Cutting insert

A cutting insert which has an approximately polygonal plate-like shape and is used for external machining and in which cutting edges are formed at respective corners of the polygonal cutting insert in the same shape and in rotational symmetry about a center axis of the polygonal cutting insert with their rake faces present on respective outer peripheral faces of the polygonal cutting insert, characterized in that:

• each of the cutting edges includes a side cutting edge formed along an intersection ridge between one polygonal face and the outer peripheral face between two adjacent corners, and an end cutting edge formed at the corresponding corner in such a manner as to form an acute angle with the side cutting edge with a nose intervening therebetween;
• the outer peripheral face between the two adjacent corners includes a rake face of a certain cutting edge associated with the outer peripheral face and a flank of the end cutting edge of another, adjacent cutting edge, and the rake face of the certain cutting edge has a negative side rake angle such that the rake face slopes downward toward the side cutting edge and upward toward a side opposite the side cutting edge between the end cutting edge of the certain cutting edge and the end cutting edge of the other, adjacent cutting edge located rearward of the end cutting edge of the certain cutting edge;
• the flank of the end cutting edge of the other, adjacent cutting edge has a positive slope in opposition to the slope having the negative side rake angle such that the flank slopes downward toward a side opposite the side cutting edge;
• and
• a breaker groove is formed on the rake face of the certain cutting edge such that the breaker groove extends rearward along the side cutting edge from a front end of the side cutting edge and cuts into the positively sloped flank of the end cutting edge of the other, adjacent cutting edge, thereby dividing the rake face into front and rear portions.