Patent Publication Number: US-2011067927-A1

Title: Drill Head for Deep-Hole Drilling

Description:
RELATED APPLICATIONS 
     This is a continuation-in-part of international application no. PCT/JP2008/072627, filed 12 Dec. 2008, which published as WO 2009/128183A1 and claims priority to JP2008-104474, filed 14 Apr. 2008. The contents of the aforementioned applications are incorporated by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a drill head for deep-hole drilling in which a cutting blade tip is mounted to face a cutting chip discharge port opened on a head distal end surface and in which a hollow inside is made into a cutting chip discharge passage communicating with the cutting chip discharge port. 
     BACKGROUND 
     As this kind of drill head for deep-hole drilling, ones as shown in  FIG. 9  and  FIG. 10  have been widely used conventionally. A drill head  50 A in  FIG. 9  has a substantially cylindrical head main body  51  with a hollow portion  52  opened at a proximal end side and a head distal end surface  51   a  in a substantially obtuse conical shape provided with one large and one small cutting chip discharge ports  53 A,  53 B communicating with the hollow portion  52 . The head main body  51  has a head shaft center axis O which defines a forward-to-rear axial direction of the drill head. A center cutting blade  54 A and a circumferential cutting blade  54 B are formed on an opening side edge of the large cutting chip discharge port  53 A along a head radial direction, and an intermediate cutting blade  54 C is formed on an opening side edge of the small cutting chip discharge port  53 B along the same head radial direction by brazing cutting blade tips, respectively. Further, guide pads  55 A,  55 B made of cemented carbide material are brazed on an outer circumferential surface  51   b  between both discharge ports  53 A and  53 B. On the other hand, a drill head  50 B in  FIG. 10  has one cutting chip discharge port  53  on the head distal end surface  51   a  and a piece of cutting blade  54  is formed on the opening side edge along the head radial direction by screw-clamping a throwaway tip. However, other configurations are almost the same as those of the drill head  50 A. 
     These drill heads  50 A,  50 B are such that a proximal portion side provided with a male thread  56  on an outer circumference is threaded into and mounted on a distal end portion of a hollow boring bar whose illustration is omitted, and the boring bar is coupled to a driving shaft such as a spindle of a machine tool to be rotatingly-driven or rotate a work material side, thereby drilling the work material by cutting blades  54 A to  54 C and  54  to form a deep-hole. In addition, it is noted that a relative rotation direction of the drill heads  50 A,  50 B is a counterclockwise direction in  FIG. 9(A)  and  FIG. 10(A) . Thus, during this deep-hole drilling work, a coolant is supplied at high pressure into a cutting region through a gap between a cutting hole and the boring bar, made to flow into the hollow portion  52  together with cutting chips generated in the cutting region from the cutting chip discharge ports  53 A to  53 B and  53 , and discharged outside through the inside of the hollow boring bar. In this manner, whole drilling is carried out in these drill heads  50 A,  50 B, and accordingly an inner end of a cutting blade responsible for cutting a center portion of a cutting hole, that is, an inner end of the center cutting blade  54 A of the drill head  50 A and an inner end of the cutting blade  54  of the drill head  50 B is arranged in a position beyond a head shaft center axis O and enters into a radially opposite side. 
     On the other hand, since these drill heads  50 A,  50 B, carry out whole drilling so that an entire cutting hole is formed by drilling, a cutting blade responsible for cutting the center portion of the cutting hole, that is, the center cutting blade  54 A of the drill head  50 A and the cutting blade  54  of the drill head  50 B is required to be set precisely such that a blade edge  54   a  matches with the radial line R passing through the head shaft center axis O and also the inner end of the blade edge  54   a  is arranged slightly beyond the head shaft center axis O. Therefore, a cutting rate becomes zero theoretically at the head shaft center axis O during deep-hole drilling work, and thus, a blade point portion in the head shaft center axis O position does not exert a cutting force at what is called a chisel edge, which results in crushing a work material, and thrust resistance is loaded. As a result, this has been a factor that cutting efficiency cannot be enhanced. Further, in a configuration such as the drill head  50 A that the cutting blade tip is brazed to a recessed portion of a head main body side, a brazing material intervenes at a fitting portion of the both, so that high precision assembly has been difficult. 
     Consequently, as for a deep-hole drilling tool employing a screw-clamping type throwaway tip as a cutting blade, the present inventor has already proposed one configured such that a sinking portion is provided to a tip side surface at a cutting blade inner end side, and the cutting blade inner end is arranged spaced apart from the head shaft center axis O, thereby forming a non-cutting zone in the vicinity of the shaft center and breaking off an uncut core of a work material generated in the non-cutting zone by press-contacting with an inclined step of the sinking portion. See Japanese Published Unexamined Patent Application No. 2003-25129, and Japanese Published Unexamined Patent Application No. 2003-236713. 
     However, when a sinking portion is provided to the cutting blade side surface and the inner end of the cutting blade is arranged spaced apart from the head shaft center axis O as in the afore-proposed throwaway tip, there is a concern that the uncut core generated in the shaft center position easily grows linearly to a position of contacting with the inclined step, whereupon a break-off size is enlarged and cutting chip dischargeability is reduced. Moreover, the cutting blade itself disadvantageously involves a great deal of trouble in processing and its manufacturing costs become expensive due to forming of the sinking portion on the side surface and the inclined step. 
     SUMMARY OF THE INVENTION 
     The present invention was made in view of the foregoing circumstances, and accordingly it is an object of the present invention to provide a drill head for deep-hole drilling in which a cutting blade tip is mounted to face a cutting chip discharge port opened on a head distal end surface and a hollow inside is made into a cutting chip discharge passage communicating with the cutting chip discharge port, wherein high cutting efficiency based on good cutting chip dischargeability can be achieved and the cutting blade itself is also easily processed in terms of form. 
     Means for achieving the aforementioned object will be described with reference numerals of the accompanying drawings. A drill head for deep-hole drilling D 1 , D 2  according to a first aspect of the present invention including one or a plurality of cutting blade tips  2 A to  2 C,  3  mounted to face cutting chip discharge ports  11  to  13  opened on a head distal end surface  1   a , and a hollow inside made into a cutting chip discharge passage  14  communicating with the cutting chip discharge ports  11  to  13 . A cutting blade tip  2 A,  3  responsible for cutting the center side (“center cutting blade tip”) has an inner end  20   a ,  30   a  of a blade edge  20 ,  30  spaced apart from a head shaft center axis O and forming a non-cutting zone Z in the vicinity of the shaft center O. The cutting blade tip  2 A,  3  also has, on an inner end side of the blade edge  20 , an inner side surface constituting an inclined side surface  21 ,  31  inclined from the inner end of the blade edge to the head shaft center axis O side. An uncut core C of a work material W generated in the non-cutting zone Z is broken off by press-contact of the inclined side surface  21 ,  31  of the cutting blade tip  2 A,  3  during deep-hole drilling work. 
     A second aspect of the present invention is configured such that the cutting blade tip  2 A,  3  responsible for cutting the center side (“center cutting blade tip”) has the inclined side surface  21 ,  31  forming an angle of 75 to 90 degrees with respect to a tip front surface  22 ,  32  in the drill head for deep-hole drilling D 1 , D 2  of the first aspect. 
     A third aspect of the present invention is configured such that the center cutting blade tip  2 A,  3  has the inner end  20   a ,  30   a  of the blade edge  20 ,  30  spaced 0.05 to 0.5 mm apart from the head shaft center axis O and also an inclination angle θ of the inclined side surface  21 ,  31  with respect to the head shaft center axis O is 5 to 30 degrees in the drill head for deep-hole drilling D 1 , D 2  of the first aspect. 
     A fourth aspect of the present invention is configured such that the blade edge  20 ,  30  of the center cutting blade tip  2 A,  3  is arranged in parallel with a radial line R passing through the head shaft center axis O and in a center-raised position 0.2 to 1.5 mm more forward in a cutting rotation direction than the radial line R in the drill head for deep-hole drilling D 1 , D 2  of the first aspect. 
     A fifth aspect of the present invention is configured such that the cutting blade tips  2 A to  2 C are brazed to recessed portions  15   a  to  15   c  provided to a head main body  1  in the drill head for deep-hole drilling D 1  of the first aspect as described above. 
     A sixth aspect of the present invention is configured such that the whole of the blade edge  20 ,  30  of the center cutting blade tip  2 A,  3  is inclined in a radially inward and axially forward direction towards the head shaft center axis O side and a guide pad  6 A,  7 A is provided in a position on the opposite side of a mounting side of the cutting blade tip  2 A,  3  on a head outer circumferential surface  1   b  in the drill head for deep-hole drilling D 1 , D 2  of any one of the first to the fifth aspects as described above. 
     Effects of the present invention will be described with reference numerals of the drawings. First, in the drill head for deep-hole drilling D 1 , D 2  according to the first aspect of the present invention, the inner end  20   a ,  30   a  of the blade edge  20 ,  30  of the cutting blade tip  2 A,  3  is spaced apart from the head shaft center axis O. Consequently, a non-cutting zone Z is formed in the vicinity of the head shaft center axis O and an uncut core C of a work material W is generated in the non-cutting zone Z during deep-hole drilling work. However, at least the inner end side on the inner side surface of the cutting blade tip  2 A,  3  constitutes an inclined side surface  21 ,  31  inclined from the inner end of the blade edge to the head shaft center axis O side. Therefore, the generated uncut core C is forcibly pushed away laterally by press-contact of the inclined side surface  21 ,  31  and is broken off in such a manner as being twisted in line with rotation of the drill head D 1 , D 2 . Thus, the inclination of the inclined side surface  21 ,  31  increases the degree of lateral displacement from the head shaft center axis O as the uncut core C becomes longer. As a result, the uncut core C is efficiently fragmented little by little without growing long, whereupon excellent cutting chip dischargeability is secured, and high cutting efficiency is attained in cooperation with an elimination of the chisel edge in the shaft center position. On the other hand, as for the cutting blade tip  2 A,  3 , itself, at least the inner end side on the inner side surface thereof has only to be a simple axially rearwardly inclined surface, so that its manufacturing can be carried out easily and at a low cost. 
     According to the second aspect of the present invention, the inclined side surface  21 ,  31  of the center cutting blade tip  2 A,  3  forms an angle in a specific range with respect to the tip front surface  22 ,  32 . Consequently, lateral displacement of the uncut core C by press-contact of the inclined side surface  21 ,  31  becomes larger, and fragmentation performance of the uncut core C is increased accordingly, while strength of a blade point at the inner end  20   a ,  30   a  side of the blade edge  20 ,  30  is increased and the blade point resists chipping, whereupon durability of the cutting blade tip  2 A,  3  is improved. 
     According to the third aspect of the present invention, the inner end  20   a ,  30   a  of the blade edge  20 ,  30  of the center cutting blade tip  2 A,  3  is spaced apart in a specific range from the head shaft center axis O, and also an inclination angle θ of the inclined side surface  21 ,  31  with respect to the head shaft center axis O is in a specific range. Consequently, the uncut core C is reliably broken off little by little at an appropriate diameter. 
     According to the fourth aspect of the present invention, the blade edge  20 ,  30  of the center cutting blade tip  2 A,  3  is arranged in an appropriate center-raised position, so that the little by little break-off of the uncut core C is carried out more reliably. 
     According to the fifth aspect of the present invention, the cutting blade tips  2 A to  2 C are brazed to the recessed portions  15   a  to  15   c  provided to the head main body  1 , but the blade edge  20  of the center cutting blade tip  2 A does not need to be made in agreement with the radial line R passing through the head shaft center axis O. Additionally, the inner end  20   a  of the blade edge  20  has only to be spaced apart appropriately from the head shaft center axis O, too. Therefore, exact positional accuracy at the time of brazing the cutting blade tip  2 A is not required, and the manufacturing of the drill head D 1  is facilitated accordingly. 
     According to the sixth aspect of the present invention, the whole of the blade edge  20 ,  30  of the center cutting blade tip  2 A,  3  is inclined in a radially inward and axially forward direction towards the head shaft center axis O side, and also a guide pad  6 A,  7 A is provided in a position on the opposite side of the mounting side of the cutting blade tip  2 A,  3  on the head outer circumferential surface  1   b , so that the radial force of the cutting reaction force is directed to the opposite side of the cutting blade tip  2 A,  3  and acts in such a manner as pressing the guide pad  6 A,  7 A on the opposite side of the cutting blade tip  2 A,  3  against a cutting hole H inner circumference. Thus, drilling accuracy of the cutting hole H is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a drill head for deep-hole drilling according to a first embodiment of the present invention, and  FIG. 1(A)  is a front view and  FIG. 1(B)  is a side view; 
         FIG. 2  is a longitudinal sectional front view showing a state of deep-hole drilling work by the drill head for deep-hole drilling; 
         FIG. 3  is a longitudinal sectional side view showing a behavior of a cutting central portion in the deep-hole drilling work; 
         FIG. 4  is an explanatory diagram showing a correlation between a center cutting blade tip and an uncut core in the deep-hole drilling work; 
         FIG. 5  shows a drill head for deep-hole drilling according to a second embodiment of the present invention, and  FIG. 5(A)  is a front view and  FIG. 5(B)  is a side view; 
         FIG. 6  is a longitudinal sectional front view showing a state of deep-hole drilling work by the drill head for deep-hole drilling work; 
         FIG. 7  is a longitudinal sectional side view showing a behavior of a cutting central portion in the deep-hole drilling work; 
         FIG. 8  is an explanatory diagram showing a correlation between a center cutting blade tip and an uncut core in the deep-hole drilling work; 
         FIG. 9  shows a configuration example of a conventional drill head for deep-hole drilling, and  FIG. 9(A)  is a front view and  FIG. 9(B)  is a side view; and 
         FIG. 10  shows another configuration example of the conventional drill head for deep-hole drilling, and  FIG. 10(A)  is a front view and  FIG. 10(B)  is a side view. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               1 : Head main body 
               1   a : Distal end surface 
               1   b : Outer circumferential surface 
               11  to  13 : Cutting chip discharge port 
               14 : Hollow portion (cutting chip discharge passage) 
               15   a  to  15   h : Recessed portion 
               2 A: Center cutting blade tip (cutting blade tip responsible for cutting the center side) 
               20 : Blade edge 
               20   a : Inner end 
               21 : Inclined side surface 
               22 : Tip front surface 
               3 : Center cutting blade tip (cutting blade tip responsible for cutting the center side) 
               30 : Blade edge 
               30   a : Inner end 
               31 : Inclined side surface 
               32 : Tip front surface 
               6 A,  7 A: Guide pad 
             C: Uncut core 
             D 1 , D 2 : Drill head for deep-hole drilling 
             H: Cutting hole 
             O: Head shaft center axis 
             R: Radial line 
             W: Work material 
             Z: Non-cutting zone 
             θ: Inclination angle 
             f: Distance (center-raised amount) 
             s: Distance (eccentric distance) 
           
         
       
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of a drill head for deep-hole drilling according to the present invention will be described in detail with reference to the drawings.  FIG. 1  shows a front and a side of a drill head for deep-hole drilling D 1  of a first embodiment,  FIG. 2  shows a state of deep-hole drilling work by the drill head D 1 ,  FIG. 3  shows a behavior of a cutting central portion in the deep-hole drilling work,  FIG. 4  shows a correlation between a center cutting blade tip and an uncut core in the deep-hole drilling work,  FIG. 5  shows a front and a side of a drill head for deep-hole drilling D 2  of a second embodiment,  FIG. 6  shows a state of deep-hole drilling work by the drill head D 2 ,  FIG. 7  shows a behavior of a cutting central portion in the deep-hole drilling work,  FIG. 8  shows a correlation between a center cutting blade tip and an uncut core in the deep-hole drilling work, respectively. 
     As shown in  FIG. 1(A)  and  FIG. 1(B) , the drill head for deep-hole drilling D 1  of the first embodiment has a substantially cylindrical head main body  1  with a hollow portion  14  opened at a proximal end side and a substantially obtuse conical-shaped head distal end surface  1   a  formed with one large and one small substantially fan-shaped cutting chip discharge ports  11 ,  12 , arranged opposed to each other in a radial direction and communicating with the hollow portion  14 , a center cutting blade tip  2 A and a circumferential cutting blade tip  2 B on an opening side edge of the large cutting chip discharge port  11  along a head radial direction, and an intermediate cutting blade tip  2 C on an opening side edge of the small cutting discharge port  12  along the same radial direction brazed to recessed portions  15   a  to  15   c  provided to the head main body  1 , respectively. The head main body  1  has a head shaft center axis O which defines a forward-to-rear axial direction of the drill head. 
     Further, on a head outer circumferential surface  1   b  at the distal end side of the head main body  1 , recessed portions  15   d ,  15   e  are formed on the opposite side of the mounting side of the center cutting blade tip  2 A and circumferential cutting blade tip  2 B, that is, on the mounting side of the intermediate cutting blade tip  2 C and the rear side of the center cutting blade tip  2 A. Guide pads  6 A,  6 B of cemented carbide material are brazed to these recessed portions  15   d ,  15   e , and respective key ways  16  for twisting operation are formed in radially opposed positions nearer to the proximal end side than the mounting positions of these guide pads  6 A,  6 B. Further, a male thread  17  is formed on a head outer circumferential surface  1   c  at the proximal end side which is rendered smaller in outer diameter than the distal end side of the head main body  1 . This proximal end side is screwed into the distal end portion having a female thread of the hollow boring bar whose illustration is omitted, thereby coupling the drill head D 1  to the distal end of the boring bar. 
     The cutting blade tips  2 A to  2 C each has a blade edge  20  inclined in a radially inward and axially forward direction towards the head shaft center axis O side and a stepped chip breaker  2   a  along the blade edge  20  at the front surface side. Thus, the center cutting blade tip  2 A is arranged such that a radially inner end  20   a  of the blade edge  20  is spaced apart by a distance s from the head shaft center axis O in a position where the blade edge  20  is parallel to the radial line R passing through the head shaft center axis O and also a center thereof is raised by a distance f more forward in the cutting rotation direction than the radial line R. Further, the inner side surface of the center cutting blade tip  2 A constitutes an inclined side surface  21  in which the whole is axially rearwardly inclined from the inner end side to the head shaft center axis O side, and the surface orientation is set at a right angle to the tip front surface  22 . On the other hand, the circumferential cutting blade tip  2 B and the intermediate cutting blade tip  2 C are both arranged in such a manner that respective blade edges  20  match with the radial line R passing through the head shaft center axis O. 
     In deep-hole drilling work by the above-described drill head for deep-hole drilling D 1 , the coolant supplied through a gap between an inner circumference of a cutting hole H and an outer circumference of the hollow boring bar and drill head D 1  is sent into a cutting region continuously while the drill head D 1  coupled to the boring bar as already described above or a work material W is rotated, cutting chips generated in the cutting region are caught in the coolant, passed through the hollow portion  14  and a hollow inside of the boring bar from the cutting chip discharge ports  11 ,  12  of the drill head D 1 , and discharged outside. 
     As described above, the inner end  20   a  of the blade edge  20  of the center cutting blade tip  2 A is spaced apart from the head shaft center axis O in this drill head for deep-hole drilling D 1 , whereby a circular non-cutting zone Z whose radius is an eccentric distance s is formed in the vicinity of the shaft center O, as shown in  FIG. 2  and  FIG. 3 , and an uncut core C of a work material W is generated in this non-cutting zone Z. Therefore, since the inner side surface of the cutting blade tip  2 A constitutes an inclined side surface  21  axially rearwardly inclined from the inner end side to the head shaft center axis O side, as shown in  FIG. 3 , the uncut core C does not grow along the head shaft center axis O as shown by a virtual line, but is forcibly pushed away laterally by press-contact of the axially rearwardly inclined side surface  21  as shown by a solid line, and is broken off in such a manner as being twisted in line with rotation of the drill head. Thus, the axially rearward inclination of the inclined side surface  21  increases the degree of lateral displacement from the head shaft center axis O as the uncut core C becomes longer. Therefore, the uncut core C is efficiently fragmented little by little without growing long, whereupon excellent cutting chip dischargeability is secured, and high cutting efficiency is attained in cooperation with an elimination of the chisel edge in the shaft center position. 
     Furthermore, in this embodiment, the blade edge  20  of the center cutting blade tip  2 A is arranged in a center-raised position by a distance f more forward than the radial line R passing through the head shaft center axis O, and the inclined side surface  21  at the inner end side is perpendicular to the tip front surface  22 . Consequently, as shown in  FIG. 4 , a side surface continuing from the inner end  20   a  of the blade edge  20  to the rearward, that is, an upper end edge  21   a  on the inclined side surface  21  comes to enter into the non-cutting zone Z by the shaded portion U as shown since the shortest distance d with respect to the head shaft center axis O becomes shorter than the eccentric distance s of the inner end  20   a  of the blade edge  20 . Therefore, the uncut core C generated by the cutting by the blade edge  20  is pushed and cut from the side by as much as the shaded portion U just after the generation, and is constricted to a circle N having a smaller radius d than the non-cutting zone Z. As a result, the uncut core C is easily broken off further little by little. 
     It is noted that the inner end  20   a  of the blade edge  20  of the center cutting blade tip  2 A is shown as an acute angle for easy understanding of the description in  FIG. 4 , but is actually round-shaped as shown in  FIG. 3  in order to resist chipping, whereby the upper end edge  21   a  on the inclined side surface  21  continuing to the inner end  20   a  also constitutes a round shape throughout its length. And, even with such a round shape, it is the same that the uncut core C is pushed and cut from the side by as much as the shaded portion U just after the generation. 
     Further, in this embodiment, the whole of the blade edge  20  of the center cutting blade tip  2 A is inclined in a radially inward and axially forward direction towards the head shaft center axis O side, so that the radial force of the cutting reaction force is directed to the opposite side of the cutting blade tip  2 A side and acts in such a manner as pressing the guide pad  6 A on the opposite side of the cutting blade tip  2 A against a cutting hole H inner circumference. Thus, there is also an advantage of improving drilling accuracy of the cutting hole H. In contrast, the radial force is directed to the outer circumferential cutting blade  54 B side when the blade edge of the center cutting blade  54 A is inclined on both sides of the head shaft center axis O as in the conventional configuration of  FIG. 9(A)  and  FIG. 9(B) . As a result, the cutting hole H tends to be enlarged, which is subject to a reduction in drilling accuracy. 
     In the drill head D 1  of this embodiment, on the other hand, the cutting blade is composed of the cutting blade tips  2 A to  2 C brazed to the recessed portions  15   a  to  15   c  provided to the head main body  1 , but the blade edge  20  of the center cutting blade tip  2 A does not need to be made in agreement with the radial line R passing through the head shaft center axis O. Additionally, the inner end  20   a  of the blade edge  20  also has only to be spaced apart from the head shaft center axis O appropriately, so that exact positional accuracy is not required at the time of brazing the center cutting blade tip  2 A, which facilitates the manufacturing of the drill head D 1  accordingly. Further, as for the center cutting blade tip  2 A itself as well, the inner side surface thereof has only to be a simple inclined side surface  21 , so that its manufacturing can be carried out easily and at a low cost. 
     A drill head for deep-hole drilling D 2  of a second embodiment has, as shown in  FIG. 5(A)  and  FIG. 5(B) , a substantially cylindrical head main body  1  with a hollow portion  14  opened at a proximal end side and a substantially obtuse conical-shaped head distal end surface  1   a  formed with a substantially fan-shaped cutting chip discharge port  13  communicating with the hollow portion  14 , and a cutting blade tip  3  composed of a throwaway tip is screw-clamped on an opening side edge along the head radial direction. Additionally, on a head outer circumferential surface  1   b  at the distal end side of the head main body  1 , recessed portions  15   h ,  15   g  are formed on the opposite side of the mounting side of the cutting blade tip  3  and the rear side of the cutting blade tip  3  in the same manner as the afore-described first embodiment. Guide pads  7 A,  7 B of cemented carbide material are brazed to these recessed portions  15   h ,  15   g , and also respective flat portions  18  for twisting operation are formed in radially opposed positions nearer to the proximal end side than the mounting positions of these guide pads  7 A,  7 B. Further, a male thread  19  is formed on the head outer circumferential surface  1   c  at the proximal end side which is rendered smaller in outer diameter. 
     The cutting blade tip  3  has a blade edge  30  forming a three-step shape and inclined in a radially inward and axially forward direction towards the head shaft center axis O side. The cutting blade tip  3  has a stepped chip breaker  3   a  at the front surface side along an inclination direction of the blade edge  30 . The cutting blade tip  3  is arranged such that a radially inner end  30   a  of the blade edge  30  is spaced apart by a distance s from the head shaft center axis O in a position where the blade edge  30  is parallel to the radial line R passing through the head shaft center axis O and also a center thereof is raised by a distance f more forward in the cutting rotation direction than the radial line R. Further, a portion at an inner end side of the inner side surface of the cutting blade tip  3  constitutes an inclined side surface  31  axially rearwardly inclined from the inner end side to the head shaft center axis O side, and the surface orientation is set at a right angle to the tip front surface  32 . 
     In deep-hole drilling work by the above-described drill head for deep-hole drilling D 2 , the drill head D 2  is coupled to the distal end of the boring bar by the male thread  19  in the same manner as the afore-described first embodiment, the coolant supplied through a gap between an inner circumference of a cutting hole H and an outer circumference of the hollow boring bar and drill head D 2  is sent into a cutting region continuously while the drill head D 2  or a work material W is rotated, cutting chips are caught in the coolant, passed through the hollow portion  14  and a hollow inside of the boring bar from the cutting chip discharge port  13  of the drill head D 2 , and discharged outside. 
     In the same manner as the afore-described first embodiment, the inner end  30   a  of the blade edge  30  of the cutting blade tip  3  is spaced apart from the head shaft center axis O in this drill head for deep-hole drilling D 2  of the second embodiment as well, whereby a circular non-cutting zone Z whose radius is an eccentric distance s is formed in the vicinity of the shaft center O, as shown in  FIG. 6  and  FIG. 7 , and an uncut core C of a work material W is generated in this non-cutting zone Z. Therefore, since the inner end side on the inner side surface of the cutting blade tip  3  constitutes the afore-mentioned inclined side surface  31 , as shown in  FIG. 7 , the uncut core C does not grow along the head shaft center axis O as shown by a virtual line, either, but is forcibly pushed away laterally by press-contact of the inclined side surface  31  as shown by a solid line, and is broken off in such a manner as being twisted in line with rotation of the drill head. Then, the uncut core C increases the degree of lateral displacement from the head shaft center axis O as it becomes longer. Therefore, the uncut core C is efficiently fragmented little by little without growing long, whereupon excellent cutting chip dischargeability is secured, and high cutting efficiency is attained in cooperation with an elimination of the chisel edge in the shaft center position. 
     Furthermore, in this embodiment as well, the blade edge  30  of the cutting blade tip  3  is arranged in a center-raised position by a distance f more than the radial line R passing through the head shaft center axis O, and the inclined side surface  31  at the inner end side is perpendicular to the tip front surface  32 . Consequently, as shown in  FIG. 8 , a side surface continuing from the inner end  30   a  of the blade edge  30  to the rearward, that is, an upper end edge  31   a  on the inclined side surface  31  comes to enter into the non-cutting zone Z by the shaded portion U as shown since the shortest distance d with respect to the head shaft center axis O becomes shorter than the eccentric distance s of the inner end  30   a  of the blade edge  30 . Therefore, the uncut core C generated by the cutting by the blade edge  30  is pushed and cut from the side by as much as the shaded portion U just after the generation, and is constricted to a circle N having a smaller radius d than the non-cutting zone Z. As a result, the uncut core C is easily broken off further little by little. Further, the whole of the blade edge  30  of the cutting blade tip  3  is inclined in a radially inward and axially forward direction towards the head shaft center axis O side, so that the radial force of the cutting reaction force is directed to the opposite side of the cutting blade tip  3  side and acts in such a manner as pressing the guide pad  7 A on the opposite side of the cutting blade tip  3  against a cutting hole H inner circumference. Thus, drilling accuracy of the cutting hole H is improved. 
     An inclination angle θ of the inclined side surface  21 ,  31  at the inner end side of the center cutting blade tip  2 A,  3  with respect to the head shaft center axis O is preferably in the range of 5 to 30 degrees in the drill head for deep-hole drilling of the present invention. If the inclination angle θ is too small or too large, break-off performance (fragmentation performance) of the uncut core C becomes low. Further, the eccentric distance s of the inner end  20   a ,  30   a  of the blade edge  20 ,  30  of the center cutting blade tip from the head shaft center axis O is preferably in the range of 0.05 to 0.5 mm. If too short, positioning of the cutting blade tip becomes difficult, whereas if too long, the uncut core C becomes thick, which requires a large force for break-off and leads to a reduction in cutting efficiency. Furthermore, a center-raised amount of the center cutting blade tip  2 A,  3 , in other words, a distance f of the blade edge  20 ,  30  with respect to the radial line R passing through the head shaft center axis O is preferably in the range of 0.2 to 1.5 mm for the purpose of further ensuring the little by little break-off of the uncut core C. 
     It is noted that the inclined side surface at the inner end side of the center cutting blade tip is perpendicular to the tip front surface in the afore-described embodiments. However, the inclined side surface has only to be in the angle range of 75 to 90 degrees with respect to the tip front surface in the present invention. In this angle range, the lateral displacement of the uncut core C by press-contact of the inclined side surface becomes sufficiently large and fragmentation performance of the uncut core is increased accordingly, while strength of a blade point at the inner end side of the blade edge of the cutting blade tip is increased, so that the blade point resists chipping, whereupon durability of the cutting blade is improved. Moreover, detailed configurations such as the shape of the cutting blade tip, the shape of the guide pad, etc., can be modified in various ways except for the embodiments in the present invention.