Throwaway insert and throwaway-type rotary cutting tool

A throwaway insert comprising:

• a insert body which defines a substantially polygonal flat plate shape;
• a rake face formed on one polygonal surface of the insert body;
• a corner cutting edge formed at a corner of the rake face and having a substantial arcuate shape from a top view facing the one polygonal surface of the insert body;
• a main cutting edge formed at a side ridge of the rake face connected to one end of the corner cutting edge;
• and
• a relief face constituted by a side face of the insert body, wherein
• the throwaway insert is to mount on a tool body which is rotated about an axis such that the rake face faces forward in a rotating direction of a tool, the corner cutting edge is positioned at an outer periphery of a distal end of the tool body, and the main cutting edge extends rearward in the rotating direction of the tool body as the main cutting edge approaches the rear end of the tool body at the outer periphery of the tool body,
• the rake face connected to the main cutting edge comprises:
• an inclined face which is inclined inward toward the other polygonal surface of the insert body serving as a seat face to seat on the tool body,
• an inclination angle of the rake face with respect to the seat face gradually decreases from one end of the main cutting edge connected to the one end of the corner cutting edge toward the other end thereof, and a rake angle which is formed by the rake face with the insert mounted to the tool body gradually decreases and then gradually increases from one end of the main cutting edge toward the other end thereof,
• the main cutting edge is formed in a shape of a convexly curved line which protrudes toward the outside of the rake face from a top view and a shape of another convexly curved line which extends toward the seat face and convexly curved from the one end of the main cutting edge to the other end thereof from a side view facing the relief face, and
• when the insert is mounted on the tool body, a projection line of a rotation locus of the main cutting edge about the axis forms a shape of a convex arc which smoothly comes, at a middle portion of the main cutting edge, into contact with a cylindrical surface having its center line corresponding to the axis and which gradually retreats inward from the cylindrical surface as it approaches both ends of the main cutting edge.