Patent Publication Number: US-2023150039-A1

Title: Drill

Description:
TECHNICAL FIELD 
     The present disclosure relates to a drill. 
     BACKGROUND ART 
     Japanese Patent Laying-Open No. 2019-181615 (PTL 1) describes a drill having an arc-shaped thinning cutting blade. 
     Citation List 
     Patent Literature 
     PTL 1: Japanese Patent Laying-Open No. 2019-181615 
     SUMMARY OF INVENTION 
     A drill according to the present disclosure rotates around a central axis, and includes a rake face, a flank face, and a thinning face. The flank face is contiguous to the rake face. The thinning face is contiguous to each of the rake face and the flank face. A ridgeline between the rake face and the flank face includes an outer cutting edge. A ridgeline between the thinning face and the flank face includes a first thinning portion contiguous to the outer cutting edge, and a second thinning portion contiguous to the first thinning portion. When viewed in a direction along the central axis, the first thinning portion is linear and the second thinning portion is curved. When viewed in the direction along the central axis, the first thinning portion, and a boundary between the first thinning portion and the second thinning portion are located rearward in a rotational direction relative to a first straight line, and are located forward in the rotational direction relative to a second straight line, the first straight line being a straight line passing through the central axis and parallel to the first thinning portion, and the second straight line being a straight line perpendicular to the first straight line. 
     A drill according to the present disclosure rotates around a central axis, and includes a rake face, a flank face, and a thinning face. The flank face is contiguous to the rake face. The thinning face is contiguous to each of the rake face and the flank face. A ridgeline between the rake face and the flank face includes an outer cutting edge. A ridgeline between the thinning face and the flank face includes a first thinning portion contiguous to the outer cutting edge, and a second thinning portion contiguous to the first thinning portion. When viewed in a direction along the central axis, the first thinning portion is linear and the second thinning portion is curved. When viewed in the direction along the central axis, the first thinning portion, and a boundary between the first thinning portion and the second thinning portion are located rearward in a rotational direction relative to a first straight line, and are located forward in the rotational direction relative to a second straight line, the first straight line being a straight line passing through the central axis and parallel to the first thinning portion, and the second straight line being a straight line perpendicular to the first straight line. When viewed in the direction along the central axis, the second thinning portion has a radius of curvature which is more than or equal to 5% and less than or equal to 30% of twice a distance between an outer end portion of the outer cutting edge and the central axis. When viewed in the direction along the central axis, a distance between the first thinning portion and the first straight line is more than or equal to 0.001 mm and less than or equal to 1.0 mm. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic plan view showing a configuration of a drill according to a first embodiment. 
         FIG.  2    is a schematic perspective view showing a configuration around a front end of the drill according to the first embodiment. 
         FIG.  3    is a schematic front view showing the configuration of the drill according to the first embodiment. 
         FIG.  4    is an enlarged schematic front view of a region IV in  FIG.  3   . 
         FIG.  5    is an enlarged schematic plan view of a region V in  FIG.  1   . 
         FIG.  6    is a schematic front view showing a configuration of a drill according to a second embodiment. 
         FIG.  7    is an enlarged schematic plan view showing the configuration of the drill according to the second embodiment. 
         FIG.  8    is a schematic front view showing a configuration of a drill according to a third embodiment. 
         FIG.  9    is an enlarged schematic plan view showing the configuration of the drill according to the third embodiment. 
         FIG.  10    is a schematic front view showing a configuration of a drill according to a fourth embodiment. 
         FIG.  11    is an enlarged schematic plan view showing the configuration of the drill according to the fourth embodiment. 
         FIG.  12    is a schematic front view showing a configuration of a drill according to a comparative example. 
         FIG.  13    is a schematic diagram of rotational projection for comparing the configurations of the drill according to the first embodiment and the drill according to the comparative example. 
         FIG.  14    is a diagram showing hole positional tolerances of a drill of a sample 1 and a drill of a sample 2. 
         FIG.  15    is a diagram showing cutting resistances of the drill of sample 1 and the drill of sample 2. 
     
    
    
     DETAILED DESCRIPTION 
     Problem to Be Solved by the Present Disclosure 
     An object of the present disclosure is to provide a drill capable of improving a hole positional tolerance. 
     Advantageous Effect of the Present Disclosure 
     According to the present disclosure, a drill capable of improving a hole positional tolerance can be provided. 
     Description of Embodiments of the Present Disclosure 
     First, embodiments of the present disclosure will be listed and described. 
     ( 1 ) A drill  100  according to the present disclosure rotates around a central axis X, and includes a rake face  1 , a flank face  2 , and a thinning face  3 . Flank face  2  is contiguous to rake face  1 . Thinning face  3  is contiguous to each of rake face  1  and flank face  2 . A ridgeline between rake face  1  and flank face  2  includes an outer cutting edge  40 . A ridgeline between thinning face  3  and flank face  2  includes a first thinning portion  31  contiguous to outer cutting edge  40 , and a second thinning portion  32  contiguous to first thinning portion  31 . When viewed in a direction along central axis X, first thinning portion  31  is linear and second thinning portion  32  is curved. When viewed in the direction along central axis X, first thinning portion  31 , and a boundary between first thinning portion  31  and second thinning portion  32  are located rearward in a rotational direction relative to a first straight line A 1 , and are located forward in the rotational direction relative to a second straight line A 2 , first straight line A 1  being a straight line passing through central axis X and parallel to first thinning portion  31 , and second straight line A 2  being a straight line perpendicular to first straight line A 1 . 
     ( 2 ) In drill  100  according to ( 1 ) above, when viewed in the direction along central axis X, second thinning portion  32  may have a radius of curvature which is more than or equal to 5% and less than or equal to 30% of twice a distance between an outer end portion  43  of outer cutting edge  40  and central axis X. 
     ( 3 ) In drill  100  according to ( 1 ) or ( 2 ) above, drill  100  may further include an outer circumferential surface  5  contiguous to each of rake face  1  and flank face  2 , and a heel face  4  contiguous to each of outer circumferential surface  5 , flank face  2  and thinning face  3 . Outer circumferential surface  5  maybe provided with a first margin  51  contiguous to outer cutting edge  40 , and a second margin  52  spaced rearward in the rotational direction from first margin  51 . When viewed in the direction along central axis X, a boundary line  6  between heel face  4  and flank face  2  may be linear. 
     ( 4 ) In drill  100  according to ( 3 ) above, a coolant hole  7  may be provided at a position spaced from boundary line  6 . 
     ( 5 ) In drill  100  according to ( 3 ) above, a coolant hole 7  may be provided so as to divide boundary line  6 . 
     ( 6 ) In drill  100  according to ( 1 ) or ( 2 ) above, drill  100  may further include an outer circumferential surface  5  contiguous to each of rake face  1  and flank face  2 , and a heel face  4  contiguous to each of outer circumferential surface  5 , flank face  2  and thinning face  3 . Outer circumferential surface  5  may be provided with a first margin  51  contiguous to outer cutting edge  40 , and a second margin  52  spaced rearward in the rotational direction from first margin  51 . When viewed in the direction along central axis X, a boundary line  6  between heel face  4  and flank face  2  may be curved. 
     ( 7 ) In drill  100  according to ( 6 ) above, a coolant hole  7  may be provided at a position spaced from boundary line  6 . 
     ( 8 ) In drill  100  according to ( 6 ) above, a coolant hole  7  may be provided so as to divide boundary line  6 . 
     ( 9 ) In drill  100  according to any one of ( 1 ) to ( 8 ) above, when viewed in the direction along central axis X, a distance between first thinning portion  31  and first straight line A 1  may be more than or equal to 0.001 mm and less than or equal to 1.0 mm. 
     ( 10 ) A drill  100  according to the present disclosure rotates around a central axis X, and includes a rake face  1 , a flank face  2 , and a thinning face  3 . Flank face  2  is contiguous to rake face  1 . Thinning face  3  is contiguous to each of rake face  1  and flank face  2 . A ridgeline between rake face  1  and flank face  2  includes an outer cutting edge  40 . A ridgeline between thinning face  3  and flank face  2  includes a first thinning portion  31  contiguous to outer cutting edge  40 , and a second thinning portion  32  contiguous to first thinning portion  31 . When viewed in a direction along central axis X, first thinning portion  31  is linear and second thinning portion  32  is curved. When viewed in the direction along central axis X, first thinning portion  31 , and a boundary between first thinning portion  31  and second thinning portion  32  are located rearward in a rotational direction relative to a first straight line A 1 , and are located forward in the rotational direction relative to a second straight line A 2 , first straight line A 1  being a straight line passing through central axis X and parallel to first thinning portion  31 , and second straight line A 2  being a straight line perpendicular to first straight line A 1 . When viewed in the direction along central axis X, second thinning portion  32  has a radius of curvature which is more than or equal to 5% and less than or equal to 30% of twice a distance between an outer end portion  43  of outer cutting edge  40  and central axis X. When viewed in the direction along central axis X, a distance between first thinning portion  31  and first straight line A 1  is more than or equal to 0.001 mm and less than or equal to 1.0 mm. 
     Details of Embodiments of the Present Disclosure 
     Next, the details of the embodiments of the present disclosure will be described with reference to the drawings. The same or corresponding parts are designated by the same reference symbols in the following drawings, and a description thereof will not be repeated herein. 
     First Embodiment 
     First, a configuration of a drill  100  according to a first embodiment of the present disclosure is described. 
       FIG.  1    is a schematic plan view showing the configuration of drill  100  according to the first embodiment. As shown in  FIG.  1   , drill  100  according to the first embodiment includes a front end  101 , a rear end  102 , a rake face  1 , a flank face  2 , an outer circumferential surface  5 , a flute  14 , and a shank  13 . Drill  100  is rotatable around a central axis X. Front end  101  of drill  100  is a portion that faces a workpiece. Rear end  102  of drill  100  is a portion that faces a tool that rotates drill  100 . Shank  13  is a portion that is attached to the tool that rotates drill  100 . In a direction along central axis X, rear end  102  is located on the opposite side to front end  101 . Central axis X passes through front end  101  and rear end  102 . The direction along central axis X is an axial direction. A direction perpendicular to the axial direction is a radial direction. As used herein, a direction from front end  101  toward rear end  102  is referred to as rearward in the axial direction. Conversely, a direction from rear end  102  toward front end  101  is referred to as forward in the axial direction. 
       FIG.  2    is a schematic perspective view showing a configuration around front end  101  of drill  100  according to the first embodiment. As shown in  FIG.  2   , flank face  2  is contiguous to rake face  1 . A ridgeline between rake face  1  and flank face  2  includes an outer cutting edge  40 . A thinning face  3  is contiguous to each of rake face  1  and flank face  2 . In the radial direction, thinning face  3  is located on the central axis X side relative to rake face  1 . Thinning face  3  is contiguous to a chisel  15 . Rake face  1  includes a first rake face portion  11  and a second rake face portion  12 . First rake face portion  11  is contiguous to second rake face portion  12 . In the radial direction, first rake face portion  11  is located on the central axis X side relative to second rake face portion  12 . Outer circumferential surface  5  is contiguous to each of rake face  1  and flank face  2 . Second rake face portion  12  is a return face, for example. Second rake face portion  12  may be tilted rearward in a rotational direction relative to first rake face portion  11 . 
     A ridgeline between thinning face  3  and flank face  2  includes a first thinning portion  31  and a second thinning portion  32 . First thinning portion  31  functions as a cutting edge. First thinning portion  31  is contiguous to outer cutting edge  40 . In the radial direction, first thinning portion  31  is located on the central axis X side relative to outer cutting edge  40 . Second thinning portion  32  is contiguous to first thinning portion  31 . In the radial direction, second thinning portion  32  is located on the central axis X side relative to first thinning portion  31 . First thinning portion  31  is located between second thinning portion  32  and outer cutting edge  40 . At least a portion of second thinning portion  32  may function as a cutting edge. 
       FIG.  3    is a schematic front view showing the configuration of drill  100  according to the first embodiment.  FIG.  3    shows the front of drill  100 , when viewed along the direction from front end  101  toward rear end  102 . As shown in  FIG.  3   , a pair of outer cutting edge  40 , first thinning portion  31  and second thinning portion  32  are provided at positions rotationally symmetric with respect to central axis X. From a different viewpoint, drill  100  according to the first embodiment has two cutting edges. Outer cutting edge  40  includes a first cutting edge  41 , a second cutting edge  42 , and an outer end portion  43 . First cutting edge  41  is contiguous to first thinning portion  31 . Second cutting edge  42  is contiguous to first cutting edge  41 . Second cutting edge  42  is located closer to the outer circumference than first cutting edge  41 . Second cutting edge  42  forms outer end portion  43 . Outer end portion  43  is located closest to the outer circumference in outer cutting edge  40 . First cutting edge  41  is located between first thinning portion  31  and second cutting edge  42 . 
     As shown in  FIG.  3   , when seen in front view, first cutting edge  41  may be arc-shaped. First cutting edge  41  may be recessed rearward in the rotational direction. When seen in front view, second cutting edge  42  may be linear or arc-shaped. Flank face  2  may include a first flank face portion  21  and a second flank face portion  22 . First flank face portion  21  is contiguous to each of outer cutting edge  40 , first thinning portion  31  and second thinning portion  32 . Second flank face portion  22  is contiguous to first flank face portion  21 . Second flank face portion  22  is located rearward in the rotational direction relative to first flank face portion  21 . Second flank face portion  22  may be tilted relative to first flank face portion  21 . 
     As shown in  FIGS.  2  and  3   , drill  100  may include a heel face  4 . Heel face  4  is contiguous to each of flank face  2  and thinning face  3 , for example. Heel face  4  may be contiguous to outer circumferential surface  5 . Heel face  4  is located rearward in the rotational direction relative to flank face  2 . Heel face  4  is contiguous to second flank face portion  22 . In the rotational direction of drill  100 , second flank face portion  22  is located between first flank face portion  21  and heel face  4 . 
     As shown in  FIG.  3   , outer circumferential surface  5  may be provided with a first margin  51  and a second margin  52 . First margin  51  is contiguous to outer cutting edge  40 . Second margin  52  is spaced from first margin  51 . Second margin  52  is located rearward in the rotational direction relative to first margin  51 . First margin  51  is contiguous to first flank face portion  21 , for example. Second margin  52  is contiguous to second flank face portion  22 , for example. Second margin  52  may be contiguous to heel face  4 . 
     As shown in  FIG.  3   , when viewed in the direction along central axis X, a boundary line  6  between heel face  4  and flank face  2  may be curved. Drill  100  may be provided with a coolant hole  7 . Coolant hole  7  may be provided so as to divide boundary line  6  between flank face  2  and heel face  4 . Boundary line  6  includes a first heel portion  61  and a second heel portion  62 . First heel portion  61  is contiguous to second thinning portion  32 . Second heel portion  62  is contiguous to outer circumferential surface  5 . Second heel portion  62  is spaced from first heel portion  61 . Coolant hole  7  is provided between first heel portion  61  and second heel portion  62 . 
       FIG.  4    is an enlarged schematic front view of a region IV in  FIG.  3   . As shown in  FIG.  4   , when viewed in the direction along central axis X, first thinning portion  31  is linear and second thinning portion  32  is curved. In the radial direction, second thinning portion  32  is curved so as to be recessed toward central axis X. Second thinning portion  32  is arc-shaped, for example. Second thinning portion  32  is contiguous to chisel  15 . Second thinning portion  32  may have a radius of curvature R which is more than or equal to 5% and less than or equal to 30% of twice a distance between outer end portion  43  of outer cutting edge  40  and central axis X. The twice the distance between outer end portion  43  of outer cutting edge  40  and central axis X corresponds to the diameter of a cutting edge of drill  100 . A lower limit of radius of curvature R of second thinning portion  32  is not particularly limited, and may be, for example, more than or equal to 10%, or more than or equal to 15% of twice the distance between outer end portion  43  of outer cutting edge  40  and central axis X. An upper limit of radius of curvature R of second thinning portion  32  is not particularly limited, and may be, for example, less than or equal to 25%, or less than or equal to 20% of twice the distance between outer end portion  43  of outer cutting edge  40  and central axis X. 
     As shown in  FIG.  4   , when viewed in the direction along central axis X, a straight line passing through central axis X and parallel to first thinning portion  31  is a first straight line A 1 , and a straight line perpendicular to first straight line A 1  is a second straight line A 2 . When viewed in the direction along central axis X, first thinning portion  31  is located rearward in the rotational direction relative to first straight line A 1 , and is located forward in the rotational direction relative to second straight line A 2 . When viewed in the direction along central axis X, a boundary (first boundary  33 ) between first thinning portion  31  and second thinning portion  32  is located rearward in the rotational direction relative to first straight line A 1 , and is located forward in the rotational direction relative to second straight line A 2 . 
     In  FIG.  4   , a region above first straight line A 1  and to the right of second straight line A 2  is a first region S 1 . A region below first straight line A 1  and to the right of second straight line A 2  is a second region S 2 . A region above first straight line A 1  and to the left of second straight line A 2  is a third region S 3 . A region below first straight line A 1  and to the left of second straight line A 2  is a fourth region S 4 . First thinning portion  31  is located in second region S 2 . The boundary (first boundary  33 ) between first thinning portion  31  and second thinning portion  32  is located in second region S 2 . A boundary (second boundary  34 ) between boundary line  6  between flank face  2  and heel face  4  and second thinning portion  32  is located in third region S 3 . 
     As shown in  FIG.  4   , when viewed in the direction along central axis X, second thinning portion  32  may intersect each of first straight line A 1  and second straight line A 2 . Specifically, second thinning portion  32  may intersect a boundary line between first region S 1  and second region S 2 , and a boundary line between first region S 1  and third region S 3 . Second thinning portion  32  is located in first region S 1 , second region S 2  and third region S 3 . From a different viewpoint, second thinning portion  32  includes a second portion  72  located in first region S 1 , a first portion  71  located in second region S 2 , and a third portion  73  located in third region S 3 . 
     First portion  71  is contiguous to each of first thinning portion  31  and second portion  72 . First portion  71  is located between first thinning portion  31  and second portion  72 . Second portion  72  is contiguous to each of first portion  71  and third portion  73 . Second portion  72  is located between first portion  71  and third portion  73 . Third portion  73  is contiguous to each of second portion  72  and boundary line  6 . Third portion  73  is located between second portion  72  and boundary line  6 . Second portion  72  is contiguous to chisel  15 . 
     As shown in  FIG.  4   , when viewed in the direction along central axis X, a distance D between first thinning portion  31  and first straight line A 1  is, for example, more than or equal to 0.001 mm and less than or equal to 1.0 mm. A lower limit of distance D between first thinning portion  31  and first straight line A 1  is not particularly limited, and may be, for example, more than or equal to 0.15 mm, or more than or equal to 0.2 mm. An upper limit of distance D between first thinning portion  31  and first straight line A 1  is not particularly limited, and may be, for example, less than or equal to 0.95 mm, or less than or equal to 0.9 mm. Distance D between first thinning portion  31  and first straight line A 1  is a distance between first thinning portion  31  and first straight line A 1  in a direction perpendicular to first straight line A 1 . 
       FIG.  5    is an enlarged schematic plan view of a region V in  FIG.  1   . As shown in  FIG.  5   , a portion of coolant hole  7  opens to second flank face portion  22 , and the rest of coolant hole  7  opens to heel face  4 . From a different viewpoint, coolant hole  7  is provided to extend from second flank face portion  22  to heel face  4 . As shown in  FIG.  5   , when viewed in a direction perpendicular to central axis X, outer cutting edge  40  is located on the rear end  102  side relative to first thinning portion  31 . When viewed in the direction perpendicular to central axis X, second cutting edge  42  is located on the rear end  102  side relative to first cutting edge  41 . When viewed in the direction perpendicular to central axis X, boundary line  6  is located on the rear end  102  side relative to second thinning portion  32 . 
     Second Embodiment 
     Next, a configuration of drill  100  according to a second embodiment of the present disclosure is described. Drill  100  according to the second embodiment is different from drill  100  according to the first embodiment in that coolant hole  7  is provided at a position spaced from boundary line  6 , and is otherwise similar in configuration to drill  100  according to the first embodiment. A configuration different from that of drill  100  according to the first embodiment is mainly described below. 
       FIG.  6    is a schematic front view showing the configuration of drill  100  according to the second embodiment. The schematic front view shown in  FIG.  6    corresponds to  FIG.  3    of drill  100  according to the first embodiment.  FIG.  7    is an enlarged schematic plan view showing the configuration of drill  100  according to the second embodiment. The enlarged schematic plan view shown in  FIG.  7    corresponds to  FIG.  5    of drill  100  according to the first embodiment. 
     As shown in  FIGS.  6  and  7   , coolant hole  7  is at a position spaced from boundary line  6 . Specifically, coolant hole  7  is provided in flank face  2 . Coolant hole  7  may be provided in second flank face portion  22 , or in first flank face portion  21 , or in a boundary between first flank face portion  21  and second flank face portion  22 . Coolant hole  7  is spaced from heel face  4 . As shown in  FIG.  6   , when viewed in the direction along central axis X, coolant hole  7  may have a substantially elliptical shape. As shown in  FIG.  7   , coolant hole  7  opens only to flank face  2 , and does not open to heel face  4 . 
     Third Embodiment 
     Next, a configuration of drill  100  according to a third embodiment of the present disclosure is described. Drill  100  according to the third embodiment is different from drill  100  according to the first embodiment in that boundary line  6  between heel face  4  and flank face  2  is linear, and is otherwise similar in configuration to drill  100  according to the first embodiment. A configuration different from that of drill  100  according to the first embodiment is mainly described below. 
       FIG.  8    is a schematic front view showing the configuration of drill  100  according to the third embodiment. The schematic front view shown in  FIG.  8    corresponds to  FIG.  3    of drill  100  according to the first embodiment.  FIG.  9    is an enlarged schematic plan view showing the configuration of drill  100  according to the third embodiment. The enlarged schematic plan view shown in  FIG.  9    corresponds to  FIG.  5    of drill  100  according to the first embodiment. 
     As shown in  FIG.  8   , when viewed in the direction along central axis X, boundary line  6  between heel face  4  and flank face  2  may be linear. As shown in  FIG.  9   , coolant hole  7  is provided to extend from second flank face portion  22  to heel face  4 . Coolant hole  7  divides boundary line  6 . Boundary line  6  includes first heel portion  61  and second heel portion  62 . As shown in  FIG.  8   , when viewed in the direction along central axis X, first heel portion  61  and second heel portion  62  are each linear. When viewed in the direction along central axis X, second heel portion  62  may have a longer length than first heel portion  61 . 
     Fourth Embodiment 
     Next, a configuration of drill  100  according to a fourth embodiment of the present disclosure is described. Drill  100  according to the fourth embodiment is different from drill  100  according to the first embodiment in that boundary line  6  between heel face  4  and flank face  2  is linear, and that coolant hole  7  is provided at a position spaced from boundary line  6 , and is otherwise similar in configuration to drill  100  according to the first embodiment. A configuration different from that of drill  100  according to the first embodiment is mainly described below. 
       FIG.  10    is a schematic front view showing the configuration of drill  100  according to the fourth embodiment. The schematic front view shown in  FIG.  10    corresponds to  FIG.  3    of drill  100  according to the first embodiment.  FIG.  11    is an enlarged schematic plan view showing the configuration of drill  100  according to the fourth embodiment. The enlarged schematic plan view shown in  FIG.  11    corresponds to  FIG.  5    of drill  100  according to the first embodiment. 
     As shown in  FIG.  10   , when viewed in the direction along central axis X, boundary line  6  between heel face  4  and flank face  2  may be linear. Coolant hole  7  is provided at a position spaced from boundary line  6 . Specifically, coolant hole  7  is provided in flank face  2 . Coolant hole  7  may be provided in second flank face portion  22 , or in first flank face portion  21 , or in the boundary between first flank face portion  21  and second flank face portion  22 . Coolant hole  7  is spaced from heel face  4 . As shown in  FIG.  10   , when viewed in the direction along central axis X, coolant hole  7  may have a substantially elliptical shape. As shown in  FIG.  11   , coolant hole  7  opens only to flank face  2 , and does not open to heel face  4 . 
     Next, functions and advantageous effects of drill  100  according to the above embodiments are described. 
       FIG.  12    is a schematic front view showing a configuration of a drill  200  according to a comparative example. As shown in  FIG.  12   , drill  200  according to the comparative example includes first thinning portion  31  and second thinning portion  32 . When viewed in the direction along central axis X, a straight line passing through central axis X and parallel to first thinning portion  31  is first straight line A 1 , and a straight line perpendicular to first straight line A 1  is second straight line A 2 . As shown in  FIG.  12   , when viewed in the direction along central axis X, first thinning portion  31  is located forward in the rotational direction relative to first straight line A 1 . 
     In drill  100  according to the above embodiments, on the other hand, when viewed in the direction along central axis X, first thinning portion  31 , and the boundary between first thinning portion  31  and second thinning portion  32  are located rearward in the rotational direction relative to first straight line A 1 , where first straight line A 1  is a straight line passing through central axis X and parallel to first thinning portion  31 , and second straight line A 2  is a straight line perpendicular to first straight line A 1  (see  FIG.  4   ). 
       FIG.  13    is a schematic diagram of rotational projection for comparing the configurations of drill  100  according to the first embodiment and drill  200  according to the comparative example. In  FIG.  13   , drill  100  according to the first embodiment is shown by solid lines, and drill  200  according to the comparative example is shown by dashed lines. As shown in  FIG.  13   , the width around chisel  15  in drill  100  according to the first embodiment is smaller than the width around chisel  15  in drill  200  according to the comparative example. As compared to drill  200  according to the comparative example, drill  100  according to the first embodiment can allow a front end portion of drill  100  to be pointed. As compared to drill  200  according to the comparative example, therefore, drill  100  according to the first embodiment can have an improved function of biting into a workpiece in drilling. As a result, hole positional tolerance can be improved. 
     In addition, when first thinning portion  31  and outer cutting edge  40  are each curved so as to be recessed rearward in the rotational direction, a portion around a boundary between first thinning portion  31  and outer cutting edge  40  is pointed. In this case, swarf that has been cut at first thinning portion  31  and swarf that has been cut at outer cutting edge  40  are divided and separately generated. As a result, the resulting swarf increases in length, causing degradation in swarf discharging performance. 
     In drill  100  according to the above embodiments, on the other hand, when viewed in the direction along central axis X, first thinning portion  31  is linear. As compared to the example where first thinning portion  31  and outer cutting edge  40  are each curved so as to be recessed rearward in the rotational direction, the boundary portion between first thinning portion  31  and outer cutting edge  40  is pointed to a lesser degree. In this case, swarf that has been cut at first thinning portion  31  and swarf that has been cut at outer cutting edge  40  are integrated. As a result, an increase in length of the swarf can be suppressed, leading to improvement in swarf discharging performance. 
     Further, in drill  100  according to the above embodiments, when viewed in the direction along central axis X, second thinning portion  32  may have a radius of curvature which is more than or equal to 5% and less than or equal to 30% of twice the distance between outer end portion  43  of outer cutting edge  40  and central axis X. By setting the radius of curvature of second thinning portion  32  to more than or equal to 5% of twice the distance between outer end portion  43  of outer cutting edge  40  and central axis X, an excessive reduction in length of second thinning portion  32  can be suppressed. As a result, an increase in cutting resistance can be suppressed. By setting the radius of curvature of second thinning portion  32  to less than or equal to 30% of twice the distance between outer end portion  43  of outer cutting edge  40  and central axis X, an excessive reduction in thickness in the vicinity of second thinning portion  32  can be suppressed. As a result, a reduction in strength of drill  100  can be suppressed. 
     Further, in drill  100  according to the above embodiments, outer circumferential surface  5  may be provided with first margin  51  contiguous to outer cutting edge  40 , and second margin  52  spaced rearward in the rotational direction from first margin  51 . When viewed in the direction along central axis X, boundary line  6  between heel face  4  and flank face  2  may be linear. As compared to the example where boundary line  6  between heel face  4  and flank face  2  is curved, it is unnecessary to perform a curving process when boundary line  6  between heel face  4  and flank face  2  is linear, and machining of heel face  4  is thereby facilitated. 
     Further, in drill  100  according to the above embodiments, outer circumferential surface  5  may be provided with first margin  51  contiguous to outer cutting edge  40 , and second margin  52  spaced rearward in the rotational direction from first margin  51 . When viewed in the direction along central axis X, boundary line  6  between heel face  4  and flank face  2  may be curved. As compared to the example where boundary line  6  between heel face  4  and flank face  2  is linear, the width of second margin  52  can be increased when boundary line  6  between heel face  4  and flank face  2  is curved. Thus, second margin  52  can perform a guiding function from an early stage of drilling. 
     Further, in drill  100  according to the above embodiments, when viewed in the direction along central axis X, the distance between first thinning portion  31  and first straight line A 1  may be more than or equal to 0.001 mm and less than or equal to 1.0 mm. By setting the distance between first thinning portion  31  and first straight line A 1  to more than or equal to 0.001 mm, the function of biting into a workpiece in drilling can be further improved. By setting the distance between first thinning portion  31  and first straight line A 1  to less than or equal to 1.0 mm, an excessive reduction in length of first thinning portion  31  can be suppressed. 
     Example 
     Preparation of Samples 
     First, drill  200  of a sample  1  and drill  100  of a sample  2  were prepared. Drill  200  of sample  1  is a comparative example. The configuration of drill  200  of sample  1  is shown in  FIG.  12   . In drill  200  of sample  1 , when viewed in the direction along central axis X, first thinning portion  31  is located forward in the rotational direction relative to first straight line A 1 . Drill  100  of sample  2  is an example. The configuration of drill  100  of sample  2  is shown in  FIG.  3   . In drill  100  of sample  2 , when viewed in the direction along central axis X, first thinning portion  31  is located rearward in the rotational direction relative to first straight line A 1 . 
     Evaluation Conditions 
     Next, drilling was performed on a workpiece using drill  200  of sample  1  and drill  100  of sample  2 . The workpiece used was ADC 12, an Al-Si-Cu-based die casting material as specified by Japanese Industrial Standards (JIS) H5302: 2006. The equipment used was a vertical machining center (ROBODRILL α-T14iF La  manufactured by FANUC CORPORATION). The rotational speed was set to 80 m/minute and 180 m/minute. The feed was set to 0.6 mm/rotation and 1.2 mm/rotation. The depth of a hole was set to 30 mm. Oil was internally supplied. 
     Evaluation Results 
       FIG.  14    is a diagram showing hole positional tolerances of drill  200  of sample  1  and drill  100  of sample  2 . In  FIG.  14   , the center of three concentric circles is a point at a theoretically correct position. The hole positional tolerance is the diameter of a geometric circle passing through a central position of a hole actually formed, with the theoretically correct position as the center. In  FIG.  14   , a point indicated by a black circle is a central position of a hole actually formed. As the distance between the position of the black circle and the center of the concentric circles increases, the hole positional tolerance increases. It is desirable that the hole positional tolerance be small. 
     As shown in  FIG.  14   , when compared under conditions of the same rotational speed and the same feed, the hole positional tolerance of drill  100  of sample  2  was smaller than the hole positional tolerance of drill  200  of sample  1 . In other words, it was confirmed that drill  100  of sample  2  could improve the hole positional tolerance as compared to drill  200  of sample  1 . 
       FIG.  15    is a diagram showing cutting resistances of drill  200  of sample  1  and drill  100  of sample  2 . In  FIG.  15   , the abscissa represents time, and the ordinate represents cutting resistance. Solid lines and dashed lines each show cutting resistance in a horizontal direction perpendicular to a thrust direction. The solid line indicates cutting resistance in a first direction. The dashed line indicates cutting resistance in a second direction. The first direction is orthogonal to the second direction. 
     As shown in  FIG.  15   , when compared under conditions of the same rotational speed and the same feed, the cutting resistance of drill  100  of sample  2  was smaller than the cutting resistance of drill  200  of sample  1 . In other words, it was confirmed that drill  100  of sample  2  could reduce the cutting resistance as compared to drill  200  of sample  1 . 
     It should be understood that the embodiments and example disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     Reference Signs List 
       1  rake face;  2  flank face;  3  thinning face;  4  heel face;  5  outer circumferential surface;  6  boundary line;  7  coolant hole;  11  first rake face portion;  12  second rake face portion;  13  shank;  14  flute;  15  chisel;  21  first flank face portion;  22  second flank face portion;  31  first thinning portion;  32  second thinning portion;  33  first boundary;  34  second boundary;  40  outer cutting edge;  41  first cutting edge;  42  second cutting edge;  43  outer end portion;  51  first margin;  52  second margin;  61  first heel portion;  62  second heel portion;  71  first portion;  72  second portion;  73  third portion;  100 ,  200  drill;  101  front end;  102  rear end; A 1  first straight line; A 2  second straight line; D distance; R radius of curvature; S 1  first region; S 2  second region; S 3  third region; S 4  fourth region; X central axis.