Throwaway insert and throwaway-type cutter

摘要

A rhombic cutting insert (1) includes a pair of cutting edges (6) merging with each other at a nose part (8) disposed at one of diagonal acute-angle corners of an upper face (3) of the insert (1). The pair of cutting edges (6) are individually formed in convex-curved shapes and are inclined to a mounting face of the insert (1) from the nose part (8) toward portions away from the nose part (8) of the respective cutting edges (6). The pair of cutting edges (6) individually include linear sub-cutting-edges (6a) disposed in the vicinity of the nose part (8), the sub-cutting-edges (6a) forming a substantially right angle therebetween. Rake faces (9) formed on the upper face (3) individually have positive rake angles and have twisted-inclined surfaces so that the rake faces (9) become closer to the mounting face along the respective cutting edges (6) from the nose part (8) toward a region disposed along a diagonal line between obtuse-angle corners (Q). The insert (1) is mounted to a tool part (21) so as to dispose the nose part (8) at the periphery of an end of the tool part (21). One of the pair of cutting edges (6) serves as a peripheral cutting edge (6A), a sub-cutting-edge (6Aa) of the peripheral cutting edge (6A) being disposed substantially parallel to an axis (O) of the tool part (21), and the other one of the pair of cutting edges (6) serves as a facing cutting edge (6B), a sub-cutting-edge (6Ba) of the facing cutting edge (6B) being disposed substantially perpendicular to the axis (O).