JPH0976109A - Throwaway ball end mill - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the damage of an end mill main body caused by contact force in attaching tips in a throwaway two-flute ball end mill. SOLUTION: A pair of throwaway tips 14, 15 provided with circular arc-like cutting edges are mounted on the tip of an end mill main body 11, and the rotational loci of cutting edges 18, 19 around an axis O are formed into approximately hemispherical shapes. One throwaway tip 14 of a pair of throwaway tips 14, 15 is so arranged that the cutting edge 18 may extend from the vicinity of the rotational center C, and the other throwaway tip 15 is so arranged that the cutting edge 18 may extend from the position separated from the rotational center C to the outer peripheral side, and also from the position deviated to the rotational direction of the end mill main body 11 in relation to a virtual line L passing the rotational center C as seen from the tip in the axis O direction and to be in contact with the cutting edge 18 of the other throwaway tip 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a throw-away type ball end mill in which a pair of throw-away tips (hereinafter referred to as "tips") having arcuate cutting edges are detachably attached to the tip of an end mill body. It is a thing.

[0002]

2. Description of the Related Art In this type of ball end mill,
For example, a so-called two-blade throw-away type ball end mill as shown in FIGS. 8 to 10 is known. This is disclosed in Japanese Patent Application Laid-Open No. 2-114418, and at the tip of the substantially cylindrical end mill body 1 rotated around the axis O, the axis O is provided on the opposite sides with the axis O interposed therebetween. When viewed from the front end side in the direction, as shown in FIG.
2 are formed, and the chips 4 and 5 are respectively clamp screws to the chip mounting seats 3 and 3 formed on the wall surface facing the end mill rotation direction (white arrow direction in FIG. 10) of these chip pockets 2 and 2. It is configured such that it is detachably attached by 6.

Here, each of these chips 4 and 5 has a pair of substantially arc-shaped cutting blades 7 and a pair of linear cutting blades 8 extending in the tangential direction of the cutting blade 7, one of which is provided. Is attached by abutting the side surface connected to the cutting edges 7, 8 with the wall surface 3A of the tip mounting seat 3, and the other arc-shaped cutting edge 7 extends from the rotation center C side of the tip of the end mill body 1 to the rear end outer circumference. The other linear cutting edge 8 that is in contact with the side is disposed so as to extend toward the side and extends toward the rear end side on the outer circumference of the end mill body 1. Further, among these chips 4 and 5, one chip 4 is the other chip 5
Is formed in a similar shape to and slightly larger than the other tip 5, and the other arcuate cutting edge 7 is arranged so as to extend from the rotation center C. The other tip 5 is formed such that the other arcuate cutting edge 7 extends from a position slightly separated from the center of rotation C to the outer peripheral side as shown in FIG. It should be noted that the other arcuate cutting edges 7, 7 of these tips 4, 5 are arranged so as to be substantially in contact with one virtual line L passing through the rotation center C when viewed from the front end in the direction of the axis O as shown in FIG. It is located in.

[0004]

However, in the ball end mill having the above structure, the arcuate cutting edges 7, 7 of the tips 4, 5 are both imaginary lines L passing through the rotation center C when viewed from the tip in the direction of the axis O. In the direction of the imaginary line L, in the direction of the virtual line L, between the wall surfaces 2A, 2A facing the outer peripheral side of the end pockets of the chip pockets 2, 2 and the tip mounting seat 3 on the side opposite to the wall surface 2A. Wall surface 3A
It is not possible to secure a sufficient wall thickness between and. Therefore, the contact force received by the wall surface 3A when the chips 4, 5 are mounted on the chip mounting seats 3, 3 or the pressing force acting on the wall surface 3A from the chips 4, 5 by the load during cutting, etc. The virtual line L between the wall surfaces 2A and 2A
There is a risk that the end mill body 1 may be damaged due to cracks along the edges.

The present invention has been made in view of such circumstances, and in the above-mentioned two-blade throw-away type ball end mill, it is possible to prevent damage to the end mill body due to a contact force or the like when mounting the tip. It is an object.

[0006]

In order to solve the above problems and to achieve the above object, the present invention provides one of a pair of tips equipped with an arcuate cutting blade attached to the tip of an end mill body. The throw away tip is arranged so that its cutting edge extends from the vicinity of the center of rotation, and the other tip is positioned such that its cutting edge is separated from the center of rotation toward the outer peripheral side and when viewed from the axial tip. It is characterized in that it is arranged so as to extend from a position eccentric to the rotation direction side of the end mill main body with respect to a virtual line that contacts the cutting edge of one of the tips, and the cutting edge of the other tip is the virtual one. The wall surfaces of both tip mounting seats overlap in the direction orthogonal to the above-mentioned imaginary line when viewed from the axial tip end, because the wall surfaces of both tip mounting seats are arranged to be offset from the line. With contact force of the-up is canceled, it becomes possible to secure a sufficient strength against the pressing force by the cutting load.

Here, if the deviation of the other chip is too small, the amount of overlap of the wall surfaces of both chip mounting seats becomes too small, and the above-mentioned action may not be sufficiently achieved. If the bias is too large,
The inner peripheral edge of the cutting edge of the other tip may be too far away from the center of rotation, and the load acting on the center of rotation of the cutting edge of one tip may become excessive. Further, if the position of the inner peripheral end of the cutting edge of the other tip is too close to the center of rotation in the direction parallel to the imaginary line, the wall thickness between the wall surfaces of both tip mounting seats becomes absolutely small. On the contrary, if it is too far from the center of rotation, the load acting on one of the chips may become excessive as in the above case. For this reason,
The cutting edge of the other tip is 0.5 in the direction perpendicular to the imaginary line from the imaginary line in the axial direction tip view.
It is desirable to be arranged so as to extend from a position deviated in the range of up to 2.0 mm, and to extend from a position separated in the range of 0.5 to 3.0 mm in the direction parallel to the imaginary line from the center of rotation. It is desirable that they are arranged as follows.

On the other hand, the arcuate cutting edge of the one tip and the arcuate cutting edge of the other tip are not cut along with the feed of the end mill body even if the rotation trajectories around the axis are not completely matched. Since the blade also moves forward while rotating, if the cutting edge of one tip extending from the center of rotation is located slightly outside the cutting edge of the other tip in the rotation trajectory, the cutting edge of the other tip After cutting by the cutting method, the cutting edge of one of the chips is finished, and it becomes possible to accurately form a groove having a hemispherical bottom surface. However, in this case, in the rotation locus around the axis, the cutting edge of the other tip is arranged radially inward of the hemisphere within a range of up to 0.05 mm with respect to the cutting edge of the one tip. Desirably, if the cutting edge of the other tip exceeds the range and is inside, the other tip may not participate in the cutting at a general rotation speed and feed amount of the end mill body.

Further, in order to secure the strength of the cutting edge on the rotation center side, the cutting edge of at least one of the tips is directed toward the inner peripheral edge side of the cutting edge and toward the rear side in the axial direction as the distance from the inner peripheral edge increases. It is desirable to form a small cutting edge that is inclined to extend beyond the axis. However, in this case, the inner peripheral edge of the cutting edge of this one tip is 0.15 to 1.
It is desirable to arrange the cutting edge so as to be positioned on the outer peripheral side with respect to the axis in the range of 0 mm, and the inner peripheral end of this cutting edge, that is, the intersection of the arcuate cutting edge and the small cutting edge exceeds the above range. If it is located on the outer circumference side, the portion left uncut by the small cutting edge may be conspicuously shown on the bottom surface of the groove, deteriorating the finishing accuracy.On the other hand, if it is below the above range, it may be located near the tool rotation center on the inner circumference side. Then, the peripheral speed is 0 at this tool rotation center.
Therefore, a large cutting load acts on the cutting edge,
The cutting edge may be damaged.

[0010]

1 to 7 show an embodiment of the present invention. In the present embodiment, the end mill main body 11 has a columnar shape whose diameter decreases toward the tip side as shown in FIG. 1, and the tip part thereof is formed in a substantially hemispherical shape. As shown in FIG. 5, a pair of chip pockets 12 and 12 are formed on opposite sides of the axis O so that the outer periphery of the end mill body 1 is cut out in an L shape. Then, the chips 14 and 15 are respectively attached to the chip mounting seats 13 and 13 formed on the wall surfaces of the chip pockets 12 and 12 facing the end mill rotation direction (the direction of the white arrow in the drawing) by the clamp screws 16 and 16, respectively. It is detachably attached.

Here, these chips 14 and 15 are flat plates made of a hard material such as cemented carbide,
At the side edge of the upper surface 17, which is the rake face, approximately 1/4
A pair of arc-shaped cutting blades 18 and linear cutting blades 19 that are in smooth contact with the arc-shaped cutting blades 18 are formed symmetrically with respect to a screw hole 20 into which the clamp screw 16 is inserted. In addition, the arcuate cutting edge 18 and the linear cutting edge 19 are provided at a pair of end portions that intersect with each other at an acute angle.
8 and 19, small cutting edges 21, so that they intersect obtusely
21 are formed. In addition, these chips 14, 1
Reference numeral 5 is a so-called positive tip in which a flank is formed on a side surface 22 which is a flank in succession to the cutting edges 18 and 19, and the cutting edges 18, 19 are provided on the periphery of the upper surface 17.
A raised surface 23 is formed so as to be spaced apart from and to gradually rise toward the inside of the chip so as to circulate around the peripheral edge of the upper surface 17 along the cutting edges 18 and 19.

Further, a pair of concave grooves 25, 25 are formed on the lower surface 24, which is a seating surface of these chips 14, 15, symmetrically with respect to the screw hole 20 and parallel to each other. On the bottom surface 13A of the chip mounting seat 13 on which the lower surface 24 is seated, a pair of ridges 26, 26 are formed in parallel with each other, and these recessed grooves 25, 25 are formed.
The chips 14 and 15 are seated on the chip mounting seats 13 and 13 while being engaged with the ridges 26 and 26. Furthermore, as shown in FIG. 3, the two chips 14, 15 are attached to the chip mounting seats 13, 13 so that the rake angles in the axial direction of the cutting blades 18, 19 are directed toward the right angle side as shown in FIG. It is installed at an angle with respect to.
Further, at the intersection of the bottom surface 13A of the tip mounting seat 13 and the outer surface of the tip end portion of the end mill body 11 forming a hemispherical surface,
A chamfered portion 27 is formed so as to form a right angle with the bottom surface 13A.

Of the tips 14, 15, one of the tips 14 is formed so that the arcuate cutting edge 18 has a substantially quarter arc shape, and the end mill body 1
While it is arranged so as to extend from the vicinity of the rotation center C of the one tip toward the outer peripheral rear end side of the end mill body 11, in the other tip 15, the arcuate cutting edge 18 has a quarter arc. However, it is arranged so as to extend from the position slightly separated from the rotation center C toward the outer peripheral rear end side. Further, the cutting edge 18 of the other tip 15 is
As shown in FIG. 7, an imaginary line L passing through the center of rotation C and coming into contact with the cutting edge 18 of the one tip 14 in the tip view in the direction of the axis O
On the other hand, it is arranged so as to extend perpendicularly to the virtual line L from a position deviated by the distance X toward the rotation direction side. Also,
The cutting edge 18 of the other tip 15 is arranged so as to extend from a position separated from the rotation center C by a distance Y in a direction parallel to the imaginary line L, as shown in FIG. Further, in the present embodiment, the distance X is 0.5 to 2.0.
Within the range of mm, and the distance Y is 0.5 to 3.0 mm.
Are set within the range of.

In the present embodiment, the arcuate cutting edge 18 of the one tip 14 is arranged so as to extend from the vicinity of the center of rotation C as described above, and strictly speaking, it is rotated. It has not reached the center C. That is,
As shown in FIG. 6, the inner peripheral edge 1 of the cutting edge 18 of this tip 14
8A is arranged so as to extend from a position that is separated from the axis O in the radial direction by a distance Z, and a small cutting edge 21 that intersects the inner peripheral end 18A of this cutting edge 18 is separated from the inner peripheral end 18A. Accordingly, it is arranged so as to incline rearward in the direction of the axis O and extend beyond the axis O. The distance Z is set within the range of 0.15 to 1.0 mm in this embodiment. Further, in the present embodiment, the arcuate cutting edges 18, 18 of both tips 14, 15 are not arranged so as to be completely coincident with each other in the rotation trajectory around the axis O, and as shown in FIG. With respect to the rotational locus of the cutting edge 18 of the tip 14, the rotational locus of the cutting edge 18 of the other tip 15 is inward in the radial direction of the hemisphere formed by the rotational locus.
It is arranged so as to be located within the range of the distance d up to 05 mm.

In the throw-away type ball end mill constructed as described above, when viewed from the front end in the direction of the axis O, on the extension line of the cutting edge 18 of one of the tips 14, that is, on the imaginary line L, as shown in FIG. Tip 15 exists, and conversely on the extension line of the cutting edge 18 of the other tip 15,
There is one of the above chips 14. That is, since the cutting edge 18 of the other tip 15 is arranged at a position deviated from the imaginary line L by the distance X, the wall surfaces 13B and 13B of both tip mounting seats 13 and 13 are arranged in a direction orthogonal to the imaginary line L. Since they overlap, the side surface 22 that abuts the wall surface 13B of the other tip 15 faces the imaginary line L extending from the cutting edge 18 of the one tip 14, and conversely the cutting of the other tip 15 is performed. On the extension line of the blade 18, one tip 14
The side surfaces 22 are arranged so as to face each other.

Therefore, when these chips 14, 15 are attached to the respective chip mounting seats 13, 13, the contact force generated by the chips 14, 15 contacting the wall surface 13B of the chip mounting seat 13 is the wall surface 13B. They will cancel each other out in the overlapping portions. In addition, the load acting on both chips 14, 15 from the cutting edges 18, 19 toward the radially inner peripheral side with respect to the axis O during cutting is also the wall surface 13B with which the side faces 22, 22 of the opposite chips 14, 15 abut. Clamp screw 1 through 13B
Since it is received by the tips 14 and 15 fixed by 6, the end mill body 11 can be given sufficient strength against the pressing force due to such cutting load. Therefore, according to the ball end mill configured as described above,
It is possible to prevent the situation where the end mill main body 11 is damaged due to such a contact force at the time of mounting the tips 14 and 15 and a load acting at the time of cutting, thereby extending the tool life and performing stable cutting work. It becomes possible to do.

By the way, in the present embodiment, the arcuate cutting edge 18 of the other tip 15 is used as the one tip 1.
The imaginary line L which is in contact with the cutting edge 18 of No. 4 is displaced from a position deviated by a distance X in the direction perpendicular to the imaginary line L, and in the direction parallel to the imaginary line L with respect to the rotation center C. Although they are arranged so as to extend from positions separated from the outer peripheral side by Y, if the distances X and Y are too large, the inner peripheral end 18A of the cutting edge 18 of the other tip 15 is separated from the rotation center C. Too far away. Then, the cutting edge 18 of the other tip 15 does not participate in the cutting around the rotation center C, and on the contrary, the cutting load or the like acting on the cutting edge 18 of the one tip 14 may become excessive. On the other hand, if the distances X and Y are too small, both chip mounting seats 1
The amount of overlap between the wall surfaces 13B and 13B of the wall surfaces 3 and 13 may be too small, or the wall thickness between the wall surfaces 13B and 13B may be too small, so that the above-described damage prevention effect may not be sufficiently achieved. Occurs. Therefore, the distances X and Y are each 0.5 as described above.
It is desirable to set it within the range of up to 2.0 mm and 0.5 to 3.0 mm.

On the other hand, in the present embodiment, as shown in FIG. 7, both chips 14 and 15 are moved in the rotation locus about the axis O.
The arcuate cutting edges 18 and 18 do not completely coincide with each other, and the cutting edge 18 of the one tip 14 extending from the vicinity of the rotation center C is different from the cutting edge 18 of the other tip 15 extending from the position separated from the rotation center C. The cutting edge 18 is positioned slightly inside the hemisphere formed by the above-mentioned rotation locus. Then, as a result of the feeding of the end mill body 11,
The tip of the other tip 15 cut by the arcuate cutting edge 18 is one tip 14 whose rotation trajectory is a substantially complete hemisphere.
The cutting edge 18 is used for finishing so that highly accurate processing is possible. However, in the case of adopting such a configuration, if the cutting edge 18 of the other tip 15 is too inside the rotation locus of the cutting edge 18 of the one tip 14, the other tip will be used at a normal feed amount. There is a risk that the fifteen cutting edges 18 will not participate in cutting. Therefore, as described above, the cutting edge 18 of the other tip 15 is separated from the cutting edge 18 of the one tip 14 by the distance d.
It is desirable that the hemisphere is arranged radially inward within a range of up to 0.05 mm.

Further, in this embodiment, as shown in FIG. 6, the arcuate cutting edge 18 of the one tip 14 extending from the vicinity of the rotation center C intersects the inner peripheral end 18A of the cutting edge 18 at an obtuse angle. Then, the small cutting blade 21 is formed so as to extend backward beyond the axis O toward the rear side in the direction of the axis O as it is separated from the inner peripheral end 18A, and the peripheral speed becomes zero. In the vicinity of the center of rotation C where a great amount of cutting resistance acts, it is possible to prevent the inner peripheral end 18A, which is the acute-angled end of the cutting edge 18, from being damaged. Moreover, in the present embodiment, such a small cutting edge 21 is formed in the other tip 15 even though it does not exceed the axis O, and similarly, it is possible to prevent the sharp edge portion of the cutting edge 18 from being damaged. There is.

Here, when such a structure is adopted, the small cutting edge 21 of the one tip 14 is the cutting edge 1 as described above.
8 is inclined so as to extend beyond the axis O toward the rear side in the direction of the axis O as being separated from the inner peripheral end 18A of the cutting edge 21 in the range where the small cutting edge 21 is formed. The rotation locus of 18 does not become a hemisphere, and as a result, a portion left uncut by the small cutting blade 21 is formed on the bottom surface of the groove or the like formed by the ball end mill. Therefore, if the range Z in which the small cutting edge 21 is formed, that is, the distance Z of the inner peripheral end 18A of the cutting edge 18 of the one tip 14 from the axis O is too large, the above-mentioned uncut portion becomes too remarkable. As a result, processing accuracy may be deteriorated. On the other hand, conversely, this distance Z is too small and
If 8A is too close to the rotation center C, a large cutting load acts in the vicinity of the rotation center C as described above.
The cutting edges 18 and 21 are likely to be damaged. Therefore, it is desirable that the distance Z is set in the range of 0.15 to 0.1 mm as described above.

By the way, as in the present invention, an imaginary line L passing through the center of rotation C and contacting the cutting edge 18 of the one tip 14 is provided.
On the other hand, when the cutting edge 18 of the other tip 15 is arranged so as to extend from the position deviated in the rotation direction side, the cutting edge 18 of the other tip 15 is in a so-called center-up state and its diameter increases. Since the direction rake angle is set to the negative angle side, the cutting resistance acting on the cutting edge 18 tends to increase. However, in contrast, in the ball end mill of the above-described embodiment, the cutting edge 1 of the tips 14 and 15 is
Since both tips 14 and 15 are mounted so as to be inclined so that the axial rake angle of 8 and 19 is directed to the positive angle side, in particular, the radial rake angle that is a negative angle of the cutting edge 18 of the other tip 14 is provided. The cutting blade 18 is designed so that excessive cutting resistance does not act on the cutting blade 18 in balance with the above.

Further, in this embodiment, the tip mounting seat 13 is used.
A chamfer 27 is formed at the intersection of the bottom surface 13A of the end mill and the outer surface of the tip portion of the end mill body 11 so as to form a right angle with the bottom surface 13A, and the strength of this intersection is secured to cause chipping or the like. To prevent this. further,
In this embodiment, the chips 14 and 15 are attached by engaging the recessed grooves 25 and 25 formed on the lower surface 24 of the chips 14 and 15 with the protrusions 26 and 26 formed on the chip mounting seat 13. As a result, the mounting rigidity and the like of the chips 14 and 15 are secured. Furthermore, in the present embodiment, a pair of tips 14, 1 is attached to the tip of the end mill body 11.
Although the ball end mill in which only No. 5 is mounted has been described, an auxiliary chip for cutting the wall of the groove is provided at the rear end side of the pair of cutting blades 14 and 15, and the rotation locus of the cutting blade is the straight line described above. The cutting blade 19 may be attached so as to be continuous.

[0023]

As described above, according to the present invention,
By overlapping the wall surfaces of the chip mounting seats where a pair of chips are mounted to each other, it is possible to cancel the abutting force that the wall surfaces receive from the chips when mounting both chips, and to reduce the cutting load that acts in the radial direction of the end mill body. Since it can be received by the tip on the opposite side, it is possible to prevent damage to the part between the two wall surfaces of the end mill body due to these abutting forces and cutting loads, which prolongs tool life and stabilizes the work. It is possible to promote the cutting work.

[Brief description of drawings]

FIG. 1 is a plan view showing an embodiment of the present invention.

FIG. 2 is an enlarged view of a tip portion of the embodiment shown in FIG.

FIG. 3 is a side view of the tip portion of the embodiment shown in FIG.

FIG. 4 is a front view of the embodiment shown in FIG.

5 is a cross-sectional view taken along the line AA in FIG.

FIG. 6 is a view showing the periphery of an inner peripheral end 18A of a cutting edge 18 of one tip 14.

FIG. 7: Cutting edge 1 of chips 14, 15 around axis O
It is a figure which shows the rotation locus of 8 and 18.

FIG. 8 is a plan view showing a conventional throw-away type ball end mill.

9 is a side view of the ball end mill shown in FIG. 8. FIG.

10 is a front view of the ball end mill shown in FIG. 8 as viewed from the front end in the direction of the axis O. FIG.

[Explanation of symbols]

11 End Mill Main Body 13 Tip Mounting Seat 13B Tip Mounting Seat 13 Wall Surface 14, 15 Chips 18 Arc Cutting Edge 19 Linear Cutting Edge 21 Small Cutting Edge O Rotation Axis of End Mill Main Body C Rotation Center of End Mill Main Body 11 L A virtual line passing through the center of rotation C and in contact with the cutting edge 18 of one of the tips 14 A distance from the virtual line L in a direction perpendicular to the X virtual line L to the cutting edge 18 of the other tip Y A direction parallel to the virtual line L From the center of rotation C to the cutting edge 18 of the other tip 15 Z The one tip 1 from the axis O in the radial direction of the axis O
Distance to the inner peripheral end 18A of the cutting edge 18 of No. 4 d Distance between the cutting edges 18 and 18 of the chips 14 and 15 in the rotation trajectory around the axis O

Claims (5)

[Claims] 1. A pair of throw-away tips each having a cutting edge having an approximately 1/4 circular arc shape at the tip of an end mill body rotated about an axis, on opposite sides of the axis and the cutting edge. A throw-away device which is detachably mounted so as to extend from the center of rotation of the tip of the end mill body toward the outer periphery of the rear end, and the rotation path of the cutting edge around the axis is substantially hemispherical. In the type ball end mill, of the pair of throw-away tips, one of the throw-away tips is arranged such that the cutting edge extends from the vicinity of the rotation center, and the other throw-away tip has the cutting edge. , A position away from the rotation center to the outer peripheral side, and passing through the rotation center in the axial direction distal end view of the one throw-away tip. To an imaginary line contacting the edge, indexable ball end mill which is characterized in that it is arranged to extend from a position deviated in the rotation direction of the end mill body. 2. The position of the cutting edge of the other throw-away tip deviated from the imaginary line in a direction perpendicular to the imaginary line in the range of 0.5 to 2.0 mm in the axial direction tip end view. The throw-away type ball end mill according to claim 1, wherein the throw-away type ball end mill is arranged so as to extend therefrom. 3. The cutting insert of the other throw-away tip is separated from the center of rotation in a direction parallel to the imaginary line in the range of 0.5 to 3.0 mm in the axial tip end view. The throw-away type ball end mill according to claim 1 or 2, wherein the throw-away type ball end mill is arranged so as to extend from the position. 4. The cutting edge of the other throw-away tip is arranged radially inward of the hemisphere within a range of up to 0.05 mm with respect to the cutting edge of the one throw-away tip in the rotation locus about the axis. The throw-away type ball end mill according to any one of claims 1 to 3, which is characterized by being provided. 5. A small cutting edge that is inclined toward the rear side in the axial direction as it is separated from the inner peripheral edge is formed on at least the inner peripheral edge side of the cutting edge of the one throw-away tip, The one throw-away tip is such that the inner peripheral edge of the cutting edge is located on the outer peripheral side with respect to the axis in the range of 0.15 to 1.0 mm, and the small cutting edge extends beyond the axis. The throw-away type ball end mill according to any one of claims 1 to 4, wherein the throw-away type ball end mill is arranged.