An insert cutter includes a body having at least one generally radially outwardly-opening recess disposed adjacent a forward end face thereof. A cutter insert is received in the recess. The insert is a polygonal shape having a front face, a rear face and side faces. Each corner portion where adjacent side faces intersect each other is removed to provide first and second corner faces. The insert has main cutting edges each defined by the front face and a respective one of the side faces, auxiliary cutting edges each defined by the front face and a respective one of the first corner faces, and corner cutting edges each defined by the front face and a respective one of the second corner faces. Each side face serves as a flank of a respective one of the main cutting edges with a positive clearance angle A. Each first corner face serves as a flank of a respective one of the auxiliary cutting edges with a positive clearance angle B. Each second corner face serves as a flank of a respective one of the corner cutting edges with a positive clearance angle C. The angles A, B and C are so selected as to satisfy ABC. The disposition of the insert is such that one set of the main, auxiliary and corner cutting edges are indexed in their machining positions, and such that an effective clearance angle of the indexed corner cutting edge with respect to a workpiece is generally equal to an effective clearance angle of the indexed main cutting edge with respect to the workpiece.
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