Stepped drill construction.

摘要

A drill bit has a series of cylindrical surfaces of increasing diameter from the outer end to the inner end of the drill bit. A conical surface interconnects each pair of adjacent cylindrical surfaces. Two flutes are machined at circumferentially spaced locations to provide cutting edges (20) and (22) on each cylindrical surface (14) and corresponding cutting edges on the conical surfaces. The cutting edges of each cylindrical surface have a raised portion b which is relieved over the angle c, the remainder of the surface over the angle r being cylindrical. The cutting edges on the conical surfaces similarly have a raised portion with a back relief. The flutes are preferably helical or incline to the drill axis.