Abstract: A drill comprising a shank part (11), which is adapted to be received in a drill holder of a drilling machine, and an opposite cutting end part (13). An intermediate fluted part (12) is provided, which defines at least one helical flute or channel (16) having a bottom surface (19) and a pair of opposite side surfaces (20) defined by the ribs or lands (17). The shape of a cross-section of the at least one flute or channel, taken at right angles to the course of the flute or channel, is such that the distance from a reference plane (24) to the bottom surface or the flute is decreasing from a central part (22) of the bottom surface towards the adjacent side surface (20) of the flute. The reference plane is at right angles to a radial line (25) extending from the longitudinal axis of the drill and through the centre of the bottom surface width. A helically extending groove (21) is formed in the middle of the central part (22) of the bottom surface (19).

Abstract: An apparatus including a bore bit comprising a bottom part having an active bottom face and a top face substantially parallel to the active face; a top part fixed to suspension cables and having a bottom face facing the top face of the bottom part; a member for limiting the maximum spacing between the two parts; a cylindrical axial bore passing through both parts; a cylindrical guide penetrating into said bore; and at least one ramp-forming recess and at least one ramp-forming extension for co-operating with said ramp-forming recess in such a manner that movement of the top part towards the bottom part is converted into rotary movement of said top part about the vertical axis, with the end of said rotary movement of the top part imparting rotary movement to the bottom part.

Abstract: A bit head retaining system includes a bit head having an upper end and a working face, and a retention sleeve adapted to be connected to an end of a drill casing and having an internal opening in which at least a portion of the bit head above the working face is disposed, the bit head being secured to the retention sleeve so as to be axially immovable relative to the retention sleeve. A method of installing a bit head in a percussion drill is also disclosed.

Abstract: A percussion drill bit is provided that has at least two different pluralities of inserts extending from the bit head to better match the conditions seen on the bit head during drilling. The preferred embodiment has a plurality of large inserts with a polycrystalline diamond layer located at least in the gage row and a plurality of smaller inserts located at least in the inner rows.

Abstract: A rock drill for rotary hammers is proposed in which a conveying helix (3) having main and secondary webs (6 to 9) is used. A one-piece carbide crown, with its wing-shaped main and secondary lips (24 to 27), is butt-welded to the conveying helix.

Abstract: A rotary drill bit for drilling subterranean formations configured with at least one protective structure proximate to the rotationally leading and trailing edges of a gage trimmer, wherein the at least one protective structure is positioned at substantially the same exposure as its associated gage trimmer. Particularly, the apparatus of the present invention may provide protection for gage trimmers during drilling, tripping, and/or rotation within a casing; i.e., when changing a drilling fluid. Protective structures may be configured and located according to anticipated drilling conditions including helix angles. In addition, a protective structure may be proximate to more than one gage trimmer while having a substantially equal exposure to each associated gage trimmer. Methods of use and a method of rotary bit design are also disclosed.

Abstract: A bit head retaining system includes a bit head having an upper end and a working face, and a retention sleeve adapted to be connected to an end of a drill casing and having an internal opening in which at least a portion of the bit head above the working face is disposed, the bit head being secured to the retention sleeve so as to be axially immovable relative to the retention sleeve. A method of method of installing a bit head in a percussion drill is also disclosed.

Abstract: A drive steel member for driving engagement with a rotary drilling machine, including a chuck adapter having main body and chuck seating means and being constructed and arranged in fixed, sealed relation on one end of the drive steel member.

Abstract: A drill crown (1) used for drilling purposes, especially for rotary percussion drilling of a mouth of a blast furnace. The drill crown (1) consists of a drilling head (4) provided with a plurality of hard material or hard metal inserts (6) and a base body (2) for connection to a drive element etc. in order to drive or impinge upon said drill crown. The hard material or hard metal inserts (6) are received in bores or openings (5) of the drill head (4). According to the invention, at least one hard material or hard metal insert (6) is provided with an at least partially curved or domed surface contour (11) on the end which is received in the drill head (4).

Abstract: A percussion apparatus and method of using the percussion apparatus. The apparatus may be used for delivering an impact to a tubular string. The apparatus comprises a cylindrical member having an internal bore containing an anvil and a first guide profile. The apparatus further includes a rotor disposed within the internal bore, and wherein the rotor member comprises a body having an outer circumference with a second guide profile thereon, and wherein the rotor contains a radial hammer face. In a first position, the second external guide profile of the rotor will engage with the first guide profile of the cylindrical member so that the radial hammer face can contact the anvil. In a second position, the second guide profile of the rotor will engage with the first guide profile of the cylindrical member so that the radial hammer face is separated from the anvil shoulder. Multiple rotors and multiple stators may be employed. The rotor may be operatively associated with a stator that directs flow into the rotor.

Abstract: A drill bit (1) comprises, on its face, either only curved cutting edges (2) or a combination consisting of at least one straight cutting edge (2′) and at least two curved cutting edges (2). The cutting edges (2,2′) on the face of the drill bit (1) are arranged such that they start from the axis (4) of the drill bit (1) or end directly adjacent thereto. Three channels (10) start from a blind hole (5) serving to attach the drill bit (1) to an anchor rod of an injection boring anchor, and their openings (11) are not located in the area of the face of the drill bit (1) but are situated in the area of the lateral surface (13) of the same, said lateral surface conically expanding toward the cutting edges (2, 2′). The openings (11) of the channels (10) are placed in the area of concave recesses (12) of the lateral surface (13) of the drill bit (1) in such a manner that they are recessed with regard to the periphery of the face of the drill bit (1).

Abstract: Apparatus is provided for introducing a consistent series of small and localized rotary impacts to a PDC bit during drilling, to improve PDC drill bit performance. Rotary impact supplements the nominal torque supplied by the rotary drive thereby avoiding lockup and potentially damaging energy storage in the drill string following windup, should the bit slow or hang up when drilling in difficult formations. The apparatus comprises a rotary hammer which is rotated about a bit shaft's anvil, preferably by a drilling fluid driven turbine. As the hammer rotates, potential energy is built up. When the hammer and anvil connect, the energy is released into the bit shaft and thus into the bit, increases its instantaneous torque and allows it to more effectively cut through difficult formations.

Abstract: The present invention relates to a downhole-hammer drill-bit assembly for removable attachment to a substantially tubular casing of a downhole hammer. The drill-bit assembly has a substantially tubular chuck, a drill-bit, and a substantially tubular retainer. The chuck has a step. The drill-bit has a bit shank, a bit shoulder extending from the shank and a bit head. A 6first portion of the bit shank is in a bore of the chuck. The retainer has a split retainer sidewall, a first end and a second end. The first end has a split retainer collar. The second end has a split retainer ring. The split retainer sidewall is around a portion of the chuck and the bit shoulder. The split retainer collar is around a first surface of the step. The split retainer ring is around a portion of the bit shank between the bit shoulder and the bit head.

Abstract: A concrete drill for a rotary and percussively driven hand tool machine with a cutting part (1) and a striking part (2) that is axially and limited displaceable relative to the cutting part (1). Each of the cutting part (1) and the striking part (2) have a respective facial surface work area for machining a bore hole surface. The respective facial surface work area of the cutting part (1) and the striking part (2) radially overlap upon rotation of the concrete drill.

Abstract: A rock drill bit for percussive drilling, especially top hammer drilling, includes a drill head provided with rock removing members. The rock drill bit is provided with a number of external grooves for conducting cuttings, the grooves extending in the axial direction of the rock drill bit. Formed between adjacent grooves are lands which also extend in the axial direction of the rock drill bit. The rock drill bit has a thread for connection of the rock drill bit to a drill rod. Rear end portions of at least some of the lands define retrac teeth. Each retrac tooth has at least one cutting edge. At least some of the lands comprise a chip surface and a clearance surface that form therebetween an edge angle not greater than 100°.

Abstract: The invention relates to a method for horizontal ground drilling, in particular for rock drilling, by means of a ground drilling apparatus which can be driven forward in rotation and percussively, the apparatus having compressed air and drilling medium applied to it alternately or simultaneously, and the compressed air and the drilling medium being supplied via one channel.

Abstract: A male portion for percussive rock drilling includes sections of reduced cross-sectional area. At least one thread for percussive rock drilling is provided at an end of the male portion. The end of the male portion comprises an abutment surface for the transfer of impact waves. The length of the portion is defined as the length of an imaginary cylinder from a plane of the impact surface to a point where the plane ceases to contact a crest of the thread, wherein the length divided by the external diameter of the cylinder lies within a range of 1-2.

Abstract: There is provided drilling apparatus (10) comprising a hammer casing (11), a free piston motor piston guide and porting sleeve (12), a drive sub (13), a drill bit (15) splined by shank (16) to the drive sub (13). Below the splined portion (17) the drive sub is relieved at (23) to form an annular air space for motor exhaust air from the splines (16) and (17). The drive sub (13) retains a gauge sleeve (24) via an annular flange (25) between the hammer casing (11) at (26) and a shoulder (27) on the drive sub (13). Below the shoulder (27) the outer diameter of the drive sub (13) is provided with eight grooves that form conduits (30) with the lower portion (31) of the gauge sleeve (24) extending parallel to the drill axis in communication with the space (23) via ports (32). The lower portion (31) extends down the side of the drive sub (13) and terminates with it at (34) adjacent the shoulder (33) at the transition from the bit shank (16) and the bit head (35).

Abstract: A down hole rock drill bit and method of manufacture of the same comprising a cast metal drill bit body having a plurality of hardened carbide studs partially cast in the drill bit body. The drill bit is cast by means of a foam pattern replicating the drill bit, typically made from polystyrene within which a plurality of carbide studs are partially inserted into the grinding surface of the foam drill bit model. The model is then subsequently supported within a vessel of sand and molten metal is poured over the foam, vaporizing it and taking the exact form of the foam pattern and permanently retaining the carbide studs within the metal drill bit.

Abstract: In a directional drilling apparatus in which a sensor/transmitter or sonde is located in one section of a drill string and an element of an adjacent section is configured to “steer” the bore drilled in a predetermined lateral direction, a device is provided for automatically ensuring correct registry or alignment of the sensor/transmitter and the steerable element, this device comprising an azimuth member on the section carrying the sensor/transmitter engageable with a reference member of the adjoining section to present, after such engagement, relative rotation of the azimuth member, which is rotatable in the section carrying it, with the sensor/transmitter, but non-rotatably connected with the sonde. The reference member or the azimuth member is resiliently displaceable axially to allow the two sections to be screwed up together.

Abstract: A rock drilling apparatus includes a drill string terminating in a drill chuck (32) and a drill bit (16) detachably fitted into the drill chuck, the drill bit having a shank (18) received in the drill chuck, the drill chuck (32) and the drill bit shank (18) having complementary splines and grooves allowing rotation or movement of the drill chuck to be transmitted to the drill bit whilst allowing limited longitudinal movement of the drill chuck relative to the drill bit. The drill bit has a drill head (22) at its lower end which is of greater diameter than said shank, said head having a part (26) above that lower end and below the splined region.

Abstract: A guide tube for use in a rock drill string includes a main portion, a sleeve disposed at a first end of the main portion, and a male thread portion disposed at a second end of the main portion. The sleeve forms a recess having a female thread. A flush channel extends through the guide tube. The main portion includes a section of reduced cross-section disposed adjacent the sleeve for defining a generally radially extending shoulder between the waist and the sleeve. The shoulder serves to support the guide tube on a rig as percussion is applied to the guide tube for loosening a threaded joint between the guide tube and another member.

Abstract: The invention provides a bore bit comprising a bottom part having an active bottom face and a top face substantially parallel to the active face; a top part fixed to suspension cables and having a bottom face facing the top face of the bottom part; means for limiting the maximum spacing between the two parts; a cylindrical axial bore passing through both parts; a cylindrical guide penetrating into said bore; and at least one ramp-forming recess and at least one ramp-forming extension for co-operating with said ramp-forming recess in such a manner that movement of the top part towards the bottom part is converted into rotary movement of said top part about the vertical axis, with the end of said rotary movement of the top part imparting rotary movement to the bottom part.

Abstract: This specification involves combining automatic fluid driven bit hammering apparatus with generic rotary drilling motor assemblies used on non-rotating drill strings to produce more effective well and earth borings.

Abstract: A rock drill including a drilling rod (2), a cup-shaped drilling head (5) secured to the drilling rod (2) at one of its ends, and connection elements including at least one radially projecting, entraining web (8) associated with the drilling rod (2) and received in at least one axially extending entrain slot (9) which is formed in an entrain surface of the cup-shaped drilling head (5) for securing the drilling head (5) to the drilling rod (2) for joint rotation therewith and for a limited axial displacement relative thereto.

Abstract: A helical drill bit (1) for rock has an axially extending shank (2) located between a chuck insert end (3) and an opposite drilling head (4) with at least one axially extending helical discharge groove (7) extending radially inwardly from the outer circumferential surface of the shank. A radially projecting groove reinforcement (9) is arranged in the base of the radial groove (8), with the reinforcement changing its geometric form or its position within the discharge groove (7) along the discharge groove (7).

Abstract: A rock drill head including a radially extending groove (2a) formed in a head body, a recess (5) formed inside the groove (2a); and at least two, substantially plate-shaped, hard-material inserts located in the groove (2a) and having each a radially extending, segment (4a, 4b) engaging in the recess (5).

Abstract: A bit retainer for a percussion bit assembly has a bore and upper and lower lips extending into the bore. Multiple slots extend from the lower end of the retainer member to a terminal end disposed intermediate the upper and lower ends, forming multiple cantilever beam segments. The cantilever beam segments are resiliently, radially deflected by the retention shoulder of the percussion bit to mount and retain the bit.

Abstract: A drill bit for drilling rock has an elongated shaft and at least one cutting plate at one end. A clearance angle &bgr; of the cutting edges, at least in segments, increases with increasing distance from the rotary axis. A wedge angle &ggr; of the cutting edges, at least in segments, decreases with increasing distance from the rotary axis.

Abstract: A drilling and/or chiseling tool (1) comprises a shaft (2) and a cylindrical insertion end (4) connected with the shaft (2) with at least one elongated recess (5 to 8), open toward the outer surface of and toward the free end of the insertion end (9) and forming a rotationally locking groove. A collar (3) is disposed between the shaft (2) and the insertion end (4) and extends at least partially around the tool (1) and projects at least radially outwardly of the shaft (2) and of the insertion end (4). The cylindrical insertion end (4)has a diameter not less than 18 mm and preferably larger than 22 mm. The at least one recess (5 to 8) has a cross-sectional area, extending perpendicularly to the axis of the tool (1), and expands toward the outer surface and is provided essentially in the form of an arc of a circle.

Abstract: A drill bit suited for impact-assisted directional boring in rock includes a bit body having a frontwardly facing, circular main cutting surface having a number of main cutting teeth disposed thereon. A gage cutter extends radially outwardly from the main cutting surface, which gage cutter mounts at least one frontwardly facing gage cutting tooth thereon suitable for cutting in a circle during straight boring and over an angle defined by less than a full rotation of the bit in order to steer the bit. A heel on an outer side surface of the bit body at a position opposite to and behind the gage tooth provides a reaction surface for the gage cutter. The bit body and gage cutter have a nearly circular side profile along a front portion thereof such that a maximum radial clearance between the side profile and a side wall of a hole being drilled is not greater than about 10 percent of the radius of a circle cut by the gage tooth.

Abstract: A drill head for an apparatus for horizontal directional drilling. The drill head includes a device for detecting angular orientation, a holder for the device for detecting angular orientation, the device for detecting angular orientation being disposed therein, a hammer driven by a liquid and a drill bit. The holder, the hammer and the drill bit are connected head to tail along a longitudinal axis of a drill string with the holder at a proximate end of the drill head and the drill bit at a distal end of the drill head.

Abstract: The invention relates to a chisel (1) or a hammer drill, in particular insertable chisel or insertable drill for power-driven drill and chisel hammers, with an insertion end (2), with a chisel shank (3) or drill shank and with a chisel head (4) or drill shank located opposite the insertion end (2). In this case, the diameter of the shank (3) increases in the direction of the chisel head (4) at least once from a first, small diameter to a second, large diameter and decreases again from this to a small diameter or forms at least one bead (8).

Abstract: An earth borer including a thin-walled tubular shaft (4) having smooth inner and outer surfaces, a shank (2) provided at one end of the tubular shaft (4), a drilling head (5) provided at another, opposite end of the tubular shaft, projecting radially beyond the tubular shaft (4), and having at least one cutter (6, 6a, 6b, 6c, 6d) extending, at least partially, radially inward and at least one internal groove (7, 7a, 7b) associated with the at least one cutter, and a releasable tubular connection member (8) for drivingly connecting the shank (2) with tubular shaft (4) for joint rotation with each other.

Abstract: A down-the-hole percussive hammer for rock drilling includes a cylindrical casing and a drill bit disposed at a front end of the casing. The drill bit includes a forwardly facing cutting surface and a center longitudinal passage extending forwardly through a rearwardly facing rearwardmost end surface of the drill bit. The passage includes a rearwardly facing impact surface. A piston is mounted in the casing longitudinally behind the drill bit for reciprocation in a longitudinal direction. The piston includes a front portion sized to enter the center passage of the drill bit and strike the impact surface of the drill bit during each forward stroke of the piston. The impact surface of the drill bit is spaced forwardly from the rearwardmost end surface of the drill bit by a distance of at least ten percent of a total longitudinal length of the drill bit.

Abstract: An insert for a rolling cone earth-boring bit has a cylindrical base that interferingly presses into a mating hole formed in a cone of the bit. The insert has a convex end that extends from the base. A polycrystalline diamond cap is bonded to the convex end. The body is formed of at least two layers of carbide material having different mechanical properties, particularly a different modulus of elasticity. The first layer may have a metallic binder with a lesser percentage than the binder of the second layer to reduce the stress at the interface between the first layer and the diamond cap. The layers may have different average carbide grain sizes, with finer average grain sizes adjoining the diamond cap. Further, the layers may have different binders, with cobalt being the binder in the layer adjoining the diamond cap and either nickel or a nickel-cobalt alloy in another layer.

Abstract: A rock drill bit for percussive drilling, especially top hammer drilling, includes a bit head provided with front rock crushing buttons and a skirt. The bit head is provided with a number of grooves formed in the outside of the skirt and extending in the axial direction of the drill bit. Protruding lands are defined between the grooves, which likewise extend in the axial direction. Axially rearward ends of at least some of the lands are formed with retrac teeth. Each retrac tooth has a cutting edge extending along an outer circumference of the skirt.

Abstract: A twist drill including a spiral-forming hollow shaft (3) having a spiral-shaped inner surface, and drilling head(5) and shank (6) connected with the shaft (3) at its opposite ends, respectively; and a method for forming such a twist drill.

Abstract: Apparatus is provided for introducing a consistent series of small and localized rotary impacts to a PDC bit during drilling, to improve PDC drill bit performance. Rotary impact supplements the nominal torque supplied by the rotary drive thereby avoiding lockup and potentially damaging energy storage in the drill string following windup, should the bit slow or hang up when drilling in difficult formations. The apparatus comprises a rotary hammer which is rotated about a bit shaft's anvil, preferably by a drilling fluid driven turbine. As the hammer rotates, potential energy is built up. When the hammer and anvil connect, the energy is released into the bit shaft and thus into the bit, increases its instantaneous torque and allows it to more effectively cut through difficult formations.

Abstract: A percussion bores for forming borer in a stone material and including a chisel stem (2) having a prismatic cross-section and provided with a chiseling head (3) and at least one radial opening (4), a twist drill (6) extending through the cylindrical through-bore (3) of the chisel stem and projecting beyond the chiseling head (7), a drilling head (10) with cutting elements (9) provided at the projecting end of the twist drill and a shank (12) provided at another end of the twist drill, and a stop (13) provided between the shank (12) and the chisel stem (2), with a drill stem being offset radially inwardly with respect to an outer diameter of the shank in the region of the stop.

Abstract: The present invention relates to a V-notched web formed on a drill insert of a drill tool assembly. The V-notched web allows greater stability and feed rates, while decreasing the heat generated at the insert tip. The present invention is provided by a drill insert comprising a drill insert body having at least two cutting edges, a web between the two cutting edges, and a web thinning notch formed on either side of the web. Each web thinning notch forms a notch cutting edge having a positive rake angle.

Abstract: A drill including a body having mortises (7), and a solid hard metal head (4) having a plurality of bits (3) and secured to the drill body over the end surface (5) of the head (4) and additionally secured to the drill body over portions of side surfaces (6) of the bits, which are located in a radially outer region of the head (4), with the side surfaces (6) extending into mortises and including first surfaces, which are associated with at least some of the bits (3) and are subjected to pressure acting in a rotational direction of the drill during operation of the drill.

Abstract: A drilling tool (1), especially a rock drilling tool, has an axially extending shaft (2) with one end arranged to seat in a chuck of a drilling device and the opposite end securely connected to a drilling head (5). The drilling head has at least three hard metal cutting members (7, 8 and 9) with at least two adjacent cutting members located at the outer circumferential surface of the drilling head and preferably projecting radially outwardly from the circumferential surface. The cutting members are spaced circumferentially apart at an angle in the range of about 100° to about 150°. Drilled material grooves (10, 11 and 12) separate the cutting members (7, 8 and 9) in the circumferential direction. In turn the drilled material grooves (10, 11 and 12) discharge into at least one discharge groove extending helically around the shaft. A circle (E) encircling the drilling head (5) is formed by the radially outer edges of the outwardly projecting cutting members.

Abstract: Drills, particularly rock drills, with a chisel shank and a drill head with at least one drill bit or tool bit on a face pointing in a feed direction have the tool bit formed of at least one cutting edge provided on the face side between a rake surface and a free surface with the rake surface being in the direction of rotation of the drill and being ahead of the cutting edge. The free surface has at least a portion which extends into the drill bit over at least one sub-area to form a concave depression.

Abstract: A drilling head of a rock drill for use with a hand-held power tool and formed completely of a hard material and including a main bit (2), and at least one auxiliary bit (3a, 3b) provided exclusively in a radially outer region of the drilling head and having an arcuate cutting edge (4a, 4b).

Abstract: A drill including a stem (2) having at least two discharge grooves (3,4) for discharging drilling dust and helically circumscribing the stem (2), a drill head (5) provided at an of the stem (2) opposite the end provided with the shank and having a plurality of bits (7-9), which are formed of hard metal and project radially beyond the drill head (5), with the drill head having a plurality of drilling dust grooves (10-12) which separate the bits and number of which exceeds a number of the discharge grooves (3,4), with at least one pair of the drilling dust grooves (10,12) passing into a common discharge groove (4), and with the drill further having a helical groove section (13) forming a common transitional region between the at least one pair of the drilling dust grooves (10,12) and the common discharge groove and the passing cross-section of which is smaller than the passing cross-section of the common discharge groove.

Abstract: A percussion drill 10 has a tubular fluid transmitting body 14, with a drill bit 16 mounted on a drill bit support 20 itself mounted in body 14 via a spring 22 and salines 24. A mass 28 is spring mounted in the body 14 and is movable to impact on a face 26 of the support 20. Mounted to the mass 28 is a rotating valve 32 comprising two valve plates 34, 36. Each plate 34, 36 defines a slot 38, 40, such that rotation of one of the valve plates 36 relative to the other moves the slots 38, 40 into and out of alignment; the change in alignment of the slots 38, 40 alters the flow of drilling fluid through the valve 32, so altering the drilling fluid pressure force acting on the mass 28, so causing mass 28 to push against the spring 30, and impact on the drill bit support 20 providing a percussive act ion at the drill bit 16.

Abstract: A drill including a stem (2,22) having at least one discharge groove (3) helically circumscribing the stem (2,22), and a drill head (4,24) provided at the end of the stem (2,22) opposite the end at which a shank is provided and having at least two hard metal bits (5,25) projecting radially beyond a drill head circumference, with the drill head further having drilling dust grooves (7,27) extending at least partially between the bits (5,25) and opening into the at least one discharge groove (3) and having, at least in a region of the at least two hard metal bits (5,25), an extended circumferential profile (8,28) projecting radially beyond a circumference (9,29) of the stem (2,22).

Abstract: An insert for a drill bit is disclosed which includes an exposed surface having a contact portion adapted to periodically contact earthen formation. The contact portion includes a polycrystalline diamond material having a hardness in the range of about 2000 HV to about 3000 HV. In some embodiments, the polycrystalline diamond material includes a composite having a first phase material including polycrystalline diamond and a second phase material selected from the group of oxide particulates metal carbides, and metallic particulates, nitrides, and mixtures of thereof.

Abstract: There is provided a rotary drag-type drill bit comprising a bit body having a leading surface comprising a plurality of pads, at least some of the pads each having a wearable abrasive surface including particles of superhard material and defining an outer profile of the pad, at least a part of the outer profile of at least one of the pads being disposed inwardly or outwardly of the outer profile of the other pads. With this arrangement, during initial drilling through softer formation, most of the removal of formation will be effected by the outermost of the abrasive pads and little or no formation will be removed by the more inward pads. Accordingly, the bit will act as a lighter set bit and good rates of penetration may be achieved.