PURPOSE: To facilitate abrasion of an edge tip by combining four cutter edges in a cross shape, each of which has a circular recess on an inner circular surface of a rotating shaft hole and has a cross-shaped recessed or projected groove outside the shaft hole.

CONSTITUTION: An end and side cutting edge 1 is divided into four pieces. A circular recessed shape is formed on an inner circular surface 7 of the rotating shaft. A circular projected shape is formed on another inner circular surface. A recessed shape 8 and a projected shape 3 are formed in a cross shape. The four cutter edges 9, 9 and 10, 10 are combined in a cross shape to be formed into the side cutting edge 1. In this manner, the combination cutter edge 1 can be decomposed to pieces at the time of abrasion and, thus, easily and efficiently abraded.