| 摘要 |
A twist drill wherein each of the chisel edges at the drill point is provided with a specific twisted wall surface functioning as the positive rake surfacethereof and efficient in cutting. The surface is defined as an additional interior helical groove united to the original helical groove so as to extend over the full length of the drill body and to have the same lead of helix as the original helical groove of the drill body. It is preferable that the intersecting corners of the lips forming the main cutting edge and the chisel edge be chamfered or rounded off. |
