JPH0733525U - Indexable cutting tool - Google Patents

Abstract

(57) [Summary] [Structure] The tool body 1 is formed with a chip mounting seat 8 having a mounting seat bottom surface 8A and a mounting seat wall surface 8B standing upright against the mounting seat bottom surface 8A. The throw-away tip 3 having a lower surface 3B seated on the bottom surface 8A and a side surface 3C abutting on the mounting seat wall surface 8B is detachably mounted. The lower surface 3B and the side surface 3C of the throw-away tip 3
In the ridge line portion 3D intersecting with, the groove-shaped concave portion 21 that intersects the lower surface 3B and the side surface 3C at an obtuse angle across the intersecting ridge line 3D.
The chip mounting seat 8 is provided with a convex portion 22 that fits into the concave portion 21 at the intersection of the mounting seat bottom surface 3A and the mounting seat wall surface 3B. [Effect] It is possible to prevent the throw-away tip 3 from shifting due to rotation and the like and maintain the cutting edge accuracy, and also to suppress the reduction of the tip strength and prevent cracks and damages.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention relates to a throw-away type cutting tool in which a throw-away tip (hereinafter referred to as a tip) having a cutting edge is detachably attached to a tip mounting seat formed on a tool body.

[0002]

[Prior art]

 As a throw-away type cutting tool of this type, for example, a throw-away type ball end mill for forming a groove having a semicircular bottom surface on a work material is known as shown in FIGS. 10 and 11. ing. This is described in Japanese Utility Model Laid-Open No. 2-31618, in which a main tip 3 having an arcuate cutting edge 2 at the tip of a substantially cylindrical tool body 1 rotated around an axis O and The sub chip 4 and the rectangular flat plate-shaped auxiliary chips 5 and 5 are attached to the base end side of the sub chip 4 by clamp screws 6. Here, a pair of chip pockets 7, 7 are formed on the tip of the tool body 1 on opposite sides of the axis O, and the tip pockets 7, 7 face the tool rotation direction. Chip mounting seats 8 and 8 are formed on the front ends of the wall surfaces 7A and 7A, and the main chip 3 and the sub chip 4 are mounted on these chip mounting seats 8 and 8, respectively.

These tip mounting seats 8 and 8 are formed so as to be recessed one step toward the rear side in the tool rotation direction with respect to the wall surfaces 7A and 7A, and a mounting seat bottom surface 8A that faces the tool rotation direction side, respectively. , A mounting seat wall surface 8B standing upright with respect to the mounting seat bottom surface 8A and connected to the wall surface 7A. Further, as shown in FIG. 11, the main chip 3 is in the form of a flat plate having a substantially elliptical shape in a plan view from the upper surface 3A side. A pair of the arc-shaped cutting edge 2 and the linear cutting edge 9 are arranged point-symmetrically so that one chip 3 can be used twice. Has been done.

The main chip 3 is closely attached to the mounting seat bottom surface 8A with the lower surface 3B as a seating surface, and of the side surfaces 3C and 3C connected to the pair of cutting blades 2 and 9, the side surface that is not used for cutting. 3C is abutted on the mounting seat wall surface 8B and fixed to the chip mounting seat 8. However, in this example, the side surface 3C of the tip 3 is composed of an arc-shaped curved side surface portion 3a connected to the arc-shaped cutting blade 2 and a flat side surface portion 3b connected to the linear cutting blade 9, while The mounting seat wall surface 8B faces the tip side of the tool body 1 and extends from the outer peripheral side to the inner peripheral side of the tool body 1, and a curved wall portion 8a extends from the inner peripheral side of the wall portion 8a toward the tip of the tool body 1. And a flat wall surface portion 8b extending toward the outer peripheral side, of which the side surface portion 3b of the side surface 3C of the chip 3 is in close contact with the wall surface portion 8b of the mounting seat wall surface 8B. The side surface portion 3a and the wall surface portion 8a are in contact with each other only on their outer peripheral sides. Further, the sub-tip 4 is formed in a similar shape smaller than the main tip 3, and like the main tip 3, the sub-tip 4 is provided with a pair of arcuate cutting blades 2 and linear cutting blades 9. The chip mounting seat 8 to be mounted has the same structure as the chip mounting seat 8 to which the main chip 3 is mounted.

[0005]

[Problems to be solved by the device]

 By the way, in order to perform high-precision machining with such a throw-away type cutting tool, the tips 3 and 4 having the cutting edges 2 and 9 must be securely fixed to the tool body 1 and the cutting edges 2 and 9 must be fixed. It is necessary to prevent the displacement from occurring. In particular, in the case of the throw-away type ball end mill, in the case where the curved side surface portion 3a of the side surface 3C of the tip 3 and the wall surface portion 8a of the mounting seat wall surface 8B are in contact only with each other on the outer peripheral side. If there is a large molding error of the chip 3 or the chip mounting seat 8 on the side surface portion 3a or the wall surface portion 8a, or if the tightening and fixing force of the clamp screw 6 is insufficient, the curved side surface portion is formed. 3a and the wall surface portion 8a, the side surface portion 3b is rotated around the clamp screw 6 in a direction in which the side surface portion 3b is separated from the wall surface portion 8b, so that the processing accuracy is significantly impaired, or the chip 3 is loose. As a result, the processing itself may become impossible.

Therefore, in the throw-away type ball end mill described in the above publication, the lower surface 3B of the main chip 3 is crossed in the width direction of the chip 3 and opened on both side surfaces 3C, 3C of the chip 3. Thus, while forming the key groove-shaped concave portion 10, the protrusion-shaped convex portion 11 that fits into the concave portion 10 is formed on the mounting seat bottom surface 8A of the chip mounting seat 8 on which the main chip 3 is mounted. Then, with the main chip 3 mounted and positioned at a predetermined position, the concave portion 10 is fitted into the convex portion 11 to restrain the rotation of the chip 3. The concave portion 10 and the convex portion 11 are also provided on the lower surface of the sub-chip 4 and the mounting seat bottom surface of the chip mounting seat 8 on which the sub-chip 4 is mounted.

However, in such a throw-away type ball end mill, since the concave portion 10 for restraining the rotation of the tip 3 is formed over the entire width direction of the lower surface 3B of the tip 3, the tip 3 has the concave portion. It is unavoidable that the thickness of 3 becomes small. For this reason, the strength of the chip 3 is impaired at the portion where the recess 10 is formed, and when an excessive cutting load is applied, a crack may run from the recess 10 and the chip 3 may be damaged. there were. In addition, the concave portion 10 must be formed over the entire width direction of the lower surface 3B of the chip 3, and the convex portion 11 on the chip mounting seat 8 side is also formed correspondingly over the entire width direction of the mounting seat bottom surface 8A. Therefore, the molding of the concave portion 10 and the convex portion 11 becomes complicated, and if the concave portion 10 and the convex portion 11 are not fitted securely over their entire lengths, the chip 3 is placed at a predetermined position. There is also a problem that relatively high molding accuracy is required because positioning becomes impossible.

[0008]

[Means for Solving the Problems]

 The present invention has been made to solve such a problem, and a tool mounting body is provided with a tip mounting seat having a mounting seat bottom surface and a mounting seat wall surface standing upright with respect to the mounting seat bottom surface. A throw-away type cutting tool in which a tip having a seating surface that sits on the bottom surface of the mounting seat and a side surface that intersects the seating surface and abuts the wall surface of the mounting seat is detachably mounted on the tip mounting seat. , A groove-shaped recess is formed at the intersection of the seating surface and the side surface of the chip, intersecting the intersectional ridgeline, and intersecting the seating surface and the side surface at an obtuse angle. A convex portion that fits into the mounting seat is provided at the intersection of the mounting seat bottom surface and the mounting seat wall surface.

[0009]

[Action]

 In the throw-away type cutting tool having such a structure, the recessed portion for restraining the rotation of the insert and preventing the displacement is formed in the ridge line crossing the seating surface and the side surface of the insert with obtuse angles on the seating surface and the side surface. That is, the recess is formed only partially on the outer peripheral ridge of the seating surface, and the recess is formed in the entire width direction of the lower surface that becomes the seating surface like the conventional chip. It is not formed over time. Therefore, it is possible to minimize the reduction in the thickness of the chip due to the provision of such a recess, and it is possible to maintain the strength of the chip and prevent damage due to cracks or the like. Further, as the recess is partially formed on the outer peripheral ridge of the seating surface as described above, the projection on the chip mounting seat side that fits into this recess also fits on the bottom surface of the mounting seat and the wall surface of the mounting seat. It suffices if they are formed so as to slightly project at the intersections, so that the concave portions and the convex portions can be easily molded, and high positioning accuracy can be obtained. Further, a convex portion and a concave portion for restraining the rotation of the chip are formed on the outer peripheral side of the chip and the outer peripheral side of the bottom surface of the chip mounting seat, which are separated from the clamp screw which is the center of the rotation of the chip. Therefore, as described above, a high rotation restraining force can be obtained even by fitting the small convex portion and the concave portion.

[0010]

【Example】

 1 to 9 show a throw-away type ball end mill as an embodiment of the present invention. However, since the basic structure of the ball end mill in this embodiment is substantially the same as that of the ball end mill in FIGS. 10 and 11 described above, the same parts are designated by the same reference numerals and the description thereof will be omitted. . 6 to 9 show a chip (main chip) 3 mounted on the ball end mill of this embodiment. As shown in these figures, the chip 3 is a plane from the upper surface 3A side. It has a substantially elliptical flat plate shape, and a pair of arcuate cutting blades 2 and 2 and linear cutting blades 9 and 9 are provided on the outer peripheral ridge of the upper surface 3A in the circumferential direction of the upper surface 3A. Are formed alternately and at point symmetry positions. Then, in the ridge line portions 3D and 3D intersecting the lower surface 3B serving as the seating surface of the chip 3 and the side surfaces 3C and 3C serving as the flanks, the recess 21 for restraining the displacement due to the rotation of the chip 3 or the like is formed. , 21 are formed.

As shown in FIG. 9, the recess 21 is in the shape of a key groove having a “U” -shaped cross section, and as shown in FIG. 6, it is continuous with the arcuate cutting edge 2 of the side surface 3 C. It is formed on the intersection ridge 3D between the curved side surface portion 3a and the lower surface 3B, and as shown in FIG. 8, it intersects both the lower surface 3B and the side surface 3C at an obtuse angle so as to cross the intersection ridge 3D. Is formed in. Here, as shown in FIG. 6, the angle α formed by the direction in which the recess 21 crosses the intersecting ridge portion 3D and the direction in which the linear cutting edge 9 extends in the plan view is 15 ° to 90 ° in the present embodiment. It is set within the range of. Further, in this embodiment, the side surface 3C is provided with a clearance angle β so as to gradually recede toward the lower surface 3B side with respect to a straight line L passing through the cutting edges 2 and 9 and orthogonal to the lower surface 3B as shown in FIG. Therefore, the concave portion 21 is formed to be inclined so as to recede with respect to the straight line L at an inclination angle γ larger than the clearance angle β as it goes toward the lower surface 3B side. However, in this embodiment, the angle γ-β at which the concave portion 21 is inclined with respect to the side surface 3C is set within the range of 15 ° to 60 °. Furthermore, the width W of this recess 21 is preferably set within the range of 2.0 mm to 10 mm, though it depends on the outer diameter of rotation of the cutting edge 9 of the throw-away type ball end mill. With the outside diameter of 25 mm, it is set to 4.0 mm.

On the other hand, the tip mounting seat 8 of the tool body 1 on which such a tip 3 is mounted has a crossing portion 8C between the bottom surface 8A of the mounting seat and the curved wall portion 8a of the wall surface 8B of the mounting seat. As shown in FIGS. 4 and 5, a key-shaped convex portion 22 that fits into the concave portion 21 is provided at a portion corresponding to the concave portion 21 in the mounted state of the chip 3. Here, in this embodiment, a recess 23 is formed in the above-mentioned portion corresponding to the recess 21 of the intersecting portion 8C so as to be recessed with respect to the mounting seat bottom surface 8A and the mounting seat wall surface 8B, and the recess 23 is formed. Is fitted with a key member 24, and the convex portion 22 is formed as a portion of the key member 24 protruding into the chip mounting seat 8. Further, the convex portion 22 is formed so that its upper surface 22A is gradually inclined toward the inside of the chip mounting seat 8 toward the mounting seat bottom surface 8A side. The inclination angle of the upper surface 22A with respect to the direction perpendicular to the mounting seat bottom surface 8A is set to be equal to the inclination angle γ formed by the recess 21 with respect to the straight line L in the chip 3. However, in the chip mounted state, a slight clearance is formed between the upper surface 22A of the convex portion 22 and the bottom surface 21A of the concave portion 21 of the chip 3.

The cutting edge 2 of the tip 3 of the present embodiment is formed so as to form a substantially 1/4 circular arc on the virtual plane P parallel to the above 3A and 3B, and the above-mentioned linear cutting edge 9 is formed. Is also formed so as to extend on this virtual plane P. Further, at the end 2A side of the cutting edge 2 located on the tool rotation center C side at the tip of the tool body 1 in the mounted state of the tip 3, the cutting edge 2 and the linear cutting edge 9 on the side not used for cutting are provided. A small cutting edge 25 is formed to intersect at an obtuse angle. Furthermore, at the side edge portion of the upper surface 3A of the tip 3, the arc-shaped cutting blade 2 and the straight-line cutting blade 9 are connected to each other, and the virtual plane is formed toward the inner side of the tip 3 in the plan view. A raised surface 26 that gradually rises with respect to P is formed. This raised surface 26 orbits the side ridge portion of the upper surface 3A with a constant width m and a constant raised angle θ with respect to the virtual plane P as shown in FIG. 7 in a cross section orthogonal to the cutting edges 2 and 9. Is formed as follows. In addition, in the portion of the upper surface 3A which is continuous with the small cutting edge 25, there is also formed an inclined flat second raised surface which gradually rises with respect to the virtual plane P as it goes inward of the chip 3.

Further, in this embodiment, such a tip 3 is shown in FIG. 3 so that the arcuate cutting edge 2 slightly moves rearward in the tool rotation direction as the arcuate cutting edge 2 moves from the one end 2A to the other end 2B. Further, the virtual plane P is arranged so as to be inclined in a direction oblique to the axis O of the tool body 1. Here, the angle A 1 formed by the virtual plane P with respect to the axis O is set to a regular angle of 15 ° or less in this embodiment. Therefore, the imaginary plane P is parallel to the axis O, or the imaginary plane P is inclined to the axis O so that the arcuate cutting edge 2 moves from the one end 2A to the other end 2B toward the tool rotation direction. They are not placed side by side. The sub-tip 4 in this embodiment is substantially the same as the tip 3 provided with the arc-shaped cutting edge 2 having the same radius as the arc-shaped cutting edge 2 of the main chip 3 and the linear cutting edge 9 connected to the arc-shaped cutting edge 2. The same shape is used, and the rotation loci of the cutting edges 2 and 9 of these chips 3 and 4 about the axis O are overlapped with each other. Further, recesses 21 and 21 similar to those of the main chip 3 are formed also on the ridge line where the lower surface and the side surface of the sub chip 4 intersect, and the recess 21 is formed in the chip mounting seat on which the sub chip is mounted. A convex portion 22 that fits in the is formed.

In the ball end mill having such a structure, the tip 3 has the lower surface 3B in close contact with the mounting seat bottom surface 8A, and has a flat surface out of the side surface 3C continuous with the cutting blades 2 and 9 not used for cutting. The side surface portion 3b is brought into contact with the flat wall surface portion 8b of the mounting seat wall surface 8B, and the outer peripheral side portion of the curved side surface portion 3a of the side surface 3C is connected to the curved wall surface portion 8a of the mounting seat wall surface 8B. The concave portion 21 formed on the lower surface side of the cutting blade 2 which is not brought into contact with the outer peripheral side and is not subjected to the cutting is fitted into the convex portion 22 and is mounted on the chip mounting seat 8. Therefore, the fitting of the concave portion 21 and the convex portion 22 restrains the displacement of the tip 3 due to the rotation around the clamp screw 6, and the tip 3 is always positioned at a predetermined position. , 9 position accuracy can be maintained, which enables high-precision machining.

In the ball end mill having the above structure, the recess 21 is formed so as to intersect the lower surface 3B and the side surface 3C of the chip 3 and intersect the lower surface 3B and the side surface 3C at an obtuse angle. That is, it is only partially formed on the outer peripheral portion of the lower surface 3B, and is not formed over the entire width direction of the lower surface unlike the conventional chip described above. Therefore, by providing such a recess 21, the thickness of the chip 3 is reduced only to the minimum in the outer peripheral portion of the lower surface 3B, and thereby the strength of the chip 3 can be maintained. Therefore, for example, even when an excessive cutting load is applied to the tip 3 during cutting, it is possible to prevent a situation in which a crack is generated from the recess 21 and the chip 3 is damaged. .

Further, since the concave portion 21 is formed so as to intersect the lower surface 3B and the side surface 3C at an obtuse angle, the portion of the concave portion 21 is formed toward the inside of the chip 3 as it goes inward. Thickness gradually increases, and its strength approaches a portion where the recess 21 is not formed. Therefore, according to the present embodiment, even if the recess is formed only on the outer peripheral portion of the same lower surface 3B, higher chip strength is secured as compared with, for example, the case where the recess is formed in the direction parallel to the lower surface 3B. be able to. Further, since the concave portion 21 is only partially formed on the outer peripheral portion of the lower surface 3B of the chip 3 as described above, the concave portion 21 can be formed as compared with the conventional case where the concave portion is formed over the entire width direction of the lower surface. In addition to being able to facilitate the formation itself, in the past, high molding precision was required over the entire length of the recess extending in the width direction, while molding only a small portion with high precision. There is also the advantage that it is sufficient. Further, as the concave portion 21 is partially formed in this way, the convex portion 22 on the side of the chip mounting seat 8 also slightly projects to the intersection 8C of the mounting seat bottom surface 8A and the mounting seat wall surface 8B. It only has to be formed, and the protrusion 22 can be easily formed.

Furthermore, in this embodiment, the concave portion 21 that fits into the convex portion 22 is formed on the outer peripheral portion of the lower surface 3 that is relatively distant from the clamp screw 6 that is the center of rotation of the chip 3. Even if the small concave portion 21 and the small convex portion 22 are fitted together, a large rotation restraining force can be obtained, and the chip 3 can be firmly and reliably prevented from being displaced. Further, in this embodiment, the convex portion 22 is formed on the key member 24 fitted in the concave portion 23 formed in the chip mounting seat 8, and therefore, the convex portion 11 of the conventional ball end mill is formed. As compared with directly forming on the chip mounting seat 8 as described above, the formation of the convex portion 22 becomes easier, and even if the convex portion 22 is damaged, it is only necessary to replace the key member 24. The advantage of being sufficient is obtained.

On the other hand, in the present embodiment, on the upper surface 3 A of the tip 3, a raised surface 26 is formed which is raised at an elevation angle θ with respect to the virtual plane P including the arc drawn by the cutting edge 2. Despite the clearance angle β on the side surface 3C, as shown in FIG. 7, the cutting edge angle δ of the cutting blades 2 and 9 is set large to secure high cutting edge strength and The advantage of being able to suppress wear and chipping is also obtained. Moreover, the imaginary plane P is oriented at an angle A with respect to the axis O of the tool body 1 so that the tip 3 moves toward the rear side in the tool rotation direction as the cutting edge 2 moves from the one end 2A to the other end 2B. The chips are arranged in a diagonal direction, and the chips generated during cutting are guided by the cutting edge ridges as the tool body 1 rotates and are sequentially discharged toward the tool base end side. It is also possible to obtain.

In the present embodiment, the angle α formed by the direction in which the concave portion 21 crosses the intersecting ridge line portion 3D and the direction in which the linear cutting edge 9 extends in the plan view from the upper surface 3A side of the chip 3 is 15 ° to 90 °. However, if the angle α is less than 15 °, the recess 21 is too close to the one end 2A side of the cutting edge 2, which is not preferable. On the contrary, when the angle α exceeds 90 °, in the tip 3 in which the cutting edge 2 is formed in a 1/4 circular arc shape as in the present embodiment, the recess 21 is located on the lower surface 3B side of the linear cutting edge 9. If it is formed, it may not be possible to obtain a sufficient restraining force for the rotation around the clamp screw 6. Further, in this embodiment, the angle γ-β formed by the bottom surface 21A of the recess 21 with respect to the side surface 3C, that is, the angle γ-β formed by the recess 21 with respect to the side surface 3C of the chip 3 is set within the range of 15 ° to 60 °. However, this is because when the angle γ-β is less than 15 °, the bottom surface 21A of the recess 21 becomes too shallow with respect to the side surface 3C, and conversely when the angle γ-β exceeds 60 °, the recess 21 has a lower surface 3B. This is because it may be formed on the upper side for a long time and the above effect may not be obtained.

Further, in the present embodiment, the width W of the concave portion 21 is set in the range of 2.0 mm to 10 mm. This is because if the width W is too small, the width of the convex portion 22 also becomes small. This is because the portion 22 may be easily damaged, and if the width W is too large, it may result in an unnecessarily high labor for forming the concave portion 21 and the convex portion 22. Further, in the present embodiment, the concave portion 21 is formed so that the bottom surface 21A thereof has an inclined plane shape, but it may be formed so as to have, for example, a concave curved surface which is concave toward the inside of the chip. In this case, the upper surface 22A of the convex portion 22 may be formed into a convex curved surface. Furthermore, in the present embodiment, the case where the present invention is used for the throw-away type ball end mill has been described, but the present invention is not limited to such a thing, and for example, other throw-away type end mills can be used. If it is a throw-away type cutting tool with a structure such that a chip equipped with a cutting edge is attached to the tip mounting seat provided on the tool body, such as a turning tool such as an end mill or cutter, or a turning tool such as a bite. It is possible to apply.

[0022]

[Effect of device]

 As described above, according to the present invention, the fitting of the concave portion provided on the ridge line intersecting the lower surface and the side surface of the chip with the convex portion provided on the chip mounting seat side allows the chip to be rotated or the like. It is possible to prevent misalignment, maintain the cutting edge accuracy, and perform high-precision cutting. In addition to this, it is possible to minimize the deterioration of the strength of the tip due to the formation of the recess, and even if an excessive load acts during cutting, the chip will crack or break. It is possible to prevent this from occurring and to extend the life of the tool, and it is possible to obtain a sufficient rotation restraining force even if the small convex portion and the concave portion are fitted together.

[Brief description of drawings]

FIG. 1 is a plan view showing a ball end mill as an embodiment of the present invention.

FIG. 2 is a front view of the ball end mill shown in FIG.

3 is a side view of the ball end mill shown in FIG. 1. FIG.

4 is a front perspective view with a part of the tool body 1 of the ball end mill shown in FIG. 1 omitted.

5] Tip mounting seat 8 of the ball end mill shown in FIG.
It is an expanded sectional view of a part.

6 is a plan view from the upper surface 3A side of the tip 3 mounted on the ball end mill shown in FIG. 1. FIG.

7 is a cross-sectional view of the chip 3 shown in FIG.

8 is a cross-sectional view of a recess 21 portion of the chip 3 shown in FIG.

9 is a perspective view of the chip 3 shown in FIG. 6 with a part thereof omitted.

10A and 10B are a plan view and a side view, respectively, showing a conventional ball end mill.

FIG. 11 is a plan view of the tip 3 mounted on the conventional ball end mill shown in FIG.
(B) It is a side view.

[Explanation of symbols]

1 Tool Body 2, 9 Cutting Edge 3, 4, 5 Tip 3A Top 3 of Tip 3B Bottom 3 of Tip 3C Side 3C of Tip 3 3D Cross Ridge between Bottom 3B and Side 3C Tip Mounting Base 8A Mounting Bottom 8B Mounting Seat wall surface 21 Recess 22 Convex 24 Key member O Rotation axis α of tool body 1 Recess 2 in plan view from the upper surface 3A side of the tip 3
The angle formed by the direction 1 crossing the intersecting ridge 3D and the direction in which the linear cutting edge 9 extends β The clearance angle of the side surface 3C γ The concave portion with respect to the straight line L passing through the cutting edges 2 and 9 of the tip 3 and orthogonal to the lower surface 3B Inclination angle formed by 21 W Width of recess 21

Claims (1)

[Scope of utility model registration request] 1. A tool mounting body is provided with a chip mounting seat having a mounting seat bottom surface and a mounting seat wall surface standing upright with respect to the mounting seat bottom surface, and a seat seating on the chip mounting seat bottom surface. A throw-away type cutting tool comprising a surface and a side surface that intersects the seating surface and abuts the mounting seat wall surface in a detachable manner. In the intersecting ridge line portion, a groove-shaped concave portion that intersects the seating surface and the side surface at an obtuse angle is formed across the intersecting ridge line,
A throw-away cutting tool, wherein the tip mounting seat is provided with a convex portion that fits into the concave portion at an intersection of the mounting seat bottom surface and the mounting seat wall surface.