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 plurality of preform primary cutting elements mounted on the bit body and defining a primary cutting profile having a downwardly convex nose portion. There are associated with at least certain of the primary cutting elements respective secondary elements which are spaced inwardly of the primary profile. The distance of the secondary elements from the primary profile, when measured in a direction perpendicular to said profile, is generally greater for secondary elements nearer the nose portion than it is for secondary elements further away from the nose portion, and is preferably such that the vertical distance of the secondary elements from the profile is substantially constant.

Abstract: A rotary drill bit comprises a bit body carrying a plurality of preform cutting elements, and a plurality of circumferentially spaced gauge pads which, in use, engage the surrounding formation. Some of the gauge pads carry cutting elements and others are free of cutting elements. To reduce cost, each gauge pad which is free of cutting elements has an outer bearing surface which is more abrasion resistant than the outer surfaces of the gauge pads which carry cutting elements.

Abstract: A preform cutting element for a rotary drill bit for use in drilling or coring holes in subsurface formations includes a cutting table 12, 13 of superhard material such as polycrystalline diamond. The rear face of the cutting table is bonded to a substrate 11 of less hard material, such as cemented tungsten carbide. A front portion 12 of the cutting table comprises a form of superhard material which is less wear-resistant than the superhard material forming the remainder 13 of the cutting table. The provision of the less wear-resistant superhard material at the front cutting faces reduces the tendency of the cutting table to spall.

Abstract: A preform cutting element for a rotary drill bit includes a layer of polycrystalline diamond material, the rear face of which is bonded to the front face of a backing layer of less hard material, such as tungsten carbide. The rear face of the backing layer is substantially greater in area than the rear face of the diamond layer. A method of manufacturing such cutting elements comprises forming an intermediate structure comprising polycrystalline diamond material bonded between two outer layers of tungsten carbide, and then cutting the intermediate structure to provide two pieces, each of the pieces including part of the diamond material and at least a major part of a respective one of the two outer layers.

Abstract: A preform cutting element for a rotary drill bit includes a layer of polycrystalline diamond material, the rear face of which is bonded to the front face of a backing layer of less hard material, such as tungsten carbide. The rear face of the backing layer is substantially greater in area than the rear face of the diamond layer. A method of manufacturing such cutting elements comprises forming an intermediate structure comprising polycrystalline diamond material bonded between two outer layers of tungsten carbide, and then cutting the intermediate structure to provide two pieces, each of the pieces including part of the diamond material and at least a major part of a respective one of the two outer layers.

Abstract: A cutting element for a rotary drill bit comprises a layer of polycrystalline diamond material having a front face and a rear face, a backing layer of less hard material bonded to the rear face of the diamond layer, and a front layer of less hard material bonded to the front face of the diamond layer and extending across at least the major part thereof. The front layer is thinner than the backing layer, at least in the vicinity of the cutting edge of the cutting element.

Abstract: A rotary drill bit for use in drilling or coring deep holes in subsurface formations comprises a bit body having a leading face and a gauge region, cutting elements mounted at the leading face of the bit body, and a passage in the bit body communicating with nozzles in the leading face of the bit body for supplying drilling fluid to the face for cooling and cleaning the cutting elements. Certain of the cutting elements each comprise a preform cutting element having a superhard front cutting face providing a cutting edge, and others of the cutting elements each comprise particles of hard material embedded in a front layer of a less hard material to form a cutting layer having a cutting edge.