Indexable rotary cutting tool and insert used therein

An indexable rotary cutting tool having inserts detachably attached to a tool holder,

• (a) said insert comprising an upper surface, a lower surface opposing said upper surface and coming into contact with a seat surface of said tool holder, side surfaces connecting said upper surface to said lower surface, a center hole penetrating said insert from said upper surface to said lower surface for receiving a fastening screw, and cutting edges formed along ridgelines of said upper surface and said side surfaces;
• said insert having in a polygonal shape and being rotationally symmetric around a center axis P of said center hole, with corner portions having angles of 90° or less;
• each of said cutting edges being constituted by a curved corner cutting edge formed in each corner portion, and a bottom cutting edge and a peripheral cutting edge formed along ridgelines on both sides of said corner cutting edge;
• a flank of said bottom cutting edge and a flank of said peripheral cutting edge having different clearance angles;
• a flank of said corner cutting edge having an angle-changing surface portion having a clearance angle continuously changing from a clearance angle of the flank of said bottom cutting edge to a clearance angle of the flank of said peripheral cutting edge;
• and
• a ridgeline of said upper surface extending from an upper end of said peripheral cutting edge being provided with a receding portion determining one end of said peripheral cutting edge;
• and
• (b) in a state where said inserts are attached to said tool holder,
• each of said bottom cutting edges and said corner cutting edges having a negative radial rake angle;
• each of said peripheral cutting edges having a radial rake angle changing from negative to positive;
• and
• the length of a region having a positive radial rake angle in said peripheral cutting edge being 50% or more of the entire length of said peripheral cutting edge.