A cutting insert based on a non-limiting aspect has a first member and a second member joined to the first member. The second member has an upper surface and a side surface adjacent to the upper surface. The upper surface includes a first side, a second side and a corner. The upper surface has a first convex portion extending toward the corner, and a second convex portion extending from the first convex portion toward the first side. The second convex portion has a first end portion and a second end portion located closer to the corner than the first end portion. A first length from the first end to the first side is less than a second length from the second end portion to the first side in a top view.

A cubic boron nitride polycrystal includes cubic boron nitride, the cubic boron nitride having an average grain size of not more than 150 nm, the cubic boron nitride polycrystal having a crack generation load of not less than 25 N in a breaking strength test in which an R200 m diamond indenter is used to apply a load at a rate of 100 N/min.

A cubic boron nitride polycrystal includes cubic boron nitride, the cubic boron nitride having an average grain size of not more than 150 nm, the cubic boron nitride polycrystal having a crack generation load of not less than 25 N in a breaking strength test in which an R200 m diamond indenter is used to apply a load at a rate of 100 N/min.

In a general aspect, a machining tool includes a diamond crystal having a working section configured to machine a workpiece. The machining tool includes a body and a shank extending from the body. The shank defines a cylindrical slot outside the body. The machining tool includes an assembly carried by the shank. The assembly includes a cylindrical pin that resides in the cylindrical slot. The assembly also includes a diamond crystal secured to an end of the cylindrical pin. The diamond crystal has a curved perimeter that defines a working section, which contacts a workpiece during operation of the machining tool. The shank is adapted to allow rotation of the cylindrical pin within the cylindrical slot to modify the working section.

In a general aspect, a machining tool includes a diamond crystal having a working section configured to machine a workpiece. The machining tool includes a body and a shank extending from the body. The shank defines a cylindrical slot outside the body. The machining tool includes an assembly carried by the shank. The assembly includes a cylindrical pin that resides in the cylindrical slot. The assembly also includes a diamond crystal secured to an end of the cylindrical pin. The diamond crystal has a curved perimeter that defines a working section, which contacts a workpiece during operation of the machining tool. The shank is adapted to allow rotation of the cylindrical pin within the cylindrical slot to modify the working section.

A turning tool includes: a holder portion; and a cutting edge portion fixed to the holder portion, wherein the cutting edge portion is composed of a synthetic single-crystal diamond, and the cutting edge portion has a nose curvature having a curvature radius of more than or equal to 0.1 mm and less than or equal to 1.2 mm, and the nose curvature satisfies at least one of a condition that a direction of a line of intersection between the rake face and a bisecting cross section of a vertex angle of the nose curvature is within ?10? relative to a <110> direction of the synthetic single-crystal diamond, and a condition that the direction of the line of intersection between the rake face and the bisecting cross section of the vertex angle of the nose curvature is within ?10? relative to a <100> direction of the synthetic single-crystal diamond.

A turning tool includes: a holder portion; and a cutting edge portion fixed to the holder portion, wherein the cutting edge portion is composed of a synthetic single-crystal diamond, and the cutting edge portion has a nose curvature having a curvature radius of more than or equal to 0.1 mm and less than or equal to 1.2 mm, and the nose curvature satisfies at least one of a condition that a direction of a line of intersection between the rake face and a bisecting cross section of a vertex angle of the nose curvature is within ?10? relative to a <110> direction of the synthetic single-crystal diamond, and a condition that the direction of the line of intersection between the rake face and the bisecting cross section of the vertex angle of the nose curvature is within ?10? relative to a <100> direction of the synthetic single-crystal diamond.

A diamond coated tool includes a base material and a diamond layer provided on the base material. The diamond layer has a boron content of 1?10.sup.3 ppma or more and 1?10.sup.6 ppma or less and an oxygen content of 1?10.sup.2 ppma or more and 1?10.sup.5 ppma or less in a first region surrounded by a surface of the diamond layer and a first imaginary plane located at a distance of 1 ?m from the surface in a thickness direction.

A diamond coated tool includes a base material and a diamond layer provided on the base material. The diamond layer has a boron content of 1?10.sup.3 ppma or more and 1?10.sup.6 ppma or less and an oxygen content of 1?10.sup.2 ppma or more and 1?10.sup.5 ppma or less in a first region surrounded by a surface of the diamond layer and a first imaginary plane located at a distance of 1 ?m from the surface in a thickness direction.

A cutting insert may include a first surface, a second surface, a third surface and a cutting edge. The first surface may include a corner, a first side, a first inclined surface located along the corner, a second inclined surface located further inside than the first inclined surface, a third inclined surface located along the first side, a fourth inclined surface located further inside the first surface than the third inclined surface, and a protruded part located further inside than the second inclined surface and the fourth inclined surface. An inclination angle of the protruded part may be smaller than an inclination angle of the second inclined surface in a cross section along a bisector of the corner. The inclination angle of the protruded part may be larger than a third inclination angle in a cross section orthogonal to the first side.