drill

Abstract

[Problem] The present invention addresses the problem of providing a drill that suppresses the generation of burrs around a machined hole. [Solution] A drill 10 according to the present invention comprises two or more cutting edges 1 to 4 applied outward from a central axis O, two or more flanks formed outward from the central axis O, and a thinning face 7 applied on the central axis O side, wherein the cutting edges are formed from a first cutting edge 1 which is adjacent to the thinning face 7 and which extends from a chisel edge 1 toward the outside of the drill 10, a second cutting edge 2 having a straight-line part that extends from an end portion of the first cutting edge 1 toward the outside of the drill 10, a third cutting edge 3 which extends from an end portion of the second cutting edge 2 toward the rear in a circumferential direction and toward the outside of the drill 10, and a fourth cutting edge 4 which extends from an end portion of the third cutting edge 3 toward the rear of the drill 10 and is connected to a leading edge. Furthermore, a tip angle (first tip angle) α1 formed by the first cutting edge 1 is equal to or less than a tip angle (second tip angle) α2 formed by the second cutting edge 2, and the tip angle (first tip angle) α1 formed by the first cutting edge 1 lies in a range of 90° to 140°.

Description

Technical Field

[0001] The present invention relates to a drill that can suppress burrs generated around a processed hole on the penetration side of a workpiece after drilling.

Background Art

[0002] Conventionally, in drilling of metal materials typified by steel materials using a drill, burrs are generated around the hole on the penetration side after processing. Therefore, a deburring process is added after the drilling process to remove the burrs. Therefore, drills capable of reducing burrs generated after drilling are disclosed in Patent Documents 1 to 3.

[0003] However, these drills have a problem that burrs still remain around the hole on the penetration side after drilling depending on the type of workpiece material. Therefore, Patent Document 4 discloses a plurality of types of drills classified according to each workpiece material and having different forms.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the drill disclosed in Patent Document 4 has a problem in that an inflection point occurs at the connection point (joint portion) between the cutting edge at the tip of the drill and the margin at the rear of the drill, which can cause burrs or rough edges to form around the machined hole on the through side of the workpiece depending on the cutting conditions such as high rotation speed or high feed rate.

[0006] Therefore, the object of the present invention is to provide a drill that suppresses the generation of burrs around the machined hole, regardless of cutting conditions such as high rotation speed or high feed rate. [Means for solving the problem]

[0007] To solve the aforementioned problems, the drill of the present invention has a plurality of cutting edges extending outward from the central axis, two or more relief surfaces adjacent to each of these cutting edges, and a plurality of thinning surfaces on the central axis side, wherein the cutting edges are formed from a first cutting edge (thinning cutting edge) extending outward from the chisel edge adjacent to the thinning surface, a second cutting edge (main cutting edge) having a straight portion extending outward from the end of the first cutting edge, a third cutting edge extending outward, circumferentially and rearward from the end of the second cutting edge, and a straight or curved fourth cutting edge extending from the end of the third cutting edge toward the rear side (shank side) of the drill and connected to the leading edge. The tip angle formed by the first cutting edge shall be less than or equal to the tip angle formed by the second cutting edge, and the tip angle formed by the first cutting edge shall be in the range of 90° to 140°. Furthermore, if both the third and fourth cutting edges are curved, the radius of curvature r4 of the fourth cutting edge can be made larger than the radius of curvature r3 of the third cutting edge.

[0008] Furthermore, the fourth cutting edge can be connected to the leading edge via the outermost corner of the drill, and the flank adjacent to the fourth cutting edge can be formed continuously with the drill's margin, so that the angle between the ridge, which is the boundary between the flank adjacent to the fourth cutting edge and the margin, and the leading edge is in the range of 4° to 25°. Furthermore, the relief angles of the third and fourth cutting edges are preferably in the range of 5° to 11°. [Effects of the Invention]

[0009] The drill of the present invention maintains the centering of the drill during cutting (drilling) regardless of the type of workpiece or processing conditions such as high rotation speed or high feed rate, while also breaking up and discharging chips to the outside, and further suppressing the generation of burrs around the processed hole. [Brief explanation of the drawing]

[0010] [Figure 1] This is a front view of the drill 10 of the present invention. [Figure 2] Figure 1 is a right side view of the drill 10. [Figure 3] Figure 1 is a view of drill 10 as seen through arrow A. [Figure 4] This is a schematic projection view of the drill 10 of the present invention in the longitudinal direction. [Figure 5] This is a schematic enlarged view (first embodiment) of the vicinity of the third cutting edge 3 and the fourth cutting edge 4 of the drill 10 of the present invention. [Figure 6] This is a schematic enlarged view (second embodiment) of the vicinity of the third cutting edge 3 and the fourth cutting edge 4 of the drill 10 of the present invention. [Figure 7] This is a schematic enlarged view of the tip of drill 100, which is comparative material 1 (tip angle: 180°) used in the example. [Figure 8] This is a schematic enlarged view of the tip of the drill 200, which is comparative material 2 (tip angle: 135°) used in the example. [Figure 9] This graph shows the results of the machining test for the example. [Figure 10] This is a schematic diagram showing a cap-shaped burr BR that formed on the through-hole H side of the workpiece in the embodiment. [Modes for carrying out the invention]

[0011] One embodiment of the drill of the present invention will be described with reference to the drawings. A front view of the drill 10 according to one embodiment of the present invention is shown in FIG. 1, a right side view thereof is shown in FIG. 2, and a view taken along arrow A of the drill 10 shown in FIG. 1 is shown in FIG. 3. The drill 10 of the present invention includes first to fourth cutting edges 1 to 4, a thinning surface 6, and relief surfaces 11(11A, 11B), 51(51A, 51B), 52(52A, 52B) formed adjacent to the first to fourth cutting edges 1 to 4, which extend from the chisel edge T located at the tip of the drill 10 in the radial direction (outward) of the drill 10 as shown in FIGS. 1 and 2.

[0012] In particular, the relief surface (first relief surface) 51(51A, 51B) adjacent to the first cutting edge 1(1A, 1B) and the relief surface (second relief surface) 52(52A, 52B) adjacent to the second cutting edge 2(2A, 2B) are formed as separate and independent relief surfaces as shown in FIGS. 1 to 3.

[0013] Further, in the drill of the present invention, the first cutting edge (so-called inner edge) 1(1A, 1B) formed adjacent to the chisel edge T on the center axis O side as shown in FIG. 2 protrudes in the tip direction of the drill 10 as it approaches the center axis O (chisel edge T) with respect to the second cutting edge (so-called outer edge) 2(2A, 2B) formed on the outer peripheral side.

[0014] That is, by configuring the relief surface (first relief surface) 51(51A, 51B) adjacent to the first cutting edge 1(1A, 1B), the relief surface (second relief surface) 52(52A, 52B) adjacent to the second cutting edge 2(2A, 2B), and the first cutting edge (so-called inner edge) formed adjacent to the chisel edge T on the center axis O side to protrude in the tip direction of the drill with respect to the second cutting edge (so-called outer edge) formed on the outer peripheral side, while maintaining the centering of the drill during cutting (drilling), the chips can be separated and discharged to the outside.

[0015] The first cutting edge 1(1A, 1B) is continuously formed from the chisel edge T in the radial direction of the drill 10 and is also called a "thinning cutting edge" because it is adjacent to the thinning surface 6. The first cutting edge 1 plays a role in preventing drill runout, or in other words, maintaining the centering of the drill, during hole drilling with the drill of the present invention. Furthermore, a second cutting edge 2 (2A, 2B) is continuously formed from the end of the first cutting edge 1, having a straight section that extends outward from the drill 10. This cutting edge is also called the "main cutting edge".

[0016] The second cutting edge 2 (2A, 2B) shown in Figures 1 to 3 is a cutting edge that extends linearly outward from the end of the first cutting edge 1, but the second cutting edge 2 (2A, 2B) is not limited to this form. For example, when the workpiece is a light metal such as aluminum alloy or a difficult-to-machine material such as stainless steel, the second cutting edge 2 (2A, 2B) can be configured to include a straight section in part or to be a combination of a straight section and a curved section.

[0017] As shown in Figures 2 and 3, this second cutting edge 2 (2A, 2B) has a rake face 7 (7A, 7B) and a groove (helical groove) 8 (8A, 8B) formed continuously therewith. Since the second cutting edge 2 has a straight section, it plays a role in breaking up and discharging the chips generated during hole drilling with the drill of the present invention, without continuously generating chips. Furthermore, the drill 10 of this embodiment is equipped with two second cutting edges 2A and 2B, and the tip angle (second tip angle) α2 formed by these two second cutting edges 2A and 2B is greater than or equal to the tip angle (first tip angle) α1 formed by the two first cutting edges 1A and 1B, as shown in Figures 2 and 3.

[0018] Here, "tip angle" refers to "the angle obtained when the cutting edge is projected parallel to a plane parallel to the drill shaft," as defined in Japanese Industrial Standards (JIS) B0171. In other words, the tip angle α1 formed by the first cutting edges 1A and 1B is the angle projected parallel to the axis (central axis O) of the drill 10 onto a plane parallel to the axis (central axis O), as shown in Figure 3, and the tip angle α2 formed by the second cutting edges 2A and 2B is the angle projected parallel to the axis (central axis O) of the drill 10 onto a plane parallel to the axis (central axis O), as shown in Figure 2.

[0019] Furthermore, by setting the tip angle (first tip angle) α1 formed by the first cutting edge 1 (1A, 1B) to a range of 90° to 140°, it is possible to improve the centering of the drill 10 (prevention of drill runout) and the bite (initial machinability on flat workpieces) at the start of hole drilling.

[0020] Next, Figure 4 shows a schematic projection of the drill 10 in the longitudinal direction (axial direction) of this embodiment, and Figures 5 and 6 show schematic enlarged views of the vicinity of the third cutting edge 3 and the fourth cutting edge 4 of the drill 10 in the first and second embodiments, respectively. As shown in Figures 2 and 3, the drill 10 of this embodiment has a third cutting edge 3 (3A, 3B) extending outward and circumferentially from the end of the second cutting edge 2 (2A, 2B) and toward the rear of the drill 10, and a fourth cutting edge 4 (4A, 4B) extending toward the rear of the drill 10 from the end of the third cutting edge 3 (3A, 3B) and connected to the leading edge. The formation of the third cutting edge 3 (3A, 3B) and the fourth cutting edge 4 (4A, 4B) suppresses the generation of burrs around the through-hole during through-hole machining.

[0021] The third cutting edge 3 (3A, 3B) can also have a curved cutting edge shape as shown in Figures 1 to 4, so for example, two or more cutting edges with different radii of curvature can be combined. In this case, the radius of curvature of the cutting edge connected to the end of the second cutting edge 2 (2A, 2B) can be made smaller than the radius of curvature of the cutting edge connected to the end of the fourth cutting edge 4 (4A, 4B). In other words, the radius of curvature of the third cutting edge 3 (3A, 3B), which is a curved cutting edge, can be gradually increased from the tip side to the rear end side of the drill 10.

[0022] The fourth cutting edge 4 is either curved or straight (not shown) and is connected to the leading edge 5 via an outer circumferential corner toward the rear side of the drill 10, as shown in Figure 5. In other words, the connection point between the fourth cutting edge 4 and the leading edge 5 corresponds to the outer corner of the drill 10 in this embodiment, and is the outermost position of the drill 10. Furthermore, as shown in Figure 4, if the third cutting edge 3 and the fourth cutting edge 4 are formed in a curved shape, the radius of curvature r4 of the fourth cutting edge 4 may be made larger than the radius of curvature r3 of the third cutting edge 3.

[0023] Furthermore, as shown in Figure 5, the relief surface 11 adjacent to the fourth cutting edge 4 is formed continuously with the margin 12 of the drill 10 in this embodiment, and the connection from the relief surface 11 of the fourth cutting edge 4 to the margin 12 changes gradually in a gradual inclination from the tip side to the rear end side of the drill 10. The straight ridge line 20 (first embodiment), which is the boundary between the relief surface 11 and the margin 12 adjacent to the fourth cutting edge 4, can have an angle θ (θ1) with the leading edge 5 in the range of 4° to 25°.

[0024] Furthermore, as a different embodiment of the boundary between the relief surface 11 and the margin 12, the ridge line 21 (second embodiment), which is the boundary between the relief surface 11 and the margin 12, may be composed of a straight portion and a curved portion, as shown in Figure 6. In this case as well, it is preferable that the angle θ(θ2) that the ridge line 21 makes with the leading edge 5 is in the range of 4° to 25°.

[0025] Furthermore, the angle θ(θ1,θ2) can be optimized according to various specifications such as the diameter (drill diameter) φD0 of the drill 10, the angle of the rake face 7 (7A, 7B), the angle of the groove (helix groove) 8 (8A, 8B), the axial length L1 of the third cutting edge 3 (3A, 3B) and the fourth cutting edge 4 (4A, 4B), and the radii of curvature r3, r4.

[0026] Furthermore, the axial length L1 of the third cutting edge 3 (3A, 3B) and the fourth cutting edge 4 (4A, 4B) in the drill 10 of the present invention is preferably in the range of 0.10 to 0.40 × D0, with respect to the diameter D0 of the drill 10, as shown in Figure 4.

[0027] Furthermore, in the drill 10 of the present invention, the radial distance (shortest distance) E1 from the end of the second cutting edge 2 (2A, 2B) to the outermost part of the drill 10 (a virtual straight line parallel to the axial direction of the drill 10 and passing through the outer corner) is preferably in the range of 0.01 to 0.20 × D0 when the diameter D0 of the drill 10 is used as the reference, as shown in Figure 4. [Examples]

[0028] A cutting test (hereinafter referred to as "this test") was conducted using a drill according to an embodiment of the present invention and a conventional drill, and the test results will be described below. The drill according to the present invention (hereinafter referred to as the "inventive material") used in this test was the same as the drill shown in Figures 1 to 4 (first tip angle: 135°, second tip angle: 135°, radius of curvature r3 of the third cutting edge: 3.8 mm, angle θ = 10° between the leading edge and the straight edge which is the boundary between the relief face and margin of the fourth cutting edge).

[0029] In contrast, two types of conventional drills (hereinafter referred to as "comparative materials") were used: Comparative Material 1 (drill 100 shown in Figure 7), in which the cutting edge is mainly composed of a cutting edge (main cutting edge) corresponding to the second cutting edge of the inventive material, and the tip angle formed by this cutting edge is approximately 180°; and Comparative Material 2 (drill 200 shown in Figure 8), in which the tip angle is 135°. Furthermore, both the inventive material and comparative materials 1 and 2 share a common specification of a diameter (drill diameter) of 6 mm, and the surfaces of the inventive material and comparative materials 1 and 2 are coated with an AlTi-based hard coating.

[0030] The workpiece material and machining conditions used in this test are as follows. • Workpiece material: 24mm thick carbon steel (S50C) • Rotational speed: 4670 min-1 (inventive material), 3979 min-1 (comparative material 1), 5305 min-1 (comparative material 2) Feed rate: 1120 mm / min (inventive material), 418 mm / min (comparative material 1), 1273 mm / min (comparative material 2) • Cutting speed: 88.0 m / min (inventive material), 75.0 m / min (comparative material 1), 100.0 m / min (comparative material 2) • Feed rate: 0.24 mm / rev (inventive material and comparative material 2), 0.105 mm / rev (comparative material 1) • Step processing: None • Projection length: 47mm · Processing form: Through hole (processing length 24mm) • Cooling method: External lubrication using water-soluble cutting fluid. ·Equipment used: Vertical M / C (BT40)

[0031] In this test, three types of drills, including the inventive material and two comparative materials, were used to machine the workpiece (S50C) under the aforementioned machining conditions until the total number of machined holes reached 6000 (cumulative number of machined holes). The height of the burrs generated around the machined holes (exit side) was measured using a non-contact 3D shape measuring instrument. Figure 9 shows the results of measuring the burr height around the machined hole after cutting in this test. At the same time, Table 1 shows the results of measuring the cutting resistance (X component force, Y component force) during drilling using each drill in the inventive material and comparative material 2, using a dynamometer.

[0032] [Table 1]

[0033] First, we will explain the measurement results of the cutting resistance values ​​(X component force, Y component force, amplitude) during drilling when using each drill in the inventive material and comparative material 2 in this test. As shown in Table 1, the X component force of the cutting resistance amplitude generated when using the drill of comparative material 2 was 46 N, while the X component force of the cutting resistance amplitude generated when using the drill of the inventive material was 35 N, resulting in a 24% reduction in the measured X component force compared to the X component force of the drill of comparative material 2.

[0034] Similarly, while the Y component force of the cutting resistance amplitude generated when using the drill of comparative material 2 was 53 N, the Y component force of the cutting resistance amplitude generated when using the drill of the inventive material was 37 N, representing a 30% reduction in the measured Y component force compared to the Y component force of the drill of comparative material 2.

[0035] Based on the above measurement results, it was confirmed that by setting the tip angle α1 formed by the first cutting edge (first tip angle) α1, as in the drill of the invention, to be less than or equal to the tip angle α2 formed by the second cutting edge (second tip angle), and by setting the first tip angle α1 in the range of 90° to 140°, the centering of the drill at the start of hole drilling (prevention of drill runout) and the bite (initial machinability on flat workpieces) were improved.

[0036] Next, in the graph shown in Figure 9, the horizontal axis shows the change in the number of machined holes using the three types of drills described above that were used in this test, and the vertical axis shows the maximum value (in μm) of the burr height at the edge of the machined hole for each number of machined holes. As shown in Figure 9, the burr height around the holes after machining using the inventive material was 13 μm for the first hole up to a total of 5500 holes. Subsequent burr heights ranged from 19 to 39 μm, reaching a total of 97 μm when the total number of machined holes reached 6000.

[0037] In contrast, in tests using comparative materials 1 and 2, a cap-shaped burr BR, as shown in Figure 10, was observed from the beginning in the processed hole H (through-hole side). Furthermore, the burr heights around the holes after machining were 98 μm and 180 μm for the first hole, and thereafter, the burr height increased as the number of machined holes increased, reaching 125 μm and 205 μm when the total number of machined holes reached 2000. Furthermore, when the total number of holes drilled using the drill for comparative material 1 reached 2000, a defect occurred in the corner (outer corner), and therefore the cutting test using the drill for comparative material 1 was terminated at that point.

[0038] Ultimately, the cutting test using the drill on comparative material 2 was carried out until the total number of machined holes reached 6000. As a result, wear progressed significantly in the corner areas (outer corners), and the burr height became 451 μm, as shown in Figure 9. From the above cutting test results, it was confirmed that the drill made of the inventive material significantly reduces the burr height generated during hole drilling compared to the drills made of the two comparative materials. Thus, in the drill of the invention, the third cutting edge 3 (3A, 3B) and the fourth cutting edge 4 (4A, 4B) were found to have the effect of suppressing the generation of burrs around the through-hole when machining through-holes. [Industrial applicability]

[0039] The drill according to the present invention exhibits excellent centering during drilling, regardless of the type of workpiece or processing conditions, and suppresses the generation of burrs around the processed hole, making it widely applicable as a cutting drill. [Explanation of Symbols]

[0040] 1. First cutting edge 2. Second cutting edge 3. Third cutting edge 4. Fourth cutting edge 5 Leading Edge 6. Thinning surface 7 Scoop surface 8 grooves (helical grooves) 10 Drills 11 Escape 12 margin 20,21 Ridge that is the boundary between the escape face and the margin 51 First escape route 52 Second escape route D0 Drill diameter (drill size) O center axis T Chisel Edge r3 Radius of curvature of the third cutting edge r4 Radius of curvature of the fourth cutting edge α1 Tip angle formed by the first cutting edge (first tip angle) α2 The tip angle formed by the second cutting edge (second tip angle) θ(θ1,θ2) Angle between the ridge and the leading edge

Claims

1. Two or more cutting edges extending outward from the central axis, Two or more relief surfaces formed outward from the central axis, A drill having a thinning surface applied to the central axis side, The cutting blade is A first cutting edge extending outward from the chisel edge of the drill, adjacent to the thinning surface, A second cutting edge having a straight portion extending outward from the end of the first cutting edge, A straight or curved third cutting edge extends from the end of the second cutting edge outward, circumferentially and rearward from the drill, A straight fourth cutting edge extends from the end of the third cutting edge opposite to the first cutting edge toward the rear of the drill and is connected to the leading edge, It is formed from, When the third cutting edge is straight, the fourth cutting edge has a different inclination angle than the third cutting edge. If the tip angle is defined as the angle obtained when the cutting edge is projected parallel to a plane parallel to the drill's axis, The tip angle α1, which is the angle formed between the two first cutting edges, is less than or equal to the tip angle α2, which is the angle formed between the two second cutting edges. Furthermore, the tip angle α1 is in the range of 90° to 140°. The fourth cutting edge is connected to the leading edge via the outer corner, Furthermore, the relief surface adjacent to the fourth cutting edge is formed continuously with the margin of the drill. A drill characterized in that the angle θ formed between the leading edge and the ridge line that forms the boundary between the relief surface adjacent to the fourth cutting edge and the margin is in the range of 4° to 25°.

2. Two or more cutting edges extending outward from the central axis, Two or more relief surfaces formed outward from the central axis, A drill having a thinning surface applied to the central axis side, The cutting blade is A first cutting edge extending outward from the chisel edge of the drill, adjacent to the thinning surface, A second cutting edge having a straight portion extending outward from the end of the first cutting edge, A curved third cutting edge extends from the end of the second cutting edge outward, circumferentially and rearward from the drill, A straight fourth cutting edge extends from the end of the third cutting edge toward the rear of the drill and is connected to the leading edge, It is formed from, If the tip angle is defined as the angle obtained when the cutting edge is projected parallel to a plane parallel to the drill's axis, The tip angle α1, which is the angle formed between the two first cutting edges, is less than or equal to the tip angle α2, which is the angle formed between the two second cutting edges. Furthermore, the tip angle α1 is in the range of 90° to 140°. The fourth cutting edge is connected to the leading edge via the outer corner, Furthermore, the relief surface adjacent to the fourth cutting edge is formed continuously with the margin of the drill. A drill characterized in that the angle θ formed between the leading edge and the ridge line that forms the boundary between the relief surface adjacent to the fourth cutting edge and the margin is in the range of 4° to 25°.

3. Two or more cutting edges extending outward from the central axis, Two or more relief surfaces formed outward from the central axis, A drill having a thinning surface applied to the central axis side, The cutting blade is A first cutting edge extending outward from the chisel edge of the drill, adjacent to the thinning surface, A second cutting edge having a straight portion extending outward from the end of the first cutting edge, A curved third cutting edge extends from the end of the second cutting edge outward, circumferentially and rearward from the drill, It is formed from a curved fourth cutting edge that extends from the end of the third cutting edge toward the rear side of the drill and is connected to the leading edge, and has a different curvature from the third cutting edge, If the tip angle is defined as the angle obtained when the cutting edge is projected parallel to a plane parallel to the drill's axis, The tip angle α1, which is the angle formed between the two first cutting edges, is less than or equal to the tip angle α2, which is the angle formed between the two second cutting edges. A drill characterized in that the tip angle α1 is in the range of 90° to 140°, the third cutting edge is a curved cutting edge, and the radius of curvature r4 of the fourth cutting edge is larger than the radius of curvature r3 of the third cutting edge.

4. Two or more cutting edges extending outward from the central axis, Two or more relief surfaces formed outward from the central axis, A drill having a thinning surface applied to the central axis side, The cutting blade is A first cutting edge extending outward from the chisel edge of the drill, adjacent to the thinning surface, A second cutting edge having a straight portion extending outward from the end of the first cutting edge, A curved third cutting edge extends from the end of the second cutting edge outward, circumferentially and rearward from the drill, It is formed from a curved fourth cutting edge that extends from the end of the third cutting edge toward the rear side of the drill and is connected to the leading edge, and has a different curvature from the third cutting edge, If the tip angle is defined as the angle obtained when the cutting edge is projected parallel to a plane parallel to the drill's axis, The tip angle α1, which is the angle formed between the two first cutting edges, is less than or equal to the tip angle α2, which is the angle formed between the two second cutting edges. Furthermore, the tip angle α1 is in the range of 90° to 140°. The radius of curvature r4 of the fourth cutting edge is greater than the radius of curvature r3 of the third cutting edge, and the fourth cutting edge is connected to the leading edge via an outer corner. Furthermore, the relief surface adjacent to the fourth cutting edge is formed continuously with the margin of the drill. A drill characterized in that the angle θ formed between the leading edge and the ridge line that forms the boundary between the relief surface adjacent to the fourth cutting edge and the margin is in the range of 4° to 25°.

5. Two or more cutting edges extending outward from the central axis, Two or more relief surfaces formed outward from the central axis, A drill having a thinning surface applied to the central axis side, The cutting blade is A first cutting edge extending outward from the chisel edge of the drill, adjacent to the thinning surface, A second cutting edge having a straight portion extending outward from the end of the first cutting edge, A straight or curved third cutting edge extends from the end of the second cutting edge outward, circumferentially and rearward from the drill, It is formed from a curved fourth cutting edge that extends from the end of the third cutting edge opposite to the first cutting edge toward the rear side of the drill and is connected to the leading edge, If the third cutting edge is straight, the fourth cutting edge has a curved shape that is different in shape from the third cutting edge. If the tip angle is defined as the angle obtained when the cutting edge is projected parallel to a plane parallel to the drill's axis, The tip angle α1, which is the angle formed between the two first cutting edges, is less than or equal to the tip angle α2, which is the angle formed between the two second cutting edges. Furthermore, the tip angle α1 is in the range of 90° to 140°, and the fourth cutting edge is connected to the leading edge via the outer corner. Furthermore, the relief surface adjacent to the fourth cutting edge is formed continuously with the margin of the drill. A drill characterized in that the angle θ formed between the leading edge and the ridge line that forms the boundary between the relief surface adjacent to the fourth cutting edge and the margin is in the range of 4° to 25°.

6. The drill according to any one of claims 1 to 5, characterized in that the relief angles of the third and fourth cutting edges are in the range of 5° to 11°.

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