| 摘要 |
The drill thins towards its tip and has helicoidal grooves round it. The value of its cutting angle ( alpha ), between the tangent at the cutting edge and the perpendicular to this edge, is greater than, or equal to, zero. The clearance angle ( beta ) of the drill, between the plane of the cutting surface and the tangent to the non-active surface of the cutting edge, is greater than zero. The cutting edge is slightly thinned to form a slightly rounded edge. The tip can be bevelled and have a flat or concave cutting edge. |
