JPH1058291A - Drill grinding method and drill - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a drill grinding method which can generate a sharp crest in the conical rotating locus of each cutting edge at the tip of a drill. SOLUTION: In the central cutting edge forming process, a pair of central cutting edges 16a and 16b constituting a convex arc of circle having a radius approx. 25-35% of the drill diameter are formed in point symmetry by means of thinning process in the rotational direction in the neighborhood of the rotational center O; in the straight cutting edge forming process, straight cutting edges 18a and 18b are formed as stretching straight to the peripheral end in continuity to the central cutting edges 16a and 16b. Accordingly there is no concave at the crests in the conical rotating loci of cutting edges 14a and 14b located at the tip of the drill 10, and a sharp crest form is established. This eliminates the problem of axis deflection at the time of starting the boring operation with a drill.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a grinding method for grinding a cutting edge of a drill.

[0002]

2. Description of the Related Art Various improvements have been proposed for drills used as drilling tools. For example, Japanese Patent Publication No. S39-16589 proposes a drill having an S-shaped cutting edge in order to solve the problem of the occurrence of runout at the start of drilling of a drill having a straight chisel edge. In addition, 48-5
Japanese Patent No. 3679 discloses an intersection of a center side cutting edge and an outer peripheral side cutting edge, that is, a corner having a predetermined radius of curvature in order to prevent loss of a corner present at a corner of a chisel edge formed by so-called X thinning. Drills rounded with arcs have been proposed. However, with a drill having a straight chisel edge as described above, the run-out of the drill at the start of drilling can be reduced to some extent, but it is still insufficient.

On the other hand, Japanese Patent Publication No. Sho 64-166 discloses that the cutting edge at the tip of a drill has a curve protruding in the direction of rotation near its center of rotation, and the curvature of the convex curve continuously decreases. A so-called spiral drill has been proposed. According to this, it is intended to eliminate the tendency of the apex of the conical rotation locus of the tip cutting edge to dent, which is likely to occur when the clearance angle of the tip cutting edge is large.

[0004]

However, in the grinding method described in Japanese Patent Publication No. Sho 64-166, the above-mentioned grinding method is used.
The dent tendency of the apex of the conical rotation trajectory cannot be actually eliminated, and this publication discloses at all a drill grinding technique for removing the dent of the apex of the conical rotation trajectory of the drill tip cutting edge. Did not.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method of grinding a drill which can make the top of a conical rotation locus of a tip cutting edge pointed. Is to do.

In order to achieve the above object, the present inventors have made various studies, and as a result, have found that the radius of curvature of the cutting edge near the rotation center in the bottom view of the drill can be determined in relation to the core thickness of the drill. It has been found that, when the dimension is within a predetermined ratio to the diameter of the cutting edge, the depression at the top of the conical rotation locus of the cutting edge is preferably eliminated. The present invention has been made based on such findings.

[0007]

First Means for Solving the Problems That is, the first means
The gist of the invention is a method of grinding a drill provided with a pair of cutting edges at a leading end, the starting end of which is near the center of rotation in bottom view, and extending to the outer peripheral end, and (a) rotating around the center of rotation. A center cutting edge portion forming step of forming a pair of center cutting edge portions forming a convex circular arc having a radius of about 25 to 35% of a drill diameter with respect to a direction by thinning, so as to be point-symmetric with each other; A straight cutting edge portion forming step of forming a straight cutting edge portion connected to the central cutting edge portion and linearly extending to the outer peripheral end portion.

[0008]

According to the first aspect of the present invention, in the step of forming the central cutting edge portion, a pair of convex arcs having a radius of approximately 25 to 35% of the diameter of the drill near the rotation center with respect to the rotation direction near the rotation center. The central cutting edge portions are formed symmetrically with respect to each other by thinning, and in the straight cutting edge portion forming step, a straight cutting edge portion that is connected to the pair of center cutting edge portions and linearly extends to the outer peripheral end portion is formed. Therefore, there is no dent at the top of the conical rotation locus of the cutting edge at the tip of the drill,
It has a pointed shape. As a result, misalignment at the start of drilling is eliminated. Here, when the radius of the arc of the pair of center cutting edges is less than 25% of the drill diameter, the chips generated by the center cutting edges are not smoothly discharged, and furthermore, the above-described generated chips are not generated. Chips are easily welded to the rake face of the central cutting edge, which causes chipping of the central cutting edge and increases cutting resistance, so that the feed rate per rotation of the drill must be reduced. When the radius of the arc of the pair of center cutting edges exceeds 35% of the drill diameter, the length of the straight cutting edge connected to the pair of center cutting edges and linearly extending to the outer peripheral end portion is increased. Becomes shorter, the portion corresponding to the straight cutting edge portion of the conical rotation trajectory of the cutting edge is reduced, the substantial drill tip angle is increased, and the center runout at the start of drilling of the drill is induced, The diameter of the processed hole is enlarged and the roundness of the hole is reduced.

[0009]

Preferably, the straight cutting edge portion is formed along a radial direction of the drill and parallel thereto. This facilitates the processing of the straight cutting edge, that is, the drill, without impairing the cutting performance of the drill.

Preferably, in the step of forming the center cutting edge portion, the pair of center cutting edge portions are connected to each other in an S-shape near the rotation center, and the curvature of the pair of center cutting edge portions is set. The center interval is formed so as to be approximately twice the radius of the pair of center cutting edges. In this case, since the pair of center cutting edges are connected in the tangential direction of the arc, there is an advantage that the connecting points are smooth.

Preferably, a tangent line passing through the center of rotation and contacting the arcs of the pair of center cutting edges in the bottom view, and a straight line passing through the center of rotation and perpendicular to the straight cutting edge. A flank grinding step of grinding the flank of each of the pair of cutting edges using a grinding direction parallel to the angle therebetween is further included. This has the advantage that the flank of one of the cutting edges is ground without damaging the other cutting edge on the bottom surface of the drill.

Preferably, the drill has a core thickness of 22 to 32% of a diameter of the drill and a linear core thickness of 18 to 28% (interval dimension in a perpendicular direction between a pair of straight cutting edges). It has. If the core thickness of the drill is less than 22% and the straight core thickness is less than 18%, the rigidity and strength of the drill become insufficient. Also, the core thickness of the drill is 3
If it exceeds 2% and the thickness of the straight core exceeds 28%, the cross-sectional area of the chip discharge groove becomes insufficient and chip clogging occurs.

[0013]

A second feature of the present invention is that a pair of cutting edges having a pair of cutting edges having a starting end near the center of rotation and an outer peripheral end when viewed from the bottom are provided at the front end. A method of grinding a drill, comprising: (a) forming a pair of central cutting edges having a radius of approximately 5 to 15% of a drill diameter with respect to a rotation direction in the vicinity of the rotation center and a central angle forming a convex arc with an acute angle; A center cutting edge portion forming step of forming points symmetrically with respect to each other by thinning, and (b) an inner circumferential side forming an inner circumferential side straight cutting edge portion connected to the pair of center cutting edge portions and linearly extending to the outer circumferential side. A straight cutting edge portion forming step, and (c) forming an outer circumferential side straight cutting edge portion connected to the inner circumferential side straight cutting portion at a predetermined angle and linearly extending radially toward the outer circumferential end portion. And forming the outer peripheral side straight cutting edge portion.

[0014]

According to the second aspect of the present invention, in the center cutting edge portion forming step, the radius of the drill is approximately 5 to 15% of the diameter in the rotation direction near the center of rotation, and the central angle becomes a convex with an acute angle. A pair of center cutting edge portions forming an arc are formed symmetrically with respect to each other by thinning, and in the inner circumferential side straight cutting portion forming step, the pair of center cutting edge portions are connected to the pair of center cutting edge portions and linearly extend to the outer peripheral side. A circumferential side straight cutting edge portion is formed, and in the outer circumferential side straight cutting edge portion forming step, the inner circumferential side straight cutting edge portion is connected to the inner circumferential side straight cutting edge portion at a predetermined angle and straight along the radial direction toward the outer circumferential end portion. The outer peripheral side straight cutting edge portion is formed to extend. For this reason, there is no dent at the top of the conical rotation locus of the cutting edge at the tip of the drill, and the cutting edge has a sharp shape. As a result, misalignment at the start of drilling is eliminated. Here, when the radius of the arc of the pair of center cutting edges is less than 5% of the drill diameter, the chips generated by the center cutting edges are not smoothly discharged, and furthermore, the generated chips are not used. Chips are easily welded to the rake face of the central cutting edge, causing chipping of the central cutting edge and increasing cutting resistance,
The feed rate per rotation of the drill must also be reduced. When the radius of the arc of the pair of center cutting edges exceeds 15% of the drill diameter, the length of the straight cutting edge connected to the pair of center cutting edges and extending linearly to the outer peripheral end portion is increased. Becomes shorter, the portion corresponding to the straight cutting edge portion of the conical rotation trajectory of the cutting edge is reduced, the substantial drill tip angle is increased, and the center runout at the start of drilling of the drill is induced, The diameter of the processed hole is enlarged and the roundness of the hole is reduced.

[0015]

Preferably, in the step of forming the central cutting edge portion, the pair of central cutting edge portions are connected to each other in an S shape near the center of rotation so as to be continuous with each other. Are formed so that the distance between the centers of curvature of the central cutting edge portions is approximately twice the radius of the pair of center cutting edge portions. In this case, since the pair of center cutting edges are connected in the tangential direction of the arc, there is an advantage that the connecting points are smooth.

Preferably, a tangent line passing through the center of rotation and contacting the arcs of the pair of center cutting edges in a bottom view, and a straight line passing through the center of rotation and perpendicular to the outer peripheral side straight cutting edge. And a flank grinding step of grinding the flank of each of the pair of cutting edges using a grinding direction parallel to the angle between the flank and the flank. This has the advantage that the flank of one of the cutting edges is ground without damaging the other cutting edge on the bottom surface of the drill.

Preferably, the drill has a core thickness of 28 to 35% of a diameter of the drill and a linear core thickness of 25 to 30% (interval dimension in a perpendicular direction between a pair of straight cutting edges). It has. If the core thickness of the drill is less than 28% and the straight core thickness is less than 25%, the rigidity and strength of the drill become insufficient. Also, the core thickness of the drill is 3
If it exceeds 5% and the linear core thickness exceeds 30%, the cross-sectional area of the chip discharge groove becomes insufficient and chip clogging occurs.

[0018]

Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 shows a drill 10 according to an embodiment of the present invention.
FIG. 2 is a bottom view as viewed from the bottom surface, and FIG.
It is a side view which shows the front-end | tip part. The drill 10 is made of a well-known material such as tool steel, high-speed steel, cemented carbide, cermet, and diamond ultra-high pressure sintered body. 1 and 2, a pair of spirally formed chip discharge grooves 12 a and 12 b are opened in the conical tip surface of the drill 10, and the starting end is near the rotation center O in bottom view. A pair of cutting edges 14a and 14b whose ends reach the outer peripheral edge are formed symmetrically with respect to the rotation center O. The pair of cutting edges 14a,
The rake face of the chip 14b corresponds to the chip discharge grooves 12a, 12b.
The relative positions of the chip discharge grooves 12a, 12b and the pair of cutting edges 14a, 14b are set so as to be directly or indirectly connected to the surface corresponding to the rotation direction of.

The pair of cutting edges 14a and 14b form a pair of convex arcs having a radius R of approximately 25 to 35% of the diameter D of the drill 10 in the rotation direction of the drill 10 near the rotation center O. Center cutting edges 16a, 16b
And straight cutting edges 18a, 18 connected to the pair of center cutting edges 16a, 16b and extending straight to the outer peripheral edge.
b.

The pair of central cutting edges 16a, 16b
Are formed in an S-shape near the rotation center O and are continuous with each other, and the pair of central cutting edges 16a, 16b
Are formed so that the distance between the centers of curvature Oa and Ob is approximately twice (2R) the radius R of the pair of center cutting edges 16a and 16b. Thereby, the pair of center cutting edge portions 1
Since the vicinity of the center of rotation O of 6a, 16b connects the centers of curvature Oa, Ob to each other and is in a direction perpendicular to the straight line, the pair of center cutting edges 16a, 16b are connected in the tangential direction of the arc. Therefore, the mutual contact points are smoothly connected.

The drill 10 has a diameter of 22 to 3 mm.
It has a core thickness of 2% (diameter of a circle in contact with the bottom of the chip discharge grooves 12a and 12b) and a linear core thickness of 18 to 28% (interval dimension in the perpendicular direction between a pair of linear cutting edges). I have. If the core thickness of the drill is less than 22% and the straight core thickness is less than 18%, the rigidity and strength of the drill will be insufficient, and the drill core thickness will be more than 32% and the straight core thickness will be 28%. If the ratio exceeds the above, the chip discharge groove becomes insufficient in cross-sectional area and chip clogging occurs.

The main part of the manufacturing process of the drill 10 configured as described above will be described below. First, the semi-finished drill 20 shown in FIGS. 3 and 4 is prepared through well-known steps. This semi-finished product 20 has a chip discharge groove 12
While a and 12b are already formed, the tip surface, that is, the bottom surface, is ground in a conical shape, and a chisel edge E having the same shape as the chisel edge is formed near the rotation center O as in a normal drill.

Next, in the flank forming step, the first flank 26a, 26b, the second flank 28a, 28b,
The third flank 30a, 30b is formed by a grinding wheel, respectively. Note that the second flank 28a, 28b and the third flank 30a, 30b may not be provided, or the first flank 26a, 26b may be formed as a curved surface.

In the following straight cutting edge forming step, a pair of center cutting edges 16 passing through the rotation center O in bottom view
a, tangent line A that comes into contact with the arc of 16b
And the straight cutting edge portions 18a, 1
By grinding each of the first flank surfaces 26a in a grinding direction parallel to an angle in a range C between the straight line B and the straight line B perpendicular to the straight line 8b, the pair of central cutting edges 16a, 16b And straight cutting edge portions 18a and 18b linearly extending in the radial direction parallel to the outer peripheral end portion are formed. By this grinding, the straight cutting edges 18a and 18b are sharply finished and their burrs and the like are removed. FIG.
The diagonal lines of the first flank surfaces 26a, 26b indicate an example of the above-described grinding direction, and are ground from the straight cutting edge portions 18a, 18b toward the first flank surfaces 26a, 26b. Incidentally, the tangent line A is substantially the same as the direction of the chisel edge of the drill without thinning shown in FIGS.

In the center cutting edge portion forming step, thinning is sequentially performed in the vicinity of the center of rotation of the tip end surface of the semi-finished drill 20, as shown in FIGS. 1 and 2.
Second thinning (gash) surfaces 24a, 24b shown in FIG.
Are formed, and then the first thinning surfaces 22a and 22b are formed to form a convex arc having a radius R of approximately 25 to 35% of the drill diameter with respect to the rotation direction of the drill 10 in the vicinity of the rotation center O. A pair of center cutting edges 16a, 16
b is formed point-symmetrically with respect to the rotation center O. The second thinning surfaces 24a and 24b are formed efficiently using a grindstone for relatively rough cutting, and the first thinning surfaces 22 forming the center cutting edges 16a and 16b having the positive rake angles.
a and 22b are formed by thinning using a grindstone for relatively precision grinding, and the first thinning surfaces 22a and 22b and the second thinning surfaces 24a and 24b are formed by thinning.
4b may be formed at once. In FIG. 2, the first thinning surface 22a and the second thinning surface 24a are not shown.

As described above, according to this embodiment, in the center cutting edge portion forming step, a convex arc having a radius of approximately 25 to 35% of the drill diameter in the rotation direction near the rotation center O is formed. The pair of center cutting edges 16a and 16b are formed symmetrically with respect to each other by thinning. In the step of forming a straight cutting edge, a pair of center cutting edges 16a and 16b are formed.
Straight cutting edge 18 extending straight to the outer peripheral end in connection with the
Since the a and 18b are formed, the conical rotation trajectories of the cutting edges 14a and 14b at the tip of the drill 10 have a sharp shape without a dent at the top. As a result, misalignment at the start of drilling is eliminated. Here, when the radius of the arc of the pair of center cutting edges is less than 25% of the drill diameter, the chips generated by the center cutting edges 16a and 16b are not smoothly discharged, and furthermore,
The generated chips are the center cutting edges 16a, 16b.
Easily adheres to the rake face, and the central cutting edge 16
Since the chipping of a and 16b is induced and the cutting resistance increases, the feed rate per rotation of the drill must be reduced. When the radius of the arc of the pair of center cutting edges 16a, 16b exceeds 35% of the drill diameter, the straight line extending straight to the outer peripheral end portion is connected to the pair of center cutting edges 16a, 16b. Cutting edge 18
a, 18b are shortened, and the portion corresponding to the straight cutting edge portions 18a, 18b in the conical rotation trajectory of the cutting edge is reduced, so that a substantial drill tip angle is increased to drill a hole. The runout at the start is induced, the diameter of the processed hole is enlarged, and the roundness of the hole is reduced.

Further, according to the present embodiment, the step of forming the center cutting edge portion comprises the step of forming the pair of center cutting edge portions 16a and 16b.
Are formed in an S shape near the rotation center O and are continuous with each other, and the interval between the centers of curvature Oa and Ob of the pair of center cutting edges 16a and 16b is
a and 16b are formed so as to be approximately twice the radius R of the circular arc.
Since the rotation center O-side end portions of a and 16b are connected in the tangential direction of the circular arc, there is an advantage that mutual connection points become smooth.

According to this embodiment, the pair of center cutting edges 16a, 16a passes through the center of rotation O in bottom view.
A pair of grinding directions parallel to an angle within an angle range C between a tangent line A respectively tangent to the arc of b and a straight line B passing through the center of rotation O and perpendicular to the straight cutting edges 18a, 18b. Flank surfaces 26a, 26 of cutting edges 14a, 14b of
A flank grinding step for grinding each b is further included. By doing so, on the bottom surface of the drill 10, the flank 26a of one cutting edge 14a is
There is an advantage that grinding is performed without impairing.

FIGS. 5, 6, and 7 are a bottom view, a perspective view, and a side view of a conventional drill 110 disclosed in, for example, Japanese Patent Publication No. 64-166. The drill 110 is provided with a cutting edge 11 according to a grinding direction indicated by a diagonal line in FIG. 5 (FIG. 5 of Japanese Patent Publication No. 64-166).
4a, 114b, the cutting edge 1
It is described that the tops of the conical rotation trajectories 14a and 114b are convex. However, as is apparent from FIG. 5, the oblique lines are arc-shaped center cutting edges 116a and 116 formed near the center of rotation O and protruding in the rotation direction.
intersects with b. In consideration of the inclination of the flank 126a, 126b, the diagonal line on the flank 126a is closer to the tip of the drill 110 to the right, and the diagonal line on the flank 126b is closer to the tip of the drill 110 to the left. Therefore, as shown in FIG. 6 and FIG.
a, the center of the arc of 116b is the most drill 11
0, the cutting edges 114a, 11
It is inevitable that a recess is formed at the top of the conical rotation locus of 4b.

FIGS. 8 and 9 show a drill 5 according to another embodiment.
0 is a bottom view and a side view. A pair of spirally formed chip discharge grooves 52a and 52b are opened in the conical tip surface of the drill 50, and the start end is near the rotation center O and the end is the outer peripheral edge in bottom view. Are formed symmetrically with respect to the center of rotation O. The pair of cutting edges 54
The rake faces of the chip discharge grooves 52a, 54b
The relative positions of the chip discharge grooves 52a, 52b and the pair of cutting edges 54a, 54b are set so as to be directly or indirectly connected to the surface corresponding to the rotation direction of 2b.

The pair of cutting edges 54a and 54b form a pair of convex arcs having a radius R of approximately 5 to 15% of the diameter D of the drill 50 in the rotation direction of the drill 50 near the rotation center O. Center cutting edges 56a, 56b
And an inner peripheral side straight cutting edge portion 58a connected to the pair of center cutting edge portions 56a and 56b and linearly extended to the outer peripheral side.
Outer peripheral side straight cutting edge portions 60a, 60 which are connected to the inner peripheral side linear cutting edge portions 58a, 58b at a predetermined angle and linearly extend radially toward the outer peripheral end portion.
b.

The pair of center cutting edges 56a, 56b
Are formed in an S-shape near the rotation center O and are continuous with each other, and the pair of center cutting edges 56a, 56b
Is formed so that the distance between the centers of curvature Oa and Ob is approximately twice (2R) the radius R of the pair of center cutting edges 56a and 56b. Thereby, a pair of center cutting edge portions 5
Since the vicinity of the rotation center O of 6a and 56b connects the centers of curvature Oa and Ob in a direction perpendicular to the straight line, the pair of center cutting edges 56a and 56b are connected in a tangential direction of the arc. Therefore, the mutual contact points are smoothly connected.

The drill 50 has a diameter of 28 to 3 mm.
It has a core thickness of 5% and a linear core thickness of 25-30%. If the core thickness of the drill is less than 28% and the straight core thickness is less than 25%, the rigidity and strength of the drill will be insufficient, and the drill core thickness will be more than 35% and the straight core thickness will be 30%. If the ratio exceeds the above, the chip discharge groove becomes insufficient in cross-sectional area and chip clogging occurs.

The main part of the manufacturing process of the drill 50 configured as described above will be described below. First, the semi-finished drill 20 shown in FIGS. 3 and 4 is prepared through well-known steps. This semi-finished product 20 has a chip discharge groove 12
a and 12b are already formed, and the tip end surface, that is, the bottom surface is ground in a conical shape, and an edge E having the same shape as the chisel edge is formed near the rotation center O as in a normal drill.

In the subsequent flank grinding step, the first surface of the drill semi-finished product 20 corresponds to the land,
The flank surfaces 66a and 66b, the second flank surfaces 68a and 68b, and the third flank surfaces 70a and 70b are formed by grinding wheels, respectively, so that the pair of inner peripheral side straight cutting portions 58a and 58b have a predetermined angle. The outer peripheral side straight cutting edges 60a, 60b are formed and connected and extend linearly in the radial direction toward the outer peripheral end, that is, parallel and linearly. In addition, the second flank 68a, 68b, the third flank 70a, 7
0b may be omitted, or the first flank 66a, 66b
May be formed on a curved surface.

Next, in the outer peripheral side straight cutting edge portion forming step, a tangent line A passing through the center of rotation O and coming into contact with the arcs of the pair of center cutting edges 56a and 56b, respectively, in a bottom view and The first flank 66 is formed by using a grinding direction parallel to an angle in a range C between a straight line B passing through and being perpendicular to the outer peripheral side straight cutting edge portions 60a, 60b.
A pair of outer peripheral side straight cutting edges 60a, 60b are formed by grinding each of a and 66b. The outer peripheral side straight cutting edge portions 60a and 60b are sharply finished by the above-described grinding, and their burrs and the like are removed. The oblique lines of the first flank surfaces 66a and 66b in FIG. 8 indicate an example of the above-described grinding direction.

Next, in the inner peripheral side straight cutting edge portion forming step, the third thinning surfaces 64a and 64 are formed by performing a predetermined thinning process using a grindstone for relatively rough grinding.
b is formed, and then the second thinning surfaces 63a and 63b are formed adjacent to the inner peripheral side of the third thinning surfaces 64a and 64b by performing thinning using a grindstone for relatively precision grinding, As a result, inner peripheral side straight cutting edges 58a, 58b are formed which are connected to the inner peripheral side of the pair of outer peripheral side straight cutting edges 60a, 60b and extend toward the center of rotation.

In the center cutting edge forming step, the second thinning surface 63a is formed by performing thinning using a grindstone for relatively precision grinding near the center of rotation of the front end surface of the drill semi-finished product 20. Since the first thinning surfaces 62a and 62b are formed adjacent to the inner peripheral side of 63b, approximately 5 to 15% of the drill diameter in the rotation direction of the drill 50 in the vicinity of the rotation center O is defined as the radius R. A pair of center cutting edges 56a and 56b having a positive rake angle forming a convex circular arc are formed point-symmetrically with respect to the rotation center O. In FIG. 9, the first thinning surface 62a,
The second thinning surface 63a and the third thinning surface 64a are not shown.

As described above, according to the present embodiment, in the center cutting edge portion forming step, a convex circular arc having a radius of approximately 5 to 15% of the drill diameter with respect to the rotation direction in the rotation direction near the rotation center O is formed. The pair of center cutting edges 56a and 56b are formed point-symmetrically with each other by thinning, and in the inner peripheral side straight cutting edge forming step, the pair of center cutting edges 56a and 56b are formed.
6b are formed on the inner peripheral side straight cutting edge portions 58a and 58b that extend linearly to the outer peripheral side in a straight line toward the outer peripheral side. Since the straight cutting edge portions 60a, 60b extending linearly in the radial direction toward the outer peripheral end portion are formed, a recess is formed at the top of the conical rotation locus of the cutting edge 54a, 54b at the tip of the drill 50. There is no point and it has a sharp shape. As a result, misalignment at the start of drilling is eliminated. Here, when the radius of the arc of the pair of center cutting edges 56a, 56b is less than 5% of the drill diameter, the chips generated by the center cutting edges 56a, 56b are not smoothly discharged, Further, the generated chips are easily welded to the rake faces of the central cutting edge portions 56a and 56b, so that chipping of the central cutting edge portions 56a and 56b is induced and the cutting resistance increases.
The feed rate per rotation of the drill must also be reduced. When the radius of the arc of the pair of center cutting edges 56a, 56b exceeds 15% of the drill diameter, the pair of center cutting edges 56a, 56b is connected to the pair of center cutting edges 56a, 56b and linearly extends to the outer peripheral end. The length of the peripheral straight cutting edge portions 58a and 58b is shortened, and the portion corresponding to the inner circumferential side straight cutting edge portions 58a and 58b in the conical rotation trajectory of the cutting edge is reduced to substantially reduce the drill tip angle. , The run-out of the drill at the start of drilling is induced, the diameter of the drilled hole is enlarged, and the roundness of the hole is reduced.

According to this embodiment, the step of forming the central cutting edge portion comprises the step of forming the pair of central cutting edge portions 56a, 56b.
Are formed in an S-shape near the center of rotation O, are continuous with each other, and the interval between the centers of curvature Oa, Ob of the pair of center cutting edges 56a, 56b is the pair of center cutting edges 56.
a and 56b are formed so as to be approximately twice the radius R of the circular arc.
Since the rotation center O-side end portions of a and 56b are connected in the tangential direction of the arc, there is an advantage that mutual connection points become smooth.

According to the present embodiment, the pair of center cutting edges 56a, 56
b, using a grinding direction parallel to an angle in an angle range C between a tangent line A respectively tangent to the arc of b and a straight line B passing through the rotation center O and perpendicular to the straight cutting edges 58a, 58b. Flank surfaces 66a, 66 of the cutting edges 54a, 54b
A flank grinding step for grinding each b is further included. In this way, on the bottom surface of the drill 50, the flank 66a of one cutting edge 54a is
There is an advantage that grinding is performed without impairing.

While the embodiment of the present invention has been described in detail with reference to the drawings, the present invention can be embodied in other forms.

For example, the drills 10 and 50 of the above-described embodiment are used.
Is a solid material, but a cemented carbide, ceramic, diamond ultra-high pressure sintered body, or CBN ultra-high pressure sintered body tip is fixed to the tip or cutting edge by brazing or the like. Is also good.

In the drill manufacturing process of the above-described embodiment, the flank grinding process, the straight cutting edge portion forming process, and the center cutting edge portion forming process have been described in this order. However, the order of these processes is changed. It can be done.

Further, the formation of the straight cutting edges 18a, 18b in the straight cutting edge forming step of the above-described embodiment and the grinding of the first flank surfaces 26a, 26b in the flank grinding step may be performed simultaneously. Absent.

Although not specifically exemplified, the present invention can be embodied with various modifications and improvements based on the knowledge of those skilled in the art.

[Brief description of the drawings]

FIG. 1 is a bottom view of a drill according to an embodiment of the present invention.

FIG. 2 is a side view showing a tip of the drill of the embodiment of FIG. 1;

FIG. 3 is a bottom view of a semi-finished drill used for manufacturing the drill of FIG. 1;

4 is a side view of the semi-finished drill of FIG. 3;

FIG. 5 is a bottom view of a conventional drill.

FIG. 6 is a perspective view showing a tip of the conventional drill shown in FIG. 5;

FIG. 7 is a side view showing the tip of the conventional drill shown in FIG.

FIG. 8 is a bottom view of a drill according to another embodiment of the present invention, and is a view corresponding to FIG.

FIG. 9 is a side view of the drill of FIG. 8, corresponding to FIG. 2;

[Explanation of symbols]

 10: drill 14a. 14b, 54a, 54b: Cutting edge 16a, 16b, 56a, 56b: Center cutting edge 18a, 18b: Straight cutting edge 58a, 58b: Inner circumference side straight cutting edge 60a, 60b: Outer circumference side straight cutting edge

Claims (7)

[Claims] 1. A method of grinding a drill having a pair of cutting edges at a leading end near a center of rotation and an outer peripheral end in a bottom view, wherein a diameter of the drill near a center of rotation with respect to a rotation direction is provided. About 2
A center cutting edge forming step of forming a pair of center cutting edges forming a convex arc having a radius of 5 to 35% symmetrically with respect to each other by thinning; and an outer peripheral end connected to the pair of center cutting edges. A step of forming a linear cutting edge portion extending linearly to form a straight cutting edge portion. 2. The method according to claim 1, wherein the step of forming the central cutting edge portion comprises forming the pair of central cutting edge portions into an S-shape near the rotation center so as to be continuous with each other, and the distance between the centers of curvature of the pair of central cutting edge portions is set to be equal. The drill grinding method according to claim 1, wherein the drill is formed so as to be approximately twice the radius of the pair of center cutting edges. 3. An angle between a tangent passing through the center of rotation and contacting the arcs of the pair of center cutting edges in the bottom view, respectively, and a straight line passing through the center of rotation and perpendicular to the straight cutting edge. Using a parallel grinding direction, a flank grinding step of grinding the flank of each of the pair of cutting edges,
3. The method for grinding a drill according to claim 1, further comprising: 4. A method for grinding a drill having a pair of cutting edges at a leading end whose starting end is near a center of rotation and which is at an outer peripheral end in bottom view, wherein a diameter of the drill near the center of rotation with respect to a rotating direction is determined. About 5
A center cutting edge portion forming step of forming a pair of center cutting edge portions having a radius of about 15% and forming a convex arc with a central angle of an acute angle by thinning to be point-symmetric with each other; Forming an inner peripheral side straight cutting edge portion that extends linearly to the outer peripheral side, and an inner peripheral side straight cutting edge portion forming step; and forming an outer peripheral end portion connected to the inner peripheral side straight cutting edge portion at a predetermined angle. A step of forming an outer peripheral side straight cutting edge portion that linearly extends radially toward the outer peripheral side cutting edge portion. 5. The center cutting edge portion forming step is such that the pair of center cutting edge portions are connected to each other in an S shape near the rotation center, and the distance between the centers of curvature of the pair of center cutting edge portions is set to be equal to each other. 5. The drill grinding method according to claim 4, wherein the drill is formed so as to have a radius substantially twice as large as the radius of the pair of center cutting edges. 6. A tangent line passing through the center of rotation and contacting the arc of each of the pair of center cutting edges in the bottom view, and a straight line passing through the center of rotation and perpendicular to the outer-side straight cutting edge. The drill grinding method according to claim 4 or 5, further comprising a flank grinding step of grinding the flank of each of the pair of cutting edges using a grinding direction parallel to the angle. 7. A drill manufactured by using the grinding method according to claim 1.