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Abstract

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 extending outwardly away from the central axis of rotation of the bit, and a number of cutters mounted side-by-side along the leading edge of each blade. The cutters include primary cutters having cutting edges which define a primary cutting profile and secondary cutters having cutting edges which define a secondary cutting profile which is disposed inwardly of the primary cutting profile with respect to the bit body. The secondary cutters may be on different blades from the primary cutters, or may be mounted on the same blades, rearwardly of the primary cutters. At least the majority of the cutters are located at different distances away from the bit axis, so that as the bit rotates each secondary cutter follows a path which lies intermediate the paths followed by two primary cutters.

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First Claim