JP7601452B2 - Ball End Mill - Google Patents

Description

The present invention relates to a ball end mill in which multiple, approximately arc-shaped bottom cutting edges are formed on the tip surface of the tool body.

In the field of precision machining, ball end mills, for example, are generally used to cut dies, parts, and the like. For example, in the ball end mill 100 shown in FIG. 10, two quarter-arc R blades 102 are formed on the tip surface of the tool body 101, arranged 180 degrees opposite each other. This ball end mill 100 is rotatable around the central axis O. A gash groove 103 that forms the rake face 102a of the R blade 102 is formed on the forward side of the rotation direction of each R blade 102.

The two R blades 102 are formed with the center raised, and each extends from the outer peripheral surface side, passing near the central axis O, to the opposite side. The two R blades 102 intersect via a chisel 104 that overlaps the central axis O. Moreover, each gash groove 103 is provided in a plan view, extending from the outer peripheral side along the R blade 102 in the direction of the central axis O to the opposite side beyond the central axis O. Therefore, a pair of gash grooves 103 intersect with each other within a range of length k. Such a ball end mill is also disclosed in, for example, Patent Document 1.

JP 2000-117522 A

When cutting a workpiece with this ball end mill 100, the cutting resistance increases in this area because the chisel 104 having a dull blade is provided near the central axis O. Moreover, in the conventional ball end mill 100, in order to prevent chipping of the chisel 104 and ensure the strength of the cutting edge, not only was the R blade 102 set to be center-up, but the width of the thick-walled portion having the chisel 104, which is an extension of the flank face of the opposing R blade 102, was made wider and the rake angle was set to be negative.
However, even in this case, the low cutting speed causes an excessive load on the chisel 104, resulting in large cutting resistance, and the problem of heat generation and abnormal wear cannot be improved.

The present invention was made in consideration of these circumstances, and aims to provide a ball end mill that improves the cutting performance of the bottom cutting edge and suppresses damage and wear of the bottom cutting edge and chisel or recess.

The ball end mill of the present invention is a ball end mill having a cutting edge portion at the tip side of a tool body rotatable around a central axis, and is characterized in that it has a plurality of bottom blades formed in an approximately arc shape on the tip surface of the cutting edge portion, a plurality of gash grooves formed intersecting each other on the front side of the bottom blades in the rotational direction, a recess formed between the plurality of bottom blades and including the central axis, through which the plurality of gashes are connected , and a second clearance surface formed on the rear side of the plurality of bottom blades in the rotational direction, the clearance angle of which changes as it moves rearward in the rotational direction .

In addition, in a ball end mill having a recess on the tip surface of the tool body, it is preferable that the multiple gash grooves are arranged to intersect with each other and communicate with the recess, and that the diameter (d) of the inscribed circle between the bottom cutting edges is set in the range of 1% to 3% of the radius R of the bottom cutting edges.
Multiple gash grooves formed forward in the direction of rotation of multiple bottom blades intersect with each other within a range of 1% to 3% of the radius R, and a recess is formed in the center between the overlapping bottom blades. This makes it difficult for the recesses between the bottom blades to wear or break during cutting, resulting in high cutting performance.

In addition, a ball end mill having a chisel on the tip surface of the tool body may have a shape in which multiple gash grooves do not intersect with each other on the tip surface of the tool body, and the chisel may be formed between the bottom cutting edges.
A chisel is provided between the bottom blades, and because the bottom blades are made of high hardness and are formed with a core-down shape, the bottom blades have high cutting ability and wear and damage to the chisel between the bottom blades can be reduced.

In addition, it is preferable that the diameter (t) of the inscribed circle of the portion having the chisel is set in the range of 0.5% to 2.0% of the radius R of the bottom cutting edge.
When cutting with the cutting edge, the part with the chisel is hard and thin, so even if it is pressed against the workpiece, wear and damage to the chisel can be reduced.

In addition, the axial rake angle of the bottom cutting edge is preferably set in the range of -10° to +15°.
The axial rake angle of the bottom cutting edge is formed on the positive side in the range of -10° to +15°, which, combined with the center-down position, provides high cutting performance.

According to the ball end mill of the present invention, a cutting edge portion made of cBN, which is a high hardness material, is used,
A chisel or recess is provided near the central axis between the bottom blades, and the amount of center depression of the bottom blade is formed to be greater than 0% and not more than 6% of the radius R of the bottom blade, which not only improves the cutting ability of the bottom blade but also provides excellent resistance to chipping and wear of the bottom blade and the chisel and recess between the bottom blades.

FIG. 2 is a tip end view showing a cutting edge portion of the ball end mill according to the first embodiment of the present invention. FIG. 2 is a side view of the ball end mill shown in FIG. 1 . FIG. 2 is a side view of a ball end mill showing the axial rake angle of the R blade. FIG. 11 is a tip surface view showing a cutting edge portion of a ball end mill according to a second embodiment. FIG. 5 is a side view of the ball end mill shown in FIG. 4 . FIG. 2 is a side view of a ball end mill showing the axial rake angle of the R blade. These show conventional examples, where (a) is a diagram of the flank near the center of the R blade before machining, (b) is a contour projection of the R blade before machining, (c) is a diagram of the flank near the center of the R blade after machining, and (d) is a contour projection of the R blade after machining. 1 shows Example 1, where (a) is a diagram of the flank of the chisel before machining, (b) is a contour projection of the R blade before machining, (c) is a diagram of the flank of the chisel after machining, and (d) is a contour projection of the R blade after machining. 1 shows Example 2, where (a) is a diagram of the flank near the center of the R blade before machining, (b) is a contour projection of the R blade before machining, (c) is a diagram of the flank near the center of the R blade after machining, and (d) is a contour projection of the R blade after machining. FIG. 1 is a view showing a tip surface of a cutting edge portion of a conventional ball end mill.

Ball end mills according to the embodiments of the present invention will be described below with reference to the accompanying drawings. Figures 1 to 3 show a ball end mill 1 according to a first embodiment of the present invention. In Figures 1 and 2, the ball end mill 1 according to this embodiment comprises a tool body 2 formed in a substantially cylindrical shape and rotated around a central axis O, and a cutting edge portion 3 formed at the tip of the tool body 2. At least the cutting edge portion 3 of the ball end mill 1 is formed of a cBN sintered compact made of a high hardness material. Alternatively, PCD may be used as the material for the cutting edge portion 3 of the ball end mill 1 instead of cBN.
This ball end mill 1 is used for cutting high-hardness materials such as machine parts, dies, etc. In this specification, the side of the cutting edge portion 3 along the central axis O of the tool body 2 is referred to as the tip side or tip, and the opposite side connected to the spindle is referred to as the base side or base.

1 and 2, the ball end mill 1 has a plurality of, for example, two R blades 5 formed at positions facing each other at 180 degrees as bottom blades having a shape of a substantially quarter arc on the tip surface 4 of the cutting blade portion 3, which is substantially hemispherical. When the outer diameter of the tool body 2 is D and the radius of the R blade 5 is R, the relationship is set such that 2R≧D. Moreover, each R blade 5 is formed so as to be center-down.
In the cutting edge portion 3, a gash groove 7 is formed on the front side in the rotation direction of each R blade 5. The gash surface formed in the gash groove 7 is used as a rake face 8 of the R blade 5, and the rake face 8 may be formed as a flat surface, a concave curved surface, or a convex curved surface. In this embodiment, the rake face 8 is, for example, a convex curved surface.

In Figure 3, the rake angle α of the rake face 8 of the R blade 5 relative to the central axis O is set in the range of -10° to +15°. The rake angle α of the R blade 5 is set to the axial rake angle α. If the rake angle α of the R blade 5 is smaller than -10°, the cutting edge strength is high but the cutting performance is significantly reduced, and if it is larger than +15°, the cutting performance is high but the cutting edge strength is reduced.
A second flank 9 and a third flank 10, each having a positive clearance angle, are formed in sequence on the rear side of the R blade 5 in the direction of rotation. The clearance angle of the second flank 9 is set to a positive angle of +10° to +25°. This clearance angle may be a constant angle, or may change so as to gradually increase or decrease from the R blade 5 side toward the rear in the direction of rotation. The clearance angle of the third flank 10 is greater than the clearance angle of the second flank 9.

A peripheral cutting edge 13 is formed on the base end side of the R blade 5. A flute groove 14 is formed on the front side in the rotation direction of the peripheral cutting edge 13. A cutting face 15 of the peripheral cutting edge 13 is formed on the flute groove 14. The peripheral cutting edge 13 and the flute groove 14 have a helix angle that gradually twists spirally backward in the rotation direction from the tip side to the base end side.
In a side view of the tool body 2, the R blade 5 and the peripheral blade 13 are twisted rearward in the direction of rotation from the tip end to the base end. The peripheral blade 13 has a twist angle twisted rearward at an angle equal to or greater than that of the R blade 5. The gash groove 7 of the R blade 5 and the flute groove 14 of the peripheral blade 13 are also twisted rearward in the direction of rotation from the tip end to the base end, similar to the R blade 5 and the peripheral blade 13.

In the tip end view of the cutting blade portion 3 shown in FIG. 1, a pair of R blades 5 are formed with the center lowered. The R blades 5 may be formed in a convex curved shape or a straight shape that curves and protrudes forward in the rotation direction. The ends of the R blades 5 on the central axis O side extend to a position close to the central axis O, and these ends are connected to a thick extended flank 18a where the extended parts of the second flank 9 are connected to each other. A chisel 18 is formed in the center of this extended flank 18a, which overlaps with the central axis O. The diameter (thickness) t of the inscribed circle of the extended flank 18a (part having the chisel 18) having the chisel 18 is set in the range of 0.5% to 2.0% of the radius R of the R blade 5. If the diameter t of the inscribed circle is smaller than 0.5%, it is significantly prone to chipping during cutting, and if it is larger than 2.0%, the cutting resistance increases, causing abnormal wear.
The chisel 18 corresponds to a dead spot. The extended flank 18a of the chisel 18 overlaps with the central axis O, and the ridge between the rake face and the extended flank 18a serves as a cutting edge for cutting when the tool body 2 rotates. However, since the chisel 18 is located near the central axis O, the cutting speed is low and cutting performance is poor.

The amount of center drop s, which is the distance between the pair of R blades 5, is set to a range greater than 0% and less than 6% of the radius R of the R blade 5. If the amount of center drop s is 0%, the R blade 5 cannot be set to center drop and the cutting performance of the R blade 5 decreases, and if it exceeds 6%, the cutting performance is high but the cutting edge strength decreases. Therefore, it is preferable to set the amount of center drop s of the pair of R blades 5 in the above range in order to ensure cutting performance and cutting edge strength. In order to ensure cutting balance by the pair of R blades 5, it is preferable that the distance from the central axis O of each of the amount of center drop s is s/2.

The two gash grooves 7 formed on the front side of each of the pair of R blades 5 in the rotational direction do not intersect with each other, and a gap of the diameter t of the inscribed circle of the extended relief surface 18a of the chisel 18 is set. The cutting edge portion 3 made of cBN has high hardness, so if the chisel 18 is present near the central axis O, it will create a large resistance and will be easily chipped. In this embodiment, the range of the diameter t of the inscribed circle of the extended relief surface 18a of the chisel 18 is narrowed to the above range, improving cutting performance and suppressing wear.

The ball end mill 1 according to this embodiment has the above-mentioned configuration, and its cutting method will now be described.
When the ball end mill 1 is rotated around the central axis O and the cutting blade 3 is fed vertically in the direction of the central axis O, the cutting resistance increases because the central chisel 18 cannot cut, and the extended flank 18a located on the tip surface 4 of the cutting blade 3 is worn. However, the diameter t of the inscribed circle of the extended flank 18a of the chisel 18 is small, ranging from 0.5% to 2.0% of the radius R, so the amount of wear on the extended flank 18a is small. The ball end mill 1 is then fed laterally to cut the vertical wall of the machining surface with the R blade 5. The ball end mill 1 is fed laterally to cut the workpiece material in the part of the chisel 18 that could not be cut during cutting with the R blade 5.

Moreover, the R blade 5 of the cutting edge portion 3 is made of cBN and has high hardness, and the diameter t of the inscribed circle of the extended flank 18a having the chisel 18 is small, so the cutting performance of the R blade 5 can be improved and wear and chipping of the extended flank 18a can be suppressed.

As described above, according to the ball end mill 1 of this embodiment, the cutting edge portion 3 is made of cBN and has high hardness, and the axial rake angle α of the pair of R blades 5 is inclined toward the positive side, and the diameter t of the inscribed circle of the extended relief surface 18a of the chisel 18 sandwiched between the R blades 5 is small. Therefore, the cutting ability of the R blades 5 is high, and the wear resistance and chipping resistance of the R blades 5 and chisel 18 are excellent.

Although the ball end mill 1 according to the embodiment of the present invention has been described above, it goes without saying that the present invention is not limited to such an embodiment, and various different forms and configurations can be adopted without departing from the spirit of the present invention. All of these are included in the technical scope of the present invention.
Other embodiments and modifications of the present invention will be described below, and the same reference numerals will be used to describe parts and components that are the same as or similar to those of the above-described embodiments.

4 to 6 show a ball end mill 20 according to a second embodiment of the present invention, which has a similar configuration to the ball end mill 1 according to the first embodiment. The ball end mill 20 has a plurality of, for example, two R blades 5 formed at positions facing each other at 180 degrees as bottom blades having a substantially quarter-arc shape on the tip surface 4 of the cutting portion 3. The outer diameter of the tool body 2 is D, and the radius of the R blade 5 is R, so that 2R≧D is satisfied. Moreover, each R blade 5 is formed so as to be center-down.
In the cutting edge portion 3, a gash groove 7 is formed on the front side in the rotation direction of each R blade 5. The gash surface of the gash groove 7 serves as a rake face 8 of the R blade 5. The rake face 8 is, for example, a convex curved surface.

The axial rake angle α, which is the rake angle of the rake face 8 of the R blade 5 relative to the central axis O, is set in the range of -10° to +15°. If the rake angle α of the R blade 5 is smaller than -10°, the cutting edge strength is high but the cutting performance is significantly reduced, and if it is larger than +15°, the cutting performance is high but the cutting edge strength is reduced.
A peripheral cutting edge 13 is formed on the base end side of the R blade 5, and a flute groove 14 is formed on the front side of the peripheral cutting edge 13 in the rotational direction. A rake face 15 of the peripheral cutting edge 13 is formed on the flute groove 14. A second flank 9 and a third flank 10, each having a positive clearance angle, are formed in this order on the rear side of the R blade 5 in the rotational direction. The clearance angle of the second flank 9 is set to a positive angle of +10° to +25°. The clearance angle of the second flank 9 is set to a fixed angle or a variable clearance angle.

A recess 21 communicating with the gash groove 7 is formed in a region including the central axis O between the two opposing R blades 5. Although the recess 21 is formed in a substantially conical shape in Fig. 4 and Fig. 5, the shape of the recess 21 is arbitrary. Therefore, no chisel 18 or dead spot is formed in the region including the central axis O between the two R blades 5.
In addition, on the tip surface 4 of the cutting blade portion 3, the gash grooves 7 formed on the front side of the rotation direction of the R blade 5 extend beyond the central axis O to the region on the opposite side in the radial direction. The two gash grooves 7 overlap and intersect with each other at the recessed portion 21 in the region of the central axis O. The diameter d of the inscribed circle (recessed portion 21) between the center side ends of the opposing R blades 5 shown in FIG. 4 is set to a range of 1% to 3% of the radius R of the R blade 5. If the diameter d of the inscribed circle between the R blades 5 on the central axis O side is smaller than 1%, there is a drawback that chips adhere to each other and abnormal wear occurs. Also, if it is larger than 3%, there is a drawback that the diameter of the recessed portion 21 during vertical wall processing by the cutting blade portion 3 becomes too large, and the diameter of the convex portion remaining without cutting increases, and the cutting load of the convex portion by the R blade 5 during lateral feeding increases, and the roughness of the processed surface deteriorates.

The ball end mill 20 according to this embodiment has the above-mentioned configuration, and its cutting method will now be described.
For example, when performing contour rough cutting by cutting vertically and feeding horizontally while rotating the cutting blade portion 3 of the ball end mill 1 around the central axis O, the recessed portion 21 formed in the center of the two R blades 5 cannot cut during vertical feeding. This results in a convex portion remaining uncut due to the recessed portion 21 on the workpiece.
Thereafter, by feeding the ball end mill 1 laterally, even if there is no blade in the recess 21 of the central axis O of the cutting blade portion 3, the remaining protrusion that could not be cut during cutting is cut by the R blade 5. Moreover, since the R blade 5 of the cutting blade portion 3 is made of cBN and has high hardness, and the diameter of the recess 21 is small, cutting performance can be improved and wear and chipping can be suppressed.

As described above, according to the ball end mill 20 of the second embodiment, the cutting edge portion 3 is made of cBN and has high hardness, and the axial rake angle of the pair of R blades 5 is inclined toward the positive side, so that the remaining cutting material in the recess 21 sandwiched between the R blades 5 can be cut by lateral feed.
Therefore, the cutting ability of the R blade 5 is high, and wear and chipping can be suppressed at the R blade 5 and the recessed portion 21. Moreover, since there is no chisel 18 in the recessed portion 21 between the R blades 5, the cutting resistance can be further reduced.

Next, a cutting test for rough contour cutting was carried out for the ball end mill 1 according to the first embodiment and the ball end mill 20 according to the second embodiment described above. In each test example, the ball end mill 1 according to the first embodiment was used as Example 1, the ball end mill 20 according to the second embodiment was used as Example 2, and the conventional ball end mill 100 shown in Fig. 10 was used as a conventional example, and die cutting was carried out on a workpiece made of a high-hardness material.
In each of the first and second embodiments, the pair of R blades 5 are center-down, while in the conventional embodiment, they are center-up. In the first embodiment, a chisel 18 having an inscribed circle diameter t is formed near the central axis O of the cutting portion 3, while in the second embodiment, a recess 21 is formed near the central axis O of the cutting portion 3. In the conventional embodiment, a chisel 104 having a thickness greater than the chisel 18 is formed near the central axis O.

The cutting conditions were: rotation speed n: 120000 min-1, feed speed Vf: 1050 mm/min, ap (cutting depth in the Z-axis direction): 0.005 mm, ae (cutting depth in the lateral direction): 0.005 mm. Oil mist was used as the coolant.
As comparative test results, contour projection photographs are shown showing the wear state of the flank near the center of rotation of each ball end mill 1, 20, 100 and the amount of wear of the R blades 5, 102 before and after cutting the workpiece for 10 hours.

Rough contour cutting of the workpiece was carried out for 10 hours for Example 1, Example 2, and the conventional example. In Figures 7 to 9, (a) and (b) show the portion of the R blade near the central axis O and a contour projection photograph of the R blade before cutting. Figures 7 to 9 (c) and (d) show the portion of the R blade near the central axis O and a contour projection photograph of the R blade after 10 hours of cutting. In the contour projection photograph of the R blade after 10 hours of cutting, the black portion indicates the remaining contour of the R blade, and the gray portion indicates the worn portion of the R blade.

In the conventional example shown in Figures 7(a) and (b), compared to the flank of the chisel 104 and the R blade 102 before cutting, after cutting as shown in Figures 7(c) and (d), the flank of the chisel 104 has worn significantly and the cutting edge of the R blade 102 has also worn significantly.
In contrast, in Example 1 shown in Figures 8(a) and (b), compared to the extended flank 18a of the chisel 18 and the R blade 5 before cutting, after cutting as shown in Figures 8(c) and (d), the extended flank 18a of the chisel 18 is slightly worn and the cutting edge of the R blade 5 is only slightly worn.

In Example 2 shown in Figures 9(a) and (b), compared to the recess 21 and R blade 5 before cutting, after cutting as shown in Figures 9(c) and (d), the second relief surface 9 of the R blade 5 on both ends of the recess 21 is slightly worn and the cutting edge of the R blade 5 is also only slightly worn.
From these test results, compared to the results of the conventional example, in Examples 1 and 2, the wear at the center portions of the pair of R blades 5 was small, and the wear at the cutting edges of the R blades 5 was also small. Therefore, it was confirmed that Examples 1 and 2 could improve cutting performance and suppress wear and chipping.

In the ball end mills 1 and 20 according to the above-mentioned embodiments, a small relief may be provided on the second flank 9 side of the R blade 5. In this case, mirror finishing can be performed without causing chipping on the machined surface. Alternatively, a honed cutting edge may be provided instead of the small relief.
In the ball end mills 1 and 20 according to the above-described embodiments, the number of R blades 5 (bottom blades) formed on the cutting edge portion 3 is not limited to two, and may be three or four or more. When three or more R blades 5 are provided, the R blades 5 are arranged at equal pitches, but may also be arranged at unequal pitches.

1, 20 Ball end mill 2 Tool body 3 Cutting edge portion 5 R-blade 7 Gash groove 8 Rake face 9 Second relief face 13 Peripheral cutting edge 14 Flute groove 15 Rake face 18 Chisel 21 Recess O Central axis

Claims (2)

A ball end mill having a cutting edge portion at the tip end of a tool body that can rotate around a central axis,
A plurality of bottom blades formed in a substantially arcuate shape on a tip surface of the cutting blade portion;
A plurality of gash grooves are formed so as to intersect with each other on the front side in the rotation direction of the bottom cutting edge;
a recess formed between the plurality of bottom cutting edges and including the central axis line, the recess communicating with the plurality of gashes;
a second flank formed on a rear side of the end cutting edges in a rotational direction, the flank angle of which changes as the flank angle moves rearward in the rotational direction ;
The diameter of an inscribed circle between the multiple bottom cutting edges centered on the central axis is set to be within the range of 1% to 3% of the radius R of each bottom cutting edge . The ball end mill according to claim 1, wherein each of the multiple bottom cutting edges is set to be center-down, and the amount of center-down between the bottom cutting edges is greater than 0% and less than or equal to 6% of the radius R of each bottom cutting edge.