Abstract: The purpose is to improve the surface roughness Rz of a cut surface to 1.6 &mgr;m or under in cutting e.g. hardened steel. The ball end mill has a cutting blade made of a polycrystalline hard sintered material containing 20 to 95 vol % of cubic boron nitride. The cutting blade has a ridge portion having a radius of curvature of 5 to 30 &mgr;m. A flank and a rake face or a negative land of the cutting blade connect smoothly with the ridge with the radius of curvature. The ridge portion of the cutting blade has a surface roughness of 0.1 to 1.0 &mgr;m in terms of 10-point average roughness (Rz).

Abstract: The purpose is to improve the surface roughness Rz of a cut surface to 1.6 &mgr;m or under in cutting e.g. hardened steel. The ball end mill has a cutting blade made of a polycrystalline hard sintered material containing 20 to 95 vol % of cubic boron nitride. The cutting blade has a ridge portion having a radius of curvature of 5 to 30 &mgr;m. A flank and a rake face or a negative land of the cutting blade connect smoothly with the ridge with the radius of curvature. The ridge portion of the cutting blade has a surface roughness of 0.1 to 1.0 &mgr;m in terms of 10-point average roughness (Rz).

Abstract: A ball nose end mill includes a holder body with a first half and a second half of a tip end thereof divided by a slot, a plate-shaped insert in the slot, a clamp screw with a screw shaft screwed into the second half and a screw head bearing against the first half, a tapered ring fitted around the screw shaft and seated in a tapered hole in the insert, and a spring between the screw head and the tapered ring pressing the ring against the insert. The insert is firmly pressed against the bottom of the slot, and the screw is frictionally secured.

Abstract: A drill has a pair of cutting edges (4) which are arranged at positions that are substantially uniformly circumferentially spaced about its rotation axis. Each of the cutting edges (4) has a first linear cutting region (4a) and a second linear cutting region (4b) which are aligned with each other on the same straight line, and a curved cutting region (4c) which is located between the first and second linear cutting regions (4a, 4b). The curved cutting region (4c) has a third linear cutting region (4e, 4f) which extends in a direction intersecting with the second linear cutting region (4b) at a prescribed angle. With this structure, it is possible to prevent chip loading etc. which may be caused during perforation drilling operations. In a throw-away tipped drill, a notch (118a) is formed in a received portion of an insert (110) which is mounted on a shank (130). A holding portion (131a, 131b, 133a, 133b) of the shank (130) corresponding to the notch (118a) is provided with a through screw hole (138).

Abstract: A drill bit structure has a drill bit body 4 and a plate-like hard tip member 9. The cutting end of the drill bit body 4 has a groove 7 extending substantially at right angles to the central drill bit axis, and the hard tip 9 is fitted and fixed in the groove 7. The web thickness is 25-35% of the drill bit diameter and the flute width ratio is 0.4:1 to 0.8:1. The rake angle of each cutting lip 10 measured at positions spaced at least 2/3 of the drill bit diameter radially outward from the drill bit center is in the range of -5.degree. to positive values.