| 摘要 |
PURPOSE:To improve the cutting performance of the center part of a drill by forming the central cutting edge and the external cutting edge contained therefrom in the point symmetry with each other with respect to the end part of the drill, forming the above cutting edge in the approximately straight line and connecting those cutting edges at an obtuse angle with each other from the bottom view. CONSTITUTION:The central cutting edge 21 and the external cutting edge 22 are formed in an approximately straight line in the bottom view of a drill and those cutting edges 21 and 22 are connected at obtuse angle with each other. The end part of a chip discharge groove 3 is a rake face 26 and the crossing face thereof with an escape face 23 forms a land face 24 at a relatively moderate slope. A face 4 curved in the direction of gaining the rake face 25 in area is formed at the part at which the land part 24 is connected to the rake face 25 of the cutting edge 21, and the cutting edge 21 and the rake face 25 are formed on the face approximately parallel to the axial direction of the drill. In this drill, the part 12 of chip generated by the cutting edge 21 is extended along the face 4 curved from the face 25, is discharged properly and has the high cutting performance. |
