drill

Abstract

To provide a drill which can make tool rigidity and chip dischargeability compatible with each other.SOLUTION: A drill of an embodiment of the present invention comprises: a body extending along a central axis; a cutting edge positioned on the tip of the body; and a chip discharge groove extending toward the rear end side of the body from the cutting edge. The body has a first outer diameter section and a second outer diameter section having outer diameters different from each other. The first outer diameter section is a section positioned in a tip portion of the body and a second outer diameter section is contiguous to the rear end side of the first outer diameter section. Each of the first outer diameter section and the second outer diameter section has a back taper shape in which an outer diameter of the body is gradually reduced toward the rear end side from the tip side. A taper amount of the first outer diameter section is larger than a taper amount of the second outer diameter section. A boundary portion between the first outer diameter section and the second outer diameter section has an outer diameter transition section of a recessed surface shape smoothly recessed to the inside in the radial direction of the body.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a drill. [Background technology]

[0002] Conventionally, in a drill for deep hole drilling, from the viewpoint of chip discharge, a configuration has been known in which the core thickness of the portion on the drill tip side is tapered gradually from the drill tip side toward the shank side, and the core thickness is constant from a predetermined position toward the shank side (see, for example, Patent Documents 1 to 3). Furthermore, Patent Document 1 discloses a configuration in which the outer diameter of the drill changes along the axial direction. Specifically, in the drill of Patent Document 1, the large diameter portion located on the drill tip side has a shape that back tapers toward the shank side to become smaller in diameter, and the small diameter portion is a cylinder with a constant diameter. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 3720010 [Patent Document 2] U.S. Pat. No. 9,844,819 [Patent Document 3] JP 2005-169600 A Summary of the Invention [Problem to be solved by the invention]

[0004] Chip discharge performance can be further improved by tapering the outer diameter of the drill toward the shank, as in the drill described in Patent Document 1. However, if a step is provided at the boundary between the large diameter section and the small diameter section of the outer circumferential surface of the drill, there is a problem that stress tends to concentrate at the step where the shape changes suddenly. [Means for solving the problem]

[0005] 1. A drill that rotates about a central axis, the drill comprising: a body extending along the central axis; a cutting edge located at a tip of the body; and a chip discharge groove extending from the cutting edge toward a rear end of the body, the body having a first outer diameter section and a second outer diameter section having different outer diameters from each other, the first outer diameter section being located at a tip portion of the body, the second outer diameter section being a section continuing from a rear end side of the first outer diameter section, the first outer diameter section and the second outer diameter section both having a back taper shape in which the outer diameter of the body gradually becomes thinner from the tip side toward the rear end side, a taper amount of the first outer diameter section being greater than a taper amount of the second outer diameter section, and a smooth outer diameter transition section that is concavely curved and concave radially inward of the body is provided at a boundary between the first outer diameter section and the second outer diameter section.

[0006] According to this configuration, since both the first outer diameter section and the second outer diameter section have a back taper shape, the clearance between the workpiece and the body can be increased, so that sludge accumulation in the machined hole can be suppressed and excellent chip removal performance can be obtained. In addition, an outer diameter transition section having a smooth concave curved surface that is concave toward the inside of the body in the radial direction is provided at the boundary between the first outer diameter section and the second outer diameter section. With this configuration, the taper amount can be gradually changed in the outer diameter transition section, so that stress concentration at the boundary between the first outer diameter section and the second outer diameter section can be suppressed during cutting using the drill. This allows the drill to have sufficient tool rigidity.

[0007] The axial length of the outer diameter transition section may be 50% or less of the outer diameter of the cutting edge, which makes it possible to reduce stress concentration in the outer diameter transition section while maintaining chip discharge performance.

[0008] The taper amount of the second outer diameter section may be 0.01 / 100 or more. By setting the taper amount in this range, the rear end portion of the second outer diameter section can be prevented from becoming excessively thin, and the strength of the drill can be maintained.

[0009] The web of the body may have a first core thickness section and a second core thickness section having different core thicknesses, the first core thickness section being located at the tip of the web, the second core thickness section being a section continuing to the rear end side of the first core thickness section, the first core thickness section and the second core thickness section both having a back taper shape in which the core thickness gradually becomes thinner from the tip side to the rear end side, and the taper amount of the first core thickness section may be greater than the taper amount of the second core thickness section. According to this configuration, in a body having a first outer diameter section and a second outer diameter section both having a back taper shape, it is easy to form a chip discharge groove having a certain depth or more over almost the entire groove length. This makes it possible to suppress the chips from protruding from the chip discharge groove into the clearance, and to smoothly discharge the chips.

[0010] The boundary between the first core thickness section and the second core thickness section may have a core thickness transition section having a smooth concave curved surface recessed inward in the radial direction of the web. With this configuration, the taper amount of the web can be gradually changed in the core thickness transition section, so that chips are less likely to get caught at the boundary between the first core thickness section and the second core thickness section during cutting using a drill. This allows for even better chip discharge.

[0011] The axial position of the core thickness transition section may overlap with the axial position of the outer diameter transition section. With this configuration, the position where the taper amount of the outer diameter of the body changes and the position where the taper amount of the core thickness of the web changes coincide in the axial direction. This makes it possible to suppress the change in the depth of the chip discharge groove in the axial direction, making it easier to discharge chips smoothly.

[0012] The taper amount of the second core thickness section may be 0.01 / 100 or more. By setting the taper amount in this range, it is possible to prevent the rear end portion of the second core thickness section from becoming excessively thin, and to maintain the strength of the drill.

[0013] The body may have a margin extending from the outer circumferential end of the cutting edge toward the rear end along the edge of the chip discharge groove, and the rear end of the margin may be located within the second outer diameter section. With this configuration, the relief surface that serves as a coolant flow path can be extended long toward the rear end, making it easier for the coolant to flow in the clearance between the workpiece and the body, and suppressing sludge accumulation.

[0014] The axial length of the first outer diameter section may be 3 to 9 times the outer diameter of the cutting edge. By setting the axial length in this range, it is easy to control the taper amount in the first outer diameter section. Also, it is possible to prevent the outer diameter of the first outer body from becoming excessively thin.

[0015] The length of the chip groove may be 13 times or more the outer diameter of the cutting edge, which can improve chip discharge performance in a drill that is prone to chip clogging. Effect of the Invention

[0016] According to one aspect of the present invention, a drill is provided that has improved chip evacuation performance while maintaining tool rigidity. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1(a) is a side view of a drill according to an embodiment, and (b) is a schematic cross-sectional view showing a straight chip discharge flute. [Diagram 2] FIG. 2 is an enlarged partial side view showing a tip portion of the drill according to the embodiment. [Diagram 3] FIG. 3 is a front view of the drill according to the embodiment. [Figure 4] FIG. 4 is an enlarged view of the boundary section and its vicinity in FIG. [Diagram 5] FIG. 5(a) is a side view showing a body of a drill of a modified example, and (b) is a schematic cross-sectional view showing a straight chip discharge flute of the drill of the modified example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Fig. 1(a) is a side view of the drill of this embodiment. Fig. 1(b) is a schematic cross-sectional view showing a straight chip flute. Fig. 2 is a partial side view showing an enlarged tip of the drill of this embodiment. Fig. 3 is a front view of the drill of this embodiment. Fig. 4 is an enlarged view of the vicinity of the boundary section of Fig. 1(b).

[0019] The drill 1 of this embodiment is a drill for deep hole drilling. The drill 1 has a generally cylindrical shape centered on a central axis O. The drill 1 has a body 10 and a shank 11, each of which extends along the central axis O. The drill 1 drills a hole in the workpiece by rotating about the central axis O with the tip face of the body 10 facing the workpiece. The drill 1 of this embodiment is a solid drill having a substrate made of cemented carbide etc. The drill 1 may have a hard coating made of metal nitride etc. that coats the surface of the substrate.

[0020] The direction along the central axis O of the drill 1 (the direction in which the central axis O extends) is called the axial direction. In the axial direction, the direction from the shank 11 to the body 10 is called the front end side, and the direction from the body 10 to the shank 11 is called the rear end side. The direction perpendicular to the central axis O is called the radial direction. Of the radial directions, the direction approaching the central axis O is called the radially inner direction, and the direction away from the central axis O is called the radially outer direction. The direction rotating around the central axis O is called the circumferential direction. In addition, within the circumferential direction, the direction in which the body 10 rotates during drilling is called the front side of the rotation direction, and the opposite side is called the rear side of the rotation direction.

[0021] The body 10 has a pair of cutting edges 15, 15 located at the tip of the body 10, and a pair of chip discharge grooves 14, 14 extending spirally around the outer periphery of the body 10 from the cutting edges 15, 15 toward the rear end side. In the drill 1, two coolant holes 9 are formed symmetrically about the central axis O from the rear end face of the shank 11 toward the tip end, twisting with the same lead as the chip discharge groove 14. These coolant holes 9 pass between the chip discharge grooves 14 in the body 10 and open to the tip flank 13. When drilling using the drill 1, coolant such as cutting oil or compressed air is sprayed from the two coolant holes 9. In addition, the tip flank 13 is formed by a two-stage flank whose relief angle becomes larger toward the side opposite the drill rotation direction T. The coolant hole 9 opens to the flank opposite the drill rotation direction T of the two-stage flanks.

[0022] The chip discharge grooves 14 are twisted rearward in the drill rotation direction T at a constant twist angle on the outer periphery of the body 10 from the cutting edge 15 toward the rear end in the direction of the central axis O. The pair of chip discharge grooves 14, 14 are disposed symmetrically with respect to the central axis O. The groove length L0 (length along the direction of the central axis O) of the chip discharge grooves 14 is in the range of 8.0D to 30.0D with respect to the outer diameter D of the cutting edge 15 of the drill 1. The outer diameter of the cutting edge 15 is the outer diameter of a circle formed by a rotation locus around the central axis O at the outer periphery of the cutting edge 15, i.e., the drill diameter.

[0023] The cutting edges 15, 15 are formed at the tip of the body 10 at the intersection ridge between the inner wall surfaces of the pair of chip discharge grooves 14, 14 facing forward in the drill rotation direction T and the tip flank 13. The pair of cutting edges 15, 15 are disposed symmetrically with respect to the central axis O. The cutting edge 15 extends toward the rear end as it approaches the outer periphery (radially outward) of the body 10, and is given a tip angle.

[0024] As shown in Fig. 1(b), the body 10 has a first outer diameter section 21 and a second outer diameter section 22 that have different outer diameters in the axial direction. In Fig. 1(b), the outer diameter and core thickness of the body 10 are shown assuming that the chip discharge groove 14 is a straight groove.

[0025] 1(b), the first outer diameter section 21 is located at the front end of the body 10, and the second outer diameter section 22 is a section that continues to the rear end side of the first outer diameter section 21. Both the first outer diameter section 21 and the second outer diameter section 22 have a back taper shape in which the outer diameter D1 of the body 10 gradually becomes thinner from the front end side to the rear end side.

[0026] The taper amount of the first outer diameter section 21 is larger than that of the second outer diameter section 22. That is, the change amount of the outer diameter D1 at both ends of the first outer diameter section 21 relative to the axial length of the first outer diameter section 21 is larger than the change amount of the outer diameter D1 at both ends of the second outer diameter section 22 relative to the axial length of the second outer diameter section 22. With this configuration, excessive burnishing by the tip of the body 10 can be suppressed, and chip discharge performance can also be improved. The taper amount of the first outer diameter section 21 is, for example, in the range of 0.1 / 100 to 1.0 / 100. The taper amount of the second outer diameter section 22 is, for example, in the range of 0.01 / 100 to 0.1 / 100.

[0027] The taper amount of the second outer diameter section 22 is preferably 0.01 / 100 or more. By setting it in this range, even in a long drill 1 in which the axial length L2 of the second outer diameter section 22 is about 50 times the outer diameter D of the cutting edge 15, for example, it is possible to prevent the end of the second outer diameter section 22 on the shank 11 side from becoming excessively thin while maintaining the back taper shape, and to maintain the strength of the drill 1.

[0028] 4, the drill 1 of this embodiment has an outer diameter transition section 24, which has a smooth concave curved surface that concaves radially inward of the body 10, at the boundary between the first outer diameter section 21 and the second outer diameter section 22. In other words, the first outer diameter section 21 and the second outer diameter section 22 are smoothly connected in the axial direction without any steps. In the outer diameter transition section 24, the taper angle of the outer circumferential surface changes continuously from the first outer diameter section 21 to the second outer diameter section 22.

[0029] In this embodiment, in the section (straight section 21a) of the first outer diameter section 21 excluding the outer diameter transition section 24, the outer diameter D1 decreases at a substantially constant rate toward the rear end of the drill. Also, in the section (straight section 22a) of the second outer diameter section 22 excluding the outer diameter transition section 24, the outer diameter D1 decreases at a substantially constant rate toward the rear end of the drill. The outer diameter transition section 24 is a section in which the taper angle of the outer peripheral surface changes continuously from the taper angle of the straight section 21a to the taper angle of the straight section 22a. Therefore, the taper angle of the outer diameter transition section 24 is in an angle range equal to or greater than the taper angle of the straight section 22a and equal to or less than the taper angle of the straight section 21a.

[0030] The axial length of the outer diameter transition section 24 is preferably 50% or less of the outer diameter of the cutting edge 15. With this configuration, it is possible to reduce stress concentration in the outer diameter transition section 24 while maintaining chip discharge performance. If the taper angle of the straight sections 21a, 22a is fixed and the axial length of the outer diameter transition section 24 is increased, the outer diameter D1 in the outer diameter transition section 24 becomes large, which reduces the gap between the wall surface of the machined hole and the outer peripheral surface of the body 10, tending to reduce chip discharge performance.

[0031] In this embodiment, the axial length L1 of the first outer diameter section 21 is preferably 3 to 9 times the outer diameter D of the cutting edge 15. If the axial length L1 is less than 3 times the outer diameter D of the cutting edge 15, the first outer diameter section 21 is too short, making it difficult to control the back taper amount in the first outer diameter section 21 and reducing manufacturability. On the other hand, if the axial length L1 exceeds 9 times the outer diameter D of the cutting edge 15, the reduction width of the outer diameter D1 at the rear end of the first outer diameter section 21 becomes excessive, making it difficult to maintain tool strength. Alternatively, if the taper amount of the first outer diameter section 21 is reduced to prevent a decrease in strength, the taper amount of the second outer diameter section 22 becomes even smaller, and the effect of improving chip discharge performance by the back taper shaped second outer diameter section 22 is reduced.

[0032] The configuration of this embodiment is suitable for a drill in which the flute length L0 is 13D or more relative to the outer diameter D of the cutting edge 15. A drill in which the flute length L0 is 13D or more is used for machining a hole depth 10 times or more relative to the outer diameter D of the cutting edge 15. In such deep hole machining, chip clogging is significantly likely to occur. According to the drill 1 of this embodiment, the excellent chip discharge performance described above can solve the problem of chip discharge in deep hole machining applications of 10D or more.

[0033] In this embodiment, the web 30 of the body 10 has a first core thickness section 31 and a second core thickness section 32 having different core thicknesses D2. A groove cut-out section 33 at the rear end of the chip discharge groove 14 is continuous with the second core thickness section 32 on the shank 11 side.

[0034] The core thickness D2 of the body 10 is the thickness (diameter) of the web 30 formed by the bottom of the chip discharge groove 14. The first core thickness section 31 is located at the tip of the web 30, and the second core thickness section 32 is continuous with the rear end side of the first core thickness section 31. Both the first core thickness section 31 and the second core thickness section 32 have a back taper shape in which the core thickness D2 gradually becomes thinner from the tip side to the rear end side. The groove cut-out section 33 is the terminal end of the chip discharge groove 14, and is a section where the chip discharge groove 14 becomes shallower toward the rear end of the drill.

[0035] The taper amount of the first core thickness section 31 is larger than the taper amount of the second core thickness section 32. That is, the change amount of the core thickness D2 at both ends of the first core thickness section 31 with respect to the axial length of the first core thickness section 31 is larger than the change amount of the core thickness D2 at both ends of the second core thickness section 32 with respect to the axial length of the second core thickness section 32. This configuration prevents the depth of the chip discharge groove 14 in the first outer diameter section 21 and the depth of the chip discharge groove 14 in the second outer diameter section 22 from differing significantly. The taper amount of the first core thickness section 31 is, for example, in the range of 0.1 / 100 to 1.0 / 100. The taper amount of the second core thickness section 32 is, for example, in the range of 0.01 / 100 to 0.1 / 100. The taper amount of the second core thickness section 32 is preferably 0.01 / 100 or more. By setting the taper amount in this range, the end of the second core thickness section 32 on the shank 11 side can be prevented from becoming excessively thin, and the strength of the drill 1 can be maintained.

[0036] 4, the drill 1 of this embodiment has a core thickness transition section 34 having a smooth concave curved surface recessed radially inward of the web 30 at the boundary between the first core thickness section 31 and the second core thickness section 32. That is, the first core thickness section 31 and the second core thickness section 32 are smoothly connected in the axial direction without any steps. In the core thickness transition section 34, the taper angle of the web 30 changes continuously from the first core thickness section 31 to the second core thickness section 32.

[0037] In this embodiment, in the section (straight section 31a) of the first core thickness section 31 excluding the core thickness transition section 34, the core thickness D2 decreases toward the rear end of the drill at a substantially constant rate. Also, in the section (straight section 32a) of the second core thickness section 32 excluding the core thickness transition section 34, the core thickness D2 decreases toward the rear end of the drill at a substantially constant rate. The core thickness transition section 34 is a section in which the taper angle of the web 30 changes continuously from the taper angle of the straight section 31a to the taper angle of the straight section 32a. Therefore, the taper angle of the core thickness transition section 34 is in an angle range equal to or greater than the taper angle of the straight section 32a and equal to or less than the taper angle of the straight section 31a.

[0038] The axial length of the core thickness transition section 34 is preferably 50% or less of the outer diameter of the cutting edge 15. This configuration makes it easier to obtain good chip discharge performance. If the taper angle of the straight sections 31a, 32a is fixed and the axial length of the core thickness transition section 34 is increased, the core thickness D2 in the core thickness transition section 34 increases, so the depth of the chip discharge groove 14 decreases, and the chip discharge performance tends to decrease.

[0039] In the drill 1 of this embodiment having the above configuration, the body 10 is rotated about the central axis O and fed toward the tip side in the direction of the central axis O, so that the cutting edges 15, 15 located at the tip of the body 10 cut the workpiece to form a machined hole in the workpiece. Chips generated from the cutting edges 15, 15 are discharged through the chip discharge grooves 14, 14 toward the rear end side in the direction of the central axis O.

[0040] In this embodiment, both the first outer diameter section 21 and the second outer diameter section 22 have a back taper shape in which the outer diameter D1 gradually decreases from the tip of the drill toward the shank 11. This configuration increases the clearance between the workpiece and the body 10, suppressing sludge accumulation in the drilled hole and providing excellent chip discharge performance.

[0041] In this embodiment, an outer diameter transition section 24 having a smooth concave curved surface recessed radially inward of the body 10 is provided at the boundary between the first outer diameter section 21 and the second outer diameter section 22. This configuration allows the taper amount to be gradually changed in the outer diameter transition section 24, thereby preventing stress from concentrating at the boundary between the first outer diameter section 21 and the second outer diameter section 22 during cutting using the drill 1. This allows the drill 1 to have sufficient tool rigidity.

[0042] In this embodiment, both the first core thickness section 31 and the second core thickness section 32 of the web 30 have a back taper shape in which the core thickness D2 gradually decreases from the drill tip toward the shank 11. This configuration makes it easier to form a chip discharge groove 14 having a certain depth or more over almost the entire groove length L0 in the body 10 having the first outer diameter section 21 and the second outer diameter section 22, both of which have a back taper shape. This makes it possible to suppress the chips from spilling out of the chip discharge groove 14 into the clearance, and to smoothly discharge the chips.

[0043] In this embodiment, a core thickness transition section 34 having a smooth concave curved surface recessed radially inward of the web 30 is provided at the boundary between the first core thickness section 31 and the second core thickness section 32. This configuration allows the taper amount of the web 30 to be gradually changed in the core thickness transition section 34, so that during cutting using the drill 1, chips are less likely to get caught at the boundary between the first core thickness section 31 and the second core thickness section 32. This allows for even better chip discharge performance.

[0044] In this embodiment, as shown in Fig. 4, the axial positions of the core thickness transition section 34 and the outer diameter transition section 24 overlap with each other. With this configuration, the position where the taper amount of the outer diameter D1 of the body 10 changes and the position where the taper amount of the core thickness D2 of the web 30 changes coincide with each other in the axial direction. This makes it possible to suppress changes in the depth of the chip discharge groove 14 in the axial direction, facilitating smooth discharge of chips.

[0045] Furthermore, in this embodiment, the taper amount of the outer diameter D1 of the first outer diameter section and the taper amount of the core thickness D2 of the first core thickness section 31 are approximately equal. The taper amount of the outer diameter D1 of the second outer diameter section 22 and the taper amount of the core thickness D2 of the second core thickness section 32 are approximately equal. With this configuration, the depth of the chip discharge groove 14 can be made approximately constant throughout the first core thickness section 31 and the second core thickness section 32. Chips are less likely to clog the inside of the chip discharge groove 14. The taper amount of the outer diameter D1 of the first outer diameter section and the taper amount of the core thickness D2 of the first core thickness section 31 can also be made different from each other. The taper amount of the outer diameter D1 of the second outer diameter section 22 and the taper amount of the core thickness D2 of the second core thickness section 32 can also be made different from each other. That is, the axial depth distribution of the chip discharge groove 14 may be adjusted by making the taper amounts of the outer diameter D1 and the core thickness D2 different.

[0046] The body 10 has lands 16, 16 between the two chip discharge grooves 14, 14. A first margin 16A is provided at the end of the land 16 on the front side in the drill rotation direction T. A second margin 16B is provided at the end of the land 16 on the rear side in the drill rotation direction T. The first margin 16A and the second margin 16B extend spirally along the edge of the chip discharge groove 14. The body 10 has a relief surface 16C recessed radially inward between the first margin 16A and the second margin 16B.

[0047] The first margin 16A and the second margin 16B are formed only at the tip portion of the body 10. In this embodiment, the axial length L3 of the cut-off surface 16C shown in FIG. 1 is about 40% of the groove length L0 of the chip groove 14. In the drill 1 of this embodiment, the rear end of the first margin 16A or the rear end of the cut-off surface 16C is located within the second outer diameter section 22. With this configuration, the cut-off surface 16C, which serves as a coolant flow path, can be extended farther toward the rear end, making it easier for the coolant to flow in the clearance between the workpiece and the body 10, and suppressing sludge accumulation.

[0048] The rear end position of the first margin 16A or the second chamfered surface 16C in the second outer diameter section 22 can be changed as appropriate. In the drill 1 of this embodiment, since the second outer diameter section 22 has a back taper shape, even if the axial length L3 of the second chamfered surface 16C is relatively short, the coolant can easily flow into the clearance of the second outer diameter section 22. By shortening the axial length L3 of the second chamfered surface 16C, the thickness of the land 16 can be ensured, and the tool rigidity can be easily maintained. The rear end position of the first margin 16A or the second chamfered surface 16C can be, for example, 5% to 40% or 10% to 30% of the axial length L2 of the second outer diameter section 22 from the tip position of the second outer diameter section 22.

[0049] The configurations of the first margin 16A and the second margin 16B are not limited to the present embodiment. For example, if the clearance between the workpiece and the body 10 can be increased, the first margin 16A or the rear end of the second chamfering surface 16C may be disposed within the first outer diameter section 21. The second margin 16B may be provided as necessary, and the body 10 may be configured without the second margin 16B.

[0050] (Modification) Fig. 5(a) is a side view showing the body 10 of the drill 1 of the modified example, and Fig. 5(b) is a schematic cross-sectional view showing the chip discharge flute 14 of the drill 1 of the modified example as a straight flute. The drill 1 shown in FIG. 5 differs from the shapes shown in FIGS. 1 to 4 in the shape of the core thickness transition section 34 of the web 30. Specifically, the core thickness transition section 34 of the modified example is smoothly connected to the straight section 32a of the second core thickness section 32 without any steps, but is connected to the straight section 31a of the first core thickness section 31 on the tip side via a step 34a.

[0051] In the drill 1 of the modified example, the taper angle, which is the inclination angle with respect to the central axis O, increases toward the tip side (straight section 31a side). The taper angle of the core thickness transition section 34 is maximum at the connection portion between the core thickness transition section 34 and the straight section 31a. The taper angle of the core thickness transition section at the connection portion between the core thickness transition section 34 and the straight section 31a is larger than the taper angle of the straight section 31a. On the other hand, the taper angle of the core thickness transition section 34 matches the taper angle of the straight section 32a at the connection portion with the straight section 32a on the rear end side.

[0052] In the configuration of this modified example, there is a step 34a on the tip side of the core thickness transition section 34, but the wall surface of this step faces the rear end side of the drill 1. Since chips generated by the cutting edge 15 move in the chip discharge groove 14 from the tip side to the rear end of the drill, the step 34a does not substantially impede the movement of the chips. Furthermore, according to the configuration of this modified example, the tip side end of the core thickness transition section 34 is stepped, so that the depth of the chip discharge groove 14 can be secured large in the second core thickness section 32. This makes it easier to discharge chips that have entered the second core thickness section 32 to the rear end side.

[0053] Furthermore, by providing the step 34a in the core thickness transition section 34, the axial length of the core thickness transition section 34 can be shortened. This makes it possible to increase the axial length of either or both of the straight section 31a of the first core thickness section 31 and the straight section 32a of the second core thickness section 32. This makes it easier to manage the taper amount of the straight sections 31a and 32a, improving manufacturability.

[0054] The shape of the outer diameter transition section 24 of this modified example is the same as the shape of the embodiment shown in Figures 1 and 4. Therefore, even during cutting using the drill 1 of the modified example, it is possible to suppress stress concentration in the outer diameter transition section 24.

[0055] In this modified example, the axial position of the core thickness transition section 34 overlaps with the axial position of the outer diameter transition section 24. The position where the taper angle of the outer diameter D1 changes coincides with the position where the taper angle of the core thickness D2 changes. Within each of the first core thickness section 31 and the second core thickness section 32, the depth of the chip discharge groove 14 is approximately constant. This makes it easier to stabilize the behavior of chips moving inside the chip discharge groove 14. [Explanation of symbols]

[0056] 1. Drill 9…Coolant hole 10. Body 11...Shank 13...Tip relief surface 14...Chip discharge groove 15…Cutting edge 16…Rand 16A…First margin 16B…Second margin 16C...Second cut surface 21…First outer diameter section 21a, 22a, 31a, 32a...Straight section 22…Second outer diameter section 24...Outer diameter transition section 30…Web 31…First core thickness section 32…Second core thickness section 33... Groove cutting section 34...Core thickness transition section 34a…Step D…Outer diameter of cutting edge D1…Outer diameter D2…core thickness L0…Groove length L1, L2, L3...Axial length O…Central axis T…Drill rotation direction

Claims

1. A drill rotated about a central axis, The cutting tool has a body extending along a central axis, a cutting edge located at a tip end of the body, and a chip discharge groove extending from the cutting edge toward a rear end side of the body, The body has a first outer diameter section and a second outer diameter section having different outer diameters, the first outer diameter section is located at a front end portion of the body, and the second outer diameter section is a section continuous with a rear end side of the first outer diameter section, the first outer diameter section and the second outer diameter section each have a back taper shape in which an outer diameter of the body gradually decreases from a front end side toward a rear end side, a taper amount of the first outer diameter section is greater than a taper amount of the second outer diameter section; a boundary between the first outer diameter section and the second outer diameter section has an outer diameter transition section having a smooth concave curved surface recessed radially inward of the body; drill.

2. The axial length of the outer diameter transition section is 50% or less of the outer diameter of the cutting edge. The drill of claim 1.

3. The taper amount of the second outer diameter section is 0.01 / 100 or more. A drill according to claim 1 or 2.

4. The web of the body has a first core thickness section and a second core thickness section having different core thicknesses, the first core thickness section is located at a front end portion of the web, and the second core thickness section is a section continuous with a rear end side of the first core thickness section, the first core thickness section and the second core thickness section each have a back taper shape in which the core thickness gradually decreases from a front end side to a rear end side, A taper amount of the first core thickness section is greater than a taper amount of the second core thickness section. A drill according to any one of claims 1 to 3.

5. A core thickness transition section having a smooth concave curved surface recessed toward the radially inward direction of the web is provided at a boundary between the first core thickness section and the second core thickness section.

5. The drill of claim 4.

6. The axial position of the core thickness transition section overlaps with the axial position of the outer diameter transition section. The drill according to claim 5.

7. The taper amount of the second core thickness section is 0.01 / 100 or more. A drill according to any one of claims 4 to 6.

8. the body has a margin extending from an outer circumferential end of the cutting edge toward a rear end along an edge of the chip discharge groove, The rear end of the margin is located within the second outer diameter section. A drill according to any one of claims 1 to 7.

9. The axial length of the first outer diameter section is 3 times or more and 9 times or less than the outer diameter of the cutting edge. A drill according to any one of claims 1 to 8.

10. The groove length of the chip discharge groove is 13 times or more the outer diameter of the cutting blade. A drill according to any one of claims 1 to 9.

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