Abstract:
The present invention provides a cutting tool insert capable of realizing a sufficiently large axial rake angle, exhibiting improved cutting ability, and conducting stable processing with little cutting vibrations, and also provides a cutting tool on which this insert is mounted. Specifically, an insert  1  and a cutting tool  100  on which the insert  1  is mounted are provided. The insert  1  is in a substantially plate form and of polygonal shape and is provided with a major cutting edge  16 A (or  16 B) extending from one corner  14 A (or  14 B) toward the other corner  15 A (or  15 B) as viewed from above. The major cutting edge  16 A (or  16 B) comprises a first edge line and a second edge line that meet to form an obtuse angle, which causes the area where the first and second edge lines meet to protrude slightly outwards, as viewed from above. The first edge line slopes down towards the second edge line so as to make the distance with a seat face  11  decrease gradually as viewed from the side, and the first edge line forms an obtuse angle with the second edge line causing the top face area to dip down towards the seat face, and, further, the first edge line is shorter than the second edge line.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an insert mounted on a cutting tool body and also relates to a cutting tool on which this insert is mounted.  
           [0003]    2. Description of the Related Art  
           [0004]    With a conventional cutting tool that uses an insert mounted on its tool body, a polygonal insert has been employed to make effective use of its corners so that as many positions in one insert as possible can be used. When an insert is mounted on a tool body, the insert is positioned so that it forms a positive axial rake angle, that is, a positive angle relative to the rotation axis of the tool body. Typically, when the axial rake angle becomes large, smooth cutting into a work piece can be realized at the time of cutting operations and the insert is able to execute cutting in more than one position, thereby enabling stable processing with little cutting vibrations. However, with the cutting tool on which the polygonal insert is mounted, it has been difficult to realize a large axial rake angle.  
           [0005]    Examples of the polygonal insert are disclosed in, for example, Japanese Patent Laid-Open (Kokai) Publication No. HEI 7-246505, Japanese Patent Laid-Open (Kokai) Publication No. HEI 9-216113, and Japanese Patent (Kohyo) Publication No. HEI 11-513315.  
           [0006]    An insert (throw-away insert) described in the Japanese Patent Laid-Open (Kokai) Publication No. HEI 7-246505 has a structure in which a minor cutting edge is provided at a position closer to one corner along an edge line between two corners, and a major cutting edge is provided that extends towards the other corner so that the major cutting edge forms an obtuse angle with the minor cutting edge. The major cutting edge slopes down from the other corner towards the minor cutting edge so as to make the distance to its seat face decrease gradually, and a minor cutting edge flank, which forms a positive relief angle, is provided on the side face extending from the minor cutting edge. Moreover, a first major cutting edge flank that extends from the major cutting edge is provided at the upper position of the side face extending from the major cutting edge, and this first major cutting edge flank is set to have a smaller relief angle than that of the minor cutting edge flank. Accordingly, it exerts no influence over the ability to cut at a right angle. Particularly, it is possible to improve the cutting blade strength of the major cutting edge portion and to extend the life of the insert chip.  
           [0007]    An insert (throw-away insert) described in the Japanese Patent Laid-Open (Kokai) Publication No. HEI 9-216113 is structured in such a manner that: a relief angle of the insert in a substantially square shape is positive; as viewed from above, a cutting edge on its top face protrudes gradually from the corners on both ends towards the middle part; and, as viewed from the side of the throw-away insert, the cutting edge gradually slopes down towards the middle part from the corners on both ends. The same insert can be used even if the cutting tool is for left-handed use. Moreover, it is possible to finish off the cutting, even without any minor cutting edge, in such a manner that the bottom part of a work piece by making a right angle cut can form an approximate 90-degree angle with the side face of the work piece.  
           [0008]    An insert described in the Japanese Patent (Kohyo) Publication No. HEI 11-513315 comprises a body part in a substantially rectangular column which has a top face, a bottom face, and at least two pairs of adjacent side faces. When viewed from above, a pair of corners along its diagonal line constitutes a cutting edge and the side faces extending respectively from the other pair of corners towards the corners constituting the cutting edges are receptacle surfaces to be received by a cutting tool.  
           [0009]    Concerning the inserts described in the Japanese Patent Laid-Open (Kokai) Publication No. HEI 7-246505 and the Japanese Patent Laid-Open (Kokai) Publication No. HEI 9-216113, it is possible to set the axial rake angle to a larger value than the inclination angle of the major cutting edge. However, if the axial rake angle is set to this large value so as to realize smooth cutting into a work piece at the time of cutting operations, to allow sufficient improvement in cutting performance, and to realize sufficiently stable processing with almost no cutting vibrations, it becomes difficult to cause the envelope of the insert formed during the rotations of the cutting tool to constitute a cylindrical surface. Accordingly, it is still difficult to form a sufficiently large axial rake angle.  
           [0010]    Concerning the insert described in the Japanese Patent (Kohyo) Publication no. HEI 11-513315, since, as viewed from above, a pair of corners along its diagonal line constitutes the cutting edges and the side faces extending respectively from the other pair of corners towards the corners constituting the cutting edges are the receptacle surfaces (referring to the fact that they are not cutting edges), the maximum cutting depth is shallow.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention aims to solve the above-mentioned conventional problems. It is the object of this invention to provide a cutting tool insert capable of forming a cylindrical envelope, realizing a sufficiently large axial rake angle, exhibiting improved cutting performance, and conducting stable processing with little cutting vibrations, and to also provide a cutting tool on which this insert is mounted.  
           [0012]    In order to achieve this object, this invention provides a cutting tool insert in a substantially plate form and of polygonal shape, wherein the edge lines on its top face opposite its seat face are formed as cutting edges, the top surface functions as a tooth face, and the side faces extending from the tooth face towards the seat face are flanks. A major cutting edge is provided that extends from one corner to the other corner as viewed from above. As viewed from above, the major cutting edge comprises a first edge line and a second edge line that meet to form an obtuse angle, which causes the area where the first and second edge line meet to protrude slightly outwards. As viewed from the side, the first edge line slopes down towards the second edge line so as to make the distance to these at face decrease gradually, and forms an obtuse angle with the second edge line causing the top face area to dip down towards the seat face, and, further, the first edge line is shorter than the second edge line.  
           [0013]    Concerning the insert having the above-described structure, since the first edge line slopes down towards the second edge line so as to make the distance to the seat face decrease gradually and the first edge line is shorter than the second edge line, even if the insert is mounted on the cutting tool body at a sufficiently large axial rake angle, it is possible to cause the envelope of the insert, which is formed by rotations of the cutting tool, to constitute a cylindrical surface. Accordingly, it is possible to enable smooth cutting into a work piece at the time of cutting operations, to sufficiently improve cutting performance, and to conduct sufficiently stable processing with almost no cutting vibrations. Moreover, even if the second cutting edge makes contact with the work piece during the cutting operation, the second edge line is available to cut the work piece. Furthermore, it is possible to reduce changes in the relief angle of the first edge line. It is desirable that the first edge line has an arc shape and constitutes a cylindrical envelope.  
           [0014]    The inclination angle of the first edge line with the seat face can be set to 10° or more as viewed from the side. Accordingly, in addition to the above-described advantages, this setting makes it possible to make the angle of the flank smaller and to further improve the cutting blade strength.  
           [0015]    When the insert is mounted on the cutting tool body, the axial rake angle of the major cutting edge is approximately 5° to 15° for a common insert. The axial rake angle of approximately 30°, as in the case of a solid end mill, is not realized regarding any common insert. In order to form an optimal axial rake angle, the insert is inclined to some degree relative to the axial direction and is mounted on the cutting tool body. At this time, if the insert is inclined too much (for example, at 20° or more) relative to the cutting tool body, the strength of the cutting tool body tends to lower. If the inclination angle of the first edge line with the seat face is set to 10° or more as viewed from the side, and even if the insert is inclined and mounted at 20° or less relative to the cutting tool body, it is possible to obtain the desirable axial rake angle of approximately 30° as mentioned above. In other words, if the inclination angle of the first edge line with the seat face is less than 10° as viewed from the side, the strength of the cutting tool body tends to lower.  
           [0016]    The tooth face can include a first edge line tooth face sloping down from the first edge line towards the center of the insert so as to make the distance to the seat face decrease gradually.  
           [0017]    The tooth face can include a second edge line tooth face sloping down from the second edge line to wards the center of the insert so as to make the distance to the seat face decrease gradually.  
           [0018]    A second flank can be provided that extends along the second edge line towards the seat face, and the second flank can form a right angle or an obtuse angle with the seat face.  
           [0019]    The second flank can be a receptacle face to be attached to a cutting tool body. Since the second edge line is longer than the first edge line, the second flank is also made long. Accordingly, when the second flank is used as the receptacle face, it is possible to attach the insert to the cutting tool body more stably.  
           [0020]    Moreover, a first flank can be provided that extends along the first edge line towards the seat face, and the first flank can form a right angle or an obtuse angle with the seat face. This structure allows the first flank not to be in contact with the cutting tool body when the insert is mounted on the cutting tool body, so it is possible to prevent the first flank from being damaged.  
           [0021]    Furthermore, concerning the insert of this invention, as viewed from above, a minor cutting edge can be provided so as to form a corner with the first edge line of the major cutting edge and extend toward the opposite side of the first edge line of the major cutting edge, and the minor cutting edge can form a right angle or an acute angle with the major cutting edge.  
           [0022]    A minor cutting edge flank can be formed on a side face extending from the minor cutting edge, and the minor cutting edge flank can form a positive relief angle with the minor cutting edge.  
           [0023]    Moreover, the tooth face can include a minor tooth face sloping down from the minor cutting edge towards the center of the insert so as to make the distance to the seat face decrease gradually.  
           [0024]    Furthermore, it is possible to employ a structure in which, as viewed from above, the minor cutting edge may form an obtuse angle with an edge line extending towards a corner which is positioned on the minor cutting edge side and which is adjacent to the corner formed with the major cutting edge and the minor cutting edge. When the cutting is performed by using the minor cutting edge, the above-described structure makes it possible to prevent the edge line, which extends towards a corner which is positioned on the minor cutting edge side and which is adjacent to the corner formed with the major cutting edge and the minor cutting edge, from becoming in contact with the work, and to make such edge line function as the receptacle face to be attached to the cutting tool. Therefore, it is possible to prevent the minor cutting edge from being damaged.  
           [0025]    This invention also provides a cutting tool that comprises the cutting tool insert described above and a tool body on which the cutting tool insert is mounted. Concerning the cutting tool having this structure, even if the insert is set at a sufficiently large axial rake angle, it is possible to cause the envelope of the insert, which is formed during the rotations of the cutting tool, to constitute a cylindrical surface. Accordingly, it is possible to realize smooth cutting into a work piece at the time of cutting operations, to sufficiently improve cutting performance, and to conduct sufficiently stable processing with almost no cutting vibrations. Moreover, even if the second edge line makes contact with the work piece during the cutting operations, the second edge line is available to cut the work piece.  
           [0026]    The insert can be mounted on the tool body in a manner such that at least the axial rake angle of the first cutting edge line of the major cutting edge may become a positive angle and a radial rake angle may become a positive angle because of a rake angle of the main cutting edge of this invention.  
           [0027]    Moreover, the major cutting edge positioned on the outer surface of the tool body can be structure in a manner such that a turning radius of the front end of the first edge line relative to the rotation axis of the tool body may be almost the same as a turning radius of the rear end of the first edge line relative to the rotation axis of the tool body.  
           [0028]    Furthermore, the major cutting edge positioned on the outer surface of the tool body can be structured in a manner such that a turning radius of the second edge line relative to the rotation axis of the tool body may be almost the same as or less than the turning radius of the front end of the first edge line. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0029]    [0029]FIG. 1 is a top view of a cutting tool insert according to Embodiment 1 of this invention.  
         [0030]    [0030]FIG. 2 is a side view of the insert of FIG. 1 as seen from the direction indicated by arrow X.  
         [0031]    [0031]FIG. 3 is a side view of the insert of FIG. 1 as seen from the direction indicated by arrow Y.  
         [0032]    [0032]FIG. 4 is a side view of a cutting tool on which the cutting tool inserts are mounted according to Embodiment 1 of this invention.  
         [0033]    [0033]FIG. 5 is a bottom view of the cutting tool shown in FIG. 4.  
         [0034]    [0034]FIG. 6 is an enlarged view of a part of the cutting tool shown in FIG. 4.  
         [0035]    [0035]FIG. 7 is an enlarged view of a part of the cutting tool shown in FIG. 5.  
         [0036]    [0036]FIG. 8 is a top view of a cutting tool insert according to Embodiment 2 of this invention.  
         [0037]    [0037]FIG. 9 is a side view of the insert shown in FIG. 8.  
         [0038]    [0038]FIG. 10 is a side view of a part of the cutting tool on which the cutting tool insert is mounted according to Embodiment 2 of this invention.  
         [0039]    [0039]FIG. 11 is a side view of a part of the cutting tool on which the cutting tool insert is mounted according to Embodiment 2 of this invention.  
         [0040]    [0040]FIG. 12 is a top view of a cutting tool insert according to another embodiment of this invention.  
         [0041]    [0041]FIG. 13 is a sectional view of the cutting tool insert taken on line XIII-XIII of FIG. 12. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]    Explanations are hereinafter given about the cutting tool insert and the cutting tool on which this insert is mounted, according to preferred embodiments of this invention with reference to the attached drawings.  
       Embodiment 1  
       [0043]    [0043]FIG. 1 is a top view of a cutting tool insert according to Embodiment 1 of this invention. FIG. 2 is a side view of the insert of FIG. 1 as seen from the direction indicated by arrow X. FIG. 3 is a side view of the insert of FIG. 1 as seen from the direction indicated by arrow Y. FIG. 4 is a side view of a cutting tool on which the cutting tool inserts are mounted according to Embodiment 1 of this invention. FIG. 5 is a bottom view of the cutting tool shown in FIG. 4. FIG. 6 is an enlarged view of a part of the cutting tool shown in FIG. 4. FIG. 7 is an enlarged view of a part of the cutting tool shown in FIG. 5.  
         [0044]    As shown in FIGS.  1  to  7 , a cutting tool insert  1  according to Embodiment 1 is in a substantially plate form and of substantially parallelogrammic (polygonal) shape as viewed from above. The insert  1  includes a seat face (or bottom face)  11 , a top face  12  opposite the seat face  11 , and side faces  13  extending from the seat face  11  towards the top face  12 . The edge lines on the top face  12  are formed as cutting edges, the top surface  12  functions as a tooth face, and the side faces  13  are flanks.  
         [0045]    On this insert  1 , a major cutting edge  16 A is formed that extends from one corner  14 A to the other corner  15 A adjacent to the corner  14 A as viewed from above (see FIG. 1). Likewise, a major cutting edge l 6 B is formed that extends from one corner  14 B to the other corner  15 B. Also, a minor cutting edge  19 A is formed on one side of the corner  14 A positioned opposite the major cutting edge  16 A side, and a minor cutting edge  19 B is formed on one side of the corner  14 B positioned opposite the major cutting edge  16 B side.  
         [0046]    The major cutting edge  16 A (or  16 B) includes a first major cutting edge  17 A (or  17 B), which is a first edge line, and a second major cutting edge  18 A (or  18 B), which is a second edge line that meets with the first edge line to form an obtuse angle α, the area around which protrudes slightly outwards, as viewed from above. As viewed from the side (see FIG. 2), this first major cutting edge (or first edge line)  17 A (or  17 B) slopes down towards the second major cutting edge (or second edge line)  18 A (or  18 B) so as to make the distance to the seat face  11  decrease gradually. The first major cutting edge  17 A (or  17 B) forms an obtuse angle β with the second major cutting edge  18 A (or  18 B) causing the top face area to dip down towards the seat face  11 , and the first major cutting edge  17 A (or  17 B) is made shorter than the second major cutting edge  18 A (or  18 B) Moreover, an inclination angle γ of the first major cutting edge  17 A (or  17 B) with the seat face  11  is set to 10° or more (about 20° in Embodiment 1) as viewed from the side.  
         [0047]    The minor cutting edge  19 A (or  19 B) is provided in a manner such that it forms an approximately 90-degree angle with the first major cutting edge  17 A (or  17 B). As viewed from above, the minor cutting edge  19 A forms an obtuse angle θ with an edge line  28 A extending from the minor cutting edge  19 A towards the corner  15 B, and the minor cutting edge  19 B forms the obtuse angle θ with an edge line  28 B extending from the minor cutting edge  19 B towards the corner  15 A. According to Embodiment 1, when the insert  1  is mounted on a cutting tool body  100 , the side face  13  extending from the edge line  28 A (or  28 B) towards the seat face  11  becomes a receptacle face to be attached to the cutting tool body  100 .  
         [0048]    The tooth face formed by the top face  12  of the insert  1  includes: a first tooth face  21 A (or  21 B) sloping down from the first major cutting edge  17 A (or  17 B) towards the center of the insert  1  so as to make the distance to the seat face  11  decrease gradually as viewed from the side; a second tooth face  22 A (or  22 B) sloping down from the second major cutting edge  18 A (or  18 B) towards the center of the insert  1  so as to make the distance to the seat face  11  decrease gradually as viewed from the side; and a minor cutting edge tooth face  26 A (or  26 B) sloping down from the minor cutting edge  19 A (or  19 B) towards the center of the insert  1  so as to make the distance to the seat face  11  decrease gradually.  
         [0049]    The flanks formed by the side faces  13  of the insert  1  includes: a first flank  23 A (or  23 B) extending from the first major cutting edge  17 A (or  17 B) towards the seat face  11 ; a second flank  24 A (or  24 B) extending from the second major cutting edge  18 A (or  18 B) towards the seat face  11 ; and a minor cutting edge flank  27 A (or  27 B) extending from the minor cutting edge  19 A (or  19 B) towards the seat face  11 . The first flank  23 A (or  23 B) forms an approximately 90-degree angle with the seat face  11 . Moreover, the second flank  24 A (or  24 B) forms an approximately 90-degree angle with the seat face  11  and functions as the receptacle face to be attached to the cutting tool body  100 . The minor cutting edge flank  27 A (or  27 B) is provided so as to form a positive relief angle λ (see FIG. 6) with the minor cutting edge  19 A (or  19 B).  
         [0050]    In the substantially center part of the top face  12  of the insert  1 , a screw hole  29  which extends from the top face  12  to the seat face  11  is made in order to fasten the insert  1  to the cutting tool body  100  with a screw.  
         [0051]    The cutting tool body  100 , on which the above-described insert  1  is mounted, has a reference rotation axis A (see FIG. 4) and includes a cutting head  101  on which a plurality of inserts  1  (eight pieces in Embodiment 1) are mounted. In the cutting head  101 , pockets  102  for receiving the respective inserts  1  are formed at evenly spaced positions. Each pocket  102  has: a support face  104  for supporting the seat face  11  of the insert  1  to be retained within the pocket  102  by a screw  103 ; a support face  105  for supporting the second flank  24 A (or  24 B); and a support face  106  for supporting the side face  13  extending from the edge line  28 A (or  28 B) towards the seat face  11 .  
         [0052]    A cutting tool  10  with the inserts  1  of Embodiment 1 mounted on its cutting tool body  100  is structured in the following manner: concerning the major cutting edge  16 A (or  16 B) positioned on the outer surface of the cutting tool body  100 , a turning radius of the front end of the first major cutting edge  17 A (or  17 B) relative to the rotation axis A of the cutting tool body  100  (see FIG. 4), is almost the same as a turning radius of the rear end of the first major cutting edge  17 A (or  17 B) relative to the rotation axis A. Moreover, a turning radius of the second major cutting edge  18 A (or  18 B) relative to the rotation axis A is almost the same as or less than the turning radius of the front end of the first major cutting edge  17 A (or  17 B)  
         [0053]    Furthermore, since the insert  1  is formed in the shape described above, even if the insert  1  is set at a sufficiently large axial rake angle Ax (see FIG. 6), it is possible to cause the envelope of the insert  1  formed during the rotations of the cutting tool  10  to constitute a cylindrical surface. It is also possible to form a radial rake angle Rd (the angle formed between the envelope and the major cutting edge tooth face) of the first major cutting edge  17 A (or  17 B) as a positive angle. Accordingly, it is possible to realize smooth cutting into a work piece at the time of cutting operations, to sufficiently improve the cutting performance, and to conduct sufficiently stable processing with almost no cutting vibrations. It is also possible to reduce changes in the relief angle of the first major cutting edge  17 A (or  17 B) Moreover, even if the second major cutting edge  18 A (or  18 B) makes contact with a work piece during the cutting operations, the second major cutting edge  18 A (or  18 B) is available to cut the work piece.  
         [0054]    The second flank  24 A and  24 B can be provided with a large area. Since these portions become the receptacle faces to be attached to the cutting tool body  100 , it is possible to enhance attachment stability of the cutting tool body  100 .  
         [0055]    Moreover, since the first flank  23 A (or  23 B) does not become the receptacle face to be attached to the cutting tool body  100 , it is possible to prevent the first flank  23 A and  23 B from being damaged, thereby enabling excellent cutting.  
         [0056]    According to Embodiment 1, the inclination angle γ of the first major cutting edge  17 A (or  17 B) with the seat face  11  is set to approximately 20° as viewed from the side. The invention is not limited to this setting, however it is desirable that the inclination angle γ be 10° or more.  
         [0057]    According to Embodiment 1, the minor cutting edge  19 A (or  19 B) is set so as to form a substantially right angle with the first major cutting edge  17 A (or  17 B). The invention is not limited to this setting, however the minor cutting edge  19 A (or  19 B) may be set to form an acute angle with the first major cutting edge  17 A (or  17 B).  
         [0058]    According to Embodiment 1, the first flank  23 A (or  23 B) is set so as to form an approximately 90-degree angle with the seat face  11 . The invention is not limited to this setting, however the first flank  23 A (or  23 B) may be set to form an obtuse angle with the seat face  11 .  
         [0059]    Moreover, according to Embodiment 1, the second flank  24 A (or  24 B) is set so as to form a substantially right angle with the seat face  11 . The invention is not limited to this setting, however the second flank  24 A (or  24 B) may be set to form an obtuse angle with the seat face  11 .  
         [0060]    Furthermore, according to Embodiment 1, as viewed from the side as in FIGS. 2 and 3, the tooth face is composed of flat planes. The invention is not limited to this structure, however, the tooth face may be composed of a curved surface as shown in FIGS. 12 and 13.  
       Embodiment 2  
       [0061]    Explanations are hereinafter given about the cutting tool insert and the cutting tool on which this insert is mounted according to Embodiment 2 of this invention, with reference to the attached drawings.  
         [0062]    [0062]FIG. 8 is a top view of a cutting tool insert according to Embodiment 2 of this invention. FIG. 9 is a side view of the insert shown in FIG. 8. FIGS. 10 and 11 are side views of a part of the cutting tool on which the cutting tool insert is mounted according to Embodiment 2 of this invention.  
         [0063]    Concerning Embodiment 2, members similar to those described in Embodiment 1 are given the same reference numerals as in Embodiment 1, and any detailed description thereof is omitted.  
         [0064]    The main difference between an insert  2  of Embodiment 2 and the insert  1  of Embodiment 1 is that the insert  2  is in a substantially triangular shape as viewed from above. Specifically, as shown in FIGS. 8 through 11, the insert  2  is a substantially plate form of substantially triangular (polygonal) shape as viewed from above. Three corners of the insert  2  can be used for cutting operations. The insert  2  includes a seat face (or bottom face)  11 , a top face  12  opposite the seat face  11 , and side faces  13  extending from the seat face  11  towards the top face  12 . The edge lines on the top face  12  are formed as cutting edges, the top surface  12  functions as a tooth face, and the side faces  13  are flanks.  
         [0065]    On this insert  2 , a major cutting edge  56 A is formed that extends from one corner  51  to the other corner  52  adjacent to the corner  51  as viewed from above (see FIG. 8). Likewise, a major cutting edge  56 B is formed that extends from one corner  52  to the other corner  53 , and a major cutting edge  56 C is formed that extends from the corner  53  to the corner  51 . Also, a minor cutting edge  59 A is formed on one side of the corner  51  positioned opposite the major cutting edge  56 A side, and a minor cutting edge  59 B is formed on one side of the corner  52  positioned opposite the major cutting edge  56 B side, and a minor cutting edge  59 C is formed on one side of the corner  53  positioned opposite the major cutting edge  56 C side.  
         [0066]    The major cutting edge  56 A (or  56 B or  56 C) includes a first major cutting edge  57 A (or  57 B or  58 C), which is a first edge line, and a second major cutting edge  58 A (or  58 B or  58 C) which is a second edge line that meets with a first edge line to form an obtuse angle α, the area around which protrudes slightly outwards as viewed from above. As viewed from the side (see FIG. 9), this first major cutting edge (or first edge line)  57 A (or  57 B or  57 C) slopes down towards the second major cutting edge (or second edge line)  58 A (or  58 B or  58 C) so as to make the distance to the seat face  11  decrease gradually. The first major cutting edge  57 A (or  57 B or  57 C) forms an obtuse angle β with the second major cutting edge  58 A (or  58 B or  58 C) causing the top face area to dip down towards the seat face  11 , and the first major cutting edge  57 A (or  57 B or  57 C) is made shorter than the second major cutting edge  58 A (or  58 B or  58 C). Moreover, an inclination angle γ of the first major cutting edge  57 A (or  57 B or  57 C) with the seat face  11  is set to 10° or more (about 20° in Embodiment 2) as viewed from the side.  
         [0067]    The minor cutting edge  59 A (or  59 B or  59 C) is provided in a manner such that it forms an acute angle with the first major cutting edge  57 A (or  57 B or  57 C) as viewed from above. As viewed from above, the minor cutting edge  59 A forms an obtuse angle θ with the second major cutting edge  58 C, and the minor cutting edge  59 B forms the obtuse angle θ with the second major cutting edge  58 A, and the minor cutting edge  59 C forms the obtuse angle θ with the second major cutting edge  58 B.  
         [0068]    According to Embodiment 2, when the insert  2  is mounted on a cutting tool body  200 , the side face  13  extending towards the seat face  11  from the remaining second major cutting edges (for example,  58 B are  58 C) that are not involved in the cutting operations, that is, second flanks (for example,  64 B and  64 C) described later in detail, become receptacle faces to be attached to the cutting tool body  200 .  
         [0069]    The tooth face formed by the top face  12  of the insert  2  includes: a first tooth face  61 A (or  61 B or  61 C) sloping down from the first major cutting edge  57 A (or  57 B or  57 C) towards the center of the insert  2  so as to make the distance to the seat face  11  decrease gradually as viewed from the side; a second tooth face  62 A (or  62 B or  62 C) sloping down from the second major cutting edge  58 A (or  58 B or  58 C) towards the center of the insert  2  so as to make the distance to the seat face  11  decrease gradually as viewed from the side; and a minor cutting edge tooth face  66 A (or  66 B or  66 C) sloping down from the minor cutting edge  59 A (or  59 B or  59 C) towards the center of the insert  2  so as to make the distance to the seat face  11  decrease gradually.  
         [0070]    The flanks formed by the side faces  13  of the insert  2  includes: a first flank  63 A (or  63 B or  63 C) extending from the first major cutting edge  57 A (or  57 B or  57 C) towards the seat face  11 ; a second flank  64 A (or  64 B or  64 C) extending from the second major cutting edge  58 A (or  58 B or  58 C) towards the seat face  11 ; and a minor cutting edge flank  67 A (or  67 B or  67 C) extending from the minor cutting edge  59 A (or  59 B or  59 C) towards the seat face  11 . The first flank  63 A (or  63 B or  63 C) forms an obtuse angle with the seat face  11 . Moreover, the second flank  64 A (or  64 B or  64 C) forms an obtuse angle with the seat face  11 . The minor cutting edge flank  67 A (or  67 B or  67 C) is provided so as to form apositive relief angle λ (see FIG. 11) with the minor cutting edge  59 A (or  59 B or  59 C).  
         [0071]    In the substantially center part of the top face  12  of the insert  2 , a screw hole  29  is made in the same manner as in Embodiment 1.  
         [0072]    The cutting tool body  200 , on which the above-described insert  2  is mounted, has a structure substantially pursuant to that of the cutting tool body  100  as described in Embodiment 1, has a reference rotation axis A (see FIG. 10), and includes a cutting head  101  on which a plurality of inserts  2  are mounted. In the cutting head  101 , pockets  102  for receiving the respective inserts  2  are formed. Each pocket  102  has: a support face  104  for supporting the seat face  11  of the insert  2 ; a support face  105  for supporting the second flank  64 B (or  64 A or  64 C); and a support face  106  for supporting the second flank  64 C (or  64 A or  64 B).  
         [0073]    A cutting tool  20  with the inserts  2  of Embodiment 2 mounted on its cutting tool body  200  is structured in the following manner: concerning the major cutting edge  56 A (or  56 B or  56 C) positioned on the outer surface of the cutting tool body  200 , a turning radius of the front end of the first major cutting edge  57 A (or  57 B or  57 C) relative to the rotation axis A of the cutting tool body  200  (see FIG. 10), is almost the same as a turning radius of the rear end of the first major cutting edge  57 A (or  57 B or  57 C) relative to the rotation axis A. Moreover, a turning radius of the second major cutting edge  58 A (or  58 B or  58 C) relative to the rotation axis A is almost the same as or less than the turning radius of the front end of the first major cutting edge  57 A (or  57 B or  57 C).  
         [0074]    Furthermore, since the insert  2  is formed in the shape described above, even if the insert  2  is set at a sufficiently large axial rake angle Ax (see FIG. 11), it is possible to cause the envelope of the insert  2  formed during the rotations of the cutting tool  20  to constitute a cylindrical surface. As in the case of Embodiment 1, it is also possible to form a radial rake angle Rd of the first major cutting edge  57 A (or  57 B or  57 C) as a positive angle. Accordingly, it is possible to realize smooth cutting into a work piece at the time of cutting operations, to sufficiently improve the cutting performance, and to conduct sufficiently stable processing with almost no cutting vibrations. It is also possible to reduce changes in the relief angle of the first major cutting edge  57 A (or  57 B or  57 C). Moreover, even if the second major cutting edge  58 A (or  58 B or  58 C) makes contact with the work piece during the cutting operations, the second major cutting edge  58 A (or  58 B or  58 C) is available to cut the work piece.  
         [0075]    The second flank  64 A,  64 B and  64 C can be provided with a large area. Since these portions become the receptacle faces to be attached to the cutting tool body  200 , it is possible to enhance attachment stability to the cutting tool body  200 .  
         [0076]    Moreover, since the first flank  63 A (or  63 B or  63 C) does not become the receptacle face to be attached to the cutting tool body  200 , it is possible to prevent the first flank  63 A,  63 B, and  63 C from being damaged, thereby enabling excellent cutting.  
         [0077]    According to Embodiment 2, the inclination angle γ of the first major cutting edge  57 A (or  57 B or  57 C) with the seat face  11  is set to approximately 20° as viewed from the side. The invention is not limited to this setting, however it is desirable that the inclination angle γ be 10° or more.  
         [0078]    According to Embodiment 2, the minor cutting edge  59 A (or  59 B or  59 C) is set so as to form an acute angle with the first major cutting edge  57 A (or  57 B or  57 C). The invention is not limited to this setting, however the minor cutting edge  59 A (or  59 B or  59 C) may be set to form an approximately 90 degree angle with the first major cutting edge  57 A (or  57 B or  57 C).  
         [0079]    According to Embodiment 2, the first flank  63 A (or  63 B or  63 C) is set so as to form an obtuse angle with the seat face  11 . However, without limitation to such setting, the first flank  63 A (or  63 B or  63 C) may be set to form a substantially right angle with the seat face  11 .  
         [0080]    Moreover, according to Embodiment 2, the second flank  64 A (or  64 B or  64 C) is set so as to form an obtuse angle with the seat face  11 . The invention is not limited to this setting, however the second flank  64 A (or  64 B or  64 C) may be set to form an approximately 90-degree angle with the seat face  11 .  
         [0081]    As described above, even if the insert of this invention is mounted at a sufficiently large axial rake angle on the cutting tool body, it is possible to cause the envelope of the insert, which is formed during the rotations of the cutting tool, to constitute a cylindrical surface. Accordingly, the cutting tool on which this insert is mounted can allow the axial rake angle to be sufficiently large. As a result, it is possible to realize smooth cutting into a work piece at the time of cutting operations, to sufficiently improve the cutting performance, and to conduct sufficiently stable processing with almost no cutting vibrations. Moreover, even if the second edge line makes contact with the work piece during the cutting operations, the second edge line is available to cut the work piece. Furthermore, it is possible to reduce changes in the relief angle of the first edge line.