JP5126205B2 - Ball end mill - Google Patents

Description

  The present invention relates to a ball end mill in which at least a pair of cutting blades whose rotational trajectory around an axis forms a hemisphere is formed at the end of an end mill main body.

  As this type of ball end mill, for example, Patent Document 1 discloses a non-ferrous iron in which a pair of ball blades (cutting blades) are provided symmetrically with respect to an axis (axis) on a hemispherical portion (end portion of the end mill body) at the tip. In the ball end mill for metal, the diameter of a circular region (heart thick circle) where a pair of ball blades do not exist at the center of the hemispherical portion is within a range of 0.02 to 0.08 mm, and It has been proposed that the overlapping dimension of the ball blades (the amount of difference between the cutting blades) is in the range of 0 to 0.08 mm, and the right angle rake angle of the pair of ball blades is in the range of 10 ° to 20 °. Yes.

  According to Patent Document 1, in such a ball end mill, since the diameter of the circular region is small, there are very few regions where the cutting action cannot be obtained, and the ball blade rake angle is large. The sharpness is improved and excellent cutting performance is obtained. On the other hand, since the overlapping dimension of the pair of ball blades is within the above range, there is no possibility that the mechanical strength of the ball blade is remarkably lowered regardless of the shrinkage of the circular region. It is said that a practically sufficient strength can be secured for soft materials such as aluminum.

JP 2000-334614 A

  However, in the ball end mill described in Patent Document 1, the diameter of the core thickness circle is within a certain range regardless of the outer diameter of the cutting edge (the diameter of the hemisphere formed by the cutting edge about the axis). When the diameter is large, the diameter of the core thick circle becomes relatively small and the strength around the axis of the end mill body tip is impaired, and such a ball end mill with a large cutting edge outer diameter is frequently used. On the other hand, there is a risk that the tip of the end mill main body may be lost in large processing, but conversely, when the outer diameter of the cutting edge is small, the diameter of the core thickness circle becomes relatively large and it becomes difficult to obtain a good sharpness. There was a fear.

  In the above ball end mill, the amount of crossing of the cutting edge is constant regardless of the outside diameter of the cutting edge. Therefore, when the outside diameter of the cutting edge is large, the amount of crossing becomes relatively small, and accordingly, generated by the cutting edge. There is a risk that the chip pocket for discharging the cut chips will be small and chip discharge performance may be impaired. As a result, the rigidity of the end mill main body becomes difficult to secure, and there is a risk that the chip will be lost.

The present invention has been made under such a background. Even in a ball end mill with a small diameter such that the outer diameter of the cutting edge is less than 2 mm, the chip end resistance of the end mill main body, the sharpness of the cutting edge, and chip dischargeability are achieved. It aims at providing the ball end mill which can be compatible.

In order to solve the above-mentioned problems and achieve such an object, the ball end mill according to the present invention has at least the end of the end mill main body rotated about the axis, and the rotation trajectory about the axis forms a substantially hemispherical shape. A pair of cutting blades are ball end mills formed at opposite ends of the end mill main body with the axis interposed therebetween, and a gash is formed at the end of the end mill main body, and the end mill rotation of the gash and forming the cutting edge to the intersecting edge line region of the flank of the wall and the end mill body tip facing direction, the outer diameter of the cutting edge and the diameter below 2 mm, in the axial direction tip view, the said axis The diameter δ (mm) of the center-thick circle inscribed in the pair of cutting edges as the center is δ = 0.03 × D ^1/2 to δ = 0.06 × with respect to the outer diameter D (mm) of the cutting blade. D ^The difference H (mm) between the cutting edges of the pair of cutting edges that cross each other beyond the contact point with the core thickness circle is H = 0.05 × D ^1/2. Gash where the cutting edge, a wall surface facing the rear side in the end mill rotation direction of the gasche, and an intersecting ridge line portion of the flank face are in a range of −0.04 to H = 0.09 × D1 ^{/ 2.} The corner portion has a radius of curvature R (mm) of 0.03 mm or more and 0.08 × D ^1/2 or less with respect to the outer diameter D (mm) of the cutting blade in the axial front end view . As a concave curve shape, the cutting edge, the wall surface facing the rear side in the end mill rotation direction of the gasche and the ridge line portion of the flank face are in smooth contact with both ends of the concave curve formed by the gasche corner portion. It is characterized by being formed .

Accordingly, in such a ball end mill, the diameter δ of the core thick circle and the amount of difference H of the cutting edge are both set within a range based on the square root of the outer diameter D of the cutting edge. Even in a ball end mill with a small diameter of D less than 2 mm , the core thickness circle is the minimum necessary while ensuring a sharp sharpness by reducing the core thickness diameter and bringing the cutting edge closer to the end axis of the end mill body. While it is possible to leave the diameter, the lack of rigidity at the tip of the end mill body due to the decrease in the diameter of the core thickness circle, thus reducing the crossing amount of the cutting blade to the minimum necessary, the required chip pocket capacity is This can be avoided by preventing the tip mill body from being greatly cut out by the tip pocket while maintaining it.

Here, it is desirable that the amount of crossing H (mm) between the cutting edges be within a range of H = 2 × δ or less with respect to the diameter δ (mm) of the core thickness circle. That is, in the small-diameter ball end mill in which the outer diameter D of the cutting edge is relatively small and the diameter δ of the core thickness circle is reduced accordingly, the cutting edge near the axis of the end mill body tip is good as described above. Therefore, it is not necessary to make the chip pocket too large because chips with good dischargeability are generated, and by keeping the crossing amount H in a smaller range, the rigidity of the end mill body tip is ensured and the cutting edge strength is maintained. can do. However , it is desirable to ensure a large chip pocket by increasing the crossing amount H to, for example, about δ−0.03 (mm) or more.

Further, in the ball end mill of the present invention , a gash is formed at the tip of the end mill body, and the cutting edge is formed at a crossing ridge line portion between a wall surface facing the end mill rotation direction of the gash and a flank at the tip of the end mill body. It is, with the cutting edge, the gash corner portion and the intersection ridge line intersects the wall face and the flank facing the end mill rotating direction rear side of the gash, curvature in the axial direction distal end view radius R (mm) Is 0.03 (mm) or more, and is a concave curve shape within the range of R = 0.08 × D ^1/2 or less with respect to the outer diameter D (mm) of the cutting blade . Note that the cutting edge, the crossing ridge line portion of the wall surface facing the rear side in the end mill rotation direction of the gasche and the flank surface intersects at an intersecting angle α in the range of 80 ° to 120 ° in the end view in the axial direction. It is desirable to be formed in the direction.

That is, if the radius of curvature R of the gash corner and the crossing angle α are too small, stress tends to concentrate on the gash corner and the end mill body may be damaged in a small-diameter ball end mill. On the other hand, if the crossing angle α is too large, the gash becomes too large and the end of the end mill main body may be greatly cut out to make it difficult to ensure its rigidity. If the curvature radius R is too large, Insufficient chip pocket capacity for the gasche cannot be secured, and good chip discharge performance is impaired, and the accuracy of the hemisphere formed by the rotation trajectory of the cutting edge may be impaired.

As described above, according to the present invention, by ensuring the rigidity of the tip of the end mill main body, a high cutting edge strength is given to the cutting edge in the vicinity of the axis to obtain excellent chipping resistance, and the cutting edge of the cutting edge is obtained. In addition, it is possible to achieve a smooth and highly accurate processing with good chip discharge performance.

It is a side view which shows one Embodiment of this invention. It is the enlarged front view which looked at the cutting-blade part 2 of embodiment shown in FIG. 1 in the axis line O direction front view. It is an enlarged front view of the axis line O vicinity in FIG. FIG. 4 is an enlarged front view in the vicinity of an axis O showing a reference example for the embodiment shown in FIGS. 1 to 3. It is the enlarged front view which looked at the cutting-blade part 2 which shows the modification of embodiment shown in FIG.

  1 to 3 show an embodiment of the present invention. In this embodiment, the end mill body 1 is formed of a hard material such as cemented carbide, a cutting edge portion 2 is formed on the tip side thereof, and a rear end portion thereof is a substantially cylindrical shaft-shaped shank portion 3. The shank portion 3 is gripped by the rotating shaft of the machine tool and is rotated in the end mill rotating direction T around the central axis O, and is used, for example, for machining a die or the like. Here, in the present embodiment, a pair of chip discharge grooves 4 that are twisted toward the rear side in the end mill rotation direction T from the front end of the cutting blade part 2 toward the rear end side on the outer periphery of the cutting blade part 2. , 4 are formed symmetrically on opposite sides with respect to the axis O, and outer peripheral blades 5 are respectively formed on the side ridges of the wall surfaces of the chip discharge grooves 4, 4 facing the end mill rotation direction T side. .

On the other hand, tip pockets 6 communicating with the chip discharge grooves 4, 4 are formed at the tip of the cutting edge 2, that is, at the tip of the end mill body 1, and each tip pocket 6 faces the end mill rotation direction T side. On the side ridge portion of the wall surface, a substantially 1/4 arc-shaped cutting blade 7 whose rotational locus around the axis O forms a hemispherical shape is also symmetrically opposite to each other across the axis O, and at the end of the end mill body 1 tip. As it goes from the vicinity of the axis O toward the outer peripheral side, it extends so as to be twisted toward the rear side in the end mill rotation direction T around the axis O, and is connected to the outer peripheral blade 5. Therefore, by being formed to be twisted in this way, in the present embodiment, the cutting blades 7 and 7 have a generally S-shape that gently curves as shown in FIG. . In addition, in the present embodiment, the outer peripheral surface of the tip of the cutting edge 2 connected to the rear side in the end mill rotation direction T of the cutting edge 7 is a flank 8 that retreats in multiple steps toward the rear side in the end mill rotation direction T. .

  Here, the vicinity of the axis O at the tip of the end mill body 1 of both the tip pockets 6 is a gash 9, and therefore, in the vicinity of the axis O, each cutting edge 7 has a wall surface facing the end mill rotation direction T side of the gash 9 and the clearance surface. 8 will be formed at the intersection ridge line portion with 8. As shown in FIG. 3, these gashes 9 and 9 are positioned on opposite sides of the virtual plane P including the axis O without intersecting the virtual plane P when viewed from the front in the direction of the axis O. In addition, they are formed so as to cross each other slightly along the imaginary plane P over the axis O. Therefore, the cutting edges 7 and 7 in the vicinity of the axis O at the end of the end mill body 1 also cross over the axis O from the outer peripheral side in parallel to each other and parallel to the virtual plane P in the end view in the direction of the axis O. Then, the wall surface of the gash 9 facing the rear side in the end mill rotation direction T intersects the intersecting ridge line portion 10 of the flank 8 (first flank 8a) connected to the other cutting edge 7.

  By forming the cutting blades 7 and 7 in this way, no gash 9 is formed between the pair of cutting blades 7 and 7 in the vicinity of the axis O at the end of the end mill body 1, and both cutting blades 7 and 7 are formed. A core thick portion 11 extending as it is is left as it is, and a circle inscribed on both the cutting edges 7 and 7 about the axis O is seen on the core thick portion 11 when viewed from the front in the direction of the axis O. Therefore, the cutting blades 7 and 7 cross each other in the tangential direction of the end mill rotation direction T at the contact point from the outer peripheral side beyond the contact point with the center thick circle C. Further, in the core thick portion 11, the flank surfaces 8 and 8 intersect each other at an obtuse angle via a chisel 12, and in the present embodiment, the chisel 12 has the axis O in the end view in the axis O direction. It passes through both cutting edges 7 and 7 in a straight line intersecting at an obtuse angle, and is formed in a substantially convex arc shape in a side view perpendicular to the virtual plane P.

The diameter δ (mm) of the thick core C is δ = 0 with respect to the diameter of the hemisphere formed by the cutting blade 7 in the rotation locus around the axis O, that is, the outer diameter D (mm) of the cutting blade 7. 0.03 × D ^{1/2 to} δ = 0.06 × D ^{1/2 and} the pair of cutting blades 7 and 7 exceed the contact point with the core thick circle C as described above. The amount of crossing H (mm) of the cutting blades 7 that cross each other is in the range of H = 0.05 × D ^1/2 −0.04 to H = 0.09 × D ^1/2 . Furthermore, in this embodiment, the amount of difference H (mm) of the cutting edge 7 is 2 × δ or less with respect to the diameter δ (mm) of the core thickness circle, and preferably δ−0.03 (mm). That's it. However, as shown in FIG. 3, the crossing amount H is an intersecting ridge line L between the first flank 8a of the pair of cutting blades 7 and the second flank 8b connected to the pair of cutting blades 7 and 7, as shown in FIG. The distance between the tangent lines S of the intersecting ridge line portion 10 at the intersection point Q with the gasche 9 is a length measured in the tangential direction between the core thickness circle C and the cutting edge 7.

Further, in this embodiment, the gash corner portion 13 at the corner of the gash 9 where the cutting edge 7 and the intersecting ridge line portion 10 intersect has a curvature radius R (mm) of 0.03 (mm) when viewed from the front in the axis O direction. ) As described above, it is a concave curved shape within the range of 0.08 × D ^1/2 or less with respect to the outer diameter D (mm) of the cutting edge 7, and the cutting edge 7 and the intersecting ridge line portion 10 (gash) 9 and the first flank 8a intersecting ridge line portion) is a straight line that is smoothly in contact with both ends of the concave curve formed by the gasche corner portion 13 when viewed from the front in the axis O direction. Furthermore, the cutting edge 7 and the intersecting ridge line 10 that are continuous through the gasche corner 13 are formed in a direction that intersects at an intersecting angle α in the range of 80 ° to 120 ° in the front view of the axis O direction. Yes.

  Therefore, in the ball end mill configured as described above, first, the flank surfaces 8 of the cutting blades 7 and 7 extending to the vicinity of the axis O intersect each other via the chisel 12 near the axis O at the tip of the end mill body 1. The core thick portion 11 is left, and in particular, the rigidity of the end mill body 1 and the strength of the cutting edge 7 in the vicinity of the axis O in which a higher load acts when the cutting speed becomes 0, for example, are compared. To prevent the end mill body 1 from being damaged in the vicinity of the axis O at the tip, even when high-speed machining of a steel material or hard alloy material with a large mechanical load is performed or when wear increases due to the progress of cutting. Can do. Further, the pair of cutting blades 7 and 7 extending in the vicinity of the axis O are formed so as to cross each other beyond the contact point with the core circle C centered on the axis O, and accordingly the cutting blade 7 Since the gash 9 communicating with the chip pocket 6 in a row is also secured so as to cross each other, a good chip discharging property can be obtained.

In the ball end mill having the above-described configuration, the diameter δ (mm) of the core thick circle C in the core thick portion 11 and the difference H (mm) between the cutting edges 7 and 7 are both the outer diameter of the cutting edge 7. Within D (mm), δ = 0.03 × D ^{1/2 to} δ = 0.06 × D ^1/2 and H = 0.05 × D ^1/2 −0.04 to H = It is set within the range of 0.09 × D ^1/2 , that is, set based on the square root of the outer diameter D. For this reason , in a ball end mill with a small diameter whose outer diameter D of the cutting edge 7 is less than 2 mm, the diameter δ of the core thickness circle C becomes smaller as the outer diameter D becomes smaller, and the cutting edge 7 becomes more axial O at the end of the end mill body 1. Since it is close to the vicinity, the cutting edge 7 is not formed at the end of the end mill body 1 and the portion not involved in cutting can be reduced, and the cutting edge 7 can be sharply sharpened up to the vicinity of the axis O, and high machining accuracy can be achieved. Can be obtained. This is particularly effective in precision machining such as a mold in which such a small-diameter ball end mill is frequently used.

  Further, as the outer diameter D of the cutting edge 7 becomes smaller in this way, the amount of difference H between the cutting edges 7 and 7 is also reduced, so that the end mill body accompanying the decrease in the diameter δ of the core thickness circle C as described above. 1 The shortage of rigidity at the tip can be compensated, that is, as the crossing amount H becomes smaller, the portion where the tip of the end mill body 1 is notched by the gash 9 and the tip pocket 6 is suppressed to a small size and the rigidity is ensured. Since the width of the thick portion 11 in the direction along the virtual plane P can be reduced, it is possible to prevent the core thick portion 11 from being damaged at the tip of the end mill body 1.

However, in such a small-diameter ball end mill, the diameter δ of the core thick circle C and the amount of difference H between the cutting edges 7 and 7 are set based on the square root of the outer diameter D of the cutting edge 7 as described above. Accordingly, it is possible to avoid becoming unnecessarily small as opposed to the case of the large diameter, as compared with the case where they are also reduced in proportion to the outer diameter D, for example. Therefore, even with a small-diameter ball end mill, the minimum necessary diameter δ can be secured for the core thick circle C, and the rigidity of the tip of the end mill body 1 is more reliably combined with the fact that the amount of crossing H can be suppressed. While this can be ensured, the crossing amount H does not become excessively small, and even with a small diameter, it is possible to secure a sufficient capacity for discharging chips in the gash 9 and the chip pocket 6. Become. That is, according to the ball end mill having the above-described configuration, it is possible to achieve both the chipping resistance of the end of the end mill body 1 and the sharpness and chip discharge of the cutting edge 7, thereby enabling smooth and high-precision processing. .

By the way, in the above-described small-diameter ball end mill having a relatively small outer diameter D of the cutting edge 7, the diameter δ of the core thickness circle C is reduced to the minimum necessary range as described above to give the cutting edge 7 a good sharpness. As a result, it is not necessary to secure a very large capacity for the gasche 9 and the chip pocket 6, and rather the cross-milling amount H is kept as small as possible. It is more desirable to ensure the rigidity of the tip 1 and the strength of the cutting edge 7 reliably .

Accordingly, in the present embodiment, as described above, the diameter δ of the core thick circle C and the difference H between the cutting edges 7 and 7 are set within a range based on the square root of the outer diameter D of the cutting edge 7. In addition, the relationship between the diameter δ (mm) of the core thick circle C and the amount of difference H (mm) between the cutting edges 7 and 7 is set so that H is 2 × δ or less. H is set to be δ−0.03 (mm) or more. Therefore, by setting the upper and lower limits of the amount of difference H with respect to the diameter δ of the thick circle C in this way, in the small-diameter ball end mill, it is more reliably avoided that the amount of difference H becomes larger than necessary. It is possible to secure further rigidity and cutting edge strength .

  Furthermore, in this embodiment, the cutting edge 7 which will be formed in the intersection ridgeline part of the wall surface which faces the end mill rotation direction T side of the said gash 9, and the flank 8 and the end mill rotation direction T back side of this gash 9 And the intersecting ridge line portion 10 of the flank 8 and the flank 8 (first flank 8a) intersect with each other via a gasche corner 13 that forms a concave curve when viewed from the front in the direction of the axis O. Accordingly, it is possible to suppress the stress during cutting from being concentrated on the gash corner portion 13 and to prevent the end mill body 1 from being damaged due to such stress concentration. Further, particularly when the outer diameter D is small, the thickness of the core in the cross section orthogonal to the axis O is gradually increased from the chisel 12 at the most distal end of the end mill body 1 where the rigidity and strength become weakest toward the rear end side in the axis O direction. Since the size can be increased, the strength of the end of the end mill body 1 can be improved.

Further, if the radius of curvature R (mm) of the concave curve formed by the gasche corner portion 13 when viewed from the front in the axis O direction is too small, the above-described effects may not be obtained. The chip pocket capacity of the gasche 9 is reduced and the chip discharge performance may be impaired, and the accuracy of the hemisphere formed by the cutting blade 7 on the rotation path around the axis O, that is, the rounding accuracy of the cutting edge in a so-called ball end mill, is likely to deteriorate. Therefore, in this embodiment, it is 0.03 mm or more, and is within a range of 0.08 × D ^1/2 or less with respect to the outer diameter D (mm) of the cutting blade 7 . Incidentally, FIG. 4 is a reference example for forming the above embodiment, as the cutting blade 7 without forming the gash corner portion 13 concave-curved and the intersection ridge line 10 intersect at an angle.

  Further, in the present embodiment, the cutting edge 7 that intersects with the gash corner portion 13 in this way, and the intersecting ridge line 10 between the wall surface facing the rear side in the end mill rotation direction T of the gash 9 and the flank 8 are also in the direction of the axis O. In the front end view, the crossing angle α is in the range of 80 ° to 120 °, that is, in the direction of crossing at the opening angle of the gasche 9, but if the crossing angle α is too small, the end mill rotation direction T Since the wall surface facing the rear side is directed so as to face the cutting blade 7 side, the tip pocket at the end of the end mill body 1 is also reduced, and there is a possibility that the chip is prevented from flowing out and the smooth discharge thereof is hindered. Because. On the other hand, if the crossing angle α is too large, the end of the end mill main body 1 may be cut out largely due to the shape of the gash 9 that is wide open when viewed from the front in the direction of the axis O, and the rigidity may not be secured. Therefore, it is desirable that the crossing angle α is set within a range of 80 ° to 120 ° as in the present embodiment.

  Further, in the present embodiment, a two-blade ball end mill in which a pair of cutting blades 7 and 7 having a hemispherical rotation trajectory around the axis O is formed at the end portion (cutting blade portion 2) of the end mill body 1 will be described. However, it is also possible to apply the present invention to a four-blade ball end mill having more than two blades, for example, as shown in FIG. In the ball end mill shown in FIG. 5, a pair of cutting blades 7 and 7 positioned on opposite sides of the axis O are formed so as to extend from the vicinity of the axis O at the end of the end mill body 1 to the outer peripheral side. The remaining two cutting blades 14 and 14 are formed so as to extend to the outer peripheral side from a position separated from the outer peripheral side of these cutting blades 7 and 7, and the rotation trajectory of these cutting blades 7, 7, 14 and 14. Are supposed to exhibit one hemisphere.

Next, the effects of the present invention will be described more specifically with reference to examples of the present invention. In this example, first, based on the embodiment shown in FIGS. 1 to 3, the outer diameter D (mm) of the cutting edge 7 is 1 mm , the diameter δ (mm) of the core thickness circle C is 0.04 mm, A cutting test was performed using two types of ball end mills in which the amount of difference H (mm) between the cutting edges 7 and 7 was different. The results are shown in Table 1 for each ball end mill, as well as the ratios H / δ between the above dimensions and the difference H (mm) and the diameter δ (mm) of the core thickness circle C. As shown. Further, as a comparative example for this, the outer diameter D (mm) of the cutting edge 7 and the diameter δ (mm) of the core thickness circle C are the same as those of the ball end mills of the respective embodiments , but the amount of difference H (mm A cutting test was conducted under the same cutting conditions even with one type of ball end mill with different). The results are also shown as comparative examples 1-1 in Table 1 also.

Note that the ball end mills of these examples and comparative examples both have (Al, Ti) N coating on the surface of the end mill body made of ultra-fine cemented carbide, and include the outer peripheral blade 5. The length (blade length) of the blade part 2 was twice the outer diameter D. Moreover, the work material used for the cutting test was SKD61 (52HRC), and in the cutting test, the work material was rotated to 10,000 rpm and the feed speed in Examples 1-1 and 1-2 and Comparative Example 1-1. 1800 mm / min, depth of cut radially 0.4 mm, in the axial direction 0.2 mm, was subjected to side processed by down cutting and air blowing.

From the results shown in Table 1, in Comparative Example 1-1 in which the amount of difference H in the cutting edge was increased, the tip of the cutting edge (tip of the end mill body 1) was damaged and the cutting test was stopped. Did not get .

However, with respect to the comparative example 1-1 , in the ball end mill of the example according to the present invention , the cutting edge 7 is not damaged, and the cutting resistance is kept low by smooth chip discharge, Excellent machining accuracy and finished surface were obtained. In particular, in Example 1-2 in which the amount of crossing H between the cutting edges 7 and 7 is greater than 2 × δ (H / δ is greater than 2) with respect to the diameter δ of the core thickness circle C, respectively. When cutting was attempted at a feed rate higher than the feed rate of the cutting test, chipping was observed on the cutting edge 7, whereas the difference H was 2 × δ or less with respect to the diameter δ of the core thickness circle C. In the ball end mill of Example 1-1 (H / δ was set to 2 or less), such chipping was not observed, and higher-efficiency cutting was possible.

DESCRIPTION OF SYMBOLS 1 End mill body 2 Cutting edge part 6 Tip pocket 7 Cutting edge 8 Relief surface 9 Gash 10 Gash 9 End mill rotation direction T The wall surface facing the back side of the end mill and the flank 8 11 Core thick part 12 Chisel 13 Gash corner part O End mill body 1 axis T End mill rotation direction C Center thick circle D Outer diameter of cutting edge 7 δ Diameter of core thick circle C H Misalignment amount of cutting edge R Curvature radius of gash corner 13 α Cutting edge 7 and cross ridge line Intersection angle with part 10

Claims (3)

At least a pair of cutting blades whose rotation trajectory around the axis forms a substantially hemispherical shape is formed on the end portion of the end mill body rotated about the axis on opposite sides of the axis at the end mill body end. It is a ball end mill, and a gash is formed at the tip of the end mill main body, and the cutting edge is formed at a cross ridge line portion between a wall surface facing the end mill rotation direction of the gash and a flank of the tip of the end mill main body. with the outer diameter of the cutting edge is a diameter below 2 mm, in the axial direction tip view, the diameter of the heart Atsuen inscribed in the pair of cutting edges [delta] (mm) is around the axial line該切The outer diameter D (mm) of the blade is within the range of δ = 0.03 × D ^1/2 to δ = 0.06 × D ^1/2 , and the pair of cutting blades has the above-mentioned core thickness. Beyond the point of contact with the circle, The amount of difference H (mm) of the cutting blades that cross each other is in the range of H = 0.05 × D ^1/2 −0.04 to H = 0.09 × D ^1/2 , The gash corner portion where the wall surface facing the rear side in the end mill rotation direction of the gash and the intersecting ridge line portion of the flank face intersects with the radius of curvature R (mm) of 0.03 mm or more in the axial front end view. It is made into the concave curve shape made into the range below 0.08xD1 ^{/ 2 with} respect to outer diameter D (mm), The wall surface which faces the end mill rotation direction back side of the said gasche, and the said The ball end mill characterized in that the intersection ridge line with the flank surface is formed so as to be in smooth contact with both ends of the concave curve formed by the gash corner .   2. The ball end mill according to claim 1, wherein the amount of difference H (mm) between the cutting edges is within a range of 2 × δ or less with respect to the diameter δ (mm) of the core thick circle. A direction in which the cutting edge, a wall surface facing the rear side in the end mill rotation direction of the gasche and an intersecting ridge line part of the flank face intersect at an intersecting angle α in the range of 80 ° to 120 ° in the front end view in the axial direction. The ball end mill according to claim 1 , wherein the ball end mill is formed as follows.

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