JP3515168B2 - Drill - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a drill capable of simultaneously improving the chip disposal performance and the strength of the drill tip.

[0002]

2. Description of the Related Art As a drill used for drilling a metal material, for example, those shown in FIGS. 5 and 6 have been proposed. This is disclosed in Japanese Patent Laid-Open No. 2-124208.
The pair of chip discharge grooves 2 and 2 are spirally formed on the side surface of the drill body 1 which is rotated around the axis O, and the drill rotation direction of these chip discharge grooves 2 and 2 ( A cutting edge 5 is formed so as to be centered at the ridge line where the wall surface 3 facing in the counterclockwise direction in FIG. 5 and the tip flank 4 of the drill body 1 intersect. Further, in this drill, the portion of the drill rotation center C at the tip thereof is thinned, whereby a thinning blade 6 linearly extending from the drill rotation center C is formed in the vicinity of the drill rotation center C. The thinning blade 6 is smoothly connected to the cutting blade 5 through an arcuate intersection 7.

By this thinning, the wall surface 8 facing the rear side of the chip discharge groove 2 in the direction of rotation of the drill and the tip flank 4 are provided.
The thinning surface 9 formed at the intersection with and is a rake surface (first thinning surface) 10 along the thinning blade 6 described above.
And a first sharpened surface (second thinning surface) 12 that intersects the first thinning surface 10 at an obtuse angle and extends from the drill rotation center C to the heel 11 side. Of these, the latter second thinning surface 12 has a ridge line 13 intersecting with the tip flank 4 so as to extend linearly from the drill rotation center C to the outer peripheral side as shown in FIG. It is formed, and as it goes to the rear side in the drill rotation direction from the intersecting ridge line 13, the intersecting portion of the wall surface 8 and the tip flank 4 is entirely inclined toward the base end side of the drill main body 1 and scraped off. Is formed in.

[0004]

However, in the drill having the above structure, the first thinning surface 10 and the second thinning surface 12 intersect at an obtuse angle on the thinning surface 9, and the second thinning surface 12 is Since it is formed so as to be entirely inclined toward the base end side from the intersecting ridge line 13 extending in a straight line toward the rear side in the drill rotation direction, the heel 11 side of the tip flank 4 is greatly enlarged as shown in FIG. It will be cut out, so the drill body 1
It was unavoidable that the strength and rigidity of the tip of the slab were impaired. Moreover, since the thickness of the portion of the cutting blade 5 on the rear side in the direction of drill rotation is reduced by making a large cutout on the heel 11 side in particular, the strength of the cutting blade 5 is also impaired, and the cutting blade is reduced. There was also a risk of loss or breakage. Further, the first thinning surface 10
If the intersection angle between the second thinning surface 12 and the second thinning surface 12 is set to be obtuse, the thickness of the thinning blade 6 at the rear of the drill rotation direction also becomes small, resulting in insufficient strength over the entire cutting edge. There was also a problem that it would occur.

On the other hand, the chips produced by the cutting blade 5 and the thinning blade 6 in such a drill have a higher peripheral speed around the axis O on the outer peripheral side of the drill main body 1.
As shown by the broken lines in FIGS. 5 and 6, it flows out while drawing a fan shape around the drill rotation center C, is guided into the chip discharge groove 2 and begins to curl near its bottom, and is divided. . However, like the drill with the above configuration,
When the second thinning surface 12 of the thinning surface 9 is formed so as to be entirely inclined toward the base end side toward the rear side in the drill rotation direction, chips and chip discharge at the portion where the chips begin to curl. Since the contact length of the groove 2 with the wall surface 8 is insufficient, chips are generated without being sufficiently curled, resulting in impairing the separability thereof. For this reason, spiral chips called free chips, which are particularly likely to occur at the beginning of cutting, are generated in a long continuous chain, which roughens the inner wall near the opening of the drilled hole and causes such chips to be drilled. There was a risk that it would be wrapped around the body 1.

The present invention has been made under such a background, and its object is to simultaneously improve the chip processing performance and the strength of the cutting edge at the tip of the drill at the same time. It is to provide a drill that can be designed.

[0007]

SUMMARY OF THE INVENTION In order to solve the above problems and achieve such an object, the present invention is provided on a side surface of a drill body rotated about an axis from a tip end side to a base end side of the drill body. In the drill, a chip discharge groove is formed toward, and a cutting blade is formed at a ridge line intersecting the wall surface of the chip discharge groove facing the drill rotation direction side and the tip flank of the drill body. The core thickness portion, by forming a thinning blade that extends linearly from the center of rotation of the drill and is continuous with the cutting edge by performing thinning, and a wall surface that faces the rear side in the drill rotation direction of the chip discharge groove by the thinning. At the intersection with the tip flank, a thinning surface is formed so as to reach the heel side of the drill body from the drill rotation center in the axial direction tip view, and the thinning surface is formed. The first thinning surface, which is arranged so as to face the rill rotation direction side and is continuous with the drill rotation center and which defines the thinning blade on the ridge line intersecting with the tip flank, and faces the outer peripheral side of the drill body. And a second thinning surface that is continuous to the center of rotation of the drill and that is arranged so as to be concavely curved and intersects with the first thinning surface and a heel side of the second thinning surface that faces the rear side in the drill rotation direction. is arranged to concavely curved with respect to the second thinned face, is formed of three thinned face of the third thinned face is reached on the heel side of the drill body, said second true
The length of the ridgeline intersecting the ridge face and the flank face is
0.2 × D with respect to the outer diameter D of the drill when viewed from the tip in the line direction
It is characterized by the following .

[0008]

In the drill having such a structure, the thinning surface formed at the intersection of the tip flank and the wall surface facing the rear side of the chip discharge groove in the direction of rotation of the drill by thinning is the first to third thinning surface. The third thinning surface is arranged on the heel side of the second thinning surface so as to be concave with respect to the second thinning surface facing the rear side in the drill rotation direction. Therefore, the crossing ridgeline between each thinning surface and the tip flank is
When viewed from the axial tip, it will have a "U" shape that opens to the outer peripheral side of the drill body, and the heel side of the tip flank will be completely cut off by the second thinning surface like the above-mentioned conventional drill. Rather, as the third thinning surface is arranged so as to be concavely curved with respect to the second thinning surface, it becomes possible to secure a sufficient circumferential wall thickness of the tip of the drill body on the heel side. .

Further, since the heel side of the tip flank is not entirely cut out in this way, a contact length capable of sufficiently curling the chips guided in the chip discharge groove is obtained, which is favorable. It is possible to secure chip breaking property. Moreover, since the chip dividing property is maintained even when biting, it is possible to reliably process the chips in the initial stage of cutting and prevent the generation of spiral chips.

[0010]

1 to 3 show an embodiment of the present invention. In these drawings, the drill main body 21 is made of a hard material such as cemented carbide and has a substantially cylindrical shape. A shank (not shown) is provided with a blade at its tip, and the tip of the shank is at the base end side. Direction of drill rotation toward (Fig. 1
In the counterclockwise direction), two chip discharge grooves 22 and 22 twisted rearward are formed at equal intervals in the circumferential direction, and the wall surface 2 of these chip discharge grooves 22 and 22 facing the drill rotation direction.
3 and a pair of cutting blades 25 so as to be symmetric with respect to a drill rotation center (intersection point of the tip flank 24 and the axis O) C of the tip of the drill body 21 at an intersection ridge line portion between the tip flank 24 of the drill body 21. 25 are formed.

So-called cross-thinning is applied to the core thickness portion at the tip of the drill body 21. As a result, the first portion forming the inner peripheral side of the cutting edge 25 extends linearly from the center C of rotation of the drill. A cutting blade (thinning blade) 26 is formed. The first blade 26 does not have to be strictly connected to the drill rotation center C, but rather, taking into consideration the fracture of the drill rotation center C due to stress concentration, the drill rotation center C is considered.
It is desirable to be formed from a position slightly separated from.
On the other hand, by this thinning, at the intersection of the wall surface 27 of the chip discharge groove 22 facing the rear side in the drill rotation direction and the tip flank 24, from the drill rotation center C to the heel 28 side of the drill body 21 in the axial O direction tip view. Thinning surface 29 is formed so as to reach. In the present embodiment, the thinning surface 29 is arranged so as to face the drill rotation direction side and to be continuous with the drill rotation center C, and the first cutting blade 26 is defined at the ridge line portion intersecting with the tip flank 24. The first thinning surface 30 and the second thinning surface 31 which faces the outer peripheral side of the drill body 21 and is continuous with the drill rotation center C and is arranged so as to be concavely curved and intersect with the first thinning surface 30.
And a third thinning surface 32 which is arranged on the heel 28 side of the second thinning surface 31 so as to be curved backward with respect to the second thinning surface 31, facing the rear side in the drill rotation direction,
It is formed from three thinning surfaces 30-32.

In this embodiment, the ridge line intersecting the first thinning surface 30 and the tip flank 24, that is, the first cutting edge 26, and the ridge line intersecting the second thinning surface 31 and the tip flank 24. The intersection angle α formed by 33 and 33 is in the range of 75 ° to 95 ° when viewed from the front end in the direction of the axis O. Further, the length L ₂ of the ridge line 33 of the intersection between the second thinning surface 31 and the tip flank 24 is set to 0.2 × D or less with respect to the outer diameter D of the drill in the axis O direction tip view. There is. Further, the intersection angle β formed by the ridge line 33 of intersection between the second thinning surface 31 and the tip flank 24 and the ridge line 34 of intersection between the third thinning surface 32 and the tip flank 24 is:
It is set to a range of 90 ° to 140 ° when viewed from the front end in the direction of the axis O.

On the other hand, in the present embodiment, the cutting edge 25 also
It consists of three parts. That is, the cutting edge 25
When viewed from the front end in the direction of the axis O, the first cutting blade 26 is formed at the ridge portion intersecting the first thinning surface 30 and the tip clearance surface 24 and extends linearly from the drill rotation center C,
A second cutting edge 35 extending to the outer peripheral side while drawing a convex curve that intersects an obtuse angle and swells in the drill rotation direction at the outer peripheral edge of the first cutting edge 26, and smoothly contacts the outer peripheral edge of the second cutting edge 35. It is composed of a third cutting edge 36 extending linearly on the outer peripheral side. Therefore, the third cutting edge 36 is arranged in a so-called center-up manner and its radial rake angle is set to the negative angle side. Become. The length L ₁ of the _first cutting blade 26 is 0.05 with respect to the outer diameter D of the drill when viewed from the front end in the direction of the axis O.
The radius of curvature R of the convex curve exhibited by the second cutting edge 35 is 0.18 × D to 0.3 × when viewed from the front end in the axis O direction. It is set in the D range. Further, the intersection angle γ between the tangent line S of the second cutting edge 35 and the first cutting edge 26 at the intersection P with the first cutting edge 26 is 5
It is set in the range of 20 ° to 20 °, so the first blade 2
6 and the second cutting edge 35 are bent and connected through the intersection point P without contacting each other.

In this embodiment, the center of rotation of the drill C and the outer peripheral edge of the cutting edge 25 in the end view in the direction of the axis O,
That is, assuming a virtual straight line Q connecting the outer peripheral edge of the third cutting edge 36, the second cutting edge 35 with respect to this virtual straight line Q is assumed.
The height H to the apex X is set to be in the range of 0.065 × D to 0.09 × D with respect to the drill outer diameter D. That is, the distance between the virtual straight line Q and a straight line which is parallel to the virtual straight line Q and is in contact with the second cutting edge 35 is 0.065 × D to 0.09 ×.
It is set in the range of D.

In the drill having such a structure, the first to third thinning surfaces 30 to 3 constituting the thinning surface 29 are used.
Of the two, the third thinning surface 32 is arranged so as to face the rear side in the direction of rotation of the drill and to be concavely curved with respect to the second thinning surface 31, and therefore the thinning surfaces 30 to 32 and the tip flank 24 are arranged. Crossing ridge line 26 (first blade), 33,
34 has a "U" shape that opens toward the outer peripheral side of the drill body 21 when viewed from the front end in the direction of the axis O. Therefore, in the drill having the above-described configuration, unlike the conventional drill, the heel side of the tip flank is not entirely cut out by thinning, and the third thinning surface 32 is bent to the second thinning surface 31. As they are placed,
It is possible to secure a sufficient thickness in the circumferential direction of the tip of the drill body 21 on the heel 28 side in the hatched portion in FIGS. 1 and 2. Therefore, it is possible to improve the strength and rigidity of the tip portion of the drill body 21, and in particular, the strength and rigidity of the cutting blade 25 on the rear side in the drill rotation direction are improved, so that the strength of the cutting blade 25 itself is improved. The cutting edge 25 can be prevented from being damaged.

Further, in the drill having the above construction, since the heel 28 side of the tip flank 24 is not entirely cut out in this way, the center of rotation C of the drill is set as shown by broken lines in FIGS. 1 and 2. The chips generated so as to draw a fan shape as the center are curled by contacting the wall surface 27 of the chip discharge groove 22 with a sufficient contact length. For this reason, according to the drill having the above-described configuration, it is possible to always ensure a good chip dividing property, thereby preventing the generation of chips or the like and improving the chip processing performance. Moreover, since this good chip breaking property is sufficiently exerted even in the initial stage of cutting, it is possible to surely process the chips even when the drill is biting, and thereby, the spiral free chips. Can be prevented from being generated continuously for a long time. Therefore, it is possible to avoid such a situation that the inner peripheral surface roughness of the machined hole is deteriorated by such chips and the chips are wound around the drill body 21, and smooth drilling can be performed throughout the machining. It will be possible.

In this embodiment, the thinning surface 2 is used.
9, the crossing ridgeline of the first thinning surface 30 and the tip flank 24, that is, the crossing angle of the first cutting edge 26 and the crossing ridge 33 of the second thinning surface 31 and the tip flank 24 in the direction of the axis O in the tip view. Although α is set to 75 ° to 95 °, this is because when the intersection angle α is less than 75 °, the first thinning surface 30 and the second thinning surface 3 sandwich the drill rotation center C.
1 and 1 are too close to each other, so that there is a possibility that a space for producing a portion of the chip particularly on the center side is not sufficiently secured and chip clogging may occur. On the contrary, when the crossing angle α exceeds 95 °, the circumferential wall thickness from the first cutting edge 26 to the thinning surface 29 on the rear side in the drill rotation direction becomes small, and the drill body near the drill rotation center C. This is because the strength of the cutting edge 21 and the cutting edge 25 decreases, so that there is a risk of chipping or fracture depending on the processing conditions.

Further, in the present embodiment, the ridge line 33 intersecting the second thinning surface 31 and the tip flank 24 and the ridge line 34 intersecting the third thinning surface 32 and the tip flank 24 are viewed in the direction of the axis O. The crossing angle β at 90 ° to 140 ° is set. However, if this crossing angle β seems to be larger than 140 °, the thickness of the cutting blade 25 on the rear side in the drill rotation direction is sufficiently secured. Not only that, the strength of the drill body 21 is lowered, and the chips are surely discharged from the chip discharge groove 2
This is because there is a risk that it will be difficult to guide the passengers in the area 2. On the other hand, on the contrary, when the intersection angle β is less than 90 °, the space of the portion where the second thinning surface 31 and the third thinning surface 34 intersect becomes narrow in the direction in which the central side portion of the chips is discharged. As a result, the chip discharging property is impaired and the chips may be clogged.

Further, in the present embodiment, the length L ₂ of the ridge line 33 of the intersection of the second thinning surface 31 and the tip clearance surface 24 in the direction of the axis O is 0.2 × D with respect to the outer diameter D of the drill.
Although it is set as follows, if the length L ₂ of the intersecting ridge line 33 exceeds 0.2 × D and the width of the second thinning surface 31 becomes too large, the cutting edge will result. This is because the thickness of 25 on the rear side in the direction of rotation of the drill is reduced, and the strength of the cutting blade 25 at the tip of the drill body 21 and the strength of the drill body 21 itself may be reduced.

On the other hand, in the drill of the present embodiment, the cutting edge 25 includes a linear first cutting edge (thinning blade) 26 on the drill rotation center C side and a linear third cutting edge 36 on the outer peripheral side. ,
It is composed of a convex cutting second cutting edge 35 between the first and third cutting edges 26 and 36. The length L ₁ of the _first cutting edge 26 as viewed from the front end in the direction of the axis O is 0 with respect to the drill outer diameter D.
It is set relatively short, from 05 × D to 0.15 × D,
Therefore, the range B in which the cutting edge 25 has a large tip angle θ ₂ on the drill rotation center C side can be suppressed to be small,
The biting property of the drill can be improved, and the biting can be quickly stabilized. That is, when the first cutting edge 26 extends linearly from the center of rotation C of the drill as in the present embodiment, while the third cutting edge 36 that is also linear is arranged in the center-up position, first change becomes larger in the tip angle theta ₂ of the blade shown in FIG. 3 from the tip angle theta ₁ of the third cutting edge 36 shown in FIG. 2, the pushing force of the drill is increased at a result drill rotation center C And eating becomes unstable.

In the drill of this embodiment, however, the length L ₁ of the _first cutting blade 26 is set short as described above, and the range in which the pushing force is large, that is, the cutting edge 25 has a large tip angle θ. By reducing the above range B that takes ₂
The biting property can be improved, and the length B of the range B in the axis O direction (biting length) F can be reduced by reducing the range B in which the tip angle θ ₂ is taken.
Since it is also kept small, it is possible to quickly stabilize the bite. As a result, according to the drill of this embodiment, it is possible to prevent the enlargement margin in the drilling process from increasing, and it is possible to perform the process with a higher roundness. In addition, this first blade 2
If the length L _{1 of} 6 exceeds 0.15 × D, the range B where the tip angle is large becomes large, and such an effect cannot be obtained. On the contrary, the length L _{1 of the first} cutting blade 26 is 0.05 × D
If it becomes smaller than the above, the central portion of the chips generated by the first cutting edge 26 and the second and third cutting edges 35, 36
The balance of chip generation is impaired with the outer peripheral side part generated by, the force attracting the chips to the center side is weakened, the chip outflow route changes as the chips try to flow to the outer peripheral side, and the chips become stretchable at the same time, The guideability of chips to the chip discharge groove 22 is also impaired, which may lead to chip clogging at the initial stage of cutting and deterioration of the roughness of the inner wall surface of the processed hole.

Further, in the drill of this embodiment, the radius of curvature R of the second cutting edge 35 having a convex curved shape between the first cutting edge 26 and the third cutting edge 36 in the tip view in the direction of the axis O is the above. It is set to a relatively large value of 0.18 × D to 0.3 × D with respect to the outer diameter D of the drill. Therefore, it becomes possible to give high strength to the second cutting edge 35, and the thinning surface 29 is used. In combination with the strength improving effect, it becomes possible to reliably prevent the cutting edge 25 from being damaged regardless of the processing conditions. In addition,
If the radius of curvature R of the second cutting edge 35 seems to be less than 0.18 × D, such improvement in cutting edge strength cannot be obtained, while conversely, if it becomes greater than 0.3 × D, the cutting Since the entire blade 25 has a gentle shape, it is not possible to balance the generation of chips in the central side portion and the outer peripheral side portion, and it is possible to give a sufficient braking force to curl the chips. There is a risk that the chips will be elongated and the chips will be clogged.

In addition to these, in the drill of this embodiment, the first cutting edge 26 and the second cutting edge 35 intersect at an obtuse angle, and at the intersection P of both cutting edges 26, 35, The crossing angle γ between the blade 26 and the tangent S of the second cutting edge 35 is 5
It is set in the range of 20 ° to 20 °. Therefore, as described above, the tip angle of the cutting edge 25 changes from the drill rotation center C toward the outer peripheral side from the large tip angle θ ₂ by the first cutting edge 26 to the small tip angle θ ₁ by the third cutting edge 36. It is possible to prevent the degree of change during the cutting from becoming gradual, and the distance until the biting of the cutting edge 25 stabilizes can be shortened accordingly. Therefore, the length of the first cutting edge 26 can be reduced. In combination with the effect of setting L1 short, it becomes possible to further improve the biting property and to stabilize the biting more quickly. This intersection angle γ is 5 °
If it is less than 20 ° C., such an effect may be impaired, while if it is more than 20 °, the concentration of cutting stress may be increased at the intersection point P, which may cause cutting edge defect.

Further, in the drill of the present embodiment, when viewed from the front end in the direction of the axis O, from the virtual straight line Q connecting the drill rotation center C and the outer peripheral end of the cutting blade 25, the second cutting edge 35 with respect to the virtual straight line Q is formed. The height H to the apex X is set in the range of 0.065 × D to 0.09 × D with respect to the outer diameter D of the drill, whereby the chips stored in the chip discharge groove 22 are removed. It is possible to smoothly send it to the base end side of the drill body 21 and efficiently discharge it. That is, the cutting edge 25
Since the cross-sectional shape of the chips generated by the above is substantially the same as the shape of the cutting blade 25, even if the chips are well curled and divided by the effect of the thinning surface 29 and the effect of the cutting blade 25 described above, If the height H is too large, the width of the chips indicated by the symbol W in FIG. 4 becomes large, and the chips cannot be smoothly stored in the chip discharge groove 22, and chip clogging is caused in the middle of the chip discharge groove 22. May occur.

On the other hand, in the drill of this embodiment, the height H is 0.09 × D with respect to the outer diameter D of the drill.
By the following, the width W of the chips is prevented from becoming too large and the occurrence of such clogging of chips is prevented in advance, which enables more stable chip disposal. In this embodiment, the height H of the cutting edge 25 from the imaginary straight line Q is 0.065 × D or more with respect to the outer diameter D of the drill because the height H becomes too small. Although the shape becomes flat and the width T of the chip becomes smaller, the curl of the chip cannot be promoted, so that it is not possible to obtain a chip shape that fits in the chip discharge groove 22, and as a result, chip clogging is caused. This is because there is a fear.

[0026]

As described above, according to the present invention, the thinning surface formed at the intersection of the tip flank of the drill and the wall surface of the chip discharge groove facing the rear side in the drill rotation direction,
By configuring at least three thinning surfaces that are concavely curved with respect to each other, it is possible to sufficiently secure the wall thickness of the cutting blade on the rear side in the drill rotation direction, and to improve the strength and rigidity of the cutting blade, and at the same time, the drill body itself. It is also possible to improve the strength of the. Moreover, since the heel side of the tip flank is not entirely cut out, and the contact length that allows the chips to be sufficiently curled in the chip discharge groove is provided, the chips can be reliably divided, which is excellent. It is possible to obtain good chip disposal performance. In particular, even when biting, it is possible to reliably divide the chips and prevent the spiral chips from continuing for a long time, so such chips deteriorate the surface roughness of the machined hole, and It is possible to prevent winding around the main body, and it is possible to perform smooth drilling throughout the processing.

[Brief description of drawings]

FIG. 1 is a view of a drill as viewed from the front end in the direction of the axis O showing an embodiment of the present invention.

FIG. 2 is a side view from the Y direction (direction orthogonal to the third cutting edge 36) of the embodiment shown in FIG.

FIG. 3 is a side view from the Z direction (direction orthogonal to the first cutting blade 26) of the embodiment shown in FIG.

FIG. 4 is a diagram showing chips generated by a cutting blade 25.

FIG. 5 is a view of a conventional drill as seen from the front end in the direction of the axis O.

6 is a side view of the conventional example shown in FIG.

[Explanation of symbols]

21 Drill Body 22 Chip Discharge Groove 24 Tip Relief Surface 25 Cutting Edge 26 First Blade (Thinning Blade) 29 Thinning Surface 30 First Thinning Surface 31 Second Thinning Surface 32 Third Thinning Surface 33 Second Thinning Surface 31 and Tip Relief Cross ridge line 34 with the surface 24 Cross ridge line 35 with the third thinning surface 32 and the tip clearance surface 24 Second cutting edge 36 Third cutting edge O Rotation axis D of the drill body 21 Drill outer diameter C Drill rotation center L ₁ First Length of cutting edge L ₂ Length of crossing ridge 33 α Crossing angle between first cutting edge 26 and crossing ridge β Crossing angle between crossing ridge 33 and crossing ridge 34

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Takeshi Inoue               Gifu Prefecture, Anpachi-gun, Kobe Town               Address 1528 Mitsubishi Materials Corporation Gi               Fu Factory                (56) References JP-A-62-44304 (JP, A)

Claims (2)

(57) [Claims] 1. A chip discharge groove is formed on a side surface of a drill body rotated around an axis from a tip end of the drill body toward a base end side thereof, and a wall surface facing the drill rotation direction side of the chip discharge groove and the above. In a drill in which a cutting edge is formed at a ridge line portion intersecting with a flank of the tip of the drill body, the core thickness portion of the tip of the drill body is linearly extended from the center of rotation of the drill by being thinned. A thinning blade that is continuous with the cutting edge is formed, and at the intersection of the tip flank and the wall surface that faces the drill rotation direction rear side of the chip discharge groove by the thinning,
A thinning surface is formed so as to reach the heel side of the drill body from the drill rotation center in the axial tip view, and the thinning surface is arranged so as to face the drill rotation direction side and to be substantially continuous with the drill rotation center, A first thinning surface on which the thinning blade is defined on a ridge line intersecting with the tip flank, and facing the outer peripheral side of the drill body, connected to the drill rotation center, and bent into the first thinning surface. a second thinned face that is arranged to cross, toward the drill rotation direction rear side is disposed such that the concave relative to said second thinned face on the heel side of the second thinned face, of the drill body formed of three thinned face of the third thinned face is reached on the heel side, the intersecting ridgeline between the second thinned face and the tip flank
The length corresponds to the outer diameter D of the drill when viewed from the tip in the axial direction.
However, the drill is characterized by having a diameter of 0.2 × D or less . 2. An intersection angle formed by an intersection ridgeline between the first thinning surface and the tip flank and an intersection ridgeline between the second thinning surface and the tip flank is 75 ° in the axial direction tip end view. The drill according to claim 1, wherein the drill has a range of ˜95 °. 3. An intersection angle formed by an intersecting ridgeline between the second thinning surface and the tip flank and an intersecting ridgeline between the third thinning surface and the tip flank is 90 ° in the axial direction distal end view. The drill according to claim 1 or claim 2, wherein the drill is in the range of 140 °.