Cutting insert, and cutting tool comprising the cutting insert

Abstract

To provide a cutting insert that can improve processing resistance of a corner part when drilling a hole, and a cutting tool.SOLUTION: A cutting insert 3 has a substantially polygonal plate shape, includes a first long side 11U and a second long side 12U, and a first short side and a second short side existing between the first long side and the second long side, and comprises a first end face 10U and a second end face 10L that face each other. Further, the first end face and the second end face have a first cutting edge 31 which extends from the first long side to a first corner part Cn1 existing between the first long side and the first short side, and a second cutting edge 32 which extends from the second long side to a second corner part Cn2 existing between the second long side and the first short side. A curvature radius R3 of a third corner part Cn3 positioned on a side opposite to the first corner part Cn1 with the first long side interposed therebetween is larger than a curvature radius R1 of the first corner part.SELECTED DRAWING: Figure 3

Description

Technical Field

[0001] The present disclosure relates to a cutting insert and a cutting tool including the cutting insert.

Background Art

[0002] As an end mill, which is one of cutting tools for performing turning machining of a workpiece, there is known one in which a cutting insert is detachably mounted on a mounting seat provided on its body. As an example of such an end mill, the applicant has proposed an end mill including a plurality of cutting inserts having a shape that can be used for both a center cutting edge and an outer peripheral cutting edge and can be reused by turning it over (see Patent Document 1).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The above-described end mill including the conventional cutting insert is a so-called two-sided specification center cutting edge type replaceable multifunctional machining end mill, which is particularly excellent in economy and has an advantage that the cutting insert can be stably fixed to the body of the cutting tool as compared with the conventional one.

[0005] Here, FIG. 5 is a schematic cross-sectional view schematically showing an example of a state in which a hole machining (drilling) of a workpiece is performed using a cutting insert provided in a conventional end mill. The cross section corresponds to a plane cross section including the rotation axis AX (center axis of the hole 61) of the end mill when machining a hole 61 in a solid workpiece 6. Further, the arrangement states of the cutting inserts 5A and 5B are also shown in the figure. These have the same shape as the cutting insert itself.

[0006] In this case, the cutting insert 5A functions as a center cutting edge having an inner cutting edge extending from the outer peripheral surface of the tool body to the rotation axis and a main cutting edge located on the outer peripheral surface thereof when mounted on the tool body. On the other hand, the cutting insert 5B functions as an outer cutting edge having a main cutting edge located substantially 180° opposite to the main cutting edge of the center cutting edge. As described above, the cutting inserts 5A and 5B are substantially rectangular parallelepiped in shape, and the center cutting edge and the outer cutting edge are formed to face each other at the ridge line portion between the front surface (for example, the upper surface) and the side surface, and similar center cutting edges and outer cutting edges are also formed on the back surface (for example, the lower surface). It can be said that they have cutting edges that can be used a total of four times.

[0007] As shown in FIG. 5, when the end mill is used to drill a hole in the workpiece, a core 6T having, for example, a columnar shape may remain near the center of the machined bottom surface 6B. This core 6T is a portion intentionally left uncut so that the cutting speed near the rotation axis AX by the cutting insert 5A does not become zero. Specifically, by arranging the cutting insert 5A to slightly retreat toward the back side of the paper in FIG. 5, the cutting edge does not pass through the rotation center (rotation axis AX), and as a result, the core 6T is left uncut. Such a core 6T is broken at the corner portion Cn provided on the tool center side of the cutting insert 5A as the hole drilling progresses. At this time, if the cutting insert 5A is designed with a core drop to allow for a center axis deviation, the load when removing the core 6T tends to become excessively high. Further, according to the findings of the present inventor, as the main processing mechanism of the core 6T, a form in which the core 6T is crushed and broken by the corner portion Cn is recalled. From these facts, in the hole drilling of the workpiece, it is desired to improve the machining resistance at the corner portion on the tool center side of the center cutting edge.

[0008] Therefore, an object of the present disclosure is to provide a cutting insert capable of improving the machining resistance of the corner portion on the tool center side when drilling a hole in a workpiece, thereby effectively suppressing defects and the like in this portion, and a cutting tool provided with the cutting insert.

Means for Solving the Problems

[0009] To solve the above problems, the present disclosure adopts the following configuration.

[0010] 〔1〕An example of the cutting insert according to the present disclosure has a substantially polygonal plate shape (for example, generally a rectangular parallelepiped shape), includes a first long side and a second long side, and a first short side and a second short side between the first long side and the second long side, and has a first end face and a second end face facing each other. Further, the first end face and the second end face have a first cutting edge extending from the first long side to a first corner portion between the first long side and the first short side, and a second cutting edge extending from the second long side to a second corner portion between the second long side and the first short side. And the radius of curvature R3 of the third corner portion located on the opposite side of the first corner portion across the first long side is made larger than the radius of curvature R1 of the first corner portion.

[0011] As described above, when drilling the workpiece, a core 6T as shown in FIG. 5 may remain, and such a core 6T can be broken by the third corner portion located on the tool center side of the center cutting edge. On the other hand, in the present disclosure, when the first corner portion and the third corner portion of the cutting insert are mounted on the cutting tool so as to be located on the tool outer peripheral side and the tool center side, respectively, the radius of curvature R3 of the third corner portion is made relatively larger than the radius of curvature R1 of the first corner portion, so that the machining resistance against the load when removing the core 6T is improved. Therefore, when drilling the workpiece, it is possible to effectively suppress the loss of the third corner portion and the like.

[0012] 〔2〕Specifically, it may further include short side faces that are connected to the first end face and the second end face and include the first short side and the second short side and face each other. In this case, these short side faces may be inclined obliquely with respect to a plane perpendicular to the first end face and the second end face. Further, it is preferable that the relief angle at the first short side is a positive angle, while the relief angle at the second short side is a negative angle. In the present disclosure, the “relief angle” being a “positive angle” means that the interior angle formed by two intersecting faces is an “acute angle”, and the “relief angle” being a “negative angle” means that the interior angle formed by two intersecting faces is an “obtuse angle”.

[0013] 〔3〕Furthermore, in such a configuration, it is also preferable that the first cutting edge and the second cutting edge on the first end face and the first cutting edge and the second cutting edge on the second end face are formed to be in a relationship of 180° rotational symmetry with respect to each other around a predetermined axis that virtually penetrates the cutting insert.

[0014] 〔4〕Moreover, in such a relationship of 180° rotational symmetry, more specifically, the first corner portion on the first end face and the third corner portion on the second end face, and the third corner portion on the first end face and the first corner portion on the second end face may be in a positional relationship of facing each other across the short side face. With such a configuration, the cutting insert according to the present disclosure can be used as a so-called double-sided and replaceable multi-functional end mill for machining. As a result, the cutting insert has the advantages of excellent economy and being stably fixable to the body of the cutting tool.

[0015] 〔5〕Also, in the above configuration, a chip breaker may be provided, which is formed on the rake face related to the first cutting edge and includes a first chip breaker located closer to the first corner portion side than the center of the first long side and a second chip breaker located closer to the third corner portion side than the center of the first long side. In this case, it is more preferable that the maximum width W cmax of the second chip breaker is larger than the maximum width W pmax of the first chip breaker. Here, the "width of the chip breaker" in the present disclosure indicates the width in the direction along the first short side in the plan view of the cutting insert.

[0016] In such a configuration, in the first chip breaker that can be located on the outer peripheral side of the tool, the distance to the "wall" is relatively close, so it becomes easier to forcibly curl the chips. On the other hand, in the second chip breaker that can be located on the tool center side, the distance to the "wall" is relatively far, so it becomes difficult to forcibly curl the chips. That is, in this configuration, it becomes easy to adjust the curling process according to the thickness of the chips as desired. This can contribute to the improvement of the machining performance by the cutting insert.

[0017] 〔6〕In addition, in the above configuration, more specifically, when the maximum width W of the second chip breaker cmax is formed to be 50% or less of the length of the first short side, it is more suitable in that it becomes easier to control the shape of the chips generated during drilling.

[0018] 〔7〕An example of the cutting tool according to the present disclosure can be effectively configured to include a body and the cutting insert according to the present disclosure attached to the body. That is, the cutting insert has a substantially polygonal plate shape and includes a first long side and a second long side, and a first short side and a second short side between the first long side and the second long side, and further includes a first end face and a second end face facing each other. Further, the first end face and the second end face include a first corner portion between the first long side and the first short side, and a first cutting edge extending from the first long side to the first short side, and a second corner portion between the second long side and the first short side, and a second cutting edge extending from the second long side to the first short side. And the radius of curvature R3 of the third corner portion located on the opposite side of the first corner portion across the first long side is made larger than the radius of curvature R1 of the first corner portion.

Advantages of the Invention

[0019] According to the present disclosure, the machining resistance of the third corner portion of the cutting insert can be improved, and as a result, defects and the like of the third corner portion during drilling of the workpiece can be effectively suppressed.

Brief Description of the Drawings

[0020]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying out the Invention

[0021] Hereinafter, this embodiment will be described with reference to the accompanying drawings. For ease of understanding the description, the same reference numerals are given to the same components in each drawing as much as possible, and duplicate descriptions are omitted. FIG. 1 is a perspective view schematically showing the overall configuration of the cutting tool according to this embodiment, and is a view seen from the side of the cutting insert mounted in the first state described later. Also, FIG. 2 is also a perspective view schematically showing the overall configuration of the cutting tool according to this embodiment, and is a view seen from the side of the cutting insert mounted in the second state described later. Furthermore, FIGS. 3 and 4 are a perspective view and a plan view (top view), respectively, schematically showing the configuration of the cutting insert provided in the cutting tool according to this embodiment.

[0022] <Overview of Cutting Tool 1> As shown in FIGS. 1 and 2, the cutting tool 1 is an end mill capable of performing drilling (drilling) in addition to turning of a workpiece, and for example, two sets of cutting inserts 3, 3 are mounted on a rotatably driven body 2. The body 2 is a member that generally constitutes the entire cutting tool 1, and is formed of, for example, steel, and has a tip portion 21 and a rear end portion 22 that are integrally formed and both have a substantially cylindrical shape. The cutting inserts 3, 3 are attached to insert pockets D1, D2 provided in the notched portions of the tip portion 21. The body 2 is gripped by a machine tool (not shown), and is driven to rotate around the rotation axis AX during machining of a workpiece (not shown). Note that the rotation axis AX coincides with the central axis of the body 2 having a substantially cylindrical shape. Also, in FIGS. 1 and 2, the rotation direction YA of the cutting tool 1 during machining is indicated by a round arrow.

[0023] The cutting inserts 3, 3 are each substantially rectangular parallelepiped-shaped and have the same shape that can be substituted for each other (see also FIGS. 3 and 4 described later). They can be detachably (replaceably) fastened and fixed to any of the insert pockets D1, D2 of the body 2, for example, by screws 8. Further, as shown in FIGS. 1 and 2, the cutting insert 3 mounted in the horizontal state (first state 3C) in the insert pocket D1 extends from the outer peripheral surface 2P of the body 2 along the tip side to the rotation axis AX or its vicinity. On the other hand, the cutting insert 3 mounted in the vertical state (a state perpendicular to the horizontal: second state 3P) in the insert pocket D2 is located on the outer peripheral side substantially opposite to the cutting insert 3 arranged in the first state 3C by 180°, and extends from the outer peripheral surface 2P of the body 2 along the tip side to midway toward the rotation axis AX.

[0024] The material of such a cutting insert 3 is not particularly limited. For example, it is preferable that the whole of it is formed of cemented carbide. Further, among the cutting inserts 3, at least the portions including the first cutting edge 31 and the second cutting edge 32 described later may be formed of a hard material such as a sintered body including cermet, ceramics, and cubic boron nitride, a material having a coating layer formed on the surface of these hard materials by PVD or CVD, and a material formed by including any of single crystal diamond or a sintered body including diamond.

[0025] <Specific configuration of the cutting insert 3> Next, a more specific configuration of the cutting insert 3 will be described below. As shown in FIGS. 3 and 4, the cutting insert 3 includes a first end face 10U (the upper face shown in the figure), a second end face 10L (the lower face shown in the figure), a first long side face 11, a second long side face 12, a first short side face 13, and a second short side face 14 that connect the first end face 10U and the second end face 10L. Further, each cutting insert 3 is provided with a mounting hole H for screwing and fixing it to the insert pockets D1, D2.

[0026] Furthermore, as shown in FIGS. 1 and 2, when the cutting insert 3 is mounted on the body 2 in the first state 3C, the second end face 10L, the second long side face 12, and the second short side face 14, or the first end face 10U, the second long side face 12, and the first short side face 13 are the portions that abut against and are constrained by the insert pocket D1 of the body 2. On the other hand, when the cutting insert 3 is mounted on the body 2 in the second state 3P, the second end face 10L, the first long side face 11, and the second short side face 14, or the first end face 10U, the first long side face 11, and the first short side face 13 are the portions that abut against and are constrained by the insert pocket D2 of the body 2.

[0027] The first end face 10U has a first long side 11U, a second long side 12U, a first short side 13U, and a second short side 14U that define boundaries with the first long side face 11, the second long side face 12, the first short side face 13, and the second short side face 14, respectively. Similarly, the second end face 10L has a first long side 11L, a second long side 12L, a first short side 14L, and a second short side 13L that define boundaries with the first long side face 11, the second long side face 12, the first short side face 13, and the second short side face 14, respectively. Due to such structural features, the first cutting edge 31 and the second cutting edge 32 of the first end face 10U and the first cutting edge 31 and the second cutting edge 32 of the second end face 10L have a relationship of rotational symmetry of 180° (i.e., two-fold rotational symmetry) with respect to each other around an axis BX that passes through the center of the cutting insert 3 and extends substantially along the first short sides 13U, 14L and the second short sides 14U, 13L. As such, the axis BX corresponds to an example of a "predetermined axis that virtually penetrates the cutting insert".

[0028] Here, the first cutting edge 31 on the first end face 10U side includes a first corner portion Cn1 between the first long side 11U and the first short side 13U, and extends from the first long side 11U to the first short side 13U. Similarly, the first cutting edge 31 on the second end face 10L side includes a first corner portion Cn1 between the first long side 11L and the first short side 14L, and extends from the first long side 11L to the first short side 14L. This first cutting edge 31 functions as a cutting edge that mainly cuts the machining bottom surface during the drilling of the workpiece 6 in the first state 3C. Also, as shown in FIGS. 1 and 2, when the cutting insert 3 is fixed to the body 2 in the first state 3C, the first corner portion Cn1 is located on the outer peripheral side of the tool, and the third corner portion Cn3 located on the opposite side of the first corner portion Cn1 across the first long sides 11U, 11L is located on the tool center side (near the rotation axis AX).

[0029] And in the present embodiment, the radius of curvature R3 of the third corner portion Cn3 is made relatively larger than the radius of curvature R1 of the first corner portion Cn1. Also, in the 180° rotational symmetry relationship described above, the first corner portion Cn1 on the first end face 10U and the third corner portion Cn3 on the second end face 10L are in a positional relationship of facing each other across the short side face 13, and the third corner portion Cn3 on the first end face 10U and the first corner portion Cn1 on the second end face 10L are also in a positional relationship of facing each other across the short side face 14.

[0030] On the other hand, the second cutting edge 32 on the first end face 10U side includes a second corner portion Cn2 between the second long side 12U and the first short side 13U, and extends from the second long side 12U to the first short side 13U. Similarly, the second cutting edge 32 on the second end face 10L side includes a second corner portion Cn2 between the second long side 12L and the first short side 14L, and extends from the second long side 12L to the first short side 14L. This second cutting edge 32 functions as a cutting edge that mainly cuts the machining peripheral surface during the turning of the workpiece 6 in the second state 3P.

[0031] Also, as shown in FIGS. 3 and 4, the first short side surface 13 and the second short side surface 14 are inclined surfaces, that is, they are provided obliquely with respect to the surfaces perpendicular to the first end surface 10U and the second end surface 10L. In such a configuration, the first short sides 13U, 14L are used as a part of the first cutting edge 31 and a part of the second cutting edge 32, and the relief angles of these first short sides 13U, 14L are positive angles. On the other hand, the second short sides 14U, 13L are not used as the first cutting edge 31 and the second cutting edge 32, and the relief angles of these second short sides 14U, 13L are negative angles.

[0032] Also, regarding the first cutting edge 31 of the cutting insert 3, chip breakers 3B are formed on the first end surface 10U and the second end surface 10L (both are rake surfaces). Here, it is assumed that the chip breaker 3B is composed of a virtual part (first chip breaker) located on the first corner part Cn1 side (tool outer peripheral side) from the center of the first long sides 11U, 11L and a virtual part (second chip breaker) located on the third corner part Cn3 side (tool center side) from the center of the first long sides 11U, 11L. At this time, in the present embodiment, the maximum width (W cmax ) of the second chip breaker is made larger than the maximum width (Wp max ) of the first chip breaker. More specifically, the maximum width W cmax of the second chip breaker is formed to be 50% or less (1 / 2 or less) of the length of the first short sides 13U, 14L.

[0033] According to the cutting inserts 3, 3 configured as described above and the cutting tool 1 to which they are attached, the cutting insert 3 functions as a double-sided and replaceable multi-functional end mill for machining. That is, the cutting inserts 3, 3 can be reversely inserted and replaced with each other at the first end surface 10U and the second end surface 10L and reused, and each can be replaced and reused, so it is possible to use one cutting insert 3 a total of four times. Therefore, the economy of the tool can be improved, and the inventory management of the product can be facilitated.

[0034] Further, the radius of curvature R3 of the third corner portion Cn3 that can be located on the tool center side is made relatively larger than the radius of curvature R1 of the first corner portion Cn1 that can be located on the tool outer peripheral side. As a result, even when the core 6T as shown in FIG. 5 that may remain during the drilling of the workpiece 6 is broken by the third corner portion Cn3 of the cutting insert 3 mounted in the first state 3C, the machining resistance of that portion can be significantly increased as compared with the conventional case. Therefore, when drilling the workpiece 6, it is possible to effectively suppress the occurrence of defects such as chipping of the third corner portion Cn3.

[0035] Furthermore, the maximum width W of the second chip breaker located closer to the third corner portion Cn3 than the center of the first long sides 11U, 11L cmax is made larger than the maximum width W of the first chip breaker located closer to the first corner portion Cn1 than the center of the first long sides 11U, 11L. pmax As a result, in the first chip breaker that can be located on the tool outer peripheral side, the distance to the "wall" becomes relatively close, so that it becomes easier to forcibly curl the chips. On the other hand, in the second chip breaker that can be located on the tool center side, the distance to the "wall" becomes relatively far, so that it becomes difficult to forcibly curl the chips. That is, this configuration has the advantage of easily adjusting the curling process according to the thickness of the chips as desired. As a result, the overall machining performance by the cutting insert 3 can be further improved.

[0036] As described above, the present embodiment has been described with reference to specific examples, but these are for facilitating the understanding of the present disclosure and are not for limiting the interpretation of the present disclosure. That is, the present disclosure is not limited to these specific examples, and those obtained by appropriately modifying the design by those skilled in the art to these specific examples are also included in the scope of the present disclosure as long as they have the features of the present disclosure. Further, each element, arrangement, material, condition, shape, dimensional size, scale, etc. included in each of the above-described specific examples are not limited to those exemplified unless otherwise specified, and can be appropriately changed. Furthermore, each element included in each of the above-described specific examples can be appropriately changed in combination as long as no technical contradiction occurs.

[0037] That is, for example, the cutting insert 3 does not necessarily need to be usable in both the first state 3C and the second state 3P. As long as a first cutting edge 31 having at least a first corner portion Cn1 and a third corner portion Cn3 is formed, and the relationship represented by the radius of curvature R3 > the radius of curvature R1 is satisfied. Also, the tip portion 21 and the rear end portion 22 of the body 2 may be separate components, and may be configured to be of an assembled type (modular type) or a head-exchangeable type, or may be connected by welding or the like. Furthermore, a plurality of insert pockets D1, D2 may be provided, and a plurality of cutting inserts 3 may be mounted in their respective pockets. Still further, as methods for mounting the cutting insert 3 into the insert pockets D1, D2, a screw-on type or a lever-lock type may be mentioned, but it is not limited thereto, and for example, brazing may also be used. Still further, the arrangement pitch of the plurality of cutting inserts 3 is not limited to an equal pitch, and may be arranged at an unequal pitch. Furthermore, when the cutting insert 3 is used for turning, the chip breaker 3B can be formed mainly for imparting a positive rake angle. In this case, it is not necessary to intend to hit the chips against the "wall" to fragment them.

[0038] In addition, the first cutting edge 31 on the first end face 10U side only needs to extend from the first long side 11U to the first corner portion Cn1, and does not necessarily need to be formed over the first short side 13U. Also, the first cutting edge 31 on the second end face 10L side only needs to extend from the first long side 11L to the first corner portion Cn1, and does not necessarily need to be formed over the first short side 14L. Similarly, the second cutting edge 32 on the first end face 10U side only needs to extend from the second long side 12U to the second corner portion Cn2, and does not necessarily need to be formed over the first short side 13U. Also, the second cutting edge 32 on the second end face 10L side only needs to extend from the second long side 12L to the second corner portion Cn2, and does not necessarily need to be formed over the first short side 14L. In other words, the first short sides 13U, 14L do not necessarily need to be used as part of the first cutting edge 31 and the second cutting edge 32.

Explanation of Reference Numerals

[0039] 1... Cutting tool, 2... Body, 2P... Outer peripheral surface, 3... Cutting insert, 3B... Chip breaker, 3C... First state, 3P... Second state, 5A, 5B... Cutting inserts (conventional), 6... Workpiece, 6B... Machined bottom surface, 6T... Core, 8... Screw, 10L... Second end face (rake face), 10U... First end face (rake face), 11... First long side face, 12... Second long side face, 11L, 11U... First long side, 12L, 12U... Second long side, 13... First short side face (short side face), 14... Second short side face (short side face), 13U, 14L... First short side, 13L, 14U... Second short side, 21... Tip, 22... Rear end, 31... First cutting edge, 32... Second cutting edge, 61... Hole, AX... Rotation axis, BX... Axis (predetermined axis), Cn... Corner part (conventional), Cn1... First corner part, Cn2... Second corner part, Cn3... Third corner part, D1, D2... Insert pocket, H... Mounting hole, W cmax ,W pmax ... Maximum width, YA... Rotation direction.

Claims

1. It has a slightly polygonal plate shape, including a first long side and a second long side, and a first short side and a second short side between the first long side and the second long side, and having a first end face and a second end face facing each other. The first end face and the second end face have a first cutting edge extending from the first long side to a first corner portion between the first long side and the first short side, and a second cutting edge extending from the second long side to a second corner portion between the second long side and the first short side. The radius of curvature R of the third corner portion located on the opposite side of the first corner portion across the first long side 3 is larger than the radius of curvature R of the first corner portion 1 by a certain amount Cutting insert.

2. It has short side faces that are connected to the first end face and the second end face and include the first short side and the second short side and face each other. The short side faces are inclined obliquely with respect to a plane perpendicular to the first end face and the second end face. The relief angle on the first short side is a positive angle. The relief angle on the second short side is a negative angle. The cutting insert according to Claim 1.

3. The first cutting edge and the second cutting edge on the first end face and the first cutting edge and the second cutting edge on the second end face are in a relationship of 180° rotational symmetry with respect to each other around a predetermined axis that virtually penetrates the cutting insert. The cutting insert according to Claim 1 or 2.

4. In the relationship of 180° rotational symmetry, the first corner portion on the first end face and the third corner portion on the second end face, and the third corner portion on the first end face and the first corner portion on the second end face are in a positional relationship of facing each other across the short side faces. The cutting insert according to Claim 2.

5. It is provided with a chip breaker including a first chip breaker formed on a rake face related to the first cutting edge and located closer to the first corner portion side than the center of the first long side, and a second chip breaker located closer to the third corner portion side than the center of the first long side. The maximum width W of the second chip breaker cmax is larger than the maximum width W of the first chip breaker pmax and is made larger than that. The cutting insert according to any one of Claims 1 to 4.

6. The maximum width W of the second chip breaker cmax is 50% or less of the length of the first short side, The cutting insert according to Claim 5.

7. A body, A cutting insert attached to the body, And is provided with, The cutting insert is, It has a slightly polygonal plate shape, including a first long side and a second long side, and a first short side and a second short side between the first long side and the second long side, and having a first end face and a second end face facing each other. The first end face and the second end face have a first cutting edge extending from the first long side to a first corner portion between the first long side and the first short side, and a second cutting edge extending from the second long side to a second corner portion between the second long side and the first short side. The radius of curvature R of the third corner portion located on the opposite side of the first corner portion across the first long side 3 is larger than the radius of curvature R of the first corner portion 1 and is made larger Cutting tool.

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