STEPPED DRILL

PROBLEM TO BE SOLVED: To solve the problem that the inner peripheral face of a recess is fractured by extended chips when a stepped hole is formed in the recess, because a rotational speed is low at the time of boring. SOLUTION: The tip of a tool body 1 is made as a small-diameter part 2 and its base end as a large-diameter part 3. Chip discharging grooves 4, 4a are formed on the outer peripheral face of the tool body 1, and second chip discharging grooves 5, 5a are formed at positions adjacent to the chip discharging grooves 4, 4a so as to form small-diameter lands 6, 6a and large-diameter lands 7, 7a. A gap 10 is formed at the boundary of main flanks 8, 8a at the tip center of the tool body 1 so as not to apply a large thrust load to the tip of the tool body 1. Leading edges 12, 12a of the small diameter lands 6, 6a are continuously formed by extending to boundaries of the chip discharging grooves 4, 4a and the second chip discharging grooves 5, 5a so that the chips extending at the time of boring are cut by the leading edges 12, 12a on sides of the small diameter lands 6, 6a, and are not brought into contact with the inner peripheral face of a recess H. COPYRIGHT: (C)2004,JPO