Drill

Abstract

This drill (10) is provided with a body (1) extending axially centered on the center axis (O) of the body. The body (1) has: chip discharge grooves (4) that opens onto a tip-end face (3) and an outer peripheral surface (8) of the body (1), and that extends from the tip-end face (3) toward the rear end; thinnings (5) in the form of recesses disposed at the tip-end portion of the body (1) and connected to the chip discharge grooves (4) and the tip-end face (3); and cutting edges (7) disposed at the tip-end portion of the body (1). The cutting edges (7) have a thinning edge (70) disposed at a radially inner end portion of the cutting edges (7). Ridge lines (6) connecting the thinnings (5) and the tip-end face (3) have a curved section (60) stretching to the thinning edge (70) and concave in the drill rotation direction (T) about the center axis (O), the curved section (60) having a radius of curvature that grows larger as heading radially outward.

Description

drill

[0001] This application claims priority from Japanese Patent Application No. 2024-033406, filed on March 5, 2024, the contents of which are incorporated herein by reference.

[0002] A drill described in Patent Document 1, for example, is known. The drill of Patent Document 1 includes a thinning edge provided at the tip of the drill body, extending from the inner end of the cutting edge (main cutting edge) toward the chisel portion, and a gash portion (thinning) whose ridgeline with the flank (tip surface) extends in an arc shape from the inner end of the thinning edge. Furthermore, a circular arc groove having a cross-sectional arc shape with a radius of curvature smaller than the radius of curvature of the gash portion is provided at a portion where the thinning surface, which is the rake face of the thinning edge, connects to the gash surface of the gash portion. The circular arc-shaped ridgeline of the gash portion and the thinning edge are connected via the ridgeline with a small radius of curvature of the circular arc groove (hereinafter, sometimes referred to as the angle R).

[0003] Furthermore, the drill disclosed in Patent Document 1 has a thinning edge that is center-up. That is, the radial rake angle of the thinning edge is a negative angle. The center-up thinning edge has the advantage of making it easier to ensure the strength of the drill tip when cutting a hard workpiece such as high-hardness steel.

[0004] Japanese Patent No. 6588625 (B)

[0005] In the drill of Patent Document 1, the difference between the radius of curvature of the arc flute (corner R) and the radius of curvature of the gash portion is large, and the curvature changes abruptly between them. As a result, when chips generated by the thinning edge are scooped by the thinning surface and pushed into the gash portion, stress tends to concentrate near the corner R, making it difficult to achieve the function of curling the chips. This makes it difficult to reduce cutting resistance.

[0006] Furthermore, in this type of drill, there is a demand for the thinning edge to be center-down, i.e., for the radial rake of the thinning edge to have a positive angle (positive angle), in order to improve the bite into the workpiece and enhance the cutting performance. However, if the thinning edge in the drill of Patent Document 1 is designed to be center-down, the pair of gash portions arranged with the chisel portion therebetween will connect to each other, and the chisel portion will disappear (see Figure 3 of Patent Document 1). This limits the degree of freedom in designing the drill, making it difficult to flexibly meet various demands for drills.

[0007] An object of the present invention is to provide a drill that can reduce stress concentration caused by chips extruded from the thinning edge into the thinning, that can improve chip disposal by making it easier to curl the chips, that can reduce cutting resistance, and that can flexibly respond to various demands for drills.

[0008] In order to solve the above problems, the present invention provides the following means.

[0009] [Aspect 1 of the present invention] A drill including a body extending in the axial direction around a central axis, wherein the body has a chip discharge flute that opens into a tip surface and an outer peripheral surface of the body and extends from the tip surface toward a rear end, a concave thinning that is disposed at the tip portion of the body and connected to the chip discharge flute and the tip surface, and a cutting edge that is disposed at the tip portion of the body, wherein the cutting edge has a thinning edge that is disposed at an inner end of the cutting edge in the radial direction, a ridge line that connects the thinning and the tip surface is continuous with the thinning edge and has a curved portion that is concave in the direction of rotation of the drill around the central axis, and the curved portion has a radius of curvature that increases radially outward.

[0010] In the drill of the present invention, a curved portion recessed in the direction of drill rotation is formed on the ridge line connecting the tip surface of the body and the thinning. The ridge line forms the edge of the bottom surface of the thinning in the direction of drill rotation, and therefore the shape of the ridge line represents the shape (cross-sectional shape) of the bottom surface of the thinning. In the present invention, the ridge line has a curved portion recessed in the direction of drill rotation, and therefore the bottom surface of the thinning is formed as a concave curved surface.

[0011] The radius of curvature of the curved portion increases radially outward, and accordingly the cross-sectional shape of the bottom surface of the thinning (the cross-sectional shape along the curved portion) also increases radially outward.

[0012] With the above configuration, the present invention can prevent a sudden change in curvature in the chip outflow area where chips flow from the rake face of the thinning edge (thinning rake face) over the bottom surface of the thinning and into the chip discharge groove. As a result, chips generated by the thinning edge during cutting are smoothly pushed from the thinning rake face onto the bottom surface of the thinning and discharged into the chip discharge groove while being well curled. This improves chip disposal and reduces cutting resistance.

[0013] Furthermore, in conventional drills, even when a curved portion that is concave in the direction of drill rotation is formed on the ridge line connecting the tip face of the body and the thinning, as in the above-mentioned Patent Document 1 (Patent Publication No. 6588625), stress concentration occurs at the connection part (corner R) between this curved portion and the thinning cutting edge, making it difficult to ensure strength near the center of rotation of the drill (chisel part).

[0014] In contrast, in the drill of the present invention, the radius of curvature of the curved portion decreases toward the inside in the radial direction, so even if the radius of curvature of the connection (corner R) between the curved portion and the thinning edge is small, the cutting load can be distributed from this connection to the inside edge of the curved portion in the radial direction. This reduces stress concentration near the rotation center (chisel portion) of the drill, ensuring strength.

[0015] Furthermore, by increasing the radius of curvature of the curved portion toward the outside in the radial direction, it is possible to position the apex of the curved portion (the tip of the curved portion in the direction of drill rotation) farther radially outward from the central axis. This allows for a consistent wall thickness near the center of rotation (chisel portion) of the drill, further increasing its strength. Furthermore, even when the thinning edges are designed to be center-down to improve bite into the workpiece and enhance sharpness, this prevents the thinning edges sandwiching the chisel portion from joining together (preventing the loss of the chisel portion). This allows for a high degree of freedom in drill design, allowing for flexible response to various drill requirements.

[0016] As described above, according to the present invention, it is possible to reduce stress concentration caused by chips extruded from the thinning cutting edge into the thinning, to make the chips easier to curl, thereby improving chip disposal, and to reduce cutting resistance, thereby making it possible to flexibly respond to various demands for drills.

[0017] [Aspect 2 of the Present Invention] The drill according to aspect 1, wherein the curved portion is formed by combining at least three arc portions each having a different radius of curvature.

[0018] In this case, the radius of curvature of the curved portion can be gradually increased radially outward using a simple structure, thereby more stably achieving the effects of the present invention described above.

[0019] [Aspect 3 of the present invention] The drill according to aspect 1 or 2, wherein the thinning edge has a concave curved edge that is connected to the curved portion and is curved in a direction opposite to the rotation direction of the drill.

[0020] In this case, even if the radius of curvature of the connection between the curved portion and the thinning edge (the concave curved corner R) is small, the cutting load can be distributed from this connection to the concave curved edge of the thinning edge. This reduces stress concentration near the rotation center (chisel portion) of the drill, ensuring strength. In particular, since the vicinity of the chisel portion accounts for a high proportion of the cutting resistance distribution, the above-mentioned configuration of the present invention effectively improves the strength of the vicinity of the chisel portion. Furthermore, by distributing the cutting load, cutting resistance is kept low.

[0021] [Aspect 4 of the present invention] The drill according to aspect 3, wherein the concave curved cutting edge has a radius of curvature that increases radially outward.

[0022] In this case, the radius of curvature of the concave curved blade increases as it moves radially outward from the connection portion (the corner R with the smaller radius of curvature), making it easier to distribute the cutting load more stably and reduce stress concentration.

[0023] [Aspect 5 of the present invention] The drill according to Aspect 3 or 4, wherein at least the thinning edge of the cutting edges has a honing, and the honing has a curved honing ridgeline that extends parallel to the concave curved edge and is concave in the opposite direction to the rotation direction of the drill.

[0024] [Aspect 6 of the Present Invention] The drill according to Aspect 5, wherein the curved honing ridge has a radius of curvature that increases radially outward.

[0025] In this case, the same effect as the concave curved blade can be obtained by using a curved honing ridge that is concave on the opposite side to the direction of rotation of the drill.

[0026] [Aspect 7 of the present invention] The drill according to any one of Aspects 1 to 6, wherein the thinning edge is center-down.

[0027] By setting the thinning edge to a center down position, the radial rake of the thinning edge can be set to a positive angle (normal angle). This improves the cutting performance of the workpiece. Even if the thinning edge is set to a center down position, the present invention allows for a design that maintains the thickness near the center of rotation of the drill (chisel part), ensuring the strength of the chisel part.

[0028] [Aspect 8 of the present invention] The drill according to any one of Aspects 1 to 6, wherein the thinning edge is center-raised.

[0029] By making the thinning edge center-raised, the radial rake of the thinning edge can be set to a negative angle. In this case, during drill manufacturing, it becomes easy to control the dimensions near the chisel portion formed using a grinding wheel and the honing dimensions near the center axis. Specifically, when forming the thinning edge while moving the grinding wheel in the cutting length direction of the thinning edge, it becomes easy to control the dimensions near the chisel portion. This also stabilizes the function near the center of rotation (chisel portion) of the drill.

[0030] Furthermore, it is easy to minimize the chisel portion. Specifically, in the case of a two-flute drill, the vertices (tip ends in the drill rotation direction) of the curved portions of a pair of thinnings arranged with the chisel portion between them can be made to overlap each other along the direction in which the thinning blades extend (the cutting edge length direction), thereby minimizing the size of the chisel portion. Furthermore, a larger area for arranging the thinnings (thinning space) can be secured, which further improves chip disposal and reduces cutting resistance.

[0031] According to the above-described aspects of the present invention, it is possible to reduce stress concentration caused by chips extruded from the thinning edge into the thinning, to make the chips easier to curl, to improve chip disposal, and to reduce cutting resistance, thereby providing a drill that can flexibly respond to various demands for drills.

[0032] FIG. 1 is a side view showing a drill of this embodiment. FIG. 2 is an enlarged side view showing part II of FIG. 1 . FIG. 3 is a front view showing the drill. FIG. 4 is an enlarged front view showing a portion of the drill. FIG. 5 is a front view of the drill of this embodiment, illustrating the center height of the thinning edge and the positional relationship between the vertices of the curved portion. FIG. 6 is a front view of a drill of a first modified example of this embodiment, illustrating the center height of the thinning edge and the positional relationship between the vertices of the curved portion. FIG. 7 is a front view of a drill of a second modified example of this embodiment, illustrating the center height of the thinning edge and the positional relationship between the vertices of the curved portion. FIG. 8 is a front view of a drill of a third modified example of this embodiment, illustrating the center height of the thinning edge and the positional relationship between the vertices of the curved portion. FIG. 9 is a front view of a drill of a fourth modified example of this embodiment, illustrating the definition of the direction based on the thinning edge and the positional relationship between the vertices of the curved portion. FIG. 10 is a front view of a drill of a fifth modified example of this embodiment.

[0033] A drill 10 according to one embodiment of the present invention will be described with reference to Figures 1 to 5. As shown in Figure 1, the drill 10 is cylindrical and has a central axis O as its center. In this embodiment, the drill 10 includes a body 1, a shank 2, and a coolant hole 15. The body 1 and the shank 2 are disposed at different positions in the direction in which the central axis O of the drill 10 extends. The body 1 may also be referred to as a cutting portion or the like.

[0034] [Definition of Directions] In this embodiment, the direction in which the central axis O of the drill 10 extends is referred to as the axial direction. Within the axial direction, the direction from the shank 2 to the body 1 is referred to as the axial tip side or simply the tip side, and the direction from the body 1 to the shank 2 is referred to as the axial rear end side or simply the rear end side.

[0035] Additionally, the direction perpendicular to the central axis O is referred to as the radial direction. Of the radial directions, the direction approaching the central axis O is referred to as the radially inner direction, and the direction away from the central axis O is referred to as the radially outer direction. The direction rotating around the central axis O is referred to as the circumferential direction. Of the circumferential directions, the direction in which the drill 10 is rotated during drilling (cutting) is referred to as the drill rotation direction T. Of the circumferential directions, the rotation direction opposite to the drill rotation direction T is referred to as the opposite side to the drill rotation direction T or the counter-drill rotation direction.

[0036] Furthermore, if there are any directions to be defined other than the above-mentioned directions, they will be defined separately when explaining each component.

[0037] [Shank] The shank 2 is columnar and extends axially about a central axis O, and in this embodiment, is cylindrical. The shank 2 is disposed at least at the rear end of the drill 10. The shank 2 is detachably held, for example, by a spindle of a machine tool or a chuck of a drilling press (not shown) (hereinafter abbreviated as the spindle, etc.). The drill 10 cuts into the workpiece with the body 1 to perform drilling by rotating the shank 2 in the drill rotation direction T by the spindle, etc., and feeding the shank 2 toward the tip end in the axial direction.

[0038] [Body] The body 1 is generally columnar and extends axially around a central axis O. The body 1 is disposed at least at the tip end of the drill 10, and in this embodiment, is disposed at a portion other than the rear end. As shown in FIG. 2 , the diameter (outer diameter) D of the body 1 corresponds to the diameter of the rotation locus of the cutting edge 7, which will be described later, around the central axis O. Therefore, the diameter D of the body 1 may also be referred to as the cutting edge diameter D.

[0039] As shown in Figures 2 and 3, the body 1 has a tip surface 3 facing the tip side of the body 1, an outer peripheral surface 8 facing radially outward of the body 1, a chip discharge groove 4, a thinning 5, a chisel portion 9, a cutting edge 7, a first margin 13A, a second margin 13B, a leading edge 12, and a chamfer surface 14.

[0040] The chip flutes 4, thinnings 5, cutting edges 7, first margins 13A, second margins 13B, leading edges 12, and relief surfaces 14 are each provided in plurality at intervals in the circumferential direction on the body 1. In this embodiment, two chip flutes 4, thinnings 5, cutting edges 7, first margins 13A, second margins 13B, leading edges 12, and relief surfaces 14 are each provided at equal intervals in the circumferential direction. In other words, the drill 10 of this embodiment is a two-flute twist drill provided with two sets of cutting edges 7.

[0041] The chip discharge flute 4 is a groove that opens to the tip surface 3 and the outer peripheral surface 8 of the body 1 and extends from the tip surface 3 toward the rear end. Specifically, the chip discharge flute 4 extends in a twisted manner in the direction opposite to the rotational direction of the drill as it extends from the tip surface 3 toward the rear end in the axial direction.

[0042] The thinning 5 is disposed at the tip of the body 1 and is connected to the chip discharge groove 4 and the tip surface 3. The thinning 5 is concave and is formed so as to cut out a portion of each of the chip discharge groove 4 and the tip surface 3. The thinning 5 has a thinning rake face 51 and a bottom surface 52.

[0043] The thinning rake face 51 is disposed on one of the inner surfaces of the thinning 5 (the multiple wall surfaces constituting the thinning 5) that faces the drill rotation direction T. As shown in Figure 2, the thinning rake face 51 has a substantially triangular surface shape. The axial dimension of the thinning rake face 51 increases radially outward.

[0044] The bottom surface 52 is disposed on a wall surface of the inner surface of the thinning 5 that faces in the direction opposite to the rotational direction of the drill. The bottom surface 52 is disposed in the direction of rotation T of the thinning rake face 51 of the drill and is connected to the thinning rake face 51. The bottom surface 52 extends toward the rear end as it moves in the direction opposite to the rotational direction of the drill. When viewed from the tip of the drill shown in FIG. 3 , the bottom surface 52 is substantially fan-shaped.

[0045] The bottom surface 52 has a concave curved shape. Therefore, a ridgeline 6 of the bottom surface 52 that forms an edge in the drill rotation direction T (the ridgeline 6 connecting the bottom surface 52 and the tip surface 3) has a concave curved shape that is recessed toward the drill rotation direction T. In this embodiment, this ridgeline 6 may be referred to as the "ridgeline 6 connecting the thinning 5 and the tip surface 3." The radially outer end of the ridgeline 6 is connected to a wall surface of the chip discharge flute 4 that faces in the direction opposite to the drill rotation direction. The radially outer end of the ridgeline 6, i.e., the intersection A between the ridgeline 6 and the chip discharge flute 4, is located radially inward from the outer circumferential surface 8.

[0046] The ridge 6 connecting the thinning 5 and the tip face 3 continues to a thinning edge 70 of the cutting edge 7, which will be described later, and has a curved portion 60 that is curved and concave in the drill rotation direction T. The radius of curvature of the curved portion 60 increases as it extends radially outward. The curved portion 60 is formed, for example, to form a part of a logarithmic spiral having a predetermined shape.

[0047] In this embodiment, the curved portion 60 is formed by combining a plurality of arc portions having different radii of curvature, specifically, by combining at least three arc portions. More specifically, the curved portion 60 is formed by combining three arc portions 61, 62, and 63 having different radii of curvature.

[0048] The three arc portions 61, 62, 63 include a first arc portion 61 connected to the radially inner end portion (thinning edge 70) of the cutting edge 7, a second arc portion 62 connected to the first arc portion 61, and a third arc portion 63 connected to the second arc portion 62. The first arc portion 61, the second arc portion 62, and the third arc portion 63 are arranged in this order from near the central axis O toward the radially outward side. Of the curved portion 60, at least the second arc portion 62 and the third arc portion 63 form an arc shape concave in the drill rotation direction T.

[0049] The radius of curvature of the third arc portion 63 is larger than the radius of curvature of the second arc portion 62. The radius of curvature of the second arc portion 62 is larger than the radius of curvature of the first arc portion 61. In this embodiment, the diameter dimension (blade diameter dimension) of the rotation trajectory of the cutting blade 7 about the central axis O is D, and for example, the radius of curvature of the first arc portion 61 is about 0.02D, the radius of curvature of the second arc portion 62 is about 0.17D, and the radius of curvature of the third arc portion 63 is about 0.25D.

[0050] The arc length of the third arc portion 63 is longer than the arc length of the second arc portion 62. The arc length of the second arc portion 62 is longer than the arc length of the first arc portion 61.

[0051] The cutting edge 7 and the first arc portion 61 are smoothly and continuously connected to each other so as to have a common tangent at their connecting portion (connection point). The first arc portion 61 and the second arc portion 62 are smoothly and continuously connected to each other so as to have a common tangent at their connecting portion. The second arc portion 62 and the third arc portion 63 are smoothly and continuously connected to each other so as to have a common tangent at their connecting portion.

[0052] In the view of the tip of the drill 10 shown in FIG. 5 , the symbol P denotes the apex P of the curved portion 60 that is located furthest in the drill rotation direction T. The apex P is also the point of the ridge line 6 that is located furthest in the drill rotation direction T. The apex P may alternatively be referred to as the tip of the curved portion 60 (ridge line 6) in the drill rotation direction T. In this embodiment, the apex P is located on the second arc portion 62 or near the connection point between the second arc portion 62 and the third arc portion 63. In other words, the apex P is not located on the first arc portion 61. Therefore, the apex P is located radially outward from the vicinity of the central axis O (near the chisel portion 9).

[0053] 2 and 3 , the tip end surface 3 has a first flank 31 and a second flank 32. A plurality of pairs of the first flank 31 and the second flank 32 are provided at intervals in the circumferential direction on the body 1. In the present embodiment, two pairs of the first flank 31 and the second flank 32 are provided at equal intervals in the circumferential direction.

[0054] The first flank 31 has a generally polygonal shape extending in the radial direction. The first flank 31 extends axially toward the rear end as it extends in the direction opposite to the rotation direction of the drill.

[0055] The second flank 32 is disposed adjacent to the first flank 31 in the direction opposite to the drill rotation direction of the first flank 31. The second flank 32 is generally fan-shaped, and the circumferential dimension increases radially outward. The second flank 32 also extends axially toward the rear end as it extends radially opposite to the drill rotation direction. The amount of axial displacement of the second flank 32 per unit length along the circumferential direction (the inclination corresponding to the clearance angle) is greater than the amount of axial displacement of the first flank 31.

[0056] In this embodiment, the tip end face 3 has two types of inclined surfaces (first flank 31 and second flank 32) with different clearance angles, but this is not limited to this. Although not particularly shown, the tip end face 3 may have one type of inclined surface (flank) with a constant clearance angle, or may have three or more types of inclined surfaces (flank) with different clearance angles.

[0057] The chisel portion 9 is disposed in the center (rotation center) of the tip end face 3. Specifically, the chisel portion 9 is disposed on the ridge line where a pair of flanks (a pair of the first flank 31 and the second flank 32) are connected, and is located on the central axis O. The chisel portion 9 is disposed so as to be sandwiched between the pair of thinnings 5. The chisel portion 9 extends so as to connect the pair of thinnings 5.

[0058] The cutting edge 7 is disposed at the tip of the body 1. The cutting edge 7 is disposed at a ridge line connecting the wall surfaces of the chip discharge flutes 4 and the thinning 5, which face in the drill rotation direction T, to the tip face 3. The cutting edge 7 has a thinning edge 70, a main cutting edge 75, and a connecting convex edge 76.

[0059] The thinning edge 70 is disposed at the radially inner end of the cutting edge 7. The thinning edge 70 is disposed at the ridge where the thinning rake face 51 and the first flank face 31 are connected. The thinning edge 70 extends radially outward from near the central axis O, generally along the radial direction. Specifically, in this embodiment, the thinning edge 70 is center-down. In other words, the radial rake (radial rake angle) of the thinning edge 70 is a positive angle (positive angle). Furthermore, the thinning edge 70 extends radially outward toward the rear end in the axial direction.

[0060] As shown in Figure 3, the thinning blade 70 has a concave curved blade 71 that is connected to the ridge line 6 connecting the thinning 5 and the tip face 3 and is curved in a direction opposite to the drill rotation direction T, and a straight blade 72 that is connected to the concave curved blade 71.

[0061] Specifically, the concave curved blade 71 is continuous with the curved portion 60 of the ridge line 6. The radius of curvature of the concave curved blade 71 increases as it extends radially outward. More specifically, the concave curved blade 71 is formed by combining a plurality of arc-shaped blades each having a different radius of curvature. In this embodiment, the concave curved blade 71 is formed by combining two arc-shaped blades 71a and 71b.

[0062] The two arc-shaped blades 71 a, 71 b include a first arc-shaped blade 71 a that is continuous with the ridge line 6 that connects the bottom surface 52 and the second flank surface 32, and a second arc-shaped blade 71 b that is continuous with the first arc-shaped blade 71 a. The first arc-shaped blade 71 a and the second arc-shaped blade 71 b each have an arc shape that is concave in the direction opposite to the rotational direction of the drill.

[0063] In this embodiment, the first arc blade 71a of the concave curved blade 71 and the first arc portion 61 of the curved portion 60 can be considered to each constitute a part (or the whole) of a common arc.

[0064] The radius of curvature of the second arc-shaped blade 71b is larger than the radius of curvature of the first arc-shaped blade 71a. In this embodiment, the diameter dimension (blade diameter dimension) of the rotation trajectory of the cutting blade 7 about the central axis O is D, and for example, the radius of curvature of the first arc-shaped blade 71a is about 0.02D, and the radius of curvature of the second arc-shaped blade 71b is about 0.10D. In addition, the blade length of the second arc-shaped blade 71b is longer than the blade length of the first arc-shaped blade 71a.

[0065] The ridge line 6 (curved portion 60) and the first arc-shaped blade 71a are smoothly and continuously connected to each other so as to have a common tangent at their connecting portion (connection point).Furthermore, the first arc-shaped blade 71a and the second arc-shaped blade 71b are smoothly and continuously connected to each other so as to have a common tangent at their connecting portion.

[0066] The straight blade 72 is disposed radially outward of the concave curved blade 71. The straight blade 72 extends linearly. The straight blade 72 is smoothly and continuously connected to the radially outer end of the concave curved blade 71 (the radially outer end of the second arc-shaped blade 71b in this embodiment). In this embodiment, the blade length of the straight blade 72 is shorter than the blade length of the concave curved blade 71 (the sum of the blade lengths of the first arc-shaped blade 71a and the second arc-shaped blade 71b).

[0067] Here, with reference to FIG. 5, the definitions of the cutting length direction (Y-axis direction) and cutting edge straight direction (X-axis direction) of the thinning edge 70 will be described.

[0068] 5, the direction in which the straight edge 72 of the thinning edge 70 extends when the drill 10 is viewed from the tip end side in the axial direction is referred to as the edge length direction of the thinning edge 70 or simply as the edge length direction. The edge length direction of the thinning edge 70 corresponds to the Y-axis direction.

[0069] The drill 10 of this embodiment is a two-blade drill and has a pair of cutting edges 7. The pair of cutting edges 7 are arranged at different positions in the cutting length direction of the thinning edge 70. In this embodiment, of the pair of cutting edges 7 (7A, 7B), the direction from one cutting edge 7A to the other cutting edge 7B along the cutting length direction of the thinning edge 70 is referred to as the other side (−Y side) of the cutting length direction of the thinning edge 70, and the direction from the other cutting edge 7B to one cutting edge 7A is referred to as one side (+Y side) of the cutting length direction of the thinning edge 70.

[0070] 5, the direction perpendicular to the straight edge 72 of the thinning edge 70 is referred to as the edge perpendicular direction of the thinning edge 70 or simply as the edge perpendicular direction. The edge perpendicular direction of the thinning edge 70 corresponds to the X-axis direction.

[0071] In this embodiment, the direction in which one cutting edge 7A faces the drill rotation direction T along the cutting edge straight direction of the thinning edge 70 is called one side of the cutting edge straight direction of the thinning edge 70 (+X side), and the direction in which the other cutting edge 7B faces the drill rotation direction T is called the other side of the cutting edge straight direction of the thinning edge 70 (-X side).

[0072] In this embodiment, the thinning edge 70 is centered down as described above. Therefore, the thinning edge 70 of one cutting edge 7A is disposed a distance LX away from the thinning edge 70 of the other cutting edge 7B on the other side (-X side) in the blade perpendicular direction. The distance LX is set in the range of 0<LX≦0.10D, for example, where D is the diameter dimension (blade diameter dimension) of the rotational locus of the cutting edge 7 about the central axis O.

[0073] Furthermore, in this embodiment, the vertices P of the curved portions 60 of a pair of ridgelines 6 do not overlap each other in the cutting edge length direction (Y-axis direction). In other words, the pair of thinnings 5 ​​do not overlap each other when viewed from the cutting edge direction (X-axis direction) (i.e., they do not overlap in the cutting edge length direction). The pair of vertices P do not overlap each other, but are spaced apart by a distance LY in the cutting edge length direction. The distance LY is, for example, in the range of -0.02D≦LY≦0.10D.

[0074] Note that the above "-0.02D≦LY" represents the range of the distance LY when a pair of thinnings 5 ​​overlap each other when viewed from the cutting edge direction (X-axis direction) (i.e., they overlap in the cutting edge length direction), and a pair of vertices P also overlap each other in the cutting edge length direction.

[0075] Furthermore, it is preferable that the distance M along the blade perpendicular direction (X-axis direction) between the apex P of the ridge line 6 (curved portion 60) and the thinning blade 70 (straight blade 72) is, for example, in the range of 0.02D or more and 0.10D or less.

[0076] As shown in Fig. 3, the main cutting edge 75 is disposed radially outward of the thinning edge 70. The main cutting edge 75 is disposed on a ridge line connecting the wall surface of the chip discharge flute 4 facing the drill rotation direction T and the first flank surface 31. The main cutting edge 75 is center-up. In other words, the radial rake of the main cutting edge 75 is a negative angle.

[0077] 2, the main cutting edge 75 extends radially outward and then axially toward the rear end. The leading edge 12 is connected to the radial outer end B of the main cutting edge 75. The radial outer end B of the main cutting edge 75, i.e., the connection point between the main cutting edge 75 and the leading edge 12, can also be referred to as the outer circumferential corner B. The outer circumferential corner B is a corner that is sharpened toward the outer circumferential tip.

[0078] As shown in Fig. 3, the connecting convex edge 76 is disposed between the thinning edge 70 and the main cutting edge 75. The connecting convex edge 76 has a curved shape that is convex toward the drill rotation direction T. The connecting convex edge 76 is connected to the radially outer end of the thinning edge 70 (the radially outer end of the straight edge 72 in this embodiment) and the radially inner end of the main cutting edge 75. The connecting convex edge 76 is smoothly and continuously connected to the thinning edge 70 and the main cutting edge 75. As shown in Fig. 2, the connecting convex edge 76 extends radially outward and toward the rear end in the axial direction.

[0079] As shown in Fig. 4, at least the thinning edge 70 of the cutting edge 7 has a honing. In this embodiment, the cutting edge 7 has a honing over its entire cutting length, and specifically, the honing is formed over the thinning edge 70, the connecting convex edge 76, and the main cutting edge 75. The honing extends along the cutting edge 7. The honing in this embodiment is, for example, a chamfer honing.

[0080] The honing of the thinning edge 70 has a curved honing ridge 81 and a straight honing ridge 82 .

[0081] The curved honing ridge 81 extends parallel to the concave curved blade 71 and forms a curve that is concave in the opposite direction to the drill rotation direction T. The radius of curvature of the curved honing ridge 81 increases as it extends radially outward. Specifically, the radius of curvature of the ridge portion of the curved honing ridge 81 that extends parallel to the second arc-shaped blade 71b is larger than the radius of curvature of the ridge portion that extends parallel to the first arc-shaped blade 71a.

[0082] The straight honing ridge 82 is disposed radially outward of the curved honing ridge 81. The straight honing ridge 82 extends in a straight line. The straight honing ridge 82 is smoothly and continuously connected to the radially outer end of the curved honing ridge 81.

[0083] The honing of the thinning blade 70 also has a honing width changing portion 83 located at the radially inner end of the thinning blade 70. The honing width of the honing width changing portion 83 increases and then decreases as the honing width increases along the direction in which the concave curved blade 71 extends from the ridge 6 connecting the thinning 5 and the tip surface 3. The honing width of the thinning blade 70 is greatest at the honing width changing portion 83. Note that the "direction in which the concave curved blade 71 extends" may also be rephrased as the blade length direction of the concave curved blade 71. In this embodiment, the chisel portion 9 is connected to the honing width changing portion 83.

[0084] As shown in Figures 2 and 3, the first margin 13A is disposed on the outer peripheral surface 8 and extends along the chip flute 4. Specifically, the first margin 13A is disposed on a portion of the outer peripheral surface 8 adjacent to the chip flute 4 in the counter-drill rotation direction. The first margin 13A extends in the counter-drill rotation direction as it extends axially toward the rear end. The first margin 13A has a curved surface shape that is convex radially outward. When viewed in a cross section perpendicular to the central axis O, the first margin 13A has an arc shape centered on the central axis O.

[0085] The second margin 13B is disposed on the outer peripheral surface 8 and extends along the chip discharge flute 4. The second margin 13B is disposed away from the first margin 13A in the counter-drill rotation direction. Specifically, the second margin 13B is disposed in a portion of the outer peripheral surface 8 adjacent to the chip discharge flute 4 in the drill rotation direction T. The second margin 13B extends in the counter-drill rotation direction as it moves toward the rear end in the axial direction. The second margin 13B has a curved surface shape that is convex radially outward. When viewed in a cross section perpendicular to the central axis O, the second margin 13B has an arc shape centered on the central axis O.

[0086] The leading edge 12 is disposed on a ridge line connecting the wall surface of the chip flute 4 facing the drill rotation direction T and the first margin 13A. The leading edge 12 extends along the wall surface of the chip flute 4 facing the drill rotation direction T and the first margin 13A. Specifically, the leading edge 12 extends in the direction opposite to the drill rotation direction as it moves toward the rear end in the axial direction.

[0087] The leading edge 12 may be back tapered. In this case, the leading edge 12 extends slightly radially inward as it approaches the rear end in the axial direction.

[0088] The relief surface 14 is disposed on the outer peripheral surface 8. The relief surface 14 is disposed between the first margin 13A and the second margin 13B in the circumferential direction. The relief surface 14 is formed recessed radially inward from the first margin 13A and the second margin 13B. During drilling, the relief surface 14 faces the inner peripheral surface of the machined hole in the workpiece with a radial gap therebetween.

[0089] [Coolant Hole] As shown in FIG. 1 , the coolant hole 15 extends through the shank 2 and the body 1 inside the drill 10. Note that part of the coolant hole 15 is not shown in FIG. 1 . The coolant hole 15 penetrates the drill 10 in the axial direction. A plurality of coolant holes 15 are provided at intervals in the circumferential direction; in this embodiment, two coolant holes 15 are provided at equal intervals in the circumferential direction. The coolant hole 15 extends spirally in the drill rotation direction T as it extends toward the tip side in the axial direction. As shown in FIG. 3 , in this embodiment, the tip of the coolant hole 15 opens into the second flank 32 of the tip face 3.

[0090] [Operation and Effect of the Present Embodiment] In the drill 10 of the present embodiment described above, the ridgeline 6 connecting the tip surface 3 of the body 1 and the thinning 5 has a curved portion 60 that is recessed in the drill rotation direction T. This ridgeline 6 forms the edge of the bottom surface 52 of the thinning 5 in the drill rotation direction T, and therefore the shape of the ridgeline 6 represents the shape (cross-sectional shape) of the bottom surface 52 of the thinning 5. In this embodiment, the ridgeline 6 has the curved portion 60 that is recessed in the drill rotation direction T, and therefore the bottom surface 52 of the thinning 5 is formed as a concave curved surface.

[0091] The radius of curvature of the curved portion 60 increases radially outward. Accordingly, the cross-sectional shape of the bottom surface 52 of the thinning 5 (the cross-sectional shape along the curved portion 60) also increases radially outward.

[0092] With the above configuration, in this embodiment, it is possible to prevent a sudden change in curvature in the chip outflow region where chips flow from the rake face (thinning rake face 51) of the thinning edge 70, over the bottom surface 52 of the thinning 5, and into the chip discharge groove 4. Therefore, chips generated by the thinning edge 70 during cutting are smoothly pushed out from the thinning rake face 51 onto the bottom surface 52 of the thinning 5, and are discharged into the chip discharge groove 4 while being well curled. This improves chip disposal and reduces cutting resistance.

[0093] Furthermore, in conventional drills, even when a curved portion that is concave in the direction of drill rotation is formed on the ridge line connecting the tip face of the body and the thinning, as in the above-mentioned Patent Document 1 (Patent Publication No. 6588625), stress concentration occurs at the connection part (corner R) between this curved portion and the thinning cutting edge, making it difficult to ensure strength near the center of rotation of the drill (chisel part).

[0094] In contrast, in the drill 10 of this embodiment, the radius of curvature of the curved portion 60 decreases radially inward, so even if the radius of curvature of the connection (corner R) between the curved portion 60 and the thinning cutting edge 70 is small, the cutting load can be distributed from this connection to the radially inner end of the curved portion 60. This makes it possible to alleviate stress concentration near the rotation center (chisel portion 9) of the drill 10 and ensure strength.

[0095] Furthermore, because the radius of curvature of the curved portion 60 increases radially outward, the apex P of the curved portion 60 located furthest in the drill rotation direction T (the tip of the curved portion 60 in the drill rotation direction T) can be positioned radially outward from the central axis O. This allows for a stable wall thickness near the rotation center (chisel portion 9) of the drill 10, further increasing its strength. Even when the thinning edges 70 are designed to be center-down to improve bite into the workpiece and enhance sharpness, as in this embodiment, this prevents the thinning edges 5 sandwiching the chisel portion 9 from joining together (preventing the disappearance of the chisel portion 9). This allows for a high degree of freedom in designing the drill 10, allowing it to flexibly meet various requirements for the drill 10.

[0096] As described above, according to this embodiment, it is possible to reduce stress concentration caused by chips extruded from the thinning cutting edge 70 into the thinning 5, to make the chips easier to curl, to improve chip disposal, and to reduce cutting resistance, thereby making it possible to flexibly respond to various demands for the drill 10.

[0097] Furthermore, as in this embodiment, when the distance M along the blade perpendicular direction (X-axis direction) between the apex P of the ridge line 6 (curved portion 60) and the thinning blade 70 (straight blade 72) is in the range of 0.02D or more and 0.10D or less, the apex P can be stably spaced radially outward from the vicinity of the central axis O while maintaining good chip disposal properties by the thinning 5, which is more preferable.

[0098] In this embodiment, the curved portion 60 is formed by combining at least three arc portions 61, 62, and 63, each having a different radius of curvature. In this case, the radius of curvature of the curved portion 60 can be gradually increased radially outward using a simple structure. This allows the above-described effects of this embodiment to be more stably achieved.

[0099] In this embodiment, the thinning edge 70 is connected to the curved portion 60 and has a concave curved edge 71 that is concave in the opposite direction to the rotation direction T of the drill.

[0100] In this case, even if the radius of curvature of the connection portion (concave curved corner R) between the curved portion 60 and the thinning edge 70 is small, the cutting load can be distributed from this connection portion to the concave curved edge 71 of the thinning edge 70. This reduces stress concentration near the rotation center (chisel portion 9) of the drill 10, ensuring strength. In particular, since the vicinity of the chisel portion 9 accounts for a high proportion of the cutting resistance distribution, the above-described configuration of this embodiment effectively improves the strength of the vicinity of the chisel portion 9. Furthermore, by distributing the cutting load, the cutting resistance is kept low.

[0101] In this embodiment, the radius of curvature of the concave curved edge 71 of the thinning edge 70 increases as it moves radially outward. In this case, the radius of curvature of the concave curved edge 71 increases as it moves radially outward from the connecting portion (the angle R with the small radius of curvature), which makes it easier to distribute the cutting load more stably and reduce stress concentration.

[0102] In this embodiment, the concave curved blade 71 is configured by combining a plurality of arcuate blades 71a, 71b having different radii of curvature. In this case, the radius of curvature of the concave curved blade 71 can be gradually increased radially outward using a simple structure. This allows the effects of this embodiment to be more stably achieved.

[0103] In this embodiment, the honing of the thinning edge 70 extends parallel to the concave curved edge 71 and has a curved honing ridge 81 that is concave in the opposite direction to the drill rotation direction T. The radius of curvature of the curved honing ridge 81 increases as it extends radially outward. In this case, the curved honing ridge 81 that is concave in the opposite direction to the drill rotation direction T can also achieve the same effects as the concave curved edge 71 described above.

[0104] Furthermore, in this embodiment, the honing of the thinning blade 70 has a honing width changing portion 83 located at the radially inner end of the thinning blade 70, and the honing width of the honing width changing portion 83 increases and then decreases as it moves away from the ridge line 6 connecting the thinning 5 and the tip face 3 along the direction in which the concave curved blade 71 extends.

[0105] In this case, it is easy to make the honing width (maximum value) of the honing width changing portion 83 located at the radially inner end of the thinning blade 70 wider than the honing width of the portion other than the honing width changing portion 83 (portions other than the radially inner end). This makes it possible to more stably increase the cutting edge strength of the concave curved blade 71, particularly in the vicinity of the chisel portion 9 where cutting resistance tends to be large.

[0106] Furthermore, in this embodiment, the radially outer end of the ridge line 6 connecting the thinning 5 and the tip face 3, i.e., the intersection A between the ridge line 6 and the chip discharge groove 4, is located away from the outer peripheral surface 8 and radially inward. In this case, chips generated by the thinning cutting edge 70 are prevented from flowing from the thinning 5 onto the outer peripheral surface 8. In other words, since chips are less likely to reach the gap between the outer peripheral surface 8 of the drill 10 and the inner peripheral surface of the machined hole, chip jamming can be suppressed, improving the accuracy of the machined surface. Furthermore, chips are stably discharged to the outside of the machined hole through the chip discharge groove 4, improving chip disposal efficiency.

[0107] In the case of a two-flute drill 10 as in this embodiment, the angle ∠AOB between the intersection point A centered on the central axis O and the outer peripheral corner B, as viewed from the tip of the drill as shown in FIG. 3, is preferably in the range of 70 to 100°.

[0108] In this embodiment, the thinning edge 70 is center-down. By center-downing the thinning edge 70, the radial rake of the thinning edge 70 can be set to a positive angle (positive angle). This improves the biting performance into the workpiece. Even if the thinning edge 70 is center-down, this embodiment allows for a design that maintains the thickness near the rotation center (chisel portion 9) of the drill 10, thereby ensuring the strength near the chisel portion 9.

[0109] As in this embodiment, when the thinning blade 70 is centered down and the vertices P of a pair of ridge lines 6 (curved portions 60) do not cross each other in the blade length direction (Y-axis direction) of the thinning blade 70, it is effective for drilling a wide range of workpiece materials, such as steel, stainless steel, and cast iron.

[0110] [Other Configurations Included in the Present Invention] The present invention is not limited to the above-described embodiment, and configurations can be modified within the scope of the present invention, as described below, for example. In the illustrations of the modified examples, the same components as those in the above-described embodiment are denoted by the same reference numerals, and the following mainly describes the differences.

[0111] FIG. 6 is a front view showing a drill 20 according to a first modified example of the embodiment described above. In this drill 20 according to the first modified example, the thinning edge 70 is center-raised. In other words, the radial rake of the thinning edge 70 has a negative angle. Therefore, the thinning edge 70 of one cutting edge 7A is spaced a distance LX from the thinning edge 70 of the other cutting edge 7B to one side (the +X side) in the perpendicular direction of the thinning edge 70. The distance LX is set within a range of, for example, 0.005D<LX≦0.06D, where D is the diameter (blade diameter) of the rotational locus of the cutting edge 7 around the central axis O.

[0112] Furthermore, in the drill 20 of the first modified example, the vertices P of the curved portions 60 of the pair of ridgelines 6 overlap each other in the cutting edge length direction (Y-axis direction) of the thinning edge 70. In other words, the pair of thinnings 5 ​​overlap each other when viewed from the cutting edge perpendicular direction (X-axis direction) (i.e., they overlap each other in the cutting edge length direction). The pair of vertices P are spaced apart by a distance LY in the cutting edge length direction while overlapping each other. The distance LY is, for example, in the range of 0.04D≦LY≦0.10D. Note that in the drill 20 of the first modified example, the thinning edge 70 does not have a concave curved edge 71.

[0113] In the drill 20 of the first modification, by raising the thinning edge 70 to the center, the radial rake of the thinning edge 70 can be set to a negative angle. In this case, during drill manufacturing, it is easy to control the dimensions near the chisel portion 9 formed using a grinding wheel and the honing dimensions near the central axis O. Specifically, when forming the thinning edge 70 while moving the grinding wheel in the cutting length direction (Y-axis direction) of the thinning edge 70, it is easy to control the dimensions near the chisel portion 9. This also stabilizes the function near the rotation center (chisel portion 9) of the drill 20.

[0114] It also becomes easy to minimize the chisel portion 9. Specifically, in the case of a two-blade drill 20, of the curved portions 60 of a pair of thinnings 5 ​​arranged with the chisel portion 9 therebetween, the vertices P (the tips of the curved portions 60 in the drill rotation direction T) that are located furthest in the drill rotation direction T can be made to pass each other along the direction in which the thinning blade 70 extends (the cutting edge length direction), thereby minimizing the size of the chisel portion 9. Furthermore, since a larger area for arranging the thinnings 5 ​​(thinning space) can be secured, chip disposal is further improved and cutting resistance is further reduced.

[0115] When the thinning blade 70 is centered and the vertices P of a pair of ridges 6 (curved portions 60) intersect in the blade length direction (Y-axis direction) of the thinning blade 70, as in the drill 20 of the first modified example, this is effective for drilling a wide range of workpiece materials, such as steel, stainless steel, and cast iron.

[0116] 7 is a front view showing a drill 30 according to a second modification of the embodiment. In this drill 30 according to the second modification, the thinning edge 70 is center-down. Furthermore, the vertices P of a pair of ridgelines 6 (curved portions 60) intersect with each other in the cutting length direction (Y-axis direction) of the thinning edge 70.

[0117] The drill 30 of this second modification can also achieve the same effects as those of the above-described embodiment and modification. Furthermore, the chisel portion 9 can be set to its shortest length, allowing for a large thinning space, further reducing cutting resistance. The drill 30 of the second modification is effective for drilling holes in workpieces such as non-ferrous metals, such as aluminum alloys, and non-metals, such as resins.

[0118] 8 is a front view of a drill 40 according to a third modification of the embodiment. In this drill 40 according to the third modification, the thinning edge 70 is center-raised. Furthermore, the vertices P of the pair of ridgelines 6 (curved portions 60) do not cross each other in the cutting length direction (Y-axis direction) of the thinning edge 70.

[0119] The drill 40 of the third modified example can also achieve the same effects as those of the above-described embodiment and modified examples. In particular, it is possible to stably ensure strength in the vicinity of the chisel portion 9. The drill 40 of the third modified example is effective for drilling hard work materials such as high-hardness steel.

[0120] 9 is a front view showing a drill 50 according to a fourth modified example of the embodiment. In this drill 50 according to the fourth modified example, the thinning edge 70 is center-down. Furthermore, the vertices P of a pair of ridgelines 6 (curved portions 60) coincide with each other when viewed from the perpendicular direction (X-axis direction) of the thinning edge 70. That is, the pair of vertices P are located at the same position in the perpendicular direction, and the distance LY is zero.

[0121] Furthermore, in the drill 50 of the fourth modified example, the thinning edge 70 does not have a straight edge 72. In this case, the cutting length direction (Y-axis direction) of the thinning edge 70 is defined as the direction of an imaginary line extending through, for example, the connection point (inflection point) I1 between the first arc-shaped edge 71 a and the second arc-shaped edge 71 b and the connection point (inflection point) I2 between the second arc-shaped edge 71 b and the connecting convex edge 76, as viewed from the tip of the drill as shown in Fig. 9. The cutting edge perpendicular direction (X-axis direction) of the thinning edge 70 is defined as the direction perpendicular to the imaginary line.

[0122] 10 is a front view of a drill 10A according to a fifth modification of the embodiment described above. While the main cutting edge 75 of the drill 10 according to the above embodiment has a straight cutting edge, the main cutting edge 75 of the drill 10A according to the fifth modification has a curved cutting edge, including a concave cutting edge 75a and a convex cutting edge 75b.

[0123] The concave cutting edge 75a is smoothly connected to the radial outer end of the connecting convex cutting edge 76 and has a curved shape that is concave in the opposite direction to the drill rotation direction T. The convex cutting edge 75b is disposed radially outward of the concave cutting edge 75a. The convex cutting edge 75b is smoothly connected to the radial outer end of the concave cutting edge 75a and has a curved shape that is convex in the drill rotation direction T. The radial outer end (outer peripheral corner) B of the convex cutting edge 75b is connected to the leading edge 12. The drill 10A of the fifth modified example also provides excellent effects similar to those of the drill 10 of the above-described embodiment.

[0124] In the above-described embodiment and each of the modified examples, the drills 10, 20, 30, 40, 50, and 10A are double-margin drills having two types of margins (first margin 13A and second margin 13B) spaced apart in the circumferential direction, but the drills are not limited to this. The drills may be single-margin drills having one type of margin, triple-margin drills having three types of margins spaced apart in the circumferential direction, or the like.

[0125] In the above-described embodiment and each of the modified examples, the drills 10, 20, 30, 40, 50, and 10A are two-flute twist drills having two cutting edges 7, but the present invention is not limited to this. The drill may be a three-flute or more drill having three or more cutting edges.

[0126] Although not specifically shown, when the drill is a three-blade drill, when viewed from the tip of the drill, it is preferable that the angle ∠AOB between the radial outer end (intersection point) A of the ridge line 6 centered on the central axis O and the radial outer end (outer peripheral corner) B of the main cutting edge 75 is in the range of 40 to 70° (see Figure 3, etc.).

[0127] Furthermore, the thinning blade 70 does not have to have the straight blade 72. In this case, the concave curved blade 71 of the thinning blade 70 may be directly connected to the connecting convex blade 76.

[0128] Furthermore, the cutting edge 7 does not necessarily have to have the connecting convex edge 76. In this case, the thinning edge 70 and the main cutting edge 75 may be directly connected to each other.

[0129] Furthermore, the drills 10, 20, 30, 40, 50, and 10A described in the above-described embodiment and each modified example are solid drills in which the body 1 and the shank 2 are integrally formed from a single member, but this is not limiting. For example, the body 1 and the shank 2 may be manufactured separately and integrated by brazing or the like.

[0130] Alternatively, although not specifically shown, the drill may be an indexable drill in which a body is detachably attached to a shank. In this case, the shank may be referred to as a holder. Also, the drill may have only a body and not a shank. In this case, the drill may be referred to as a drill head or the like.

[0131] The present invention may be combined with the various configurations described in the above-described embodiments and modifications, and may also include additions, omissions, substitutions, and other modifications of configurations, without departing from the spirit of the present invention. Furthermore, the present invention is not limited to the above-described embodiments, but is limited only by the scope of the claims.

[0132] The drill of the present invention can reduce stress concentration caused by chips extruded from the thinning edge into the thinning, can easily curl the chips to improve chip disposal, can reduce cutting resistance, and can flexibly meet various demands for drills, thus having industrial applicability.

[0133] REFERENCE SIGNS LIST 1 Body 3 Tip surface 4 Chip discharge groove 5 Thinning 6 Ridge line 7, 7A, 7B Cutting edge 8 Outer circumferential surface 10, 10A, 20, 30, 40, 50 Drill 60 Curved portion 61 First arc portion (arc portion) 62 Second arc portion (arc portion) 63 Third arc portion (arc portion) 70 Thinning edge 71 Concave curved edge 81 Curved honing edge O Central axis T Drill rotation direction

Claims

1. A drill having a body extending in the axial direction around a central axis, wherein the body has: a chip discharge flute that opens onto a tip surface and an outer peripheral surface of the body and extends from the tip surface toward the rear end; a concave thinning that is arranged at the tip of the body and connected to the chip discharge flute and the tip surface; and a cutting edge that is arranged at the tip of the body, wherein the cutting edge has a thinning edge that is arranged at the radially inner end of the cutting edge, a ridge line that connects the thinning and the tip surface is continuous with the thinning edge and has a curved portion that is concave in the direction of rotation of the drill around the central axis, and the curved portion has a radius of curvature that increases radially outward.

2. The drill according to claim 1, wherein the curved portion is formed by combining at least three arcuate portions each having a different radius of curvature.

3. The drill according to claim 1, wherein the thinning edge has a concave curved edge that is connected to the curved portion and is curved in a direction opposite to the rotation direction of the drill.

4. The drill according to claim 3, wherein the radius of curvature of the concave curved cutting edge increases as it extends radially outward.

5. The drill according to claim 3, wherein at least the thinning edge of the cutting edges has a honing, and the honing has a curved honing ridge that extends parallel to the concave curved edge and is recessed in the opposite direction to the rotation direction of the drill.

6. The drill according to claim 5, wherein the curved honing ridge has a radius of curvature that increases radially outward.

7. The drill according to claim 1, wherein the thinning edge is centered down.

8. The drill according to claim 1, wherein the thinning edge is centered.

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