 6  is configured including a processing system which comprises a CPU, ROM, RAM, and other peripheral circuits. The control part  30  is connected to the position detector  31  which detects a Z-axis position of the electrode holder  7  ( FIG. 1 ), a position detector  32  which detects a W-axis position of the electrode guide  8  ( FIG. 1 ), an input part  33  to which a machining program and various settings are input, a voltage detection part  34  which detects an interpolar voltage between the pipe electrode  10  and the workpiece  20 , drive parts  35  which make the spindle  6  move relative to the workpiece  20  (X-axis use drive part, Y-axis use drive part, Z-axis use drive part, arm drive part, spindle drive part, B-axis use drive part, and A-axis use drive part), and a display part  36  which displays various types of information relating to electrode replacement judgment. The control part  30  uses the signals from the position detectors  31  and  32 , input part  33 , and voltage detector  34  as the basis to perform predetermined processing and outputs control signals to the drive part  35  and the display part  36 . 
         [0033]      FIG. 7  is a flow chart which shows one example of the processing which is performed by the control part  30  according to the first embodiment. The processing which is shown in this flow chart is, for example, started by operation of the input part  33  to input a machining start command and is repeated each time an individual cooling hole  22  is formed. That is,  FIG. 7  corresponds to the formation of a single cooling hole  22 . By repeating the processing of  FIG. 7 , mutually adjoining cooling holes  22  are successively formed. 
         [0034]    At step S 1 , the machining program is followed to output control signals to the drive parts  35  whereby the position/posture of the workpiece  20 , the position of the electrode holder  7 , and the position of the electrode guide  8  are set to the machining preparation state ( FIGS. 4 and 5 ). That is, the workpiece  20  is held in the machining posture and the distance between the electrode guide  8  and the workpiece  20  is held constant. In that state, the top-bottom end parts of the pipe electrode  10  are held. In that state, the electrode holder  7  and the electrode guide  8  are integrally moved so that the bottom end face  8   a  of the electrode guide  8  is positioned at the electrode support position A above from the machining start point P by exactly a predetermined distance D 1 . 
         [0035]    At step S 2 , a pulse voltage is applied to the pipe electrode  10  and control signals are output to the drive parts  35  (Z-axis use drive part, arm drive part, and spindle drive part), and, while holding the electrode guide  8  at the electrode support position A, the pipe electrode  10  is made to rotate by a predetermined speed while making it descend toward the machining start point P. Along with this, machining fluid is ejected from the front end part of the pipe electrode  10 . 
         [0036]    At step S 3 , it is judged if electrodischarge has started between the pipe electrode  10  and the workpiece  20 . This judgment is performed by judging if the average value of the interpolar voltage which is detected by the voltage detection part  34  (average machining voltage V) has become smaller than a predetermined threshold value V 1 . In this case, the control part  30  reads the signal from the voltage detection part  34  for example every 2 msec, averages the data within the most recent predetermined time (for example 1 second), and makes this the average machining voltage V. If step S 3  is affirmative, the routine proceeds to step S 4 , while if negative, the routine returns to step S 2 . 
         [0037]    At step S 4 , the signal from the position detector  31  is read and the Z-axis position of the electrode holder  7  at the time judged to be the electrodischarge start is stored as the electrodischarge start position in a memory. 
         [0038]    At step S 5 , the electrode length L 1  is set. A new pipe electrode  10  which has a known initial length L 0  (for example 300 mm) is firstly attached to the rotary spindle  6  manually or by an electrode changing system. The electrode length L 1  is the distance from the bottom end of the electrode holder  7  to the bottom end face  10   a  of the pipe electrode  10 . This electrode length L 1  is first set to the initial length L 0 . The electrode length L 1  when using a new pipe electrode  10  to form the n-th cooling hole  22  is set as follows. That is, when the electrodischarge start position when forming the n−1-th cooling hole  22  in the previous processing is Z n-1  and the electrodischarge start position when forming the n-th cooling hole  22  in the current processing is Z n , the amount of change ΔZ of the electrodischarge start position becomes Z n -Z n-1 . This amount of change ΔZ is subtracted from the electrode length L 1  which is found by the previous processing (L 1 −ΔZ) and the remainder is set as the new electrode length L 1 . Note that the electrodischarge start position when using a new pipe electrode  10  to first form a cooling hole  22  (initial electrodischarge start position Z 1 ) may be stored in a memory, a difference ΔZ (=Z 1 −Z n ) between an electrodischarge start position Z n  when forming an n-th cooling hole  22  and the initial electrodischarge start position Z 1  may be subtracted from an initial length L 0  of the pipe electrode  10  (L 0 −ΔZ), and the remainder may be set as the new electrode length L 1 . 
         [0039]    At step S 6 , the machining program is followed to control the drive parts  35  and form a cooling hole  22  of the desired shape in the workpiece  20 . At the time of forming the cooling hole  22 , the pipe electrode  10  gradually descends. 
         [0040]    At step S 7 , it is judged if the pipe electrode  10  has penetrated through the workpiece  20 . This judgment is performed by judging if the average value of the interpolar voltages which are detected by the voltage detection part  34  (average machining voltage V) has become larger than a predetermined threshold value V 2 . If step S 7  is affirmative, the routine proceeds to step S 8 , while if step S 7  is negative, the routine returns to step S 6 . Note that, below, for convenience, the threshold value V 2  is explained as being the same value as the threshold value V 1 , but V 2  and V 1  may be values which differ from each other as well. For V 1  and V 2 , suitable values are found in advance by experiments. 
         [0041]    At step S 8 , control signals are output to the drive parts  35  to stop the descent of the pipe electrode  10 . In the present embodiment, a signal from the voltage detection part  34  is fetched at short periods (every 2 seconds), so after the workpiece  20  (blade part  21 ) is penetrated, the pipe electrode  10  can be immediately stopped and the amount of projection of the pipe electrode  10  from the inside surface  21   a  of the blade part  21  can be kept to a minimum extent. 
         [0042]    At step S 9 , a signal from the position detector  31  is read and the Z-axis position of the electrode holder  7  at the point of time when it is judged that the workpiece has been penetrated is stored as the penetration position (electrodischarge end position) in the memory. 
         [0043]    At step S 10 , the electrodischarge start position which is stored in the memory (step S 4 ) is decreased by the penetration position (step S 9 ) to calculate the amount of feed E of the pipe electrode  10  which is required from electrodischarge start to electrodischarge end (this called substantive feed amount E). The substantive feed amount E includes the plate thickness t of the workpiece  20  and the amount of consumption F (electrode consumption amount) of the pipe electrode  10 . The electrode consumption amount F is found by multiplying the plate thickness t and a preset electrode consumption ratio α. The substantive feed amount E is expressed by the following formula (I). 
         [0000]        E=t (1+α)  (I)
 
         [0044]    Note that, strictly speaking, the substantive feed amount E also includes the feed amount of the pipe electrode  10  from the workpiece penetration position, that is, the amount of projection of the pipe electrode  10  from the inside surface  21   a , but in the present embodiment, after detection of penetration, the descent of the pipe electrode  10  is immediately made to stop (step S 8 ), so this can be deemed to be 0. The electrode consumption ratio α changes depending on various conditions, but in the present embodiment, an experimentally found average value is set in advance. 
         [0045]    At step S 11 , the electrode length L 1  of step S 5  is decreased by the electrode consumption amount F (=tα) to calculate the residual length La of the pipe electrode  10 . In this case, first, the plate thickness t is found from the above formula (I) and that plate thickness t is multiplied with the electrode consumption ratio α to calculate the electrode consumption amount F. Next, the electrode length L 1  is decreased by the electrode consumption amount F to calculate the residual length La. 
         [0046]    At step S 12 , control signals are output to the drive parts  35  (Z-axis use drive part) to make the electrode holder  7 , that is, pipe electrode  10 , rise, so that the pipe electrode  10  is positioned above the machining start point P of the workpiece  20  by a safety margin. The amount of rise of the electrode holder  7  is made smaller than the substantive feed amount E by exactly an electrode consumption amount F. Due to this, as shown by the solid line in  FIG. 5 , the pipe electrode  10  sticks out from the bottom end face  8   a  of the electrode guide  8  by exactly a predetermined amount D 2 . 
         [0047]    At step S 13 , the length of the pipe electrode  10  required for forming the next cooling hole  22  (required length Lb) is calculated. The required length Lb is found by determining in advance the minimum required length of the pipe electrode  10  for stably holding the pipe electrode  10  without the electrode holder  7  and the electrode guide  8  interfering (minimum required length) and adding to this minimum required length the substantive feed amount E of step S 10 . The substantive feed amount E is used to find the required length Lb since the currently formed cooling hole  22  and the next formed cooling hole  22  are adjacent and the change in the plate thickness t is believed to be small, so even when forming the next cooling hole  22 , a substantive feed amount E the same as the current time is assumed to be necessary. Note that, the minimum required length of the pipe electrode  10  may be set to a value including a predetermined safety margin as well. The minimum required length of the pipe electrode  10  is the sum (for example 45 mm) of the minimum distance of the electrode holder  7  and the electrode guide  8  (for example 5 mm), the length D 3  of the electrode guide  8  (for example 30 mm), and the amount of projection D 2  from the electrode guide  8  (for example 10 mm). 
         [0048]    At step S 14 , it is judged if the residual length La of the pipe electrode  10  is the required length Lb or more (La≧Lb) of the pipe electrode  10 . If step S 14  is affirmative, the routine proceeds to step S 15 , while if step S 14  is negative, the routine proceeds to step S 16 . 
         [0049]    At step S 15 , it is judged that the pipe electrode  10  has sufficient length for forming the next cooling hole  22  and formation of the next cooling hole  22  is permitted. In this case, processing similar to the one explained above is repeated for the machining start point P of the next cooling hole  22 . On the other hand, at step S 16 , it is judged that the pipe electrode  10  is not sufficient in length and electrode replacement is necessary and formation of the next cooling hole  22  is prohibited. In this case, a new pipe electrode  10  is taken out from the tool magazine and processing is performed for attachment to the rotary spindle  6  (electrode replacement processing). 
         [0050]    Summarizing the operation of the first embodiment, the following is obtained. Below, the operation after forming the cooling hole  22   a  of  FIG. 2 , then forming the adjoining cooling hole  22   b  will be explained. First, a known length new pipe electrode  10  is positioned above the machining start point P (first machining start point) of the cooling hole  22   a  and the electrode guide  8  is moved to the electrode support position A (step S 1 ). Next, a pulse voltage is applied to the pipe electrode  10  while making the pipe electrode  10  descend and making the pipe electrode  10  approach the machining start point P at the workpiece surface (step S 2 ). 
         [0051]      FIG. 8  is a view which shows the relationship of the elapsed time T from when the pipe electrode  10  starts to descend and the average machining voltage V. From the point of time T 0  when the pipe electrode  10  starts to descend to when electrodischarge is started, the average machining voltage V is larger than the threshold value V 1 . If electrodischarge is started at the point of time T 1 , the average machining voltage V becomes smaller than the threshold value V 1 . After this, until the electrodischarge ends, the state where V&lt;V 1  continues. The Z-axis position (electrodischarge start position) of the electrode holder  7  at this time is stored in the memory (step S 4 ). This electrodischarge start position is used to find and set the electrode length L 1  (step S 5 ). Note that, when using a new pipe electrode  10  to form the n-th cooling hole  22 , at step S 5 , the difference ΔZ(=Z n −Z n-1 ) between the electrodischarge start position Z n-1  when forming the n−1-th cooling hole  22  and the current electrodischarge start position Z n  is subtracted from the electrode length L 1  which is found when forming the n−1-th cooling hole  22  (L 1 −ΔZ) and the remainder is set as the new electrode length L 1 . 
         [0052]    After that, if, at the point of time T 2 , the pipe electrode  10  penetrates through the workpiece  20 , the average machining voltage V becomes larger than the threshold value V 2  (=V 1 ). If V&gt;V 2  is detected, the descent of the pipe electrode  10  is stopped (step S 8 ). The Z-axis position of the electrode holder  7  at this time (penetration position) is stored in the memory (step S 9 ). In this case, the signal from the voltage detection part  34  is read by a short period (2 msec), so the pipe electrode  10  can immediately stop after penetrating through the workpiece. Therefore, as shown in  FIG. 9 , it is possible to prevent the inside surface  21   a  (part A) of the workpiece  20  at the hollow part side which faces the pipe electrode  10  from being mistakenly machined by the electrodischarge machining. 
         [0053]    When the pipe electrode  10  penetrates through the workpiece  20 , the pipe electrode  10  moves above the machining start point P (step S 12 ). At this time, the amount of decrease of the penetration position from the electrodischarge start position is calculated as the substantive feed amount E (step S 10 ), the electrode consumption amount F is calculated from the substantive feed amount E, and the value of the electrode length L 1  at the time of machining start minus the electrode consumption amount F, that is, the residual length La of the pipe electrode  10 , is calculated (step S 11 ). Furthermore, for forming the next cooling hole  22   b , it is assumed that an amount of feed of the pipe electrode  10  the same as the substantive feed amount E is necessary and the length Lb of the pipe electrode  10  which is required for forming the cooling hole  22   b  is calculated (step S 13 ). When the residual length La is the required length Lb or more, it is judged that the pipe electrode  10  is sufficient in length and the machining operation of the next cooling hole  22   b  is advanced to (step S 15 ). When the residual length La is less than the required length Lb, it is judged that the pipe electrode  10  is insufficient in length and the pipe electrode  10  is replaced without advancing to the machining operation for the next cooling hole  22   b.    
         [0054]    According to the above first embodiment, the signal from the voltage detection part  34  is used to detect the electrodischarge start position and the penetration position of the pipe electrode  10  (step S 4  and step S 9 ), and the electrode length L 1  at the time of electrodischarge machining start is decreased by the electrode consumption amount F to detect the residual length La of the pipe electrode  10  (step S 11 ). Furthermore, the difference between the electrodischarge start position and the penetration position (substantive feed amount E) is used as the basis to set the required length Lb of the pipe electrode  10  required for machining the next through hole  22  (step S 13 ) and the residual length La and the required length Lb are compared to judge a need for replacement of the pipe electrode  10  (step S 14 ). That is, it is judged that a substantive feed amount E the same as the current time is needed when forming the next cooling hole  22 , the required length Lb of the pipe electrode  10  is calculated, and this required length Lb is used as the basis to judge a need for replacement of the pipe electrode  10 . Therefore, the substantive feed amount E is found regardless of the plate thickness t of the workpiece  20 , so even in the case where the plate thickness t of the workpiece  20  changes, a need for electrode replacement can be precisely judged. 
         [0055]    Referring to  FIG. 10 , a second embodiment of the present invention will be explained. Note that, below, mainly the points of difference from the first embodiment will be explained. The second embodiment differs from the first embodiment in the processing at the control part  30 . That is, in the first embodiment, the pipe electrode  10  is made to descend until detecting penetration through the workpiece  20 , but in the second embodiment, the descending operation of the pipe electrode  10  is made to stop if the distance D ( FIG. 1 ) between the electrode holder  7  and the electrode guide  8  becomes a predetermined value D 0  or less. 
         [0056]      FIG. 10  is a flow chart which shows one example of the processing which is executed by the control part  30  according to the second embodiment and shows mainly parts which differ from  FIG. 7 . Note that, in  FIG. 10 , parts of the processing the same as in  FIG. 7  are assigned the same reference notations. As shown in  FIG. 10 , if starting the processing for forming a cooling hole  22  at step S 6 , the routine proceeds to step S 21  where the signals from the position detectors  31  and  32  are used as the basis to calculate the distance D between the electrode holder  7  and the electrode guide  8 . At step S 22 , it is judged if the calculated distance D is the preset predetermined value D 0  or less. This judgment judges if there is any interference (collision) between the electrode holder  7  and the electrode guide  8 . The predetermined value D 0  is set to a value larger than at least 0, for example, to 5 mm. If step S 22  is negative, the routine proceeds to step S 7 , then processing similar to  FIG. 7  is performed. 
         [0057]    On the other hand, if step S 22  is affirmative, the routine proceeds to step S 23  where control signals are output to the drive parts  35  and the descent of the pipe electrode  10  is stopped. At step S 24 , control signals are output to the drive parts  35  (Z-axis use drive part) so that the pipe electrode  10  is positioned above the machining start point P of the workpiece  20  and the electrode holder  7  is made to rise. Next, at step S 16 , it is judged that the pipe electrode  10  is not sufficient in length and electrode replacement is necessary and formation of the next cooling hole  22  is prohibited. 
         [0058]    In the second embodiment, if the distance D between the electrode holder  7  and the electrode guide  8  becomes a predetermined value D 0  or less, the descent of the electrode holder  7  is stopped, so even if the workpiece  20  suddenly increases in plate thickness t at the machining start point P etc., the electrode holder  7  and the electrode guide  8  can be prevented from colliding. That is, if the plate thickness t of the workpiece  20  at the time of the current machining operation suddenly increases from the plate thickness t of the workpiece  20  at the time of the previous machining operation, the substantive feed amount E of the pipe electrode  20  which is required until penetration through the workpiece increases, and the electrode holder  7  and the electrode guide  8  are liable to collide before detecting penetration through the workpiece. On this point, in the present embodiment, if the distance D between the two becomes a predetermined value D 0  or less, the descent of the electrode holder  7  is forcibly stopped, so the electrode holder  7  and the electrode guide  8  can be prevented from colliding. 
         [0059]    In the above embodiments, the electrodischarge start position and the penetration position are detected in accordance with the interpolar voltage V which is detected by the voltage detection part  34 , but the electrodischarge start position detecting means and the penetration position detecting means are not limited to this in configuration. For example, the interpolar voltage V has correlation with the feed rate of the pipe electrode  10 , so it is possible to detect the electrodischarge start position and penetration position by detecting the feed rate. In the above embodiments, processing at the control part  30  is performed to decrease the electrode consumption amount F from the electrode length L 1  at the time of start of electrodischarge machining to find the residual length La of the pipe electrode  10 , but the electrode length detecting means is not limited to this in configuration. 
         [0060]    If using the difference between the electrodischarge start position and the penetration position (substantive feed amount E) as the basis to find the required length Lb of the pipe electrode  10  which is required for machining the next through hole  22 , the required length setting means (control part  30 ) may be configured in any way. For example, instead of using the substantive feed amount E which is obtained at the time of machining the immediately preceding cooling hole  22 , it is also possible to use the average value of the substantive feed amounts E which are obtained at the time of forming several immediately preceding (for example five) cooling holes  22 . Processing at the control part  30  is used to judge the need for electrode replacement in accordance with the relative sizes of the residual length La and the required length Lb of the pipe electrode  10 , but if comparing the residual length La and the required length Lb, the judgment by the judging means need not be such simple judgment of relative sizes. 
         [0061]    In the above embodiments, the base end part of the pipe electrode  10  is held at the electrode holder  7  and the front end part of the pipe electrode  10  is held by the electrode guide  8  which can move relative to the electrode holder  7  in the length direction of the pipe electrode  10 , but the first holding part and second holding part are not limited to this configuration. The position detectors  31  and  32  are used to calculate the distance between the electrode holder  7  and the electrode guide  8 , but the distance detecting means may be any means. If, after the start of electrodischarge, the distance D between the electrode holder  7  and the electrode guide  8  becomes the predetermined value D 0  or less, the processing at the control part  30  is used to stop the descent of the electrode holder  7 , but the electrodischarge stopping means for making the electrodischarge machining operation stop is not limited to the above in configuration. 
         [0062]    In the above embodiments, if the comparison of the residual length La and the required length Lb of the pipe electrode  10  resulted in judgment that electrode replacement is necessary, the processing at the control part  30  is used to prohibit advance to the process for forming the next cooling hole (next machining process), but the electrodischarge control means is not limited to this in configuration. For example, at the time of prohibiting advance to the next machining process, an alarm may be output to the display part  36 . In the above embodiments, a pipe shaped electrode  10  is used, but the shape of the electrode which extends in the long direction may be one other than a pipe shape. The electrodischarge machine  100  is not limited to the above in configuration. In the above embodiments, as one example of a workpiece  20 , a turbine blade is used, but the replacement judgment apparatus according to the present invention may be applied even when machining another workpiece so long as the workpiece requires formation of a plurality of through holes  22 . Further, the replacement judgment apparatus according to the present invention may be applied even when there are a plurality of workpieces in each of which just a single through hole is to be formed and the workpieces are successively exchanged so as to successively form the through holes by electrodischarge machining. 
         [0063]    In the above embodiments, a replacement judgment method of an electrode for electrodischarge machining which judges a need for replacement of an electrode (pipe electrode)  10  for successively forming a plurality of through holes  2  in a workpiece  20  by electrodischarge machining is explained. That is, the residual length La of the electrode  10  is detected, the position of the electrode  10  at the time of start of electrodischarge is detected as the electrodischarge start position, the position of the electrode  10  at the time of penetration through the workpiece is detected as the penetration position, the difference between the detected electrodischarge start position and the detected penetration position is used as the basis to set the required length Lb of the electrode  10  which is required for forming the next through hole  22 , and the detected residual length La and the set required length Lb are compared to judge a need of the electrode replacement. In this case, the position of the electrode  10  includes a position of the member which has correlation with the electrode  10  (electrode holder  7  etc.) 
         [0064]    According to the present invention, the most recent actual data of the through holes, that is, the difference of the electrodischarge start position and the penetration position, is used as the basis to set the required length of the electrode which is required for formng the next through hole and this required length is used to judge a need for electrode replacement, so regardless of any change of the workpiece in plate thickness, a need for electrode replacement can be precisely judged. In particular, normally the electrode consumption amount changes depending on the machining state (extent of stability or instability), machining depth, and machining fluid pressure, but the present invention uses the most recent data, so the various conditions are substantially the same. It is possible to precisely judge the need for electrode replacement without waste. Further, compared with the method of measuring the electrode length before forming and after forming one through hole and detecting the electrode consumption amount, the cycle time can be greatly shortened. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           7  electrode holder 
           8  electrode guide 
           10  pipe electrode 
           20  workpiece (turbine blade) 
           22  cooling hole 
           30  control part 
           31 ,  32  position detector 
           34  voltage detection part 
           35  drive part 
         La residual length 
         Lb required length