Rotary drill bits

摘要

A rotary drill bit for drilling holes in subsurface formations comprises a bit body having a shank for connection to a drill string, a number of circumferentially spaced blades on the bit body each extending outwardly away from the central axis of rotation of the bit, a number of cutters mounted side-by-side along each blade, and a passage in the bit body for supplying drilling fluid to the surface of the bit for cleaning and cooling the cutters. The blades comprise alternating primary and secondary blades. The cutters on the primary blades are primary cutters which are located at different radial distances from the bit axis so as to define a cutting profile which, in use, covers substantially the whole of the bottom of the bore hole being drilled. At least the majority of the cutters on the secondary blades are secondary cutters each of which is located at substantially the same radial distance from the bit axis as an associated primary cutter on the preceding primary blade.