Indexable insert

1. An indexable insert prepared by bonding a superhard sintered body containing cubic boron nitride to at least the upper surface of an apical angle part of a tool body having a polygonal shape in top view and forming an edge and a chip breaker on said superhard sintered body, wherein

a chamfer is formed on the intersection between the upper surface and the side surface of said superhard sintered body,

said chip breaker has a substantially symmetrical shape with respect to a section bisecting the apical angle of said apical angle part, and has a protrusion and a flat part provided between said protrusion and said apical angle part,

a pair of ridges reaching said chamfer, the pair of ridges are on the apex of said protrusion and are linear or arcuate,

θ is in the range of 6/10×α≦θ≦90−1/10×α assuming that θ represents an angle formed by the linear ridges of said protrusion or a tangential line for said arcuate ridges at a point bisecting the arcuate ridges of said protrusion and a bisector for said apical angle and α represents said apical angle, and

a line segmental ratio is in the range of 0.9≦L 1 ′/L 1 ≦1.1 assuming that L 1 represents the length of a line segment connecting a first intersection between said pair of ridges on the apex of said protrusion and an extreme point of first said ridge and L 1 ′ represents the length of a line segment connecting an extreme point of second said ridge and said first intersection, and 0.2≦L 1 /L 2 0.8 assuming that L 2 represents the length of a line segment connecting a second intersection between a straight line passing through the extreme point of said first ridge from said first intersection and the outer periphery of said tool body and said first intersection.

2. The indexable insert according to claim 1 , wherein the surface roughness of said flat part of the superhard sintered body and said chamfer adjacent to said flat part is at least 0.1 μm and less than 0.5 μm in ten point height of irregularities (Rz JIS 94).

3. The indexable insert according to claim 1 , wherein an angle formed by said chamfer and the upper surface of said tool body is in the range of at least 15° and not more than 45°.

4. The indexable insert according to claim 1 , wherein the width of said chamfer on the forward end of said apical angle part is in the range of at least 0.02 mm and not more than 0.2 mm in top view.

5. The indexable insert according to claim 1 , wherein the distance between the forward end of said apical angle part and said first intersection is in the range of at least 0.1 mm and not more than 2 mm in top view.

6. The indexable insert according to claim 1 , wherein the difference between the heights of the forward end of said apical angle part and said first intersection is in the range of at least 0.02 mm and not more than 0.5 mm.

7. The indexable insert according to claim 1 , wherein the shear angle of said flat part is 0°.

8. The indexable insert according to claim 1 , wherein a coating layer consisting of at least one element selected from a group consisting of the elements belonging to the groups 4a, 5a and 6a of the periodic table, Al, Si and B, a nitride, a carbide or an oxide of at least one metal selected from said group or a solid solution thereof is formed on the surface of said superhard sintered body.

9. The indexable insert according to claim 1 , wherein the surface roughness of said flat part of said superhard sintered body formed with a coating layer on the surface thereof and said chamfer adjacent to said flat part is at least 0.1 μm and not more than 1.0 μm in ten point height of irregularities (Rz JIS 94).