| 摘要 |
PROBLEM TO BE SOLVED: To quickly carry out the formation of recessed parts by angularly disposing to each other the parts of the subsidiary cutting edges of a plurality of continuous cutting edges. SOLUTION: A subsidiary cutting edge element 5a' merges with an adjacent major cutting edge 4a via a rounded nose portion 6a and defines a first acute angleθ1 lying between 50 deg. to 70 deg. with the major cutting edge 4a. A subsidiary cutting edge element 5a" merges with a major cutting edge 4b to define an acute angleθ2 lying between 20 deg. to 40 deg. therewith. In the same way, a subsidiary cutting edge element 5b' merges with the major cutting edge 4b via a rounded nose portion 6b, and defines an acute angleθ1 therewith. On the other hand, a subsidiary cutting edge element 5b" merges with the major cutting edge 4a and defines an acute angleθ2 (20 deg. to 40 deg.) therewith. Further, the projected lengths L1 , L2 of the subsidiary cutting edge elements 5a', 5a", 5b', and 5b" of the respective subsidiary cutting edges 5a, 5b are preferably equal to 0.6L. Thereby, the drilling of a required depth, or the number of times of milling work in the transverse direction in which the number of formation of recessed parts is fewer can be achieved. |
