Cutting insert and tool holder on which cutting insert is provided

A cutting insert to be provided on a tool holder, comprising:

• a circular upper surface, a circular lower surface, a cylindrical lateral surface connecting the upper surface and the lower surface, an upper cutting edge formed between the upper surface and the lateral surface, a lower cutting edge formed between the lower surface and the lateral surface, and a plurality of rotation-preventing parts provided on the lateral surface to prevent rotation with respect to the tool holder;
• a plurality of holder contact surfaces formed on the lateral surface to contact the tool holder;
• a first relief surface formed on the lateral surface;
• and
• a second relief surface formed on the lateral surface,
• wherein each of the plurality of rotation-preventing parts comprises,
• an inner curved surface recessed toward a center of the lateral surface and convexly rounded so as to form concentric circles in conjunction with the lateral surface, and
• a rotation-preventing surface of a stepped shape, parallel to an axis direction of the insert and connecting the inner curved surface and the lateral surface,
• wherein the first relief surface is formed from the upper cutting edge to the plurality of rotation-preventing parts and the plurality of holder contact surfaces to thus configure an overall upper circumferential surface of the lateral surface, and has a positive type in which a diameter gradually decreases from the upper cutting edge toward the lower surface,
• wherein the second relief surface is formed from the lower cutting edge to the plurality of rotation-preventing parts and the plurality of holder contact surfaces to thus configure an overall lower circumferential surface of the lateral surface, and has a positive type in which a diameter gradually decreases from the lower cutting edge toward the upper surface,
• wherein the rotation-preventing surface is a plane, and
• wherein the rotation-preventing surface is formed perpendicularly to the inner curved surface.