| 摘要 |
Significantly improved resistance to breaking while drilling in metal can be realized in twist drill bits having a web thickness taper which is about twice the web thickness taper specified in Standards and in common use in the industry. This creates much shallower flutes and flies in the face of conventional wisdom in the art which holds that the web taper rate should be low to maximize the amount of flute volume available to convey chi ps and sawdust out of the hole. A flute length of about 1/2" less than the flute lengths specified by the Standards makes it possible to have such a large taper rate but still contro l variations in tolerances for the flute length. It also provides a marking zone for placing size indicia on the drill bit shank outside the portion which is gripped by the chuck jaws of a power drill. The use of the new web taper rate, in conjunction with modifications to the tip geometry of a self-centering drill bit with pilot tip, produces a drill bit optimized both for metal and wood drilling which is strong enough to withstand the forces exerted on the bit while drilling in metal using a hand-held power drill, yet which need not be retracted from a 2 X 4 to cle ar sawdust prior to drilling a hole completely through the thickness of the wood. |
