Abstract: A rotary cutting tool, e.g. a drill bit, has a generally cylindrical working head consisting of ultrahard incisive material such as cubic polycrystalline diamond or boron nitride. The working head has at least peripheral cutting edges which may extend onto an adjoining carrier of hard metal secured thereto along a transverse interface; the carrier may be bonded at another junction to a tool body or may itself constitute such a body. Also disclosed is a method of making such a tool, particularly a drill with a working head of 5 mm or less, according to which a prefabricated block, consisting of interconnected ultrahard and metallic blanks for the head and its carrier, is soldered inside an aligning sleeve onto a cylindrical extension of the tool body whereupon the extension and the block are jointly machined to form the tool.

Abstract: A multiple-toothed drill bit having at least three teeth is provided with front end cutting edges having respective outer and inner cutting edge sections oriented angularly to each other and meeting at respective frontwardmost cutting points located in a common plane, and with an axial centering spur having laterally outwardly and rearwardly inclined center cutting edges merging with the inner front end cutting edge sections via respective frontwardly concave cutting edge sections.

Abstract: A drill bit of the multigroove type has a shaft with at least three flutes forming respective lateral cutting edges and as many generally radial frontal ridges, one of them extending to or possibly beyond the shaft axis while the others terminate short of same. Each frontal ridge forms an outer and an inner cutting edge adjoining each other at an obtuse angle in a forwardly pointing peak, these peaks lying at different distances from the axis. The peak closest to the axis is axially foremost and the corresponding inner cutting edge includes the smallest angle with the axis to act as a centering blade; with more than three ridges this centering blade can be duplicated in diametrically opposite positions. The outer cutting edges all lie on a common conical surface centered on the axis.