Throwaway tip
 A throwaway cutting tip (31) has two substantially polygonal main surfaces (32, 33) and cutting edges (42, 43, 44, 45, 49, 50) in the form of a curved line projecting inwardly in the direction of the thickness of the tip. The cutting edges extend between nose portions (38, 39, 40, 41) positioned at apexes of the polygon. The main surface has a flat central portion (47) in the middle. A rake surface having two portions extends between the flat central portion and the cutting edge. The rake surface (51) has a flat portion (56) and a curved, concave portion (53). The curved concave surface portion (53) connecting the lowermost edge (52) of the flat rake surface portion to the flat central portion functions as a chip breaking surface. The chip breaking is enhanced by a plurality of projections (55) arranged along the curved concave surface portion. Each nose portion has a nose protecting surface (38a, 39a, 40a, 41a) extending parallel to the flat central portion inwardly from an end edge defined between lateral surfaces (34, 35, 36, 37) and the main surface. 
Sumitomo Electric Industries, Ltd.
Y Y B
Y10T Y10T B23B
additional additional inventional
Yoshikatsu
Mori