DRILL HEAD, TIP EXCHANGE DRILL, AND DRILL

A drill head is rotated about a center axis. The drill head includes: a mounting surface that is an end face in a direction of the center axis; a tip surface that is an opposite surface to the mounting surface in the direction of the center axis; an outer peripheral surface continuous to the mounting surface and the tip surface; a cutting edge extending from the outer peripheral surface toward the center axis in the tip surface; a flute that is formed on the outer peripheral surface and extends spirally around the center axis from the tip surface to the mounting surface; and a thinning surface continuous to the tip surface and the flute. The tip surface includes: a first flank surface continuous to the cutting edge; and a second flank surface located on a side opposite to the first flank surface with the flute interposed therebetween.

TECHNICAL FIELD

The present disclosure relates to a drill head, a tip exchangeable drill, and a drill.

BACKGROUND ART

WO 2019/049257 (PTL 1) describes a drill. The drill described in PTL 1 is rotated about an axis. The drill described in Patent Literature 1 includes an outer peripheral surface, a discharge flute, a first flank surface, a second flank surface, a cutting edge, a thinning edge, and a gash.

A discharge flute is formed on the outer peripheral surface. The discharge flute extends spirally around the axis from a tip side of the drill toward a rear end side of the drill. The discharge flute includes an inner surface facing a rotation direction side of the drill. The cutting edge is provided on a ridgeline between the inner surface of the discharge flute and the first flank surface. The second flank surface is opposite to the first flank surface with the discharge flute interposed therebetween. The thinning edge extends from the inner end of the cutting edge toward a chisel. The gash is connected to the discharge flute.

The gash includes a thinning surface continuous to the thinning edge from a side opposite to the first flank surface and a gash surface continuous to the second flank surface. The ridgeline between the gash surface and the second flank surface has an arc shape in an end face view seen along the direction of the axis from the tip side of the drill.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

A drill head of the present disclosure is rotated about a center axis. The drill head includes: a mounting surface that is an end face in a direction of the center axis; a tip surface that is an opposite surface to the mounting surface in the direction of the center axis; an outer peripheral surface continuous to the mounting surface and the tip surface; a cutting edge extending from the outer peripheral surface toward the center axis in the tip surface; a flute that is formed on the outer peripheral surface and extends spirally around the center axis from the tip surface to the mounting surface; and a thinning surface continuous to the tip surface and the flute. The tip surface includes: a first flank surface continuous to the cutting edge; and a second flank surface located on a side opposite to the first flank surface with the flute interposed therebetween. The cutting edge includes: a main cutting edge continuous to the outer peripheral surface; and a thinning cutting edge that is continuous to the main cutting edge and is located closer to the center axis than the main cutting edge. The main cutting edge includes: a first end that is an end on a side of the outer peripheral surface; and a second end that is an end on a side of the thinning cutting edge. The thinning surface includes: a thinning rake surface continuous to the thinning cutting edge from a side opposite to the first flank surface; and a thinning heel surface continuous to the second flank surface. A concave amount of the main cutting edge with respect to a virtual straight line connecting the first end and the second end is greater than or equal to −0.01 times and less than or equal to 0.02 times a diameter of a circumscribed circle of the drill head in an end face view viewed from a tip surface side along the direction of the center axis. A ridgeline between the thinning heel surface and the second flank surface has an arc shape having a radius of greater than or equal to 0.10 times and less than or equal to 0.30 times the diameter in the end face view.

DETAILED DESCRIPTION

Problem to be Solved by the Present Disclosure

In the drill described in PTL 1, the cutting edge extends in an arc shape in the end face view viewed along the direction of the axial center from the tip side of the drill. Accordingly, in the drill described in PTL 1, chips cut out of the cutting edge is curved and hardly divided.

The present disclosure has been made in view of the above-described problems of the prior art. More specifically, the present disclosure provides a drill head in which the chips are easily divided.

Advantageous Effect of the Present Disclosure

According to the drill head of the present disclosure, the chips are easily divided.

DESCRIPTION OF EMBODIMENTS

First, an embodiment of the present disclosure will be listed and described.

(1) A drill head according to the embodiment is rotated about a center axis. The drill head includes: a mounting surface that is an end face in a direction of the center axis; a tip surface that is an opposite surface to the mounting surface in the direction of the center axis; an outer peripheral surface continuous to the mounting surface and the tip surface; a cutting edge extending from the outer peripheral surface toward the center axis in the tip surface; a flute that is formed on the outer peripheral surface and extends spirally around the center axis from the tip surface to the mounting surface; and a thinning surface continuous to the tip surface and the flute. The tip surface includes: a first flank surface continuous to the cutting edge; and a second flank surface located on a side opposite to the first flank surface with the flute interposed therebetween. The cutting edge includes: a main cutting edge continuous to the outer peripheral surface; and a thinning cutting edge that is continuous to the main cutting edge and is located closer to the center axis than the main cutting edge. The main cutting edge includes: a first end that is an end on a side of the outer peripheral surface; and a second end that is an end on a side of the thinning cutting edge. The thinning surface includes: a thinning rake surface continuous to the thinning cutting edge from a side opposite to the first flank surface; and a thinning heel surface continuous to the second flank surface. A concave amount of the main cutting edge with respect to a virtual straight line connecting the first end and the second end is greater than or equal to −0.01 times and less than or equal to 0.02 times a diameter of a circumscribed circle of the drill head in an end face view viewed from a tip surface side along the direction of the center axis. A ridgeline between the thinning heel surface and the second flank surface has an arc shape having a radius of greater than or equal to 0.10 times and less than or equal to 0.30 times the diameter in the end face view.

According to the drill head of (1), the chips are easily divided.

(2) In the drill head of (1), the concave amount of the main cutting edge with respect to the virtual straight line connecting the first end and the second end is greater than or equal to 0.00 times and less than or equal to 0.01 times a diameter of the drill head in the end face view viewed from the tip surface side along the direction of the center axis.

According to the drill head of (2), the chips are further easily divided, and the stability of cutting is enhanced.

(3) In the drill head of (1) or (2), the radius of the ridgeline between the arc-shaped thinning heel surface and the second flank surface may be greater than or equal to 0.13 times and less than or equal to 0.24 times the diameter of the drill head.

According to the drill head of (3), the chips are more easily divided.

(4) In the drill heads of (1) to (3), the difference between the maximum value and the minimum value of the radial rake angle of the main cutting edge may be less than or equal to 20°.

(5) In the drill heads of (1) to (4), the angle between the direction of the center axis and the thinning heel surface may be greater than or equal to 28° and less than or equal to 32°.

According to the drill head of (5), the chips are more easily divided.

(6) In the drill heads of (1) to (5), the thinning cutting edge may extend linearly in the end face view viewed from the tip surface side along the direction of the center axis.

According to the drill head of (6), the chips are more easily divided.

(7) A tip exchangeable drill according to an embodiment includes the drill heads of (1) to (6) and a holder attached to a mounting surface.

(8) The drill according to the embodiment is rotated about the center axis. The drill includes: a tip and a rear end that are ends in a direction of the center axis; a tip surface that is located at the tip; an outer peripheral surface continuous to the mounting surface and the tip surface; a cutting edge extending from the outer peripheral surface toward the center axis in the tip surface; a flute that is formed on the outer peripheral surface and extends spirally around the center axis from the tip surface toward a rear end surface; and a thinning surface continuous to the tip surface and the flute. The tip surface includes: a first flank surface continuous to the cutting edge; and a second flank surface located on a side opposite to the first flank surface with the flute interposed therebetween. The cutting edge includes: a main cutting edge continuous to the outer peripheral surface; and a thinning cutting edge that is continuous to the main cutting edge and is located closer to the center axis than the main cutting edge. The main cutting edge includes: a first end that is an end on a side of the outer peripheral surface; and a second end that is an end on a side of the thinning cutting edge. The thinning surface includes: a thinning rake surface continuous to the thinning cutting edge from a side opposite to the first flank surface; and a thinning heel surface continuous to the second flank surface. A concave amount of the main cutting edge with respect to a virtual straight line connecting the first end and the second end is greater than or equal to −0.01 times and less than or equal to 0.02 times a diameter of a circumscribed circle of the drill in an end face view viewed from a tip surface side along the direction of the center axis. A ridgeline between the thinning heel surface and the second flank surface has an arc shape having a radius of greater than or equal to 0.10 times and less than or equal to 0.30 times the diameter in the end face view.

According to the drill of (8), the chips are easily divided. [Detailed Embodiments of the Present Disclosure] With reference to the drawings, an embodiment of the present disclosure will be described in detail. In the following drawings, the same or corresponding component is designated by the same reference numeral, and the overlapping description will be omitted.

A drill (hereinafter referred to as a “drill100”) according to an embodiment will be described. Drill100is a tip exchangeable drill.

A configuration of drill100will be described below.

FIG.1is a perspective view of drill100. As illustrated inFIG.1, drill100is rotated about a center axis A. Drill100includes a tip100aand a rear end100b. Rear end100bis an end opposite to tip100ain the direction of center axis A. Tip100aand rear end100bare ends of drill100in the direction of center axis A.

Drill100includes a drill head10, a holder20, and a fixing member30. Drill head10is located on the side of tip100aof drill100. Holder20is located on the side of rear end100bof drill100.

<Detailed Configuration of Drill Head10>

For example, drill head10is made of cemented carbide.FIG.2is a perspective view of drill head10.FIG.3is a front view of drill head10.FIG.4is a rear view of drill head10. As illustrated inFIGS.2,3, and4, drill head10is rotated about a center axis A1. Center axis A1is located on the same straight line as center axis A.

Drill head10includes a mounting surface11, a tip surface12, and an outer peripheral surface13. Mounting surface11and tip surface12are end faces of drill head10in the direction of center axis A1. Mounting surface11faces the side of holder20. Tip surface12is a surface opposite to mounting surface11in the direction of center axis A1. Tip surface12is located at tip100a. Outer peripheral surface13is continuous to mounting surface11and tip surface12.

A plurality of grooves11aare formed in mounting surface11. Mounting surface11is recessed toward tip surface12in groove11a. Groove11aextends from a central portion of mounting surface11toward outer peripheral surface13along the direction orthogonal to center axis A1.

In the end face view viewed from the side of tip surface12along the direction of center axis A1, the diameter of the circumscribed circle of drill head10is set to an outer diameter D1. InFIG.3, the circumscribed circle is indicated by a dotted line. For example, outer diameter D1is greater than or equal to 12 mm. Outer diameter D1may be greater than or equal to 10 mm.

Two flutes14are formed on outer peripheral surface13. Flute14is formed in a spiral shape about center axis A1so as to reach mounting surface11from tip surface12. From another point of view, flute14extends in a spiral shape about center axis A1from tip surface12toward the side of rear end100b. One of flutes14is located at a position symmetrical to the other flute14with respect to center axis A1in the end face view viewed from the side of tip surface12along the direction of center axis A1.

Drill head10includes two cutting edges15. Cutting edge15extends from outer peripheral surface13toward center axis A1on tip surface12. Cutting edge15includes a main cutting edge15aand a thinning cutting edge15b. Thinning cutting edge15bis located closer to center axis A1than main cutting edge15a. One of cutting edges15is located at a position symmetrical to the other cutting edge15with respect to center axis A1in the end face view viewed from the side of tip surface12along the direction of center axis A1.

Main cutting edges15aincludes a first end15aaand a second end15ab. First end15aais an end on the side of outer peripheral surface13. Second end15abis an end opposite to first end15aa. Main cutting edge15ais continuous to thinning cutting edge15bat second end15ab. Main cutting edge15aextends substantially linearly in the end face view viewed from the side of tip surface12along the direction of center axis A1. Preferably, thinning cutting edge15bextends linearly in the end face view viewed from the side of tip surface12along the direction of center axis A1.

FIG.5is a schematic diagram illustrating a concave amount of main cutting edge15a.FIG.5illustrates a shape of main cutting edge15awhen tip surface12is viewed in the direction from tip100atoward rear end100b. InFIG.5, the direction substantially perpendicular to main cutting edge15ais illustrated in an enlarged manner as compared with the direction substantially parallel to main cutting edge15a. As illustrated inFIG.5, a virtual straight line connecting first end15aaand second end15abis set to a virtual straight line L1. A distance between virtual straight line L1and main cutting edge15ais set to a distance DIS. The maximum value of distance DIS is a concave amount of main cutting edge15awith respect to virtual straight line L1.

The concave amount of main cutting edge15awith respect to virtual straight line L1is greater than or equal to −0.01 times and less than or equal to 0.02 times outer diameter D1. Preferably, the concave amount of main cutting edge15awith respect to virtual straight line L1is greater than or equal to 0.00 times and less than or equal to 0.01 times outer diameter D1. When main cutting edge15aprotrudes with respect to straight line L, the concave amount of main cutting edge15abecomes a negative value. When main cutting edge15ais recessed from straight line L, the concave amount of main cutting edge15abecomes a positive value. The concave amount of main cutting edge15awith respect to virtual straight line L1inFIG.5is the positive value.

FIG.6is a schematic diagram illustrating a radial rake angle θ1of main cutting edge15a.FIG.6illustrates the shape of main cutting edge15awhen tip surface12is viewed in the direction from tip100atoward rear end100b. InFIG.6, the direction substantially perpendicular to main cutting edge15ais illustrated in an enlarged manner as compared with the direction substantially parallel to main cutting edge15a. As illustrated inFIG.6, a specific position on main cutting edge15ais set to a position P1. A virtual straight line connecting center axis A1and position P1is set to a virtual straight line L2. An angle formed by a tangent line of main cutting edge15aat position P1and virtual straight line L2is radial rake angle θ1of main cutting edge15a. The difference between the maximum value and the minimum value of radial rake angle θ1of main cutting edge15ais preferably less than or equal to 20°.

As illustrated inFIG.3, tip surface12includes two first flank surfaces12aand two second flank surfaces12b. First flank surface12ais continuous to cutting edge15(main cutting edge15aand thinning cutting edge15b). One of first flank surfaces12ais located at the position symmetrical to other first flank surface12awith respect to center axis A1. Flute14is continuous to main cutting edge15afrom the side opposite to first flank surface12a. That is, main cutting edge15ais located on the ridgeline between first flank surface12aand flute14. Second flank surface12bis located on the opposite side of first flank surface12awith flute14interposed therebetween. One of second flank surface12bis located at the position symmetrical to the other second flank surface12bwith respect to center axis A1.

As illustrated inFIG.2, drill head10includes a thinning surface16. Thinning surface16is continuous to tip surface12and flute14. Thinning surface16includes a thinning rake surface16aand a thinning heel surface16b.

Thinning rake surface16ais continuous to thinning cutting edge15bfrom the side opposite to first flank surface12a. That is, thinning cutting edge15bis located on the ridgeline between first flank surface12aand thinning rake surface16a. Thinning heel surface16bis continuous to second flank surface12b. As illustrated inFIG.3, the ridgeline between thinning heel surface16band second flank surface12bhas an arc shape in the end face view viewed from the side of tip surface12along the direction of center axis A1. The arc shape has a corresponding curvature radius when a certain arc-shaped point is determined. The arc shape may have substantially the same curvature radius with respect to all arc-shaped points. The arc shape may have different curvature radii corresponding to arc-shaped points. In the present specification, the “radius” of the arc shape means a range of values of the curvature radius corresponding to all the arc-shaped points. The radius of the arc shape is greater than or equal to 0.10 times and less than or equal to 0.30 times outer diameter D1. The radius of the arc shape is preferably greater than or equal to 0.13 times and less than or equal to 0.24 times outer diameter D1. The ridgeline between thinning heel surface16band second flank surface12bmay be divided by a through-hole17. The shape of a portion of the ridgeline between thinning heel surface16band second flank surface12bon the side close to center axis A1may be an arc shape when the ridgeline between thinning heel surface16band second flank surface12bis divided by through-hole17. The radius of the arc shape is greater than or equal to 0.10 times and less than or equal to 0.30 times outer diameter D1. The radius of the arc shape is preferably greater than or equal to 0.13 times and less than or equal to 0.24 times outer diameter D1.

FIG.7is a sectional view taken along a line VII-VII inFIG.3.FIG.7illustrates a section of drill head10parallel to thinning rake surface16a. As illustrated inFIG.7, an angle formed by thinning heel surface16band center axis A1is set to an angle θ2. The angle θ2is preferably greater than or equal to 28° and less than or equal to 32°.

As illustrated inFIGS.3and4, two through-holes17are made in drill head10. Through-hole17penetrates drill head10along the direction of center axis A1. One of through-holes17is located at the position symmetrical to the other through-hole17with respect to center axis A1. An end on the side of tip surface12of through-hole17is opened in second flank surface12b.

<Detailed Configuration of Holder20>

For example, holder20is made of steel.FIG.8is a perspective view of holder20.FIG.9is a front view of holder20. As illustrated inFIGS.8and9, holder20is rotated about center axis A2. Center axis A2is located on the same straight line as center axis A1. Holder20includes a mounting surface21and an outer peripheral surface22. Mounting surface21is an end face of holder20in the direction of center axis A2, and faces the side of drill head10. Outer peripheral surface22is continuous to mounting surface21.

A plurality of protruding strips21aare formed on mounting surface21. Mounting surface21protrudes toward the side of drill head10at protruding strip21a. Protruding strip21aextends from the central portion of mounting surface21toward outer peripheral surface22along the direction orthogonal to center axis A2. Two screw holes21bare made in mounting surface21. Screw hole21bextends from mounting surface21toward the side of rear end100b. One of screw holes21bis located at the position symmetrical to the other screw hole21bwith respect to center axis A2.

Protruding strip21ais fitted into groove11a. In a state where protruding strip21ais fitted into groove11a, screw hole21bis located at the position overlapping through-hole17. For example, fixing member30is a screw. Fixing member30is inserted into through-hole17and then screwed into screw hole21b, thereby fastening drill head10to holder20. In this manner, holder20is attached to drill head10(mounting surface11).

Effects of the drill100will be described below.

When cutting is performed with drill100, the chips are generated from a work material by cutting edge15. The chip portion generated by thinning cutting edge15bcomes into contact with thinning heel surface16band is curved when being moved toward flute14. When the curvature reaches the limit, a crack is generated in the chip portion generated by thinning cutting edge15b. The crack generated in the chip develops to the chip portion generated by main cutting edge15aby centrifugal force of the chip, and breaks the chip.

In drill100, main cutting edge15aextends substantially linearly. That is, in drill100, the concave amount of main cutting edge15awith respect to virtual straight line L1is greater than or equal to −0.01 times and less than or equal to 0.02 times outer diameter D1. Consequently, in drill100, the chip portion generated by main cutting edge15ahas small distortion, and the crack generated in the chip portion generated by thinning cutting edge15beasily develops to the chip portion generated by main cutting edge15a.

When the ridgeline between thinning heel surface16band second flank surface12bis linear in the end face view viewed from the side of tip surface12along the direction of center axis A1, the chip portion generated by thinning cutting edge15bis hardly curved by thinning heel surface16b, so that the crack is hardly generated in the chip portion generated by thinning cutting edge15b.

However, in drill100, the ridgeline between thinning heel surface16band second flank surface12bhas an arc shape having a radius of greater than or equal to 0.10 times and less than or equal to 0.30 times outer diameter D1. Consequently, in drill100, the chip portion generated by thinning cutting edge15bis easily curved by thinning heel surface16b, so that the crack is easily generated in the chip portion generated by thinning cutting edge15b.

As described above, in drill100, the crack is easily generated in the chip portion generated by thinning cutting edge15b, and the generated crack easily develops to the chip portion generated by main cutting edge15a, so that the chips are easy to divide.

When the concave amount of main cutting edge15awith respect to virtual straight line L1is greater than or equal to 0.00 times and less than or equal to 0.01 times outer diameter D1, the chips are more easily divided. In this case, stability of cutting can be enhanced. When the radius of the ridgeline between arc-shaped thinning heel surface16band second flank surface12bis greater than or equal to 0.13 times and less than or equal to 0.24 times outer diameter D1, the chips are more easily divided.

In the case of large angle θ2, the chips stagnate within the thinning, so that there is a possibility that the crack is hardly generated in the chip portion generated by thinning cutting edge15b. On the other hand, in the case of small angle θ2, the chip portion generated by thinning cutting edge15bis hardly curved due to the contact with thinning heel surface16b(the crack is hardly generated). Consequently, the chips are more easily divided by optimizing the range of angle θ2(more specifically, angle θ2is set to greater than or equal to 28° and less than or equal to 32°).

When thinning cutting edge15bextends linearly in the end face view viewed from the side of tip surface12along the direction of center axis A1, the distortion of the chip portion generated by thinning cutting edge15bis reduced, and the generated crack easily develops in the chip portion generated by thinning cutting edge15b. Accordingly, in this case, the chips are more easily divided.

A first cutting test, a second cutting test, and a third cutting test were performed in order to check the effect of drill100.

In the first cutting test, drills100of sample numbers1-1to1-7were used. In the first cutting test, the radius of the ridgeline between arc-shaped thinning heel surface16band second flank surface12bwas set to 0.20 times outer diameter D1, and angle θ2was set to 30°. In the first cutting test, the concave amount of main cutting edge15awith respect to virtual straight line L1was changed within a range of greater than or equal to −0.01 times and less than or equal to 0.06 times outer diameter D1.

In the second cutting test, drills100of sample numbers2-1to2-8were used. In the second cutting test, the concave amount of main cutting edge15awith respect to virtual straight line L1was set to 0.00 times outer diameter D1, and angle θ2was set to 30°. In the second cutting test, the radius of the ridgeline between arc-shaped thinning heel surface16band second flank surface12bwas changed within a range of greater than or equal to 0.10 times and less than or equal to 0.40 times outer diameter D1.

FIG.10is a graph illustrating a result of the first cutting test. A horizontal axis inFIG.10is a value obtained by dividing the concave amount of main cutting edge15awith respect to virtual straight line L1by outer diameter D1, and a vertical axis inFIG.10is the chip division ratio (unit:percent). The chip division ratio is a value obtained by dividing a weight of the chip divided by one curl by a total weight of the generated chips.

FIG.11is a graph illustrating a result of the second cutting test. The horizontal axis inFIG.11is a value obtained by dividing the radius of the ridgeline between arc-shaped thinning heel surface16band second flank surface12bby outer diameter D1, and the vertical axis inFIG.11is the chip division ratio.

As illustrated inFIG.10, when the concave amount of main cutting edge15awith respect to virtual straight line L1fell within the range of greater than or equal to −0.01 times and less than or equal to 0.02 times outer diameter D1, the chip division ratio was greater than or equal to 50%. On the other hand, when the concave amount of main cutting edge15awith respect to virtual straight line L1fell within the range of greater than or equal to 0.03 times, the chip division ratio was less than 50%.

As illustrated inFIG.11, when the radius of the ridgeline between arc-shaped thinning heel surface16band second flank surface12bfell within the range of greater than or equal to 0.10 times and less than or equal to 0.30 times outer diameter D1, the chip division ratio was greater than or equal to 50%. On the other hand, when the radius of the ridgeline between arc-shaped thinning heel surface16band second flank surface12bfell within the range of greater than or equal to 0.35 times outer diameter D1, the chip division ratio was less than 50%.

From the above comparison, it has been experimentally found that the chips are easily divided when the concave amount of main cutting edge15awith respect to virtual straight line L1falls within the range of greater than or equal to −0.01 times and less than or equal to 0.02 times outer diameter D1, and when the radius of the ridgeline between arc-shaped thinning heel surface16band second flank surface12bfalls within the range of greater than or equal to 0.10 times and less than or equal to 0.30 times outer diameter D1.

When the concave amount of main cutting edge15awith respect to virtual straight line L1fell within the range of greater than or equal to 0.00 times and less than or equal to 0.01 times outer diameter D1, the chip division ratio became greater than or equal to 77%. From this, it has been experimentally found that the chips are further easily divided when the concave amount of main cutting edge15awith respect to virtual straight line L1fell within the range of greater than or equal to 0.00 times and less than or equal to 0.01 times outer diameter D1.

When the radius of the ridgeline between arc-shaped thinning heel surface16band second flank surface12bfell within the range of greater than or equal to 0.13 times and less than or equal to 0.24 times outer diameter D1, the chip division ratio became greater than or equal to 80%. From this, it has been experimentally found that the chips are further easily divided when the radius of the ridgeline between arc-shaped thinning heel surface16band second flank surface12bfell within the range of greater than or equal to 0.13 times and less than or equal to 0.24 times outer diameter D1.

In the third cutting test, drills100of sample numbers3-1to3-7were used. In the third cutting test, the concave amount of main cutting edge15awith respect to virtual straight line L1was set to 0.00 times outer diameter D1, and the radius of the ridgeline between arcuate thinning heel surface16band second flank surface12bwas set to 0.20 times outer diameter D1. In the third cutting test, angle θ2was changed within the range of greater than or equal to 25° and less than or equal to 45°.

FIG.12is a graph illustrating a result of the third cutting test. The horizontal axis inFIG.12represents angle θ2(unit:°). InFIG.12, the vertical axis represents the chip division ratio (unit:percent). As illustrated inFIG.12, when angle θ2fell within the range of greater than or equal to 28° and less than or equal to 32°, the chip division ratio became greater than or equal to 82%. From this, it has been experimentally found that the chips were more easily divided when angle θ2fell within the range of greater than or equal to 28° and less than or equal to 32°.

Although the example in which drill100is the cutting edge exchangeable drill has been described above, drill100may be a solid drill. That is, drill100may not be divided into drill head10and holder20.

It should be understood that the embodiment disclosed herein is illustrative in all respects and are not restrictive. The scope of the present invention is defined not by the embodiment but by the claims, and is intended to include meanings equivalent to the claims and all modifications within the scope.

REFERENCE SIGNS LIST