Ball end mill

Abstract

The invention discloses a ball end mill, and belongs to the technical field of cutters. The ball end mill comprises a clamping part and a ball cutting part fixedly connected with one end of the clamping part. Due to the fact that the first rear tool face in the bottom blade part is far away from the ball center of the ball head cutting part relative to the second rear tool face, namely the height difference exists between the first rear tool face and the second rear tool face in the bottom blade part. The height difference ensures that the first flank face in the bottom cutting edge part cannot directly diffuse to the second flank face in the bottom cutting edge part after being abraded, and a certain height difference exists between the two flank faces, so that the area change of the abraded flank faces after machining for a period of time is not large. And the contact area with the processed surface of the processed workpiece is prevented from being obviously increased, so that the increase of the roughness value of the processed workpiece is slowed down, and the processing quality of the ball end mill on the processed workpiece is better.

Description

Technical Field

[0001] The present application relates to the technical field of cutting tools, and in particular to a ball-end milling cutter. Background Art

[0002] Ball end mills are widely used in profiling in industries such as molds, vehicles and aviation. With the development of processing and manufacturing technology, higher requirements are placed on the service life and processing efficiency of ball end mills.

[0003] However, after the existing ball-end milling cutter has been processing the workpiece for a period of time, the workpiece processing quality is reduced due to the wear generated. Summary of the invention

[0004] The embodiment of the present application provides a ball end mill, which can solve the problem that the ball end mill in the prior art reduces the processing quality of the workpiece due to wear after processing the workpiece for a period of time. The technical solution is as follows:

[0005] In one aspect, a ball end mill is provided, the ball end mill comprising:

[0006] A clamping portion, and a ball-end cutting portion fixedly connected to one end of the clamping portion, wherein the bottom of the ball-end cutting portion has: at least three bottom cutting portions distributed in an annular manner;

[0007] Each of the bottom blade portions has: a rake face, a first flank face, and a second flank face, wherein the rake face, the first flank face, and the second flank face are distributed along the circumference of the ball-end cutting portion, and the intersection of the rake face and the first flank face forms a ball-end bottom blade;

[0008] Wherein, the first flank surface is farther away from the center of the ball end cutting portion than the second flank surface.

[0009] Optionally, there is a height difference between the first flank surface and the second flank surface along the target direction, and a value range of the height difference δ is 0.01 mm to 0.1 mm.

[0010] The target direction is a direction parallel to an extension direction of a straight line passing through the ball head bottom edge of the bottom edge portion and the ball center of the ball head cutting portion.

[0011] Optionally, the first back cutting surface is a concave arc surface or a plane.

[0012] Optionally, when the first flank surface is a concave arc surface, the first flank surface is a concave arc surface;

[0013] Wherein, the radius of the concave arc surface is greater than or equal to the blade diameter of the ball head cutting part.

[0014] Optionally, the angle range of the first back angle formed between the first back cutting edge and the cutting plane is: 2 degrees to 15 degrees; the angle range of the second back angle formed between the second back cutting edge and the cutting plane is: 0 degrees to 15 degrees; the angle range of the front angle formed between the front cutting edge and the base plane is: -5 degrees to 5 degrees.

[0015] Optionally, a central area of ​​the bottom of the ball head cutting portion has an inner recessed portion, the inner recessed portion has an opening, and the lowest ends of the plurality of bottom blade portions facing away from the clamping portion are connected to the opening edge of the inner recessed portion.

[0016] Optionally, the radius of the bottom edge of the ball head and the inner diameter of the opening of the inner recess satisfy the relationship:

[0017]

[0018] The bottom edge of the ball head is in an arc shape, R is the radius of the bottom edge of the ball head, and Φ is the inner diameter of the opening of the inner recess.

[0019] Optionally, a distance h between the bottom surface of the inner recess and the lowest end point of the bottom edge of the ball head away from the clamping portion along the central axis of the ball head cutting portion ranges from 0.01 mm to 0.05 mm.

[0020] Optionally, the plurality of bottom blade portions are distributed at equal or unequal intervals.

[0021] Optionally, the ball end mill further comprises: a circumferential cutting portion distributed between the clamping portion and the ball end cutting portion, the circumferential cutting portion having at least three circumferential cutting edges connected to the at least three bottom cutting edges in one-to-one correspondence;

[0022] Each of the circumferential blade portions has a circumferential blade that is smoothly connected to the ball head bottom blade in the corresponding bottom blade portion.

[0023] The beneficial effects brought by the technical solution provided by the embodiment of the present application include at least:

[0024] A ball end mill may include: a clamping portion, and a ball end cutting portion fixedly connected to one end of the clamping portion. Since the first flank face in the bottom blade portion is far away from the ball center of the ball end cutting portion relative to the second flank face, that is, there is a height difference between the first flank face in the bottom blade portion and the second flank face in the bottom blade portion, the height difference ensures that the first flank face in the bottom blade portion will not directly diffuse to the second flank face in the bottom blade after wear, and there is a certain height difference between the two flank faces, so that the area of ​​the worn flank face does not change much after a period of processing, thereby avoiding a significant increase in the contact area with the processed surface of the workpiece being processed, thereby slowing down the increase in the roughness value of the workpiece being processed, and ensuring that the ball end mill has a good processing quality for the workpiece being processed. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0026] Figure 1 It is a structural schematic diagram of a ball end mill provided in an embodiment of the present application;

[0027] Figure 2 yes Figure 1 A front view of a ball nose cutting portion of a ball nose end mill is shown;

[0028] Figure 3 is a side view of a ball end mill provided in an embodiment of the present application;

[0029] Figure 4 yes Figure 3 A cross-sectional view of a ball end mill at AA' is shown;

[0030] Figure 5 is a front view schematic diagram of a ball head cutting portion provided in an embodiment of the present application;

[0031] Figure 6 is a simplified cross-sectional view along the central axis of a ball-end mill provided in an embodiment of the present application;

[0032] Figure 7 The present application provides a line graph comparing the surface roughness values ​​of a workpiece in different time periods obtained by respectively processing the workpiece with two ball-end end mills.

[0033] Among them, ball end mill 000, clamping part 100, ball end cutting part 200, bottom blade part 201, rake face m1, first relief face m2, second relief face m3, circumferential direction z1 of cutting part, ball end bottom blade A1, circumferential cutting part 300, circumferential blade part 301, circumferential blade A2, circumferential blade rake face 301a, circumferential blade relief face 301b, transition part 400, height difference δ, target direction f1, curvature radius r of concave arc surface, blade diameter Dc, first clearance angle α1, blade width W1, cutting plane K1, second relief angle α2, base plane K2, rake angle γ, third relief face m4, third relief angle α3, concave part O, opening o1, radius R, inner diameter Φ, blade area A11 in center area, distance h, center axis L.

[0034] The above drawings have shown clear embodiments of the present application, which will be described in more detail later. These drawings and text descriptions are not intended to limit the scope of the present application in any way, but to illustrate the concept of the present application to those skilled in the art by referring to specific embodiments. DETAILED DESCRIPTION

[0035] In order to make the objectives, technical solutions and advantages of the present application clearer, the implementation methods of the present application will be further described in detail below with reference to the accompanying drawings.

[0036] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0037] It should be understood that the references to "one embodiment" or "an embodiment" throughout the specification mean that the specific features, structures, or characteristics associated with the embodiment are included in at least one embodiment of the present invention. Therefore, the references to "in one embodiment" or "in an embodiment" appearing throughout the specification do not necessarily refer to the same embodiment. In addition, these specific features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0038] Please refer to Figure 1 and Figure 2 , Figure 1 is a schematic structural diagram of a ball end mill provided in an embodiment of the present application, Figure 2 yes Figure 1 The front view of the ball end mill 000 is shown. The ball end mill 000 may include a clamping portion 100 and a ball end cutting portion 200 fixedly connected to one end of the clamping portion 100, and the bottom of the ball end cutting portion 200 may have at least three bottom blade portions 201 distributed in an annular manner.

[0039] Each bottom edge portion 201 in the ball-end cutting portion 200 may have a rake face m1, a first flank face m2 and a second flank face m3, and the rake face m1, the first flank face m2 and the second flank face m3 may be distributed along the circumferential direction z1 of the ball-end cutting portion. The intersection of the rake face m1 and the first flank face m2 may form a ball-end bottom edge A1.

[0040] The first flank surface m2 in the bottom blade portion 201 can be away from the center of the ball-end cutting portion 200 relative to the second flank surface m3. Here, the first flank surface m2 in each bottom blade portion 201 can be away from the center of the ball-end cutting portion 200 relative to the second flank surface m3. It should be noted that the ball-end cutting portion 200 can be in a hemispherical shape or a part of a virtual sphere, and the center of the sphere cutting portion 200 can be the center of the virtual sphere where the sphere cutting portion 200 is located.

[0041] In the embodiment of the present application, since the first back tool surface m2 in the bottom blade portion 201 is far away from the center of the ball head cutting portion 200 relative to the second back tool surface m3, that is, there is a height difference between the setting positions of the first back tool surface m2 in the bottom blade portion 201 and the second back tool surface m3 in the bottom blade portion 201, therefore, the height difference ensures that the first back tool surface m2 in the bottom blade portion 201 will not directly diffuse to the second back tool surface m3 in the bottom blade portion 201 after wear, and there is a certain height difference between the two back tool surfaces, so that the area of ​​the worn back tool surface does not change much after a period of processing, thereby avoiding a significant increase in the contact area with the processed surface of the workpiece being processed, thereby slowing down the increase in the roughness value of the workpiece being processed, and ensuring that the ball end mill 000 has a good processing quality for the workpiece being processed.

[0042] Optional, please refer to Figure 3 , Figure 3 It is a side view of a ball end mill provided in an embodiment of the present application. The ball end mill 000 may also include: a circumferential cutting portion 300 distributed between the clamping portion 100 and the ball end cutting portion 200, and the circumferential cutting portion 300 may have at least three circumferential blade portions 301 connected to at least three bottom blade portions 201 in a one-to-one correspondence. Among them, each circumferential blade portion 301 may have a circumferential blade A2 smoothly connected to the ball end bottom blade A1 in the corresponding bottom blade portion 201. For example, each circumferential blade portion 301 may have: a circumferential blade front cutting surface 301a and a circumferential blade back cutting surface 301b, and a circumferential blade A2 may be formed at the intersection of the circumferential blade front cutting surface 301a and the circumferential blade back cutting surface 301b.

[0043] In this application, if Figure 3 As shown, the ball end mill 000 may also include: a transition portion 400 located between the clamping portion 100 and the circumferential cutting portion 300, and the two ends of the transition portion 400 may be fixedly connected to the clamping portion 100 and the circumferential cutting portion 300, respectively. It should be noted that the clamping portion 100, the transition portion 400, the circumferential cutting portion 300 and the ball end cutting portion 200 in the ball end mill 000 may be an integrated structure. For example, the clamping portion 100, the transition portion 400, the circumferential cutting portion 300 and the ball end cutting portion 200 in the end mill 000 may be made of one or more materials selected from high-speed steel, cemented carbide, cubic boron nitride and polycrystalline diamond.

[0044] For example, the multiple bottom blades 201 provided on the bottom of the ball-end cutting part 200 can be distributed at equal intervals. For example, the number of the multiple bottom blades 201 provided on the bottom of the ball-end cutting part 200 can be 3, 4, 5, 6, 7 or 8, etc., and the embodiment of the present application does not specifically limit this. It should be noted that, to a certain extent, the more the number of the multiple bottom blades 201 provided on the bottom of the ball-end cutting part 200, the better the wear resistance of the ball-end end mill 000. It should also be noted that in other possible implementations, the multiple bottom blades 201 can also be distributed at unequal intervals, and the embodiment of the present application does not limit this.

[0045] In the examples of this application, please refer to Figure 3 and Figure 4 , Figure 4 yes Figure 3 The cross-sectional view of the ball end mill at A-A' is shown. There is a height difference δ between the first flank face m2 and the second flank face m3 in the bottom blade portion 201 along the target direction f1, and the numerical range of the height difference δ can be 0.01 mm to 0.1 mm. Among them, the target direction f1 can be: a direction parallel to the extension direction of the straight line passing through the ball end bottom blade A1 of the bottom blade portion 201 and the center of the ball of the ball end cutting portion 200. In this way, the height difference δ ensures that the first flank face m2 in the bottom blade portion 201 will not directly diffuse to the second flank face m3 in the bottom blade portion 201 after wear, and a certain height difference between the two flank faces can achieve that the area of ​​the worn flank face does not change much after a period of processing, avoid a significant increase in the contact area with the processed surface of the workpiece being processed, thereby slowing down the increase in the roughness value of the workpiece being processed, and ensuring that the ball end mill 000 has a good processing quality for the workpiece being processed.

[0046] It should be noted that there is a height difference δ between the first back cutting edge m2 and the second back cutting edge m3 in each bottom cutting portion 201 along the target direction f1, and the target direction f1 can be a direction parallel to the extension direction of the straight line of the ball head bottom edge A1 of the bottom cutting portion 201 and the center of the ball head cutting portion 200.

[0047] Optional, such as Figure 4 As shown, the first flank surface m2 in the bottom blade portion 201 may be a concave cambered surface or a plane. In this case, by setting the first flank surface m2 as a concave cambered surface, the contact area between the first flank surface m2 of the bottom blade portion 201 and the machined surface of the workpiece during the wear process is reduced during the machining process, thereby slowing down the increase in the roughness value of the machined workpiece.

[0048] For example, Figure 4As shown, in the case where the first flank m2 in the bottom blade portion 201 is a concave arc surface, the first flank m2 can be a concave circular arc surface. Among them, the curvature radius r of the concave circular arc surface (the central angle θ corresponding to the curvature radius r of the concave circular arc surface) can be greater than or equal to the blade diameter Dc of the ball head cutting portion 200. It should be noted that the smaller the curvature radius r of the concave circular arc surface, the further the contact area between the first flank m2 of the bottom blade portion 201 and the machined surface of the workpiece during the wear process during the machining process can be reduced. For example, the curvature radius r of the concave circular arc surface can be set to 1 to 5 times the blade diameter Dc of the ball head cutting portion 200.

[0049] In the embodiments of the present application, Figure 4 As shown, the angle range of the first clearance angle α1 formed between the first clearance face m2 in the bottom blade portion 201 and the cutting plane K1 can be: 2 degrees to 15 degrees. In this way, while ensuring that the first clearance angle α1 is relatively large, the contact area between the first clearance face m2 of the bottom blade portion 201 and the machined surface of the workpiece during the wear process can also be reduced during the machining process, thereby slowing down the increase in the roughness value of the machined workpiece. For example, the first clearance face edge width W1 of the bottom blade portion 201 can be set to 3% to 15% of the edge diameter Dc of the ball head cutting portion 200.

[0050] like Figure 4 As shown, the angle range of the second back angle α2 formed between the second back cutting edge m3 in the bottom cutting edge portion 201 and the cutting plane K1 can be: 0 degrees to 15 degrees. In this way, the smaller the second back angle α2 is, the more favorable it is for the swing angle when the grinding wheel is used to grind the tool. This can ensure the consistency of the width of the first back cutting edge W1 of the bottom cutting edge portion 201 formed by the grinding wheel from the circumferential outer side surface of the ball head cutting portion 200 to the center of the bottom cutting edge portion 201, as well as the consistency of the height difference δ between the first back cutting edge m2 and the second back cutting edge m3 of the bottom cutting edge portion 201. It should be noted that the second back cutting edge m3 in the bottom cutting edge portion 201

[0051] like Figure 4 As shown, the angle range of the rake angle γ formed between the rake face m1 in the bottom blade portion 201 and the base surface K2 can be: -5 degrees to 5 degrees.

[0052] In this application, if Figure 4 As shown, the bottom edge portion 201 may also have a third clearance face m4 located on the side of the second clearance face m3 away from the first clearance face m2. For example, the third clearance angle α3 formed between the third clearance face m4 and the cutting plane K1 may range from 50 degrees to 70 degrees.

[0053] Optional, please refer to Figure 5 and Figure 6 , Figure 5is a front view schematic diagram of a ball head cutting portion provided in an embodiment of the present application, Figure 6 It is a simplified cross-sectional view along the central axis of a ball-end end mill provided in an embodiment of the present application. The central area of ​​the bottom of the ball-end cutting portion 200 may have an inner recess O, and the inner recess O may have an opening o1, and the lowest end of each bottom blade portion 201 away from the clamping portion 100 is connected to the edge of the opening o1 of the inner recess O. Here, there is no cutting edge in the area where the inner recess O opened in the central area of ​​the bottom of the ball-end cutting portion 200 is located. In this way, by setting the inner recess O in the central area of ​​the bottom of the ball-end cutting portion 200, no cutting edge is set in the area where the inner recess O is located, so that the contact area between the central part of the ball-end cutting portion 200 and the machined surface of the workpiece after the back face is worn during the machining process is effectively reduced, thereby achieving the purpose of reducing the surface roughness value of the machined workpiece and extending the machining life of the tool. For example, the shape of the opening o1 of the inner recess O can be circular.

[0054] In this application, if Figure 5 and Figure 6 As shown, the radius R of the ball end bottom edge A1 and the inner diameter Φ of the opening o1 of the inner concave portion O disposed at the central area of ​​the bottom of the ball end cutting portion 200 may satisfy the following relationship:

[0055]

[0056] Among them, the ball head bottom edge A1 is in an arc shape, R is the radius of the ball head bottom edge A1, and Φ is the inner diameter of the opening o1 of the inner concave part O. It should be noted that the larger the opening Φ of the inner concave part O, that is, the larger the diameter of the edge A11 in the central area of ​​the ball head cutting part 200, the greater the linear speed at the lowest end point of the ball head bottom edge A1 under a fixed cutting spindle speed, and the better the surface quality of the workpiece being processed. However, the larger the inner diameter Φ of the opening o1 of the inner concave part O, the larger the gap at the bottom center area of ​​the ball head cutting part 200, and the less full the contour of the radius R value of the ball head bottom edge A1; the smaller the inner diameter Φ of the opening o1 of the inner concave part O, the fuller the radius R value of the ball head bottom edge A1, but the more difficult it is to produce the ball head end mill.

[0057] In the embodiments of the present application, Figure 5 and Figure 6As shown, the distance h between the bottom surface of the concave portion O provided at the central area of ​​the bottom of the ball head cutting portion 200 and the lowest end point of the ball head bottom edge A1 away from the clamping portion 100 along the central axis L of the ball head cutting portion 200 can range from 0.01 mm to 0.05 mm. It should be noted that the smaller the distance value h, the better the strength of the central part of the bottom edge portion 201 in the ball head end mill 000, but the shorter the processing time, the more the wear to the most central part of the bottom edge portion 201; the larger the distance value h, the longer the processing time can be extended to avoid the most central part of the bottom edge portion 201 from being worn and contacting the processed workpiece surface of the workpiece, but the strength of the central part of the bottom edge portion 201 in the ball head end mill 000 is poor.

[0058] The processing effect of the ball end mill provided in the present application is schematically illustrated below through an experimental example: a workpiece processing experiment is carried out under the same working conditions using the 5-flute ball end mill 000 of the embodiment of the present application and a normal 2-flute ball end mill in the prior art.

[0059] Please refer to Figure 7 , Figure 7 The following is a line graph comparing the surface roughness values ​​of workpieces at different time periods obtained by machining workpieces with two ball-end end mills provided in the embodiment of the present application. The experimental results of the surface roughness values ​​Ra of the workpieces after machining at different time periods are as follows: Figure 7 As shown, the experimental results show that: the surface roughness value Ra of the workpiece surface milled by the 5-edge ball end mill of the embodiment of the present application is less than that of the normal 2-edge ball end mill within 0 to 300 minutes, especially within 240 minutes, the surface roughness value Ra of the workpiece after processing also changes less, much less than that of the normal 2-edge ball end mill. In this way, within a certain processing time, the multi-edge ball end mill 000 of this embodiment can effectively obtain a better workpiece surface roughness value; or within a certain surface roughness value requirement, the multi-edge ball end mill 000 of this embodiment can effectively extend the tool life, and the surface roughness value of the processed workpiece will not increase significantly with the increase of cutting time.

[0060] In summary, the embodiment of the present application provides a ball end mill 000, which may include: a clamping portion 100, and a ball end cutting portion 200 fixedly connected to one end of the clamping portion 100. Since the first flank m2 in the bottom blade portion 201 is far away from the ball center of the ball end cutting portion 200 relative to the second flank m3, that is, there is a height difference δ between the first flank m2 in the bottom blade portion 201 and the second flank m3 in the bottom blade portion 201, the height difference δ ensures that the first flank m2 in the bottom blade portion 201 will not directly diffuse to the second flank m3 in the bottom blade after wear, and a certain height difference between the two flanks can achieve that the area of ​​the worn flank does not change much after a period of processing, avoids a significant increase in the contact area with the processed surface of the workpiece being processed, thereby slowing down the increase in the roughness value of the workpiece being processed, and ensuring that the ball end mill 000 has a good processing quality for the workpiece being processed.

[0061] In the present application, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance. The term "plurality" refers to two or more than two, unless otherwise clearly defined.

[0062] The above description is only an optional embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A ball end mill, characterized in that: include: A clamping portion, and a ball-end cutting portion fixedly connected to one end of the clamping portion, wherein the bottom of the ball-end cutting portion has: at least three bottom cutting portions distributed in an annular manner; Each of the bottom blade portions has: a rake face, a first flank face, and a second flank face, wherein the rake face, the first flank face, and the second flank face are distributed along the circumference of the ball-end cutting portion, and the intersection of the rake face and the first flank face forms a ball-end bottom blade; Wherein, the first flank surface is farther away from the center of the ball end cutting portion than the second flank surface.

2. The ball end mill according to claim 1, characterized in that: There is a height difference δ between the first flank surface and the second flank surface along the target direction, and the value range of the height difference δ is 0.01 mm to 0.1 mm; The target direction is a direction parallel to an extension direction of a straight line passing through the ball head bottom edge of the bottom edge portion and the ball center of the ball head cutting portion.

3. The ball end mill according to claim 2, characterized in that: The first flank surface is a concave arc surface or a flat surface.

4. The ball end mill according to claim 3, characterized in that: In the case where the first flank surface is a concave arc surface, the first flank surface is a concave arc surface; Wherein, the curvature radius of the concave arc surface is greater than or equal to the blade diameter of the ball head cutting part.

5. The ball end mill according to any one of claims 1 to 4, characterized in that: The angle range of the first back angle formed between the first back cutting edge and the cutting plane is: 2 degrees to 15 degrees; the angle range of the second back angle formed between the second back cutting edge and the cutting plane is: 0 degrees to 15 degrees; the angle range of the front angle formed between the front cutting edge and the base plane is: -5 degrees to 5 degrees.

6. The ball end mill according to claim 5, characterized in that: The central area of ​​the bottom of the ball head cutting part has an inner concave part, and the inner concave part has an opening. The lowest ends of the plurality of bottom blade parts away from the clamping part are connected with the opening edge of the inner concave part.

7. The ball end mill according to claim 6, characterized in that: The radius of the bottom edge of the ball head and the inner diameter of the opening of the inner concave portion satisfy the relationship: The bottom edge of the ball head is in an arc shape, R is the radius of the bottom edge of the ball head, and Φ is the inner diameter of the opening of the inner recess.

8. The ball end mill according to claim 7, characterized in that: The distance h between the bottom surface of the inner concave portion and the lowest end point of the bottom edge of the ball head away from the clamping portion along the central axis direction of the ball head cutting portion ranges from 0.01 mm to 0.05 mm.

9. The ball end mill according to any one of claims 1-4, 6-8, characterized in that: The plurality of bottom blade portions are distributed at equal or unequal intervals.

10. The ball end mill according to any one of claims 1-4, 6-8, characterized in that: The ball end mill further comprises: a circumferential cutting portion distributed between the clamping portion and the ball end cutting portion, the circumferential cutting portion having at least three circumferential cutting edges connected to the at least three bottom cutting edges in a one-to-one correspondence; Each of the circumferential blade portions has a circumferential blade that is smoothly connected to the ball head bottom blade in the corresponding bottom blade portion.

Images