Abstract:
Provided is a drilling method for forming a plurality of through-holes in a plate member, the drilling method including: a first step of fixing at least one jig, which has a clearance hole, on a bed of the machine tool; a second step of placing the plate member on the at least one jig and fixing the plate member on the bed; a third step of operating the machine tool so that a leading end portion of the drilling tool is inserted into the clearance hole, and forming a first through-hole at a position of the plate member corresponding to the clearance hole; a fourth step of passing a fixing member through the first through-hole, and fixing the plate member on the at least one jig using the fixing member; and a fifth step of operating the machine tool to form a second through-hole other than the first through-hole.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to a drilling method and a drilling jig for forming through-holes in a plate member, and to a heat exchanger. 
       BACKGROUND ART 
       [0002]    Commonly, plate members having a plurality of through-holes, such as a tube plate and a tube support plate of a condenser, are drilled with a machine tool on which a drilling tool such as a high-speed steel drill or a cemented carbide drill is mounted. During drilling, typically, the drilling tool is inserted into a plate member while the outer peripheral part of the plate member is fixed with a clamp. At this point, if the rigidity around the drilling position is low, the plate member becomes bent due to insertion of the drilling tool or the plate member vibrates due to rotation of the drilling tool, which may degrades the machining accuracy of through-holes, and may further cause breakage of the cutting edge of the drilling tool. Therefore, it is required to fix the plate member so as to provide the plate member with sufficient rigidity to withstand drilling. 
         [0003]    There is a known method in which, in order to enhance the rigidity of a plate member, a dummy plate is laid on the lower surface of the plate member during drilling, and the plate member and the dummy plate are drilled together with a drilling tool. In this case, since the dummy plate provides the plate member with a reaction force during insertion of the machining tool, significant bending of the plate member can be prevented. However, since the drilling tool rotating at a high speed may cause the plate member to bounce and break the drilling tool, it is necessary to ease the machining conditions of the drilling tool (tool rotational speed, tool feeding speed). Moreover, it takes time and effort to remove foreign matters from or take care of the dummy plate due to its large area of contact with a plate member, and since the dummy plate needs to be prepared for each of plate members having different hole arrays, storage cost is incurred. 
         [0004]    A fixing method not using a dummy plate is disclosed in Patent Literature 1, for example, in which spacers are interposed between each adjacent ones of a plurality of plate members stacked on a receiving cleat of a surface plate, and the outer peripheral part of the plate members is fixed with clamps. Patent Literature 2 discloses a method in which the outer periphery of a plate member is fixed with clamps, and the plate member is supported from the drilling tool side by means of cylinders which move on rails. Patent Literature 3 discloses a configuration in which a thrust force absorbing member made of a square bar etc. is provided between a plate member and a bed so as to absorb the thrust force of a drill and vibration during boring. In this configuration, a pressing member which presses the plate member near a part to be bored is further provided to thereby prevent breakage of the drill due to bouncing of the plate member. 
       CITATION LIST 
     Patent Literature 
     [Patent Literature 1] 
       [0005]    Japanese Patent Laid-Open No. H11-320223 
       [Patent Literature 2] 
       [0006]    Japanese Patent Laid-Open No. H5-146905 
       [Patent Literature 3] 
       [0007]    Japanese Utility Model Laid-Open No. S64-12711 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    As a tool used for drilling, a cemented carbide drill capable of high-speed machining is known. Although the cemented carbide drill rotates at a higher speed than a high-speed steel drill and allows reduction of machining time, large vibration occurs during insertion of the drill, which is likely to cause bouncing of a plate member. Moreover, while the high-rigidity, low-ductility cemented carbide drill bit is capable of high-accuracy machining, it is prone to a failure such as breakage of the cutting edge due to bouncing of a plate member etc. Thus, there is a demand for a plate member fixing method which allows smooth drilling even with a drilling tool like the cemented carbide drill for which the machining conditions are severe. 
         [0009]    On the other hand, when the periphery of a plate member is fixed with clamps alone as in Patent Literature 1, the restraining force around a machining point is weak and it is difficult to sufficiently secure the rigidity of the plate member. Therefore, vibration or bouncing of the plate member occurs during drilling, so that the accuracy of through-holes is affected and a failure such as breakage of the cutting edge is likely to occur. Although spacers are interposed between each adjacent ones of the plurality of plate members in Patent Literature 1, these spacers are required for chamfering of the holes and not intended for a fixing purpose. In terms of fixing the plate member, the spacers serve only to support the reaction force during insertion of the drilling tool and not to enhance the restraining force, so that possible bouncing of the plate member cannot be avoided. In Patent Literatures 2 and 3, too, the restraining force on the plate member is insufficient when the plate member is pressed from one side with the cylinders or the pressing member alone, so that bouncing of the plate member may occur. 
         [0010]    Moreover, in Patent Literature 2, since the spacers are laid under a tube plate, unless the number of holes is small and the hole array is a simple one with a square pitch etc., it is difficult to dispose the spacers due to possible interference between the drilling tool and the spacers. Thus, it is difficult to apply this method to drilling of plate members having a complicate hole array and a large number of through-holes, such as a tube plate and a tube support plate of a condenser. Similarly, in Patent Literature 3, since the absorbing member is disposed across the entire width of the plate member, application of this method is difficult depending on the hole array of a plate member. 
         [0011]    Furthermore, in Patent Literature 3, since it is necessary to perform a pressing operation using the pressing member on each machining point, one problem is that the machining time is long, and another problem is that chips generated during drilling may be caught between the plate member and a presser foot of the pressing member, which would cause a failure in the pressing operation. 
         [0012]    An object of at least one embodiment of the present invention is to provide a drilling method and a drilling jig, with which through-holes of an arbitrary hole array can be formed and precise through-holes can be formed while avoiding troubles such as breakage of a drilling tool, and a heat exchanger. 
       Solution to Problem 
       [0013]    According to at least one embodiment of the present invention, there is provided a drilling method for forming a plurality of through-holes in a plate member using a machine tool having a drilling tool equipped with a cemented carbide drill, the drilling method including: a first step of fixing at least one jig, which has a clearance hole, on a bed of the machine tool; a second step of placing the plate member on the at least one jig and fixing the plate member on the bed with a clamp; a third step of operating the machine tool so that a leading end portion of the drilling tool, which has penetrated the plate member, is inserted into the clearance hole, and forming a first through-hole of the plurality of through-holes at a position of the plate member corresponding to the clearance hole; a fourth step of passing a fixing member through the first through-hole, and fixing the plate member on the at least one jig using the fixing member; and a fifth step of, after the fourth step, operating the machine tool to form a second through-hole other than the first through-hole of the plurality of through-holes. 
         [0014]    The clearance hole in this specification is not limited in its shape, and holes of various shapes, for example, circular holes and polygonal holes including rectangular holes can be adopted, as long as the clearance hole has such a size that a clearance is formed between the outline of the drilling tool and the clearance hole. 
         [0015]    According to the above-described drilling method, vibration, bouncing, etc. of the plate member during insertion of the drilling tool are suppressed, so that it is possible to drill at a high speed and reduce the machining time. Moreover, it is possible to form through-holes of an arbitrary hole array and form precise through-holes while avoiding troubles such as breakage of the drilling tool. 
         [0016]    In some embodiments, in the first step, the at least one jig is positioned with reference to a machining origin of the machine tool and the at least one jig is fixed on the bed, and in the second step, the plate member is positioned with reference to the machining origin and the plate member is placed on the at least one jig. 
         [0017]    In this embodiment, since at least one jig is positioned with reference to the machining origin of the machine tool and the plate member is also positioned with reference to this machining origin, it is possible to dispose at least one jig with high accuracy in a predetermined positional relation with the plate member. 
         [0018]    In some embodiments, in the second step, the outer peripheral side of the plate member relative to a jig disposition area of the at least one jig is fixed on the bed with the clamp, and in the third step, the first through-hole is formed while the jig disposition area of the plate member is supported from below with the at least one jig and the outer peripheral side of the plate member is fixed with the clamp. 
         [0019]    Thus, since the reaction force of the plate member is received by the jig which supports the jig disposition area from below while the restraining force on the plate member is secured by the clamp which fixes the outer peripheral side relative to the jig disposition area, it is possible to drill properly even when a large number of through-holes are located in a central part of the plate member. The region where the first through-holes and the second through-holes are arrayed shall be referred to as the jig disposition area. 
         [0020]    In some embodiments, the first through-holes are through-hole of the plurality of through-holes which are closest to intersections between a virtual longitudinal division line extending in the longitudinal direction and a virtual lateral division line extending in the lateral direction which divide the jig disposition area of the plate member in a lattice shape. 
         [0021]    Thus, since the jigs are disposed at substantially regular intervals in the jig disposition area in the central part of the plate member, it is possible to enhance the rigidity of the plate member in the jig disposition area substantially uniformly, which allows more proper drilling. 
         [0022]    According to at least one embodiment of the present invention, there is provided a drilling jig which assists a forming work of a plurality of through-holes in a plate member using a machine tool having a drilling tool, the drilling jig including: a base part which can be fixed on a bed of the machine tool; and a stand part of which an upper portion comes into contact with the lower surface of the plate member while a lower portion is connected with the base part, wherein a clearance hole, into which a leading end portion of the drilling tool having penetrated the through-hole can be inserted, is formed in the upper portion of the stand part, and under the clearance hole, an engaging part is provided which engages a fixing member for fixing the plate member passed through the through-hole and the clearance hole. 
         [0023]    According to the above-described drilling jig, it is possible to receive the reaction force during insertion of the drilling tool with the stand part by drilling the plate member using the drilling tool while fixing the base part on the bed and placing the plate member on the stand part. Moreover, it is possible to fix the plate member on the bed side through the drilling jig by engaging the fixing member for fixing the plate member with the drilling jig while the fixing member is passed through the through-hole of the plate member and the clearance hole of the stand part. Thus, since the rigidity of the plate member can be enhanced, it is possible to suppress vibration and bouncing of the plate member during formation of other through-holes and form precise through-holes while preventing troubles such as breakage of the drilling tool. 
         [0024]    In some embodiments, the engaging part is a female thread of which the diameter at the root is smaller than the inner diameter of the through-hole and which is screw-engaged with a male thread provided in the fixing member. 
         [0025]    In this configuration, the fixing member is engaged with the female thread formed in the stand part and having a diameter smaller than the inner diameter of the through-hole. Thus, it is possible to fix the plate member on the bed by screw-engaging the fixing member, which has the male thread formed in it, with the female thread through the through-hole of the plate member and the clearance hole. 
         [0026]    In some embodiments, the clearance hole has an inner diameter larger than the inner diameter of the through-hole. 
         [0027]    Thus, since the clearance hole is formed in the upper portion of the stand part, it is possible to prevent the stand part from being cut by the leading end portion of the drilling tool having penetrated the through-hole. 
         [0028]    In some embodiments, a cavity portion is formed in the lower portion of the stand part under the female thread, and a chip outlet, which communicates with the cavity portion and through which chips generated by drilling of the through-holes in the plate member are discharged, is provided in the stand part or the base part. 
         [0029]    Thus, since the cavity portion for collecting chips generated by drilling is provided in the lower portion of the stand part and the chip outlet for discharging the chips collected in the cavity portion is provided in the stand part or the base part, it is possible to smoothly discharge the chips generated by drilling from the drilling jig to the outside. 
         [0030]    In some embodiment, the base part extends along a direction perpendicular to the stand part, and a slit or a long hole, through which a leading end portion of a fastening member for fastening the base part on the bed can be passed, is formed in the base part along the extension direction of the base part. 
         [0031]    Thus, it is possible to finely adjust the mounting position of the drilling jig on the base part by adjusting the position of the fixing member in the longitudinal direction of the slit or the long hole formed in the base part. 
         [0032]    A heat exchanger according to at least one embodiment of the present invention includes a tube plate or a tube support plate constituted of the plate member in which the through-holes are formed by the drilling method according to any one of claims  1  to  4 . 
       Advantageous Effects of Invention 
       [0033]    According to at least one embodiment of the present invention, it is possible to drill at a high speed and reduce the machining time. In addition, it is possible to form through-holes of an arbitrary hole array, and form precise through-holes by suppressing vibration, bouncing, etc. of a plate member during insertion of a drilling tool while avoiding troubles such as breakage of the drilling tool. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0034]      FIG. 1  is a plan view schematically showing a tube plate of a condenser to which embodiments of the present invention are applied. 
           [0035]      FIG. 2  is a side view showing the general configuration of a machine tool according to one embodiment of the present invention. 
           [0036]      FIG. 3  is a side view showing a drilling jig and a clamp mechanism according to one embodiment of the present invention. 
           [0037]      FIG. 4  is a side view showing another configuration example of the clamp mechanism. [ FIGS. 5(   a ) and  5 ( b )]  FIG. 5(   a ) and  FIG. 5(   b ) are a side view and a bottom view, respectively, of the drilling jig according to one embodiment of the present invention. 
           [0038]      FIG. 6  is a cross-sectional view along the line A-A of  FIG. 5 . 
           [0039]      FIG. 7  is a cross-sectional view of a drilling jig according to a first modified example of the present invention. 
           [0040]      FIG. 8  is a cross-sectional view of a drilling jig according to a second modified example of the present invention. 
           [0041]      FIG. 9(   a ) is a side view showing a special bolt according to the second modified example of the present invention, and  FIG. 9(   b ) is a cross-sectional view showing a wedge. 
           [0042]      FIG. 10  is a schematic view showing a configuration example of a condenser according to one embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0043]    In the following, embodiments of the present invention will be described in accordance with the accompanying drawings. The dimensions, materials, shapes, relative positions, etc. of components that will be described below as embodiments or are shown in the drawings as embodiments are merely illustrative examples, and it is not intended to limit the scope of the present invention to these examples. 
         [0044]      FIG. 1  is a plan view showing a tube plate of a condenser to which embodiments of the present invention are applied.  FIG. 2  is a side view showing the general configuration of a machine tool according to one embodiment of the present invention.  FIG. 3  is a side view showing a drilling jig and a clamp mechanism according to one embodiment of the present invention.  FIG. 4  is a side view showing another configuration example of the clamp mechanism. 
         [0045]    In the following embodiment, a tube plate of a condenser will be taken as an example of a plate member  80  to be machined. However, the object to be machined is not limited to this example, and the plate member  80  may be any plate member in which a plurality of through-holes are to be formed. 
         [0046]    As shown in  FIG. 1 , a large number of through-holes  81 ,  82  for heat transfer tubes to pass through are formed in a tube plate being the plate member  80 . In the tube plate, the hole array, in which the large number of through-holes  81 ,  82  (or places at which through-holes are to be formed) are arrayed, is typically an array of a triangular pitch or a square pitch, and a drilling method and a drilling jig  2  according to this embodiment can also be suitably applied to such arrays. As the plate member  80  having a hole array similar to that of a tube plate, a tube support plate may also be an object to be machined. In  FIG. 1 , the hole array composed of a plurality of through-holes including the first through-holes  81  and the second through-holes  82  are indicated as the shaded region in the drawing. Although the shapes of the second through-holes  82  are not shown, the plurality of second through-holes  82  are formed in a region of the shaded region except where the first through-holes  81  are present. In  FIG. 1 , clamp positions  85 , at which the plate member  80  is fixed with a clamp mechanism  4  to be described later, are shown on the outer peripheral part of the plate member  80 . 
         [0047]    In one embodiment, as shown in  FIG. 2  and  FIG. 3 , a machine tool  1  for drilling the plate member  80  includes a bed  10  on which the plate member  80  is fixed, the drilling jig  2  used for forming the first through-holes  81  and fixing the plate member  80  on the bed  10 , the clamp mechanism  4  used for fixing the plate member  80  on the bed  10 , and a drilling mechanism  12  for drilling the through-holes  81 ,  82  in the plate member  80 . 
         [0048]    The bed  10  is used for fixing the plate member  80  to be machined. More specifically, the bed  10  is provided with a jig fixing part  10   a  on which the drilling jig  2  is fixed, and a clamp fixing part  10   b  on which the clamp mechanism  4  is fixed. In addition, the drilling mechanism  12  is disposed on the bed  10 . The plate member  80  fixed on the bed  10  is drilled from above by the drilling mechanism  12 . 
         [0049]    The drilling mechanism  12  has at least one pair of support pillars  13  provided upright on the bed  10 , rails  14  suspended between the at least one pair of support pillars  13 , a tool driving part  15  which moves on the rails  14  in the longitudinal direction (the X direction shown in  FIG. 1  and  FIG. 2 ), and a drilling tool  16  mounted on the tool driving part  15 . The drilling tool  16  is rotated by the tool driving part  15 , and moves up and down in the height direction (the Z direction shown in  FIG. 2 ). As the drilling tool  16 , for example, a high-speed steel drill, a cemented carbide drill, or the like is used. The pair of support pillars  13  may be adapted to move in the depth direction (the Y direction shown in  FIG. 1 ) along with the rails  14 , the tool driving part  15 , and the drilling tool  16 . In this way, through-holes can be formed at arbitrary positions of the plate member  80  shown in  FIG. 1 . 
         [0050]    The drilling jigs  2  are fixed on the bed  10  at positions corresponding to the first through-holes  81 , and assist the forming work of the plurality of through-holes  81 ,  82  in the plate member  80 . The detailed configuration of the drilling jig  2  will be described later. 
         [0051]    The clamp mechanism  4  has a bolt  40  fixed on the bed  10 , a presser plate  42  which is mounted on the bolt  40  and presses the plate member  80 , a nut  46  for fixing the presser plate  42  at a predetermined position, and a jack  48  which supports the presser plate  42  from the lower surface on the opposite side of the plate member  42  across the bolt  40 . With the objective of preventing damage to the presser plate  42 , a washer  44  may be interposed between the presser plate  42  and the nut  46 . In addition, an actuator such as a cylinder or a motor may be used instead of the jack  48 . 
         [0052]    More specifically, the bolt  40  is implanted in the clamp fixing part  10   b  of the bed  10  so as to stand upright in the vertical direction. A male thread  41  is formed on the upper end side of the bolt  40 . The presser plate  42  is provided with a through-hole (not shown), and the upper end side of the bolt  40  is passed through this through-hole. As the washer  44  and the nut  46  are fitted sequentially and the nut  46  is screw-engaged with the male thread  41  of the bolt  40  on the upper end side of the bolt  40 , a pressing force toward the bed  10  side is imparted from the presser plate  42  to the plate member  80 , and the presser plate  42  is fixed on the bed  10  side. In order to prevent inclination of the presser plate  42 , the jack  48  is provided which supports the presser plate  42  from the lower surface on the opposite side of the plate member  42  across the bolt  40 . This jack  48  is adjusted so as to be substantially at the same level as the upper surface of the plate member  80  and supports the presser plate  42  from the lower surface on the opposite side of the plate member  42 . In this way, it is possible to impart a pressing force to the plate member  80  while retaining the presser plate  42  in the horizontal direction. 
         [0053]    As shown in  FIG. 4 , in another configuration example, the clamp mechanism  4  may have a bolt  50  which is removal from the bed  10 . The bolt  50  has male threads  51 ,  52  formed at both ends. Similarly to the example of  FIG. 3  described above, as the presser plate  42 , the washer  44 , and the nut  46  are fitted on the upper end side of the bolt  50  and the male thread  51  in the upper portion and a female thread  47  of the nut  46  are screw-engaged with each other, a pressing force is imparted from the presser plate  42  to the plate member  80 . On the other hand, a clamp fixing part  10   c  of the bed  10  has a concave shape, and a nut  54  having a T-shaped cross-section is housed in this clamp fixing part  10   c . The nut  54  is fixed so as not to rotate inside the clamp fixing part  10   c . A female thread is formed on the inner peripheral side of the nut  54 , and this female thread  56  is screw-engaged with the male thread  52  in the lower portion of the bolt  50 . In this way, the bolt  50  can be removably fixed on the bed  10 . For example, if the plurality of clamp fixing parts  10   c  and the nuts  54  are installed in advance on the outer periphery of the plate member  80 , the clamp mechanisms  4  can be installed only at those positions that are appropriately selected. 
         [0054]    Here, the configuration of the drilling jig  2  according to one embodiment of the present invention will be described in detail with reference to  FIG. 3 ,  FIG. 5 , and  FIG. 6 .  FIG. 5(   a ) and  FIG. 5(   b ) are a side view and a bottom view, respectively, of the drilling jig according to one embodiment of the present invention.  FIG. 6  is a cross-sectional view along the line A-A of  FIG. 5 . 
         [0055]    In one embodiment, the drilling jig  2  includes a base part  30  which can be fixed on the bed  10 , and a stand part  20  of which an upper portion comes into contact with the lower surface of the plate member  80  while a lower portion is connected with the base part  30 . During formation of the first through-hole  81 , the drilling jig  2  supports the periphery of the first through-hole  81  from below and serves to receive the reaction force of the drilling tool  16 , and during formation of the second through-hole  82 , the drilling jig  2  fixes the plate member  80  on the bed  10  with the plate member fixing bolt (fixing member)  34  for fixing the plate member  80 , and serves to increase the rigidity of the plate member  80 . The first through-holes  81  are those through-holes that are formed first of the plurality of through-holes, and after formation of the through-holes, the plate member fixing bolt  34  is inserted into the drilling jig  2  through the first through-hole  81  to thereby fix the plate member  80 . The second through-holes  82  are those through-holes that are formed while the plate member  80  is fixed on the drilling jig  2  through the first through-hole  81 . 
         [0056]    A clearance hole  22 , into which a leading end portion of the drilling tool  16  having penetrated the first through-hole  81  of the plate member  80  can be inserted, is formed in the upper portion of the stand part  20 . The clearance hole  22  is not limited in its shape, and holes of various shapes, for example, circular holes and polygonal holes including rectangular holes can be adopted, as long as the clearance hole has such a size that a clearance is formed between the outline of the drilling tool  16  and the clearance hole. In an intermediate portion of the stand part  20 , an intermediate cavity portion  25  is provided, in which a female thread  24  having a diameter at the root smaller than the inner diameter of the first through-hole  81  is formed. This female thread  24  is screw-engaged with the male thread formed in the plate member fixing bolt  34 , and functions as an engaging part for engaging the plate member fixing bolt  34 . In addition, a flat support portion  23 , which comes into contact with the lower surface of the plate member  80 , is provided on the peripheral edge of the clearance hole  22  at the upper end of the stand part  20 . 
         [0057]    In the above configuration of the drilling jig  2 , the base part  30  of the drilling jig  2  is fixed with bolts  38  on the jig fixing part  10   a  of the bed  10 . Thus, the drilling jig  2  is fixed while standing upright on the bed  10 . The plate member  80  is placed on the support portion  23  of the stand part  20 . During formation of the first through-hole  81 , the drilling tool  16  is inserted into the plate member  80  and the plate member  80  is drilled while being placed on the stand part  20 . Thus, the reaction force during insertion of the drilling tool  16  can be received with the support portion  23  of the stand part  20 . In addition, since the clearance hole  22  is formed in the upper portion of the stand part  20 , it is possible to prevent the inner surface of the stand part  20  from being cut by the leading end portion of the drilling tool  16  having penetrated the first through-hole  81 . 
         [0058]    After formation of the first through-holes  81 , the bolt (plate member fixing bolt)  34  with a male thread formed in it is inserted into the first through-hole  81  from above the plate member  80 , and the bolt  34  is screwed on the female thread  24  formed in the lower portion of the stand part  20  to thereby fix the plate member  80  on the drilling jig  2 . In order to prevent damage to the plate member  80 , a washer  36  may be interposed between the head of the bolt  34  and the plate member  80 . Thus, the plate member  80  can be fixed on the bed  10  side by screwing the bolt  34 , which has the male thread formed in it, on the female thread  24  through the first through-hole  81  of the plate member  80  and the clearance hole  22 . 
         [0059]    During formation of the second through-hole  82 , the second through-hole  82  is formed with the drilling tool  16  while the plate member  80  is fixed on the drilling jig  2  and the bed  10  through at least one of the first through-holes  81 . During this process, since the plate member  80  is fixed on the drilling jig  2  through at least one of the first through-holes  81 , it is possible to enhance the rigidity of the plate member  80  and suppress vibration and bouncing of the plate member  80  during formation of the second through-hole  82 . 
         [0060]    The above-described configuration makes it possible to form the precise first through-holes  81  and second through-holes  82  while preventing troubles such as breakage of the drilling tool  16 . 
         [0061]    In some embodiments, a cavity portion  26  may be formed in a lower portion of the stand part  20 . In this case, the cavity portion  26  is provided under the female thread  24 , and communicates with the hole in which the female thread  24  is formed. Chips generated by drilling fall into and collect in this cavity portion  26 . The cavity portion  26  communicates with the outside through a chip outlet  27  provided in the base portion  30  or the stand part  20 . This configuration allows the chips collected in the cavity portion  26  to be discharged through the chip outlet  27  to the outside. 
         [0062]    In one embodiment, the base part  30  extends along a direction perpendicular to the stand part  20 , and a long hole  32 , through which the leading end portion of the bolt  38  for fastening the base part  30  on the bed  10  can be passed, may be formed along the extension direction of the base part  30 . Thus, the mounting position of the drilling jig  2  on the base part  30  can be finely adjusted by adjusting the position of the bolt  38  in the length longitudinal direction of the long hole  32 . A slit may be used instead of the long hole  32 . 
         [0063]    Drilling jigs  6  according to a first modified example and a second modified example shown in  FIG. 7  and  FIG. 8  may be used. While the jig  2  shown in the above-described embodiment includes the female thread  24  in the intermediate portion of the stand part  20  in the vertical axial direction, the jigs  6  shown in the first modified example ( FIG. 7 ) and the second modified example ( FIG. 8 ) to be described below are different in that an inner wall of an intermediate portion of a stand part  60  has a smooth intermediate cavity portion  62 . Since the other configurations are the same as those of the above-described embodiment, a detailed description of the same configurations will be omitted.  FIG. 7  is a cross-sectional view of the drilling jig according to the first modified example of the present invention, and  FIG. 8  is a cross-sectional view of the drilling jig according to the second modified example of the present invention. 
         [0064]    While the plate member is fixed on the drilling jig by means of the fixing member, the embodiment and the first and second modified examples are different from each other in the configuration of the fixing member. That is, in the first modified example shown in  FIG. 7 , a double-nut bolt  66  is used for the fixing member as the means for fixing the plate member on the jig  6 . No female thread is cut on the inner surface of the intermediate cavity portion  62 , and the inner diameter of the intermediate cavity portion  62  is preferably smaller than the inner diameter of the clearance hole and slightly larger than the outer diameter of the double-nut bolt  66 . In such a configuration, when the double-nut bolt  66  is fastened, the seat surface of the nut  46  disposed in a cavity portion  64  is brought into contact with the ceiling surface of the cavity portion  64 , so that the ceiling surface of the stand part  60  can support the tensile force acting on the double-nut bolt  66 . According to the first modified example, the ceiling surface of the cavity portion  64 , with which the seat surface of the nut  46  comes into contact, functions as an engaging part for engaging the double-nut bolt (fixing member)  66 . 
         [0065]    As the horizontal cross-section of the cavity portion  64 , in which the lower portion of the intermediate cavity portion  62  is opened, various shapes such as circles and polygonal shapes including rectangular shapes can be adopted as long as the opening of the intermediate cavity portion  62  can be disposed in the cross-section. Of the nuts  46  used for fastening the double-nut bolt  66 , the nut  46  disposed in the cavity portion  64  may be tacked by spot welding etc. at the position where the intermediate cavity portion  62  is opened in the ceiling surface of the cavity portion  64 . According to the configuration of the first modified example shown in  FIG. 7 , there is no need to provide a female thread in the intermediate cavity portion  62 , which facilitates maintenance. 
         [0066]    The second modified example shown in  FIG. 8  is different from the first modified example in that a special bolt  70  and an auxiliary member (wedge  78 ) are adopted instead of the double-nut bolt as the fixing member shown in the first modified example. That is, as shown in  FIG. 8 , the second modified example is the same as the first modified example in that the drilling jig  6  is used, but is different in that the special bolt  70  shown in  FIG. 9(   a ) is used as the fixing member. A male thread part  72  is provided on one side of the special bolt  70  in the longitudinal direction, and a lower fixing part  73  is provided on the other side, and the male thread part  72  and the lower fixing part  73  together with a main body part  71  form an integrated fixing member. The lower fixing part  73  has a structure with a hollow portion  76  having an inversed conical shape formed on the inside in the axial direction and a four-leg strut part  74  formed on the outer peripheral side by dividing the outer circumference into eight parts and cutting the inside and a part of the circumference (cross-section B-B). Due to this configuration, each leg of the strut part  74  has a spring property which allows elastic deformation in the radial direction of the special bolt  70 . The strut part  74  includes at its end a collar portion  75  which expands in the radial direction. While the example of the strut part  74  having eight divisions (four legs) is shown, the strut part  74  is not limited to this example as long as the number of divisions is four or larger. According to the second modified example, the inner peripheral surface of the intermediate cavity portion  62 , with which the outer peripheral surface of the strut part  74  comes into contact, and the ceiling surface of the cavity portion  64 , with which the collar portion  75  comes into contact, function as an engaging part for engaging the special bolt (fixing member)  70 . 
         [0067]    While the jig  6  adopted in the second modified example has the clearance hole  22 , the intermediate cavity portion  62 , and the cavity portion  64  as with the other embodiment and modified example, it is preferable that a connection part between the clearance hole  22  and the intermediate cavity portion  62  has such a shape that the inner surfaces are connected smoothly through a slope etc. In addition, the clearance hole  22  may be slightly increased in diameter at the upper end and slightly reduced in diameter in the opening at the lower end of the intermediate cavity portion  62 , and may have a conical shape with a constant slope of the inner surface from the upper end of the clearance hole  22  to the opening at the lower end of the intermediate cavity portion  62 . Such a shape allows smooth movement of the special bolt  70  when it is passed, the collar portion  75  of the special bolt  70  first, through to the opening of the cavity portion  64 . 
         [0068]      FIG. 9(   b ) shows the conical wedge  78  to be inserted into the cavity portion  64 . As shown in  FIG. 8 , the special bolt  70  is inserted from above the plate member  80  into the stand part  60  through the through-hole  81  of the plate member  80  until the collar portion  75  appears in the cavity portion  64 . In this case, the special bolt  70  is inserted, the collar portion  75  side first. When the collar portion  75  reaches the cavity portion  64 , due to the spring property of the strut part  74 , the collar portion  75  expands in the radial direction of the special bolt. Further, the wedge  78  is inserted from the cavity portion  64  on the other side into a cavity portion  76  formed inside the strut part  74  of the special bolt  70 . As a result, the collar portion  75  and the strut part  74  further expand in the radial direction, and the outer peripheral circle of the collar portion  75  formed of the four legs (the length between outer surfaces of opposite legs of the collar portion) becomes larger than the opening of the intermediate cavity portion  62 . As the nut  46  in the upper portion of the special bolt  70  is fastened, the horizontal upper surface of the collar portion  75  comes into close contact with the ceiling surface of the cavity portion  64 . This operation can fix the special bolt  70  on the stand part  60  through the collar portion  75 , and can thereby fix the plate member  80  on the jig  6 . According to the configuration of the second modified example shown in  FIG. 8 , since no female thread is provided, maintenance is facilitated compared with the embodiment. 
         [0069]    In the following, a drilling method according to the one embodiment and the modified examples will be described in detail. 
         [0070]    A hole array composed of a plurality of positions of through-holes is previously set in the plate member  80  to be machined. 
         [0071]    First, positions of through-holes to be the first through-holes  81  of the plurality of through-holes are selected. More specifically, as shown in  FIG. 1 , the jig disposition area of the plate member  80  is divided in a lattice shape by a virtual longitudinal division line  90  extending in the longitudinal direction and a virtual lateral division line  92  extending in the lateral direction. For example, the jig disposition area is divided at an about 400 mm pitch of regular intervals in both the lateral and longitudinal directions. Then, through-holes closest to intersections between the virtual longitudinal division line  90  and the virtual lateral division line  92  (including through-holes on the intersections) are selected, and these through-holes are set as the first through-holes  81 . Thus, since the drilling jigs  2  are disposed at substantially regular intervals in the jig disposition area in the central part of the plate member  80 , the rigidity of the jig disposition area can be enhanced substantially uniformly, which allows more proper drilling. 
         [0072]    Next, the drilling jigs  2  are fixed on the bed  10  of the machine tool  1  as shown in  FIG. 2  and  FIG. 3 . More specifically, at least one jig  2  is positioned with reference to a machining origin O of the machine tool  1 , and the at least one jig  2  is fixed on the bed  10  with the bolt  38 . In  FIG. 2 , the machining origin O is a machining reference point determined by the machine tool side, and a tool driving part  15 ′, a drilling tool  16 ′, and the plate member are positioned with reference to this machining origin. 
         [0073]    Further, the plate member  80  is placed on the at least one jig  2 , and the plate member  80  is fixed on the bed  10  with the clamp mechanism  4 . In this process, the plate member  80  may be positioned with reference to the machining origin O and the plate member  80  may be placed on the at least one jig  2 . It is possible to dispose the at least one jig  2  with high accuracy in a predetermined positional relation with the plate member  80  by positioning the at least one jig  2  with reference to the machining origin O of the machine tool  1  and then positioning also the plate member  80  with reference to the machining origin O. 
         [0074]    Then, the machine tool  1  is operated so that the leading end portion of the drilling tool  16  having penetrated the plate member  80  is inserted into the clearance hole  22 . The drilling tool is positioned at the position of the plate member  80  corresponding to the clearance hole  22 , and the first through-hole  81  is formed. During formation of the first through-hole  81 , the plate member  80  is fixed on the bed  10  with the clamp mechanism  4 , while the periphery of the first through-hole  81 , into which the drilling tool  16  is inserted, is supported from below by the jig  2 . Therefore, it is possible to receive the reaction force during insertion of the drilling tool  16  with the jig  2  without fixing the plate member  80  on the jig  2 . In addition, since vibration, bouncing, etc. of the plate member  80  can be suppressed, it is possible to form precise through-holes and prevent troubles such as breakage of the drilling tool  16 . 
         [0075]    Subsequently, the plate member fixing bolt  34  is passed through the first through-hole  81 , and the plate member  80  is fixed on the at least one jig  2  with the plate member fixing bolt  34 . Thus, since the plate member  80  is fixed on the bed  10  using the first through-hole  81  which has been formed first of the plurality of through-holes, even the plate member  80  having a hole array of a triangular pitch, a square pitch, etc. can be easily fixed on the bed  10 . 
         [0076]    Further, the machine tool  1  is operated to form the second through-hole  82  other than the first through-hole  81  of the plurality of through-holes. During formation of the second through-hole  82 , since the plate member  80  is fixed on the bed  10  with the plate member fixing bolt  34  passed through the first through-hole  81 , the rigidity of the plate member  80  can be enhanced and vibration and bouncing of the plate member  80  can be suppressed. Accordingly, it is possible to form precise through-holes while preventing troubles such as breakage of the drilling tool  16 . In addition, it is possible to freely adjust the rigidity of the plate member  80  around the second through-hole  82  by appropriately selecting the positions of the first through-holes  81 , namely, the installation positions of the plate member fixing bolts  34 . 
         [0077]    In one embodiment, when the plate member  80  is fixed on the bed  10  with the clamp mechanism  4 , the outer peripheral side of the plate member  80  relative to the jig disposition area of the at least one jig  2  may be fixed on the bed  10  with the clamp mechanism  4 . During formation of the first through-hole  81 , the first through-hole  81  may be formed while the jig disposition area of the plate member  80  is supported from below with the at least one jig  2  and the outer peripheral side of the plate member  80  is fixed with the clamp mechanism  4 . Thus, it is possible to drill properly even when a large number of through-holes are located in the central part of the plate member  80 , by receiving the reaction force of the plate member  80  with the jig, which supports the jig disposition area from below, while securing the restraining force on the plate member  80  with the clamp mechanism  4  which fixes the outer peripheral side relative to the jig disposition area. 
         [0078]    In one embodiment, the drilling tool  16  may be a cemented carbide drill. Then, it is possible to drill at a high speed and reduce the machining time as well as to form more precise through-holes. 
         [0079]    Next, a condenser  100  according to one embodiment of the present invention will be described with reference to  FIG. 10 .  FIG. 10  is a schematic view showing a configuration example of the condenser according to one embodiment of the present invention. 
         [0080]    The condenser  100  condenses steam having been used for driving a steam turbine into water and resupplies the condensate to a boiler etc. which generates steam. In this embodiment, the condenser  100  used for a thermal power plant, a nuclear power plant, etc. is shown as an example. 
         [0081]    As shown in  FIG. 10 , the condenser  100  in one embodiment has a shell  102  forming the outer shape and a cooling tube bundle  104  which is a collection of cooling tubes  106  for cooling steam. The shell  102  is provided with a water chamber  110  which forms, together with the cooling tube bundle  104 , a passage for cooling water to circulate through, and a hot well  112  in which condensate being water condensed from steam is temporarily collected. A turbine part  114  of a steam turbine is disposed in an upper portion of the shell  102 , and the cooling tube bundle  104  is disposed under the turbine part  114 . 
         [0082]    The plurality of cooling tube bundles  104  are disposed inside the shell  102 . Each of the cooling tube bundles  104  is provided between the turbine part  114  and the hot well  112  so as to extend in the extension direction of the rotational axis of the turbine part  114 . 
         [0083]    The cooling tube bundle  104  is provided with a plurality of tube support plates  108  for supporting the cooling tubes  106 . The tube support plates  108  are disposed at intervals in the extension direction of the cooling tubes  106 , and retain the array of the cooling tube bundles  104 . 
         [0084]    Tube plates  80  are provided at the ends of the cooling tube bundle  104 . The tube plate  80  is produced using the drilling jig  2  or the drilling method as described above. The tube support plate  108 , other than the tube plate  80 , may be produced using the drilling jig or the drilling method according to the embodiment. 
         [0085]    As has been described above, according to the foregoing embodiment, it is possible to form the through-holes  81 ,  82  of an arbitrary hole array, and form the precise through-holes  81 ,  82  by suppressing vibration, bouncing, etc. of the plate member  80  during insertion of the drilling tool  16  while avoiding troubles such as breakage of the drilling tool  16 . 
         [0086]    While the embodiment of the present invention has been described in detail, it is obvious that the present invention is not limited to this embodiment and various improvements and modifications may be made within the scope of the present invention. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           1  Machine tool 
           2 ,  6  Drilling jig 
           4  Clamp mechanism 
           10  Bed 
           10   a  Jig fixing part 
           10   b ,  10   c  Clamp fixing part 
           12  Drilling mechanism 
           13  Support pillar 
           14  Rail 
           15  Tool driving part 
           16  Drilling tool 
           20 ,  60 ,  70  Stand part 
           22 ,  62  Clearance hole 
           23  Support portion 
           24 ,  56 ,  64  Female thread 
           26 ,  66  Cavity portion 
           27 ,  67  Chip outlet 
           30  Base part 
           32  Long hole 
           34  Plate member fixing bolt (fixing member) 
           62  Intermediate cavity portion 
           66  Double-nut bolt (fixing member) 
           70  Special bolt (fixing member) 
           38 ,  40 ,  50  Bolt (fastening member) 
           36 ,  44  Washer 
           41 ,  51 ,  52  Male thread 
           42  Presser plate 
           46 ,  54  Nut 
           48  Jack 
           47 ,  56  Female thread 
           100  Condenser 
           102  Shell 
           104  Cooling tube bundle 
           106  Cooling tube 
           108  Tube support plate 
           110  Water chamber 
           112  Hot well 
           114  Turbine part