Cutting insert and indexable rotary cutting tool

A cutting insert that is detachably mounted in an mounting seat formed in a front end portion of a tool main body that is rotated around a central axis, the cutting insert comprising:

• a plate-shaped insert main body;
• and
• a cutting edge portion formed in the insert main body,
• wherein the cutting edge portion includes
• a bottom cutting edge that is disposed in a front end portion of the insert main body in the central axis direction, extends along a radial direction that is orthogonal to the central axis, and forms an arc shape that protrudes toward a front end side in the central axis direction,
• a rake face of the bottom cutting edge,
• an outer peripheral cutting edge that is disposed in an outer end portion of the insert main body in the radial direction, extends along the central axis direction, and forms an arc shape that protrudes toward an outside in the radial direction,
• a rake face of the outer peripheral cutting edge,
• a cutting edge of corner R that connects an outer end of the bottom cutting edge in the radial direction and a front end of the outer peripheral cutting edge in the central axis direction and forms an arc shape that protrudes toward a front end outer circumferential side, and
• a rake face of the cutting edge of corner R,
• wherein, a curvature radius of the bottom cutting edge and a curvature radius of the outer peripheral cutting edge are equal to each other,
• the curvature radius of the bottom cutting edge and the curvature radius of the outer peripheral cutting edge is set to be larger than ½ of a maximum diameter of a rotation trajectory of the cutting edge portion respectively, and
• a curvature radius of the cutting edge of corner R is set to be smaller than ½ of the maximum diameter of the rotation trajectory of the cutting edge portion,
• when the rake face of the cutting edge of corner R is seen as a front surface,
• a straight line that passes through a front end of the bottom cutting edge in the central axis direction and extends in the radial direction is considered as a radial-direction reference line,
• a straight line that passes through an outer end of the outer peripheral cutting edge in the radial direction and extends in the central axis direction is considered as a central axis-direction reference line,
• a tangent line at a boundary point between the cutting edge of corner R and the bottom cutting edge is considered as a first tangent line,
• a tangent line at a boundary point between the cutting edge of corner R and the outer peripheral cutting edge is considered as a second tangent line,
• a rectangular triangle formed by connecting an intersection point between the first tangent line and the radial-direction reference line, an intersection point between the first tangent line and the central axis-direction reference line, and an intersection point between the radial-direction reference line and the central axis-direction reference line together using straight lines is considered as a first rectangular triangle, and
• a rectangular triangle formed by connecting an intersection point between the second tangent line and the central axis-direction reference line, an intersection point between the second tangent line and the radial-direction reference line, and the intersection point between the radial-direction reference line and the central axis-direction reference line together using straight lines is considered as a second rectangular triangle,
• the first rectangular triangle and the second rectangular triangle are congruent with each other.