Cutting insert and indexable insert-type cutting tool

A cutting insert which is attached in a detachable manner to an insert seat of an indexable insert-type cutting tool, the cutting insert comprises:

• a polygonal plate-like insert main body having a pair of polygonal faces and a plurality of side faces arranged at the periphery thereof,
• wherein a cutting edge is formed on a side ridge portion of the polygonal face, and the insert main body is formed in the shape of having front-back inversion symmetry with respect to the pair of polygonal faces,
• of the pair of polygonal faces, when one polygonal face is used as a rake face, the other polygonal face is used as a seating face which is seated on a base of the insert seat,
• a pair of contact surfaces disposed along each of a pair of imaginary flat faces of the cutting insert are formed on each of the pair of polygonal faces,
• the pair of contact surfaces are allowed to be individually in contact with a pair of contacted surfaces formed on the base of the insert seat, when the polygonal face on which the pair of contact surfaces are formed is used as the seating face,
• the pair of imaginary flat faces of the cutting insert on which the pair of contact surfaces are disposed individually include one imaginary straight line of the cutting insert for the polygonal face extending on the polygonal face on which the pair of contact surfaces are formed and are also arranged in a direction intersecting with each other in an X-letter shape, with setting the imaginary straight line of the cutting insert as an intersection point, when seen in a direction along the imaginary straight line of the cutting insert,
• on the insert main body, there is formed an attachment hole at the center of the insert central line orthogonal to the imaginary straight lines of the cutting insert of the pair of polygonal faces, the insert central line passing through the centers of the pair of the polygonal faces, and
• of the side faces of the insert main body, on one of the side faces with respect to one of the polygonal faces which faces one direction of two directions at which the imaginary straight line of the cutting insert of the pair of contact surfaces extends, there is formed a contact portion which is allowed to be in contact with a contacted portion formed on one wall face rising to the base of the insert seat.