Patent Publication Number: US-2016236282-A1

Title: Cutting tool holder and cutting tool

Description:
TECHNICAL FIELD 
     The present invention relates to a cutting tool holder that has a shaft hole and is used for inner diameter processing by turning or the like, and a cutting tool formed by inserting a boring bar (a bar-shaped cutting tool having a cutting edge formed at one side of one end thereof or at one side of each of both ends thereof) into the shaft hole of the cutting tool holder and screwing a setscrew into a fixing screw hole, which is provided so as to penetrate from the outer peripheral surface of the cutting tool holder toward the shaft hole, thereby to fix (cramp) the boring bar. 
     BACKGROUND ART 
     Conventionally, of inner diameter processing (hole processing) of a workpiece (to-be cut object), in boring in which the inner peripheral surface of a pilot hole is processed by turning or the like, a cutting tool may be used which is formed by: inserting a boring bar having a cutting edge (knife edge) at one side of a front end thereof into a shaft hole of a sleeve-shaped holder such that a rear end of the boring bar initially enters the shaft hole; and screwing a setscrew into a screw hole provided in the outer peripheral surface of the cutting tool holder (hereinafter, also referred to merely as holder), thereby to fix the boring bar. Such a cutting tool is inserted into a holder mount hole that is provided in a tool rest of a lathe and extends in a main shaft (rotation shaft) direction, the holder of the cutting tool is fixed by screwing a fixing bolt into a screw hole provided in the outer peripheral surface of the tool rest, and the cutting tool is used for processing with the lathe. As such a cutting tool, in the case where the diameter of a hole to be processed is small and the boring bar is thin, there is a cutting tool in which a coolant is not supplied from the outside, and a flow path provided in the holder itself along a direction in which the shaft hole extends is opened in a front end surface of the holder to form an ejection port (discharge port) (Patent Document 1). As shown in  FIG. 11 , in this cutting tool  400 , the ejection port  120  is provided so as to be located at a cutting edge  203  side (flank side) of the boring bar  200  inserted and fixed in the shaft hole  110  of the holder  500 , so that a coolant (shown by broken lines) C is ejected (jetted out) from the ejection port  120  under high pressure in a jet flow manner to be poured (sprayed) to the cutting edge  203 . In the present application, the coolant means a fluid (cutting fluid or gas) poured to a cutting part for the purpose of lubrication between the cutting edge (knife edge)  203  and a to-be cut object, and cooling of both (temperature rise prevention), etc. 
     In inner diameter (inner peripheral surface) processing of a hole with such a cutting tool  400 , if the inner diameter is small (e.g., Ø10 mm), the gap between the outer peripheral surface of the boring bar  200  and the inner peripheral surface of the hole becomes small due to a demand for ensuring a desired thickness of the bar, and thus dischargeability of swarf deteriorates. Still, as shown in  FIG. 12 , in processing in the case where a hole in a workpiece W penetrates, swarf generated at the cutting edge  203  can be discharged through an opening of the hole at the deep side together with the coolant C due to ejection of the coolant C from the ejection port  120  at the front end of the holder  500 . However, in inner diameter processing of a small-diameter hole that is not a through hole and is a so-called blind hole (a hole closed at the deep side) having a closed end portion at a side opposite to a boring bar entry start (inlet) side, such discharge is not achieved. Thus, in the case of processing of such a blind hole, the dischargeability of swarf extremely decreases, and swarf tends to remain at the deep side of the hole, as compared to the case of processing of a through hole. This causes a problem such as damaging the inner peripheral surface of the hole to cause a decrease in the roughness of a finished surface. 
     Under such a circumstance, an invention is known in which, in order to enhance the dischargeability of swarf even in such inner diameter processing of a blind hole, the coolant ejection port is opened not at the cutting edge side (flank), but at the side opposite to the cutting edge (at the back surface side), when the cutting tool is seen from the front end side (when the cutting tool is seen from a rotation shaft direction of the workpiece) (Patent Document 2). In this invention, as shown in  FIG. 13 , the coolant C is ejected from the ejection port  120 , which is provided at the side opposite to the cutting edge  203 , toward the deep side of the blind hole, and is caused to make a U turn at the deep side of the hole in a way, to be collected, whereby swarf generated at the cutting edge  203  side is discharged along a void such as a swarf discharge groove, provided at the cutting edge  203  side, together with the coolant C to the inlet side of the blind hole. Hereinafter, the coolant supply method in which the coolant is supplied at the cutting edge  203  side as shown in  FIGS. 11 and 12  is also referred to as “knife edge supply”, and the coolant supply method in which the coolant is supplied at the side opposite to the cutting edge  203  as shown in  FIG. 13  is also referred to as “back surface supply”. 
     Incidentally, in the cutting tool  400  including the holder  500  shown in  FIG. 11  and the boring bar  200  inserted and fixed in the shaft hole  110 , such back surface supply (method) is achieved as follows. Specifically, in the cutting tool  400  that performs inner diameter processing of a through hole with the knife edge supply, the coolant ejection port  120  opened in a front end surface  103  of the holder  500  is located at the cutting edge  203  side of the boring bar  200 , as shown in  FIG. 14 -A, when the cutting tool  400  is seen from the front end thereof. In the cutting tool  400 , for inner diameter processing of a blind hole, in a set-up operation for changing to the back surface supply, in  FIG. 14 -A, if the cutting tool  400  is fixed to a tool rest H of a lathe, a fixing bolt Vo in the tool rest H that fixes the holder  500  and setscrews  130  that fix the boring bar  200  to the holder  500  are loosened. Then, for example, only the holder  500  is inverted (rotated by 180 degrees) within a mount hole of the tool rest H without rotating the boring bar  200  within the shaft hole  110 , and the fixing bolt Vo and the setscrews  130  are screwed in. By so doing, as shown in  FIG. 14 -B, the coolant ejection port  120  opened in the front end surface of the holder  500  is located at the side opposite to the cutting edge  203  of the boring bar  200 , so that processing with the back surface supply is enabled. 
     Meanwhile, in the holder  500  of the above conventional cutting tool  400 , screw holes  135  into which the setscrews  130  for fixing the boring bar  200  are screwed are provided in one side surface of the outer peripheral surface of a shank portion of the boring bar  200  (see  FIGS. 11 and 12 ). Pressing for fixing the boring bar  200  by means of the setscrews  130  is performed at a surface that faces in the same direction as a rake face  205  of the cutting edge  203 . This is because, if such a pressing state is obtained, a major component force of cutting resistance is not received through point support by the tips of the setscrews  130 , but can be received by the inner peripheral surface of the shaft hole  110 , so that high stability of the cutting edge  203  can be obtained. On the other hand, the above conventional cutting tool  400  employs the knife edge supply in principle, and thus the pressing surface of the boring bar  200  faces in the same direction as the rake face  205  of the cutting edge  203 . Therefore, in the knife edge supply, the holes into which the setscrews  130  are screwed are provided at the side of the surface facing in the same direction as the rake face  205  of the cutting edge  203 . 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: Japanese Utility Model Publication Laid-Open (kokai) No. H05-85535 
     Patent Document 2: Japanese Patent Publication Laid-Open (kokai) No. 2007-185765 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     As is obvious from the above, in the cutting tool  400 , when the holder  500  is rotated relative to the boring bar  200  in order to shift to the back surface supply, the setscrews  130  do not press the surface facing in the same direction as the rake face  205  of the cutting edge  203  of the boring bar  200  but press a surface opposite to this surface, as shown in  FIG. 14 -B. Thus, there is a problem that, in cutting in such a fixed state, the major component force of cutting resistance is received by the tips of the setscrews  130 , so that the fixed state is inappropriate. 
     Furthermore, originally, settings are made in which the holder  500  is fixed to the tool rest H such that, in the set-up operation including such an operation of screwing the setscrews  130  and the like, an operator is allowed to screw the setscrews  130  and the like while seeing the holder  500  from above, in order to be able to visually recognize the setscrews  130  and the like. On the other hand, in changing to the back surface supply as described above, the holder  500  is rotated in the mount hole of the tool rest H by 180 degrees, and thus the operation of screwing the setscrews  130  becomes an operation in which the setscrews  130  are screwed at the lower surface of the holder  500 . Therefore, there is also a problem that the operability deteriorates. If the boring bar  200  is rotated by 180 degrees without rotating the holder  500 , to be disposed in the reverse direction (configured as a so-called reverse cutting tool), and inner diameter processing is performed, such a problem in the screwing operation is eliminated. However, in this case as well, the setscrews  130  press the surface facing in the direction opposite to the rake face  205  side, and thus the fixed state becomes inappropriate. In addition, in the case of being configured as a reverse cutting tool, a difference occurs in a position such as the edge position of the cutting edge  203 . Thus, for example, another program needs to be created in an NC lathe, and there is also a problem that the settings of the NC lathe need to be changed. 
     For these reasons, conventionally, two types of holders between which the position of the coolant ejection port  120  and the positions of the screw holes  135  are different need to be prepared in order that pressing of the boring bar  200  by the setscrews  130  can be performed at the surface facing in the same direction as the rake face  205  of the cutting edge  203  in any of the “knife edge supply” and the “back surface supply”. Therefore, since the number of holders is doubled, there is a remarkable difficulty in terms of stock of the cutting tool and control thereof on a site for such a type of machining, and this causes an increase in the processing cost. 
     The present invention has been made in view of the above-described problems in the cutting tool for inner diameter processing, and an object of the present invention is to allow a single holder to handle a difference in the position of an ejection port between knife edge supply and back surface supply of a coolant without causing the problems regarding fixing of a boring bar. 
     Means for Solving the Problem 
     The invention according to claim  1  is a cutting tool holder including: a shaft hole into which a boring bar having a cutting edge at one side of a front end thereof can be inserted; one or a plurality of screw holes for fixing the inserted boring bar by screwing setscrews thereinto, the one or the plurality of screw holes being provided at one side of an outer peripheral surface of the cutting tool holder so as to penetrate toward the shaft hole; and an ejection port formed such that a coolant can be ejected therefrom toward the front end of the boring bar inserted in the shaft hole and fixed by screwing in the setscrews, the ejection port being opened in a front end surface of the cutting tool holder, wherein a screw hole for fixing the inserted boring bar by screwing a setscrew thereinto is provided also at a side opposite to the screw holes in a radial direction of the shaft hole, so as to penetrate from the outer peripheral surface of the cutting tool holder toward the shaft hole. 
     The invention according to claim  2  is a cutting tool holder according to claim  1 , wherein, when the cutting tool holder is seen from the front end surface, the ejection port is provided so as to be present on a straight line that is drawn so as to be perpendicular to a center line of the screw hole and pass through a center of the shaft hole. 
     The invention according to claim  3  is a cutting tool holder according to claim  1 , wherein, when the cutting tool holder is seen from the front end surface, the ejection port is provided so as not to be present on a straight line that is drawn so as to be perpendicular to a center line of the screw hole and pass through a center of the shaft hole. 
     The invention according to claim  4  is a cutting tool holder according to any one of claims  1  to  3 , wherein the ejection port is opened in the front end surface so as to be recessed in an inner peripheral surface of the shaft hole. 
     The invention according to claim  5  is a cutting tool holder according to any one of claims  1  to  3 , wherein the ejection port is opened in the front end surface as an independent hole near the shaft hole via a flow path provided within the cutting tool holder without communicating with the shaft hole. 
     The invention according to claim  6  is a cutting tool holder according to any one of claims  1  to  5 , wherein the setscrews  130  are screwed in all the screw holes. 
     The invention according to claim  7  is a cutting tool including: the cutting tool holder according to claim  6 ; a boring bar being inserted into a shaft hole of the cutting tool holder; and the setscrew screwed with the screw hole located at a position where a surface facing in the same direction as a rake face, of the setscrews, thereby to fix the boring bar. 
     Advantageous Effects of the Invention 
     With the cutting tool holder according to the present invention, because of the above configuration, particular complication of the structure is not caused, and pressing for fixing the boring bar by means of the setscrews can be performed at the surface facing in the same direction as the rake face of the cutting edge, even in any of knife edge supply and back surface supply. Thus, it is not necessary to prepare two types of holders between which the positions of the screw holes are different as in the conventional art, in order to be able to handle a difference in the position of the coolant ejection port. Therefore, a remarkable merit is obtained for the effect on stock of the cutting tool and control thereof. As described above, with the cutting tool holder according to the present invention, because of the above configuration, the following remarkably excellent effects are obtained: there is no problem in pressing by screwing in the setscrews for fixing the boring bar; a convenient holder can be configured as a single holder allowed to handle a change in the position of the ejection port between the knife edge supply and the back surface supply of the coolant; and the processing cost can also be reduced. In the present invention, the ejection port is formed so as to be opened in the front end surface of the holder itself and only needs to allow a selection to be made between the knife edge supply and the back surface supply. Therefore, the ejection port itself suffices to be opened as one port in the front end surface and at one side of the shaft hole, but a plurality of ports may be opened. 
     As recited in claim  2 , the ejection port is preferably provided such that the center thereof is present on a straight line that is drawn so as to be perpendicular to the center line of the screw hole and pass through the center of the shaft hole, when the holder is seen from the front end surface. However, as recited in claim  3 , the ejection port may be provided so as not to be present on the straight line that is drawn so as to perpendicular to the center line of the screw hole and pass through the center of the shaft hole, when the holder is seen form the front end surface. 
     In the present invention, the ejection port may be recessed in the inner peripheral surface of the shaft hole as recited in claim  4 , or may be provided as a separate hole independently of the shaft hole as recited in claim  5 . If the ejection port is provided as recited in claim  4 , the ejection port can be recessed in the inner peripheral surface of the shaft hole so as to be cut thereinto. Thus, it is easy to form the ejection port, and the coolant can be effectively supplied even when the hole diameter is small and the gap between the inner peripheral surface of the hole and the outer peripheral surface of the boring bar is narrow. On the other hand, if the ejection port is provided in this manner, there is a problem regarding airtightness such as a possibility that the coolant flows around into the gap between the inner peripheral surface of the shaft hole and the outer peripheral surface of the boring bar, or the coolant leaks through the screwed face of the setscrew. In contrast, in the invention according to claim  5 , such a problem can be eliminated. 
     In the holder of the present invention, as in the invention according to claim  6 , the setscrews may be screwed in all the screw holes. This is because the setscrews are less likely to be scattered or lost, and the cutting tool can be promptly adapted to any of the knife edge supply or the back surface supply. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       [ FIG. 1 ] Partially cross-sectional view, passing through screw holes, of an embodiment of the cutting tool holder of the present invention, an enlarged view of a main part thereof, and cross-sectional views of respective parts. 
       [ FIG. 2 ] Enlarged view of a front end surface of the holder in  FIG. 1 . 
       [ FIG. 3 ] Cross-sectional view of the holder in  FIG. 1 , taken along S 4 -S 4 . 
       [ FIG. 4 ] Exploded perspective view of a cutting tool including the holder in  FIG. 1 . 
       [ FIG. 5 ] A is a cross-sectional view of a cutting tool formed by inserting a boring bar into the holder in  FIG. 1  and fixing the boring bar with “knife edge supply”, and B is a view as seen from a rake face side passing through an axial line of the cutting tool, and an enlarged view of a main part thereof. 
       [ FIG. 6 ] Enlarged view of the cutting tool in  FIG. 5 , as seen from a front end thereof. 
       [ FIG. 7 ] A is a partially cross-sectional view as seen from the front end side when the cutting tool with the “knife edge supply” in  FIG. 5  is fixed to a tool rest, and B is a partially cross-sectional view when a shift to “back surface supply” is made in A. 
       [ FIG. 8 ] Diagram illustrating another example in which an ejection port is changed in  FIG. 6 . 
       [ FIG. 9 ] Diagram illustrating another example in which the ejection port is changed in  FIG. 6 . 
       [ FIG. 10 ] Diagram illustrating modifications in which the ejection port is changed in  FIGS. 2 and 8 . 
       [ FIG. 11 ] Explanatory perspective view showing an example of a conventional cutting tool for inner diameter processing, and an enlarged view of a main part thereof. 
       [ FIG. 12 ] Explanatory plan cross-sectional view as seen from the rake face side in a state where inner diameter processing of a through hole is performed with knife edge supply. 
       [ FIG. 13 ] Explanatory plan cross-sectional view as seen from the rake face side in a state where inner diameter processing of a blind hole is performed with back surface supply. 
       [ FIG. 14 ] A is a partially cross-sectional view as seen from the front end side in a state where the cutting tool in  FIG. 11  is fixed to a tool rest with “knife edge supply”, and B is a diagram when a shift from the “knife edge supply” to “back surface supply” is made in A. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     An embodiment of the cutting tool holder of the present invention will be described in detail based on  FIGS. 1 to 4 . In the drawings, reference numeral  100  denotes a holder that has, in a round bar (cylindrical body), a hole having a circular inner peripheral surface in a transverse cross-section thereof and coaxial with a central axis G of the round bar. However, in this example, of this hole, a part from a front end surface  103  of the holder  100  to a middle position before a rear end (surface)  105  is formed as a shaft hole  110  for inserting a boring bar therein, and a part from the middle position to the rear end  105  is formed as a flow path  113 , for coolant supply, having a larger diameter than the shaft hole  110 . A thread (tapered thread for pipe)  114  is provided as a coolant supply pipe connection portion at an end portion opened in the rear end surface  105  of the holder  100 , of the inner peripheral surface of the flow path  113 . 
     Meanwhile, in an outer peripheral surface  102  of the holder  100 , flat surfaces (even surfaces)  106  having a certain width are formed at both sides in the same manner so as to be parallel to each other and extend in a front-rear direction. In this example, in a portion of each flat surface  106  which portion is present at the front end surface  103  side of the holder  100  and corresponds to the above-described shaft hole  110 , screw holes  135  for screwing fixing setscrews  130  therein are provided so as to penetrate in a radial direction of the holder  100  toward the shaft hole  110 . In this example, the screw holes  135  are provided at two locations on each side spaced apart from each other in the front-rear direction, namely, at four locations in total, and each screw hole  135  is set such that the setscrew  130  (e.g., a setscrew  130  with a hexagon socket) can be screwed thereinto. Each flat surface  106  is a surface pressed by screwing in a fixing bolt after a cutting tool including the holder  100  is inserted into a mount hole of a tool rest of a lathe. That is, the holder  100  has a transverse cross-section that can rotate in a slide contact state within a mount (hole) having a circular transverse cross-section in the tool rest of the lathe, and is configured such that the rotation is stopped when the holder  100  is fixed. 
     Meanwhile, the shaft hole  110  is formed as a circular hole into which an intermediate portion (shank portion)  207  between cutting edges  203  provided at one sides of front and rear ends, respectively, as in a boring bar  200  shown in the lower drawing in  FIG. 4  can be inserted. The boring bar  200  shown in  FIG. 4  has rake faces  205  in side surfaces that are opposite to each other and at the front and rear ends. The inner diameter of the shaft hole  110  is sized so as to allow the intermediate portion  207  to rotate about an axial line thereof in a slide contact state with a minute gap. In this example, a recessed groove  116  is recessed in the inner peripheral surface of the shaft hole  110  and on a straight line L 1  that is drawn so as to be perpendicular to an axial line (center line) Ls of the screw hole  135  for screwing the setscrew  130  therein and pass through the center of the shaft hole  110 , when the holder  100  is seen from the front end surface  103  side (see  FIG. 2 ). The recessed groove  116  extends in the front-rear direction over the overall length of the shaft hole  110  (see each transverse cross-sectional view in  FIG. 1 , and  FIG. 3 ). In addition, when the holder  100  is seen from the front end surface  103 , that is, when the holder  100  is cut along a cross-section perpendicular to the axial line G of the shaft hole  110 , the recessed groove  116  is formed as a recess in a circular arc shape (crescent shape) and provided such that the center of the recessed groove  116  in a groove width direction thereof is present on the straight line L 1 . The center of the recessed groove  116  in the groove width direction thereof may not be present on the straight line L 1 . A rear end  117  of the recessed groove  116  is connected to the flow path  113  for coolant supply at the rear side, and a front end of the recessed groove  116  is opened in the front end surface  103  of the holder  100  to form an ejection port  120 . 
     In such a holder  100  of this example, the predetermined boring bar  200 , as shown in the lower drawing in  FIG. 4 , corresponding to the diameter of the shaft hole  110  is inserted into the shaft hole  110  by a predetermined amount. Of the intermediate portion (shank portion)  207 , surfaces facing in the same direction as the rake faces  205  of the cutting edges  203  are pressed by the setscrews  130  screwed into the screw holes  135 , so that the boring bar  200  is fixed. By so doing, a cutting tool  300  for inner diameter processing as shown in  FIGS. 5 and 6  is obtained. In this case, in the case of making the cutting tool  300  into a cutting tool with “knife edge supply”, as shown in  FIG. 6 , when seen from the front end surface  103  of the holder  100 , the cutting edge  203  at the one side of the front end of the boring bar  200  becomes the coolant ejection port  120  side. Thus, in this case, cramping may be performed by screwing in, with predetermined torque, the setscrews  130  present at the side of the surface facing in the same direction as the rake face  205  of this cutting edge  203  (the upper setscrews  130  in  FIGS. 5 -A and  6 ). Prior to this screwing, the setscrews  130  at the opposite side may be moderately screwed back such that the tips thereof do not project from the inner peripheral surface of the shaft hole  110 . In the intermediate portion  207  (shank portion) of the boring bar  200  fixed by means of the setscrews  130 , a flat surface  206  having a predetermined width and extending in the front-rear direction is formed in a surface facing in the same direction as the rake face  205 , similarly as in the holder  100 , such that the rake face  205  can be recognized and fixing by means of the setscrews  130  can be stably performed without rotation. In this regard, since the shown boring bar  200  is of a type having the cutting edges  203  at the front and rear ends and at the different sides from each other, the flat surfaces  206  are provided at both sides on the outer peripheral surface based on a circular cross-section so as to be parallel to each other, similarly as in the holder  100 . 
     Thus, in the cutting tool  300 , if a coolant supply pipe is connected to the connection portion (thread for pipe)  114  of the rear end  105  of the flow path  113  for coolant supply at the rear end surface  105  of the holder  100  and a coolant (e.g., a cutting fluid) is supplied under high pressure, the coolant is poured as a jet flow from the ejection port  120 , which is opened in the front end surface  103  of the holder  100 , toward the cutting edge  203 . Accordingly, when the cutting tool  300  of this example is inserted into a mount hole of a tool rest H of a predetermined lathe with the rake face  205  facing upward and is fixed by means of a fixing bolt Vo as shown in  FIG. 7 -A, desired inner diameter processing (inner diameter processing of a through hole) with the “knife edge supply” can be performed. 
     Meanwhile, in such a cutting tool  300  using the holder  100  of this case, at the time of set-up for changing the cutting tool  300  to “back surface supply” in order to allow for inner diameter processing of a blind hole, the fixing bolt Vo fixing the holder  100  in the tool rest H and all the setscrews  130  screwed into the holder  100  are screwed back to be loosened, and, for example, the holder  100  is inverted (by 180 degrees) around the boring bar  200  while the attitude of the boring bar  200  is maintained as it is. By so doing, as shown in  FIG. 7 -B, the ejection port  120  comes to the position for the “back surface supply”. Therefore, at that position, similarly to the above, the setscrews  130  are screwed in, and the fixing bolt Vo is screwed in, so that inner diameter processing with the back surface supply is enabled to be performed. 
     That is, as described above, for example, when the holder  100  is inverted about the axial line of the boring bar  200  without rotating the boring bar  200 , the coolant ejection port  120  is located at the side opposite to the cutting edge  203  side. At this time, the setscrews  130  that are present at the rake face  205  side of the boring bar  200  and have not played a role of fixing before the inversion come to the same position as the setscrews  130  that have performed fixing at the surface facing in the same direction as the rake face  205  before the inversion. Therefore, the boring bar  200  can be cramped again as if by screwing again the setscrews  130  that have played a role of fixing before the inversion. As described above, with the holder  100  according to this example, even in a use mode of the back surface supply, instability of fixing of the boring bar  200  by means of the setscrews  130  is not caused, and the screwing operation can be performed in an attitude in which the operator looks down from above similarly as described above. Thus, for the operator, the operation can be simplified and sped up. As a matter of course, also in the case of returning to the knife edge supply later, the operation can be performed in the exactly same manner. The operation of changing between the knife edge supply and the back surface supply can be performed naturally after the cutting tool  300  is removed from the tool rest H. In this case as well, the same advantageous effects can be obviously obtained. 
     In the above example, the ejection port  120  is provided by recessing the inner peripheral surface of the shaft hole  110  and the shape of the ejection port  120  is a circular arc shape (crescent shape). Thus, in any of the knife edge supply and the back surface supply, the coolant can be supplied toward the front end of the boring bar  200  along the boring bar  200 , efficiently even if the gap with the inner peripheral surface of a machined hole is small. However, even when the ejection port  120  is provided (opened) by recessing the inner peripheral surface of the shaft hole  110 , the flow path cross-section of the ejection port  120  is not limited to the circular arc shape (crescent shape), and the ejection port  120  may have an appropriate cross-sectional shape such as a rectangular shape. In addition, in the above example, the case where the coolant is supplied from the rear end surface  105  of the holder  100  via the flow path  113  provided coaxially with the shaft hole  110  has been described. However, for supply of the coolant from a pressure feed source (pump), a connection port (screwing portion) for a supply pipe may be provided, for example, at a portion that does not interfere with fixing to the tool rest and is close to the front end, of the outer peripheral surface  102  of the holder  100 , and the flow path  113  may be provided so as to be connected to the ejection port  120 . 
     As shown in  FIG. 8 , the ejection port  120  may be opened in the front end surface  103  of the holder  100  and at a position away from the shaft hole  110  even by a small distance, and may be provided as an independent hole via a flow path (not shown) provided without communicating with the shaft hole  110 . That is, in this case, a flow path for coolant supply may be formed within the holder  100  in a tunnel shape and independently of the shaft hole  110  so as to be connected to the ejection port  120 , and a connection port for supply of the coolant from the pressure feed source may be provided in the rear end surface  105  or the outer peripheral surface of the holder  100 . With this configuration, the coolant can be ejected as an independent jet flow from the ejection port  120  as also described above, and thus a problem such as leak of the coolant through the screwed faces of the setscrews  130  (the gaps between the threads of the screw holes  135  and the threads of the setscrews  130 ) can be avoided. Even if a hole to be machined has a small diameter, this configuration is effective at a relatively large hole. 
     The cutting tool holder  100  of the above example is based on a round bar having a circular transverse cross-section and provided with the flat surfaces  106 , which are parallel to each other, on the outer peripheral surface. The shape of the outer peripheral surface of the cutting tool holder  100  itself may be appropriate one. The outer peripheral surface may be, for example, smaller in diameter at a portion close to the front end and connected to the front end surface  103  than at the other portion continuous rearward therefrom (a portion mounted to the tool rest). In addition, the shaft hole  110  only needs to allow the position of the ejection port  120  to be selectable from a position for the knife edge supply or a position for the back surface supply in accordance with the boring bar. Therefore, the transverse cross-sectional shape of the shaft hole  110  is also not limited to a circle. 
     In the above example, the case has been described in which the center of the recessed groove  116 , connected to the ejection port  120 , in the groove width direction thereof when the holder  100  is seen from the front end surface  103  side (see  FIGS. 2 and 6 ) is present on the straight line L 1  that is drawn so as to be perpendicular to the axial line (center line) Ls of the screw hole  135  for screwing the setscrew  130  therein and pass through the center of the shaft hole  110 . However, the position of the center may be changed as follows. Specifically, in the above example, the case has been described in which, when the holder  100  is seen from the front end surface  103  side (see  FIGS. 2 and 6 ), the ejection port  120  is present on the straight line L 1  that is drawn so as to be perpendicular to the axial line (center line) Ls of the screw hole  135  and pass through the center of the shaft hole  110 . However, in the present invention, as shown in  FIG. 9 , the ejection port  120  may not be present on the straight line L 1  that is drawn so as to be perpendicular to the axial line (center line) Ls of the screw hole  135  and pass through the center of the shaft hole  110 . 
     In  FIG. 9 , the ejection port  120  and the center of the recessed groove  116  in the groove width direction thereof are located on a straight line L 2  that is drawn so as to pass through the center of the shaft hole  110  and be inclined at an inclination angle α (e.g., 45 degrees) relative to the axial line (center line) Ls of the screw hole  13  when the holder  100  is seen from the front end surface  103  side. In this case, as shown in  FIG. 9 , when the holder  100  is seen from the front end surface  103  side, the ejection port  120  is not present on the straight line L 1  that is drawn so as to be perpendicular to the axial line (center line) Ls of the screw hole  135  and pass through the center of the shaft hole  110 .  FIG. 9  illustrates the “knife edge supply”. In this case, for example, the “knife edge supply” is achieved due to the positional relationship between the ejection port  120  and the fixed boring bar  200 , and the case where the coolant is supplied to the cutting edge  203  as the “knife edge supply” closer to the rake face  205  is shown. 
     Even with the “knife edge supply”, this is suitable for the case where supply of the coolant to the rake face  205  side in a large amount is demanded. In the holder  100 , in the case of shifting to the “back surface supply”, even at the back surface side, the ejection port  120  is located in a portion close to the side opposite to the rake face  205 . Thus, in processing of a blind hole, when the coolant makes a U turn to be returned, an effect of causing generated swarf to rise from the rake face  205  by the flow of the coolant is obtained. Thus, the dischargeability of swarf also can be enhanced depending on a processing condition. That is, even in any of the “knife edge supply” and the “back surface supply”, a portion surrounding the shaft hole  110  at which portion the coolant should be ejected to the front end side depends on a processing condition. In accordance with this, the position of the ejection port  120  (the position of the ejection port around the shaft hole  110  relative to the axial line of the screw hole  135 ) may be set, and, therefore, the above angle α may be set as appropriate. In  FIG. 9 , the case has been described in which, in the above example, the ejection port  120  is recessed in the inner peripheral surface of the shaft hole  110  and the shape of the ejection port  120  is a circular arc shape (crescent shape). This configuration is similarly applicable to the case where the ejection port  120  is opened in the front end surface  103  of the holder  100  and independently at a position away from the shaft hole  110  as shown in  FIG. 8 . 
     In the above example, when seen from the front end surface  103  of the holder  100 , the ejection port  120 , which should achieve either the “knife edge supply” or the “back surface supply”, is composed of a single port. However, for example, even in the case where the ejection port  120  is provided in the inner peripheral surface of the shaft hole  110 , the ejection port  120  may be divided into two or more ports, and may be divided into two ejection ports  120  as shown in  FIG. 10 -A. In addition, the same applies to the case where the ejection port  120  is an independent hole, as shown in  FIG. 10 -B. That is, in the present invention, as long as the ejection port is opened in the front end surface of the holder itself and at one side of the shaft hole, even if the ejection port is composed of a plurality of ejection ports or an ejection port group, either the “knife edge supply” or the “back surface supply” may be selectable at the plurality of ejection ports. In any of  FIGS. 10 -A and  10 -B, the two divided ejection ports (two ejection ports)  120  are provided such that the straight line L 1 , which is drawn so as to be perpendicular to the center line Ls of the screw hole  135  and pass through the center of the shaft hole  110  when the holder is seen from the front end surface  103 , is present between the two ejection ports  120 . Thus, the two divided ejection ports  120  are not present on the straight line L 1 . Meanwhile, in any of  FIGS. 10 -A and  10 -B, obviously, either of the two divided ejection ports (two ejection ports)  120  may be provided so as to be present on (overlap) the straight line L 1 . The ejection port may be composed of three or more ports. 
     Regarding the holder according to the present invention, as long as the boring bar can be fixed at a fixed part thereof on the surface facing in the same direction as the rake face side, by means of the setscrews, and the single holder can handle the difference in the position of the ejection port between the “knife edge supply” and the “back surface supply” of the coolant, there are no limitations on the other structure and shape of the holder itself. Regarding fixing of the boring bar, the “surface facing in the same direction as the rake face” at which surface the fixing by means of the setscrews is performed includes, in addition to the surface parallel to the rake face, a surface that is not parallel to the rake face and has an inclination angle. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           100 : cutting tool holder 
           102 : outer peripheral surface of holder 
           103 : front end surface of holder 
           110 : shaft hole of holder 
           120 : ejection port of holder 
           130 : setscrew 
           135 : screw hole of holder 
           200 : boring bar 
           203 : cutting edge 
           205 : rake face 
           300 : cutting tool