Abstract: A preform element, suitable for use as a cutting element on a rotary drill bit, includes a polycrystalline diamond facing table bonded to a tungsten carbide substrate, the rear surface of the facing table being formed with an array of elongate ribs which project from the facing table into the substrate. The array of ribs includes a plurality of pairs of ribs which extend inwardly away from the periphery of the facing table, each pair of ribs being in a generally V-shaped formation so that the ribs diverge as they extend inwardly away from said periphery. Adjacent pairs of ribs may intersect to provide a generally star-shaped formation.

Abstract: A preform element includes a facing table of super hard material having a from face, an outer peripheral surface, and a rear surface bonded to a substrate which is less hard than the super hard material. The facing table comprising a peripheral region surrounding an inner region, the peripheral region having an inner surface which is inclined at an angle of greater than 90.degree. to the rear surface of the facing table so as to face in a direction having a component rearwardly away from the facing table as well as inwardly towards said inner region. The inner region of the facing table may be formed with projections extending into the substrate.

Abstract: A rotatable pressure seal between a relatively rotatable shaft and body structure comprises two annular sealing discs concentric with the shaft, one disc being mounted on the body structure and the other disc being carried on the shaft the discs having engaging sealing faces formed of polycrystalline diamond or other superhard material. The pressure seal is particularly for use in a modulated bias unit, for controlling the direction of drilling of a rotary drill bit when drilling boreholes in subsurface formations. The bias unit comprises a number of hydraulic actuators spaced apart around the periphery of the unit, and a selector control valve modulates the fluid pressure supplied to each actuator in synchronism with rotation of the drill bit so that, as the drill bit rotates, a thrust member of each actuator is displaced outwardly at the same selected rotational position so as to bias the drill bit laterally and thus control the direction of drilling.

Abstract: A preform element, such as a cutting element for a drag-type drill bit, includes a facing table of superhard material having a front cutting face, a peripheral surface, and a rear surface bonded to a substrate which is less hard than the superhard material. The rear surface of the facing table is integrally formed with a plurality of protuberances, such as ribs or studs, which project into the substrate. The depth to which the protuberances project into the substrate varies in irregular manner across the facing table, so as to inhibit the propagation of cracks across the substrate to the rear of the facing table.

Abstract: A preform element, such as a cutting element for a drag-type drill bit, includes a facing table of superhard material having a front face, a peripheral surface, and a rear surface bonded to a substrate which is less hard than the superhard material. The rear surface of the facing table is integrally formed with a plurality of ribs which project into the substrate and extend in directions outwardly away from an inner area of the facing table towards the peripheral surface thereof.

Abstract: A modulated bias unit, for controlling the direction of drilling of a rotary drill bit when drilling boreholes in subsurface formations, comprises a number of hydraulic actuators spaced apart around the periphery of the unit. Each actuator comprises a movable thrust member which is hydraulically displaceable outwardly and a formation-engaging pad which overlies the thrust member and is mounted on the body structure for pivotal movement about a pivot axis located to one side of the thrust member. A selector control valve modulates the fluid pressure supplied to each actuator in synchronism with rotation of the drill bit so that, as the drill bit rotates, each pad is displaced outwardly at the same selected rotational position so as to bias the drill bit laterally and thus control the direction of drilling. The pivot axis of the formation-engaging member is inclined to the longitudinal axis of rotation of the bias unit so as to compensate for tilting of the bias unit in the borehole during operation.

Abstract: A modulated bias unit, for controlling the direction of drilling of a rotary drill bit when drilling boreholes in subsurface formations, comprises a number of hydraulic actuators spaced apart around the periphery of the unit. Each actuator comprises a movable thrust member which is hydraulically displaceable outwardly and a pivotally mounted formation-engaging pad which overlies the thrust member. An inlet passage supplies fluid under pressure to the chamber, and an outlet passage delivers fluid from the chamber to a lower pressure zone. A selector control valve modulates the fluid pressure supplied to each actuator in synchronism with rotation of the drill bit so that, as the drill bit rotates, each pad is displaced outwardly at the same selected rotational position so as to bias the drill bit laterally and thus control the direction of drilling.

Abstract: A modulated bias unit, for controlling the direction of drilling of a rotary drill bit when drilling boreholes in subsurface formations, comprises a number of hydraulic actuators spaced apart around the periphery of the unit, each having a movable thrust member which is hydraulically displaceable outwardly for engagement with the formation of the borehole being drilled. A selector control valve modulates the fluid pressure supplied to each actuator in synchronism with rotation of the drill bit so that, as the drill bit rotates, each movable thrust member is displaced outwardly at the same selected rotational position so as to bias the drill bit laterally and thus control the direction of drilling. The control valve is a disc valve comprising two relatively rotating elements having contiguous surfaces formed of polycrystalline diamond.

Abstract: A method of forming a cutter assembly for a rotary drill bit comprises locating in a mould a preform polycrystalline diamond cutting element of a non-thermally stable type, packing powdered matrix-forming material, such as powdered tungsten carbide, around at least part of the cutting element within the mould, and then infiltrating the powdered material with a metal alloy in a furnace to form a body of solid infiltrated matrix in which the cutting element is at least partly embedded. The metal alloy is selected to provide an infiltration temperature, for example of up to about 850.degree., which is not greater than the temperature at which significant thermal degradation of the cutting element would occur.

Abstract: A cutting element for a rotary drill bit comprises a thin cutting table of polycrystalline diamond having a front cutting face and a rear surface bonded to a tungsten carbide substrate. The substrate incorporates one or more thin barrier layers which extend at least partly through the thickness of the substrate in a direction transverse to its front surface. Each barrier layer is formed of a material, such as nickel, which is more ductile than the material of the substrate, but is metallurgically compatible with it.

Abstract: A drill bit has a plurality of radial blades on its leading face which define fluid channels leading to junk slots in the gauge region, cutting elements being mounted along each blade. At least two of the channels are in communication with one another at their inner ends and there is provided, at the communicating inner ends of the channels, an inner nozzle which is angled to direct the majority of drilling fluid emerging from the nozzle outwardly along one of the channels. An outer nozzle is located in the other channel, adjacent its junk slot, and is angled so that the majority of fluid emerging from the outer nozzle flows inwardly along said other channel and towards the inner nozzle.

Abstract: A system for maintaining a downhole instrumentation package in a roll stabilised orientation with respect to a drill string comprises an instrument carrier which is mounted within a drill collar for rotation about the longitudinal axis of the collar. An impeller is mounted on the instrument carrier so as to rotate the carrier relatively to the drill collar as a result of the flow of drilling fluid along the drill collar during drilling. The torque transmitted to the instrument carrier is controlled, in response to signals from sensors in the carrier which respond to the rotational orientation of the carrier, and input signals indicating the required roll angle of the carrier, so as to rotate the carrier in the opposite direction to the drill collar and at the same speed, so as to maintain the carrier non-rotating in space and hence roll stabilised.