Abstract:
A gun drill in which coolant delivered to a cutting part can be discharged intensively to a cutting chip discharge groove along with the chips, damage on the inner circumference of a drilled hole or fall of machining precision due to clogging with chips can be prevented, and cutting efficiency is enhanced by reducing rotary load and long lifetime can be attained. Coolant supply paths ( 10, 20 ) are provided in a tool shank ( 1 ) and a cutting head ( 2 ), one cutting chip discharge groove ( 3 ) is provided in the outer circumferential surface from the proximal side of the tool shank ( 1 ) to the distal end of the cutting head ( 2 ), discharge ports ( 22   a,    22   b ) communicating with coolant delivery ports ( 21   a,    21   b ) and a cutting chip discharge groove ( 3 ) are provided in the distal end face ( 2   c ) of the cutting head ( 2 ), and annular protrusions ( 4   a ) are formed in the outer circumferential surface of the cutting head ( 2 ) continuously over the entire circumference while bridging over the cutting chip discharge groove ( 3 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a gun drill used for deep-hole drilling. 
       BACKGROUND OF THE INVENTION 
       [0002]    Although various systems, such as a gun drilling system, an ejector system (double tube system), a single tube system, etc., are known as deep-hole drilling systems, the gun drilling system is suitable for deep-hole drilling of a small diameter. That is, generally, a gun drill system employs a gun drill, in which, a hollow tool shank, having a cross section with a shape of ⅔ to ¾ of a circle, has disposed at a distal end thereof a cutter head with an outer cross section of the same shape, and with this system, cutting is performed while supplying a coolant, supplied through an interior of the tool shank, to a cut portion from a coolant delivery port at a distal end face of the cutter head and cutting chips, which are generated in accordance with the cutting, are discharged to the exterior along with the coolant through a cutting chip discharge groove, having a V-shaped cross section and extending along a lengthwise direction of an outer circumference of the tool shank, and because, in addition to enabling a large flow path cross-sectional area to be secured for the cutting chip discharge groove even in a small diameter system, a feeding pressure of the coolant is determined by a tool length and the feed pressure does not have to be made high even if a cut hole becomes deep, the system is suitable for deep-hole drilling of small diameter. 
         [0003]    In one such type of gun drill, a main discharge port, at which a cutting chip discharge groove opens to a distal end face of the cutter head, an auxiliary discharge port, opening to the distal end face at a substantially opposite position in a radial direction with respect to the main discharge port, a bypass flow path port, leading from the auxiliary discharge port to the cutting chip discharge groove through the interior of the head, and two coolant delivery ports, opening to the distal end face at substantially opposite positions in a radial direction, are disposed at the distal end face of the cutter head, and a plurality of blades are formed in a distributive manner so as to face the main discharge port and the auxiliary discharge port. Because the coolant is distributed and supplied to the cut portion from the two coolant delivery ports at the distal end face of the cutter head and the cutting chips are efficiently discharged along with the coolant from the main discharge port and the auxiliary discharge port at the distal end face to the cutting chip discharge groove, such a gun drill provides a benefit that a high cutting efficiency can be obtained based on the good cutting chip discharge performance. 
         [0004]    FIGS. 13A, 13B and 14 of Japanese Published Unexamined Patent Application No. 2005-118940 show an example of a gun drill having a pair of coolant delivery ports and a pair of discharge ports disposed at a distal end face of a cutter head as described above. With this gun drill, a cutter head  52  has a connecting shaft portion  52   b  spigot-fitted and coaxially connected to a distal end portion of a tool shank  51 , a cutting chip discharge groove  53 , with a substantially V-shaped cross section, is formed rectilinearly from a proximal side of the tool shank  51  to a distal end of the cutter head  52 , and a coolant supply path  54 , communicating from an interior of the tool shank  51  to an interior of the connecting shaft portion  52   b  of the cutter head  52 , branches into two inside a main head body  52   a  and opens as two coolant delivery ports  55   a  and  55   b  that are oppositely positioned in a radial direction at a distal end face of the head. At the distal end face of the head, a main discharge port  56   a , which is a distal end opening of the cutting chip discharge groove  53 , and an auxiliary discharge port  56   b , opening at a substantially opposite position in a radial direction with respect to the main discharge port  56   a , are disposed, a bypass flow path port  57 , leading from the auxiliary discharge port  56   b , through the interior of the head, and to the cutting chip discharge groove, is formed, inner and outer blades  58   a  and  58   c  are fixed facing the main discharge port  56   a , and an intermediate blade  58   b  is fixed facing the auxiliary discharge port  56   b .  59  indicates guide pads that are fixed to a distal end circumferential face of the cutter head  52 . 
         [0005]    However, with the conventional gun drill, because in a drilling process, the outer blade  58   c  and the guide pads  59  on the cutter head  52  slidingly contact an inner circumference of a cut hole H and thereby gives rise to a gap t between the circumferential face of the cutter head  52  and the inner circumference of the cut hole H as shown in  FIG. 14 , in many cases, a portion of the coolant that contains cutting chips flows from the distal end face of the head into the gap t and causes clogging by the cutting chips at an outer circumferential portion of the tool shank  51  at the rear side, and this clogging causes flawing of the inner circumference of the cut hole H, lowering of the processing precision as well as increase of a rotational load that leads to breakage of the tool. Such a problem is especially prominent with gun drills having a pair of coolant delivery ports  55   a  and  55   b  and a pair of discharge ports  56   a  and  56   b  at the distal end face of the cutter head  52  as illustrated and also occurs frequently with other general gun drills as well. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention has been made in view of the above circumstances and an object thereof is to provide a gun drill that can intensively discharge a coolant, delivered to a cut portion, to a cutting chip discharge groove along with cutting chips to thereby prevent flawing of an inner circumference of a cut hole and lowering of processing precision due to clogging by cutting chips and enable improvement of the cutting efficiency and elongation of tool life by lightening of a rotational load. 
         [0007]    In order to achieve the aforementioned object, a gun drill according to a first aspect of the present invention, described with reference symbols in the drawings, includes; a cutter head  2  installed on a distal end portion of a tool shank  1 , a coolant supply paths  10  and  20  communicating both of the cutter head  2  and the tool shank  1  in interiors of the cutter head  2  and a single cutting chip discharge groove  3  disposed rectilinearly in a lengthwise direction along an outer circumferential face extending from a proximal side of the tool shank  1  to the distal end of the cutter head  2 ; wherein, the cutter head  2  having coolant delivery ports  21   a  and  21   b  in communication with the coolant supply path  20  at a distal end face  2   c  thereof and discharge ports (main discharge port  22   a  and auxiliary discharge port  22   b ) in communication with the cutting chip discharge groove  3 , and having an outer circumferential face provided with annular protrusion portions  4   a  continuously over the entire circumference thereof while bridging over the cutting chip discharge groove  3 . 
         [0008]    According to a second aspect of the present invention, in the gun drill according to the first aspect, a plurality of the annular protrusion portions  4   a  are disposed in parallel and form a labyrinth seal portion  4 . 
         [0009]    According to a third aspect of the present invention, in the gun drill according to the first aspect, the cutter head  2  has a main discharge port  22   a , at which the cutting chip discharge groove  3  opens to the distal end face, an auxiliary discharge port  22   b , opening to the distal end face at a position substantially opposite the main discharge port  22   a  in a radial direction, a bypass flow path port  23 , leading from the auxiliary discharge port  22   b  to the cutting chip discharge groove  3  through the interior of the head, and two coolant delivery ports  21   a  and  21   b , opening to the distal end face at substantially opposite positions in a radial direction, a plurality of blades  5   a  to  5   c  are formed and distributed so as to face the main discharge port  22   a  and the auxiliary discharge port  22   b , and the annular protrusion portions  4   a  are positioned closer to the proximal side of the head than a merging section at which the bypass flow path port  23  joins the cutting chip discharge groove  3 . 
         [0010]    According to a fourth aspect of the present invention, in the gun drill according to the first aspect, a proximal end portion (connecting shaft portion  2   b ) of the cutter head  2  is detachably and coaxially connected to the distal end portion of the tool shank  1 . 
         [0011]    According to a fifth aspect of the present invention, in the gun drill according to any of the first to fourth aspect, the tool shank  1  has a main shank body  11  having a C-shaped cross-section at least at a forming part of a cutting chip discharge groove  3  inside thereof, and a pipe member  12 , which is inserted and fitted in the main shank body  11 , an interior of which constitutes a coolant supply path  10 ; wherein a front end portion of the pipe member  12  is attached to the main shank body  11  side via an external screw  12   a  on its outer circumference. 
         [0012]    Effects of the present invention shall now be described using the reference numbers provided in the drawings. Firstly, the gun drill according to the first aspect of the present invention, in a drilling process, although a part of the coolant containing cutting chips that is delivered from the coolant delivery ports  21   a  and  21   b  at the distal end face of the cutter head  2  flows into a gap t between a circumferential face of the cutter head  2  and an inner circumference of a cut hole H, instead of flowing into the discharge ports  22   a  and  22   b , because this gap t is plugged at positions of annular protrusion portions  4   a  formed at the cutter head  2  preventing from further entry in a rearward direction, in consequence, substantially all of the coolant is concentrated in the cutting chip discharge groove  3  at the positions of the annular protrusion portions  4   a  and the cutting chips riding on the flow of the coolant are thus powerfully discharged rearward through the cutting chip discharge groove  3 , without giving rise to a clogging by cutting chips. Accordingly, damages on the inner circumference of the cut hole or a deterioration of machining precision due to clogging with chips can be prevented, and cutting efficiency is enhanced by reducing rotary load and long lifetime the gun drill itself can be attained. 
         [0013]    According to the second aspect of the present invention, because the labyrinth seal portion  4 , in which the plurality of annular protrusion portions  4   a  of the cutter head  2  are disposed in parallel, is formed, a coolant entry prevention effect of the annular protrusion portions  4   a  is high and clogging by cutting chips is prevented more reliably. 
         [0014]    According to the third aspect of the present invention, the cutter head  2  has the pair of coolant delivery ports  21   a  and  21   b  and the pair of discharge ports  22   a  and  22 , and conventionally such arrangement gives rise readily to a clogging by cutting chips in spite of achieving a high cutting efficiency. However, clogging by cutting chips is prevented by the annular protrusion portions  4   a  formed at the cutter head  2  only the benefit of high cutting efficiency of the arrangement can be provided. 
         [0015]    According to the fourth aspect of the present invention, because the cutter head  2  is an independent member that is attachable/detachable with respect to the tool shank  1 , exchange with a new head upon wear or damage and exchange with different types of head according to drilling conditions are enabled and a benefit of enabling forming and processing of the annular protrusion portions  4   a  to be readily and inexpensively performed according to each head is provided. 
         [0016]    According to the fifth aspect of the present invention, because the coolant supply path  10  inside the tool shank  1  is constituted by the pipe member  12  and the front end side of the pipe member  12  is attached to the main shank body  11  side via a screw, there is a beneficial point in that processing and manufacture of the tool shank  1  can be readily and inexpensively performed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  shows a gun drill according to a first embodiment of the present invention, with  FIG. 1A  being a side view of an entirety of the gun drill and  FIG. 1B  being a front view. 
           [0018]      FIG. 2  is a longitudinal sectional side view of a cutter head side of the gun drill during a drilling process. 
           [0019]      FIG. 3  is a sectional view taken on line V-V of  FIG. 2 . 
           [0020]      FIG. 4  is a sectional view taken on line W-W of  FIG. 2 . 
           [0021]      FIG. 5  is a side view of the cutter head used in the gun drill. 
           [0022]      FIG. 6  is a sectional view taken on line X-X of  FIG. 5 . 
           [0023]      FIG. 7  is a sectional view taken on line Y-Y of  FIG. 6 . 
           [0024]      FIG. 8  is a longitudinal sectional side view of a tool shank used in the gun drill. 
           [0025]      FIG. 9  is a sectional view taken on line Z-Z of  FIG. 8 . 
           [0026]      FIG. 10  shows a procedure of connecting a connecting shaft portion of the cutter head to a main shank body of the tool shank of the gun drill, with  FIG. 10A  being a longitudinal sectional side view of the main shank body and the connecting portion before fitting,  FIG. 10B  being a longitudinal sectional side view of a rotation operation after fitting, and  FIG. 10C  being a longitudinal sectional side view of an operation of fixing by a side lock method. 
           [0027]      FIG. 11  shows a pipe stopping member used in the tool shank, with  FIG. 11A  being a front view,  FIG. 11B  being a longitudinal sectional side view, and  FIG. 11C  being a sectional view taken on line C-C of  FIG. 11B . 
           [0028]      FIG. 12  is a longitudinal sectional side view of an operation of connecting a pipe member to the pipe stopping member of the tool shank. 
           [0029]      FIG. 13  shows an arrangement example of a conventional gun drill, with  FIG. 13A  being a side view of an entirety of the gun drill and  FIG. 13B  being a front view. 
           [0030]      FIG. 14  is a longitudinal sectional side view of a state of drilling by the conventional gun drill. 
       
    
    
     DESCRIPTION OF THE SYMBOLS 
       [0000]    
       
         
           
               1  tool shank 
               10  coolant supply path 
               11  main shank body 
               11   a  connecting recess portion 
               12  pipe member 
               12   a  external thread 
               13  pipe stopping member 
               13   b  internal thread 
               2  cutter head 
               2   a  main head body 
               2   b  connecting shaft portion (base end portion) 
               20  coolant supply path 
               21   a  coolant delivery port 
               21   b  coolant delivery port 
               22   a  main discharge port 
               22   b  auxiliary discharge port 
               23  bypass flow path port 
               24  bridging portion 
               3  cutting chip discharge groove 
               4  labyrinth seal portion 
               4   a  annular protrusion portion 
               5   a  inner blade 
               5   b  intermediate blade 
               5   c  outer blade 
           
         
       
     
       DETAILED DESCRIPTION 
       [0055]    An embodiment of a gun drill according to the present invention shall now be specifically described with reference to the drawings.  FIGS. 1A and 1B  show a side view and a front view of an entirety of a gun drill according to the embodiment,  FIG. 2  shows a state of drilling by the gun drill,  FIGS. 3 and 4  show sectional views of principal portions in  FIG. 2 ,  FIGS. 5 to 7  show a cutter head of the gun drill,  FIGS. 8 and 9  show a tool shank of the gun drill,  FIG. 10  shows an operation of connecting the cutter head and the tool shank,  FIG. 11  shows a pipe receiving member of the tool shank, and  FIG. 12  shows an operation of connecting a main shank body of the tool shank and a pipe member. 
         [0056]    As shown in  FIG. 1A , this gun drill is constituted of an elongate tool shank  1 , an interior of which is arranged as a coolant supply path  10 , a cutter head  2 , coaxially connected to a distal end of the tool shank  1 , and a large-diameter, cylindrical driver  6 , inserted and fitted and fixed to a proximal end portion of the tool shank  1 , a coolant supply path  20 , communicating with the coolant supply path  10  of the tool shank  1 , is disposed inside the cutter head  2 , and a single, rectilinear, cutting chip discharge groove  3  that extends in a lengthwise direction is disposed on an outer circumferential face from a proximal side of the tool shank  1  to a distal end of the cutter head  2 . As shown in  FIG. 1B , the cutting chip discharge groove  3  has a fan-shaped cross section with an opening angle θ of 100° to 130° from centers of the tool shank  1  and the cutter head  2 . 
         [0057]    As shown in  FIG. 1B  and in detail in  FIGS. 5 to 7 , the cutter head  2  is constituted of a main head body  2   a  at a front side and a thin-diameter, connecting shaft portion  2   b , protruding coaxially from a rear end of the main head body  2   a , and has in the interior thereof the coolant path  20 , in communication with the coolant supply path  10  inside the tool shank  1 . As shown in  FIGS. 6 and 7 , the coolant path  20  branches into two at the main head body  2   a  side, and the branch paths  20   a  and  20   b  open to a distal end face of the head as coolant delivery ports  21   a  and  21   b  at opposite side positions in a radial direction. In the cutter head  2 , a bottom portion of the cutting chip discharge groove  3  forms a convex arcuate face from a rear portion of the main head body  2   a  to the connecting shaft portion  2   b  due to the presence of the coolant supply path  20  of circular cross section at an axial center position ( FIG. 6 ). 
         [0058]    On the main head body  2   a  are formed a main discharge port  22   a , with which the cutting chip discharge groove  3  opens to a head distal end face  2   c , an auxiliary discharge port  22   b , opening to the head distal end face  2   c  at a position substantially opposite the main discharge port  22   a  in a radial direction, a bypass flow path port  23  as shown in  FIG. 2 , leading from the auxiliary discharge port  22   b  to the cutting chip discharge groove  3  through the interior of the head, and two coolant delivery ports  21   a  and  21   b , opening to the distal end face at substantially opposite positions in a radial direction. At a distal end side of the main head body  2   a , inner and outer blades  5   a  and  5   c , constituted of throwaway tips, are fixed by screwing so as to face the main discharge port  22   a , an intermediate blade  5   b  is fixed facing the auxiliary discharge port  22   b , and guide pads  7 , are fixed by screwing at opposite side positions of the circumferential face that sandwich the auxiliary discharge port  22   b.    
         [0059]    At a rear side of the main head body  2   a , a bridging portion  24  that arcuately spans the cutting chip discharge groove  3  is formed as an extension of an outer circumferential portion, and on an outer circumferential face passing along the bridging portion  24 , a plurality (four in the figure) of annular protrusion portions  4   a  are disposed in parallel to form a labyrinth seal portion  4 . At each annular protrusion portion  4   a , an outer diameter of the labyrinth seal portion  4  is set substantially equal to a cutting diameter of the outer blade  5   c  and an outer circumferential face of each annular recess  4   b  between the annular protrusion portions  4   a  is at the same surface level as the outer circumferential face at the head distal end side of the labyrinth seal portion  4 . 
         [0060]    By a recessed step  26 , as shown in  FIG. 7 , formed circumferentially at a central portion in an axial direction of the connecting shaft portion  2   b  of the cutter head  2 , a rear end side of the connecting shaft portion  2   b  is arranged as a spigot protrusion  27  of the same outer diameter as a base  25  side (front end side), and mortar-shaped latching recesses  27   a  are disposed at two locations that are separated by 90 degrees in a circumferential direction on an outer circumferential face of the spigot protrusion  27 . Thus a groove portion  3   a  ( FIG. 5 ), constituting a bottom side of the cutting chip discharge groove  3 , is formed across a total length in the axial direction of a circumferential face portion of the connecting shaft portion  2   b.    
         [0061]    Meanwhile, the tool shank  1 , as shown in  FIG. 12 , has a main shank body  11 , which, besides a cylindrical proximal end side, has a C-shaped diametrical section, a pipe member  12 , which is inserted and fitted in close contact in an inner side of the main shank body  11  and the interior of which constitutes the coolant supply path  10 , and a pipe stopping ring  13 , which is internally fitted to a proximal end side of the main shank body  11 , and a bottom portion of the cutting chip discharge groove  3  takes on the form of a convex arcuate face due to exposure of a circumferential face of a portion of the pipe member  12 . Thus an outer diameter of the main shank body  11  is set equal to an outer diameter of a portion of the main head body  2   a  of the cutter head  2  at a front side of the labyrinth seal portion  4 . An outer shape and outer dimension (outer diameter and width in the axial direction) of the pipe stopping ring  13  are set equal to those of the spigot protrusion  27  of the connecting shaft portion  2   b  of the cutter head  2 . 
         [0062]    As shown in  FIGS. 8 and 9 , a connecting recess portion  11   a , in which the connecting shaft portion  2   b  of the cutter head  2  and the pipe stopping ring  13  are fitted, is formed at an inner circumference at a distal end side of the main shank body  11  of the tool shank  1 . The connecting recess portion  11   a  is constituted, from the distal end side, of a wide-mouth portion  15 , corresponding to the base  25  of the connecting shaft portion  2   b  of the cutter head  2 , a protruding step  16 , corresponding to the recessed step  26 , and an inner wide portion  17 , being of the same inner diameter as the wide mouth portion  15  and corresponding to the spigot protrusion  27  and the pipe stopping ring  13 , and in the inner wide portion  17 , threaded holes  17   a , are bored in radial directions at two locations, that is, front and rear locations and at a phase difference of 90 degrees. 
         [0063]    To connect the tool shank  1  and the cutter head  2 , the connecting shaft portion  2   b  of the cutter head  2  is fitted from the side into the connecting recess portion  11   a  of the main shank body  11  and fixed by a side lock method. That is, as shown in  FIG. 10A , whereas the inner diameter φ 1  of the inner wide portion  17  of the connecting recess portion  11   a  is substantially equal to the outer diameter φ 2  of the spigot protrusion  27  of the connecting shaft portion  2   b , a width d of the opening facing the cutting chip discharge groove  3  is wider than a minimum width w, passing through a center of the spigot protrusion  27 , and because the same relationship holds between the wide mouth portion  15  and the protruding step  16  of the connecting recess portion  11   a  and the base  25  and the recessed step  26  of the connecting shaft portion  2   b , the connecting shaft portion  2   b  is put in an orientation that differs by approximately 90 degrees from a proper orientation as illustrated and fitted from the side into the connecting recess portion  11   a , the connecting shaft portion  2   b  that has been fitted is then relatively rotated as shown in  FIG. 10B  so that the groove part  3   a  matches the cutting chip discharge groove  3 , and side locking screws  18  are then screwed into the threaded holes  17   a  of the main shank body  11  and distal end portions of the screws are fitted in and put in pressed contact with the latching recesses  27   a  of the connecting shaft portion  2   b  as shown in  FIG. 10C  to fix both components  1  and  2  in a manner disabling relative displacement. Such a connection method is disclosed in detail in Japanese Published Unexamined Patent Application No. 2005-118940 and Japanese Published Unexamined Patent Application No. 2006-234030 filed by the present applicant. 
         [0064]    Because the pipe stopping ring  13  is the same in outer shape and outer dimensions as the spigot protrusion  27  of the connecting shaft portion  2   b  of the cutter head  2  and has mortar-shaped latching recesses  13   a  disposed at two locations on an outer circumferential face as shown in  FIG. 11 , the pipe stopping ring  13  is fixed to the connecting recess portion  11   a  of the main shank body  11  at a position more to the inner side than the spigot protrusion  27  by a side lock method in the same manner as the spigot protrusion  27 . Thus the pipe stopping ring  13  has an internal thread  13   b , which is formed on an inner circumference across half an axial direction width and onto which an external thread  12   a , at a distal end side of the pipe member  12  that is fitted into the inner side of the main shank body  11 , is fixed by screwing as shown in  FIG. 12 , and has a sealing ring  19 , formed of a short, cylindrical elastic material, internally fitted in the remaining half of the inner circumference. The sealing ring  19  seals, in a liquid tight manner, contact surfaces of the pipe stopping ring  13  and the rear end of the connecting shaft portion  2   b  of the cutter head  2  and the portion at which the internal thread  13   b  of the pipe stopping ring  13  and the external thread  12   a  of the pipe member  12  are screwed together and thereby serves a function of preventing coolant of high pressure that passes through the interior of the coolant supply path from leaking to the exterior. 
         [0065]    In a drilling process using the gun drill of the above arrangement, cutting of a workpiece M is performed while supplying the coolant, supplied through the coolant supply paths  10  and  20  inside the tool shank  1  and the cutter head  2 , to the cut portion from the coolant delivery ports  21   a  and  21   b  at the head distal end face  2   c , and at the same time, cutting chips that are generated in accordance with the drilling are made to flow along with the coolant into the main and auxiliary discharge ports  22   a  and  22   b  and discharged to the exterior through the cutting chip discharge groove  3  on the circumferential face, and as shown in  FIGS. 2 and 3 , at the head distal end side, because a gap t is present between the outer circumferential face of the cutter head  2  and an inner circumferential face of a cut hole H at a position away from the main discharge port  22   a , a portion of the cutting chips flows along with the coolant into this gap t. However, because the gap t is substantially plugged by the annular protrusion portions  4   a  at the labyrinth seal portion  4  of the cutter head  2 , the coolant, containing the cutting chips, that had flowed into the gap T is also made to flow into the cutting chip discharge groove  3 , and because substantially all of the coolant supplied to the cut portion is thus concentrated into the cutting chip discharge groove  3  and all of the cutting chips are made to ride on this flow and be discharged powerfully rearward through the cutting chip discharge groove  3 , clogging by cutting chips does not occur at the periphery of the tool shank  1 , flawing of the inner circumference of the cut hole H and lowering of the processing precision due to clogging by cutting chips are prevented, and by the lightening of a rotational load, the cutting efficiency is improved and the life of gun drill itself is elongated. 
         [0066]    Although an example where the respective pairs of the coolant delivery ports  21   a  and  21   b  and discharge ports  22   a  and  22   b  are disposed at the distal end face  2   c  of the cutter head  2  was described with the embodiment above, the present invention is also applicable to a gun drill with which there is just one of either or both the coolant delivery port and discharge port. However, because conventionally with an arrangement having respective pairs of the coolant delivery ports and discharge ports as in the embodiment, clogging by cutting chips occurs readily even though a high cutting efficiency is obtained, the effect of application of the present invention is especially high with such an arrangement. 
         [0067]    The gun drill according to the present invention also includes an arrangement where a single annular protrusion portion  4   a  that bridges the cutting chip discharge groove  3  and is continuous along the entire circumference is disposed on the outer circumferential face of the cutter head  2 . However, by providing the labyrinth seal portion  4 , in which the plurality of annular protrusion portions  4   a  are disposed in parallel, as in the embodiment, the coolant entry prevention effect by the annular protrusion portions  4   a  is made high and a benefit that clogging by cutting chips is prevented more reliably is provided. 
         [0068]    Furthermore, although the present invention also includes an arrangement where the cutter head  2  is brazed to a distal end portion of the shank  1 , by making the cutter head  2  an independent member that is attachable/detachable with respect to the tool shank  1  as in the present embodiment, exchange with a new head upon wear or damage and exchange with different types of head according to cutting conditions are enabled, and forming and processing of the annular protrusion portions  4   a  can be performed readily and inexpensively according to each head. In the case where the cutter head  2  is made detachably exchangeable, various methods besides that employed in the embodiment may be applied as the method for connection to the tool shank  1 . 
         [0069]    Also, although the tool shank  1  may be arranged as a single member, by employing the structure where the coolant supply path  10  of the tool shank  1  is constituted of the pipe member  12  and the front end side of the pipe member  12  is attached by screwing to the main shank body  11  side, the benefit that processing and manufacturing the tool shank  1  can be performed readily and inexpensively is provided. Although in the embodiment, the front end portion of the pipe member  12  is fixed by screwing by the pipe stopping member  13 , a method, where an internal thread is provided in the main shank body  11  and the front end portion of the pipe member  12  is directly fixed by screwing to the main shank body  11  may be employed in place of using the pipe stopping member  13 . Besides this, with the present invention, blades may be formed integrally to the main head body  2   a  in place of using throwaway tips as the blades of the cutter head  2  as in the present embodiment, the number of blades may be set variously, and various other design changes besides those of the embodiment may be applied to other arrangement details.