Abstract: An insert rotary cutter for processing a metal workpiece includes a cutter body of a generally circular cross-section having an axis of rotation therethrough. The cutter includes one or more cutter inserts releasably mounted on a periphery of the cutter body. The insert includes a generally quadrilateral plate defined by a front face, a rear face disposed generally parallel to the front face and four side faces. The insert has a pair of main cutting edges defined by the front face and one pair of the opposite side faces. The pair of opposite side faces serve as respective rake surfaces for the main cutting edges. Each of the rake surfaces is convexly curved in such a manner that the width of the insert between the rake surfaces is larger at a central portion of each main cutting edge than at opposite ends of the main cutting edge.