ROTARY DRILL BITS

摘要

"Improvements in or relating to rotary drill bits" A rotary drill bit comprises a bit body having a shank for connection to a drill string and a passage for supplying drilling fluid to the face of the bit, which carries a plurality of polycrystalline diamond preform cutting elements . The cutting elements on one side of a diameter of the bit have positive side rake and the cutting elements on the other side of the diameter have negative side rake so that the vectorial sum of the reaction forces between the formation being drilled and the cutting elements provides a resultant lateral imbalance force acting on the bit body as it rotates in use. The gauge of the bit body includes low friction bearing pads so located as to transmit the resultant lateral force to the sides of the borehole. Since the bearing pads are of low friction, they slide around the surface of the formation and any tendency for bit whirl to be initiated is reduced. The lateral imbalance force may also be provided by varying the back rake of the cutting elements on different parts of the bit, or by including an asymmetrical mass of material in the bit body.