PURPOSE: To wear a base metal flank in the same degree as in the case of a cutting tip and to keep a relief angle almost constant by forming the base metal flank on the circumference of a base metal continuing to the rear of the rotational direction of a tip flank of each cutting tip, into a irregular form.

CONSTITUTION: A lot of mounting groove 3 are formed on the circumference of a base metal 1, and a blade edge 5 is formed in a nose edge of each cutting tip 4. In the rotational direction of the blade edge 5 of each cutting tip 4, a rake face 6 is formed in the front of X and a tip flank 7 in the rear, respectively, and each tip flank 7 is crossed at a specified flank α to a first virtual circle A passing through all blade edges 5. A base metal flank 8 formed on the circumference of a base metal 1 continuing to the rear of X in the rotational direction of the tip flank 7 is formed into an irregular form provided with two almost circular recess parts 9 separating from each other and three projecting parts 10 adjoining before and behind these recess parts 9. Thus, the base metal flank is worn out at the same degree as the cutting tip, and the relief angle is kept almost constant, through which cutting performance is favorably maintainable.