GB2208672A - Rotary drag drill bit - Google Patents

Abstract

A rotary drag drill bit (10) is seen wherein cutter bodies (22) are rotatively connected to a main body structure (12) at a fully offset position. The fully offset position is defined by a rotational axis of each cutter body (22), a longitudinal axis (17) of the drill bit (10) and end support points or positions of the cutter bodies (22). The rotational axes of the cutter bodies (22) are perpendicular to the longitudinal axis (17) of the drill bit (10). In a primary rotational direction (19) of the drill string and drill bit (10) rows (25) of cutting elements on the cutter body abrade the bottom and side walls of a well bore as the cutter body (22) attacks the earth formation as the drill bit (10) is rotated. The impingement of the cutting elements of the cutter body (22) on the earth formation imparts a secondary rotation to the cutter bodies (22), which secondary rotation is induced by the primary rotation. The secondary rotation allows the rows (25) of cutting elements to engage the side wall of the bore (13) and gauge the hole as well as abrading away material from the bottom of the well bore (13). A lubrication system is included in the main body structure (12) of the drill (10). <IMAGE>

Description

DRILL BIT WITH FULL OFFSET CUTTER BODIES The present invention is divided out of Patent Application No. 8527556 (Publication No. 2182692), and pertains to rotary drill bits employed for coring and drilling well bores in an earth formation. More particularly, the present invention pertains to an improved arrangement and shape of cutter bodies, and attached cutter elements, in which arrangement the cutting elements are oriented to impart a secondary rotation to the cutter body, while each cutter body is simultaneously rotating with a primary rotation around a longitudinal axis of a drill string, cutting away and crushing the earth formation being drilled.

The cutter body employed in prior art rotary drill bits is rotatively connected, in groups of two or more, to a main body structure. Each cutter body is generally of conical or frustoconical-shape, a base of the cutter body positioned at the outside of the drill bit to gauge the size of the hole being drilled.

Cutting elements or teeth are attached to or formed on the exterior surface of each cutter body. The cutting elements are rotated about a rotational axis of the cutter body, and in a rotation about a longitudinal axis of a drill string, to engage on earth formation being drilled. The contact of the cutting elements with the earth formation during rotation loosens particulate matter or cuttings for removal by drilling fluids in a conventionally known manner.

In some of the prior art, the rotational axis of each cutter body passes through the longitudinal axis of the drill string, the drill string being rotated to drill the earth formation. Each cutter body rotates in direct proportion to the primary rotation of the drill string about the longitudinal axis. Very little abrasive cutting action bears on the well bore being drilled. The proportional relationship is virtually the same relationship as exists between a wheel and axle, as the drill string rotates, the cutter body rotates. The primary force acting to abrade away and remove the particulate matter is the weight of the drill bit and drill string producing a crushing type of action.

Little abrasion is imparted to the formation by the rotation of each cutter body.

It is known in the prior art to move or offset the rotational axis of each cutter body so that the axis does not pass through the longitudinal axis of the drill string. Offsetting the rotational axis of each cutter body increases the abrasion, or scraping, force of the rotary drill bit against the earth formation. The offset cutter body of the prior art has heretofore been small enough that a primary component of the rotation of the cutter body remains proportional to the rotation of the drill string.

An axis of rotation of the cutter body perpendicular to the primary rotational axis and with the rotational end supports of each cutter body equi-distant from the longitudinal axis of the drill string, is seen in a well reamer, U.S. Pat. No.

2,174,587 to D. Love. As used herein, this orientation is defined as fully offset. The well reamer of Love is not a rotary drill bit, but rather is used to enlarge the bore hole formed by a rotary drill bit. Love therefore does not act on the bottom of the bore hole itself but rather only at the periphery thereof, after the hole has been drilled.

The patent to C. Reed, U.S. Pat. No. 1,236,982, shows rotational axes for a pair of cutter bodies perpendicular to the longitudinal axis of a drill stem in a rotary drill bit. The cutter body elements are not fully offset, in that the cutter body end support points are not equi-distant from the longitudinal axis of the drill string. Reed does show spiral teeth or cutter elements that apparently impart a rotation to each cutter body. The imparted rotation of each cutter body remains proportional to the rotation of the drill string. The two cutter bodies of Reed have cutter elements of opposite angular orientation along the cutter body surface so that one cutter body does not track the previously made cutting pattern. Other cutter body cutter elements of spiral tooth orientation are known. Skewed teeth are seen in H. Mitchell, U.S. Pat.

No. 4,161,225.

A rotary cutting action as well as a crushing or abrading action is seen in H. Hughes, U.S. Pat. No.

1,124,242. Hughes uses both crushing rollers and cutter bodies to provide both rotary and crushing action. A combination rotary and percussion drill bit is known from S. Skidmore, U.S. Pat. No. 3,885,638, wherein helical grooves extend the length of the generally frustoconically-shaped bit. Skidmore apparently would be used with a drill string but is not a multiple cutter body rotary bit.

The patent to B. Munson, U.S. Pat. No. 4,408,671 shows variation of taper and angle of the teeth between cutter bodies. Different numbers of teeth between cutter bodies are seen in J. Strauss et al, U.S. Pat.

No. 1,045,756, while F. Phelps, U.S. Pat. No. 4,187,922 varies the angle of the various teeth or cutter elements.

Of general interest in rotary drill bits are the patents to R. Evans, U.S. patent. No. 4,148,368, B.

Austin, U.S. Pat. No. 3,468,583, H. Bovenkerk, U.S. Pat.

No. 4,109,737 and W. Daniels et al U.S. Pat. No.

4,333,540.

The principal object of the invention is to provide a rotary drill bit with improved abrasive force at a side wall and bottom of a well bore or core made through an earth formation.

It is a related object of the invention to provide a rotary drill bit that, during a drag method of operation, moves each cutter body of a group of cutter bodies by reason of the rotation of a drill string, resulting in the abrasive force.

It is a further related object of the invention to provide a rotary drill bit with a cutter body rotational axis perpendicular to a radial reference from the primary rotational axis of a drill string which primary rotation moves the cutter body.

It is a still further object of the invention to provide a drill bit having a primary abrasive action imparted by the primary rotation of the drill string and secondary abrasive action imparted by the secondary rotation of each cutter body. The secondary rotation results from the primary rotation of the drill string, a full offset orientation of each cutter body and the orientation of cutting elements of the cutter body.

It is a still further related object of the invention to provide a rotary drill bit wherein the cutter bodies are fully offset, i.e., perpendicular to the primary rotational axis of the drill string with cutter body end support points equi-distant from the primary rotational axis.

It is another object of the invention to provide a rotary drill bit bearing system for the cutter bodies applying abrasive force in the primary rotational direction and also for the secondary rotation about the cutter body axis.

It is another related object of the invention to provide a lubrication system for the bearing system of the cutter body, extending the life and wear of the cutter bodies of the drill bit.

The invention comprises a drag type rotary drill bit having abrasive cutting elements for drilling a bore into an earth formation by shearing cutting action on the earth formation, comprising means for periodically moving each of said abrasive cutting elements about said rotational axis and into and out of contact with said earth formation during primary rotation of said drill bit in contact with the earth formation, said rotational axis being offset from and perpendicular to said longitudinal axis, said periodic moving means further imparting both a radial component of abrading movement in the direction of the primary rotation along the rotational axis and a circumferential component of abrading movement about said rotational axis to each cutting element in contact with the earth formation, the circumferential component being greater than the radial component.

The invention also comprises a drag type rotary drill bit having abrasive cutting elements for drilling an earth formation by shearing cutting action on the earth formation, comprising at least one cutter body rotatably mounted on a main body structure of said drill bit, said cutter body having helical flutes extending along the length of said cutter body, said flutes having abrasive cutting elements operatively associated therewith, said cutter bodies being rotated during a primary rotation of said drill bit into contact with the earth formation, said flutes contacting said earth formation to move said cutter body about a rotational axis of said cutter body offset from a perpendicular to a longitudinal axis of said drill bit, said primary rotation and said movement about said rotational axis imparting both a radial and circumferential component of abrading movement to each cutting element while in contact with the earth formation, the circumferential component being greater than the radial component.

Figure 1 is a perspective view of a rotary drag bit of the present invention.

Figure 2 is a bottom plan view of the rotary drag bit seen in Figure 1.

Figure 3 is a sectional view taken in the plane of line 3-3 of Figure 2.

Figure 4 is a sectional view taken in the plane of line 4-4 of Figure 3.

Figure 5 is a sectional view taken in the plane of line 5-5 of Figure 3, arrows indicating a primary direction of rotation.

Figure 6 is a diagrammatic view of a cutter body of the invention, arrows indicating a primary rotational direction and a secondary rotational direction of a cutter body of the rotary drag bit seen in Figure 1.

Figure 7 is an exploded perspective view of a cutter body of the invention showing a bearing assembly and a lubrication system seen in a fragmentary sectional view.

Figures 8-11 are perspective views of alternative cutter elements arranged on the cutter body of the rotary drag bit seen in Figure 1.

Figure 12 is a bottom view of an alternative embodiment of the rotary drag bit with three cutter bodies.

Figure 13 is a fragmentary side elevational view of an alternative embodiment of the bit for a core bit.

Figure 14 is a bottom view of the core bit seen in Figure 13.

A rotary bit 10 particularly useful in a drag mode of cutting is seen in Figures 1-5. The rotary bit 10 (Figures 1, 2- and 3) has a main body structure 12 with a threaded connector 14 and an integral generally cylindrical portion 15 extending downwardly away from the connector 14. A diameter of gauge of a well bore 13 in an earth formation is established by rotation of the drag bit 10 about a longitudinal or rotational axis 17 in a primary rotational direction indicated by arrow 19. In a conventional manner, lengths of drill pipe (not specifically shown) are connected together to comprise a drill string, which drill string is threadably connected to the rotary bit 10 at the threaded connector 14. A drilling fluid passageway 16 extends along the primary rotational axis 17 into an interior opening 18 of the main body structure 12.The passageway 16 terminates in a drilling fluid nozzle 11 (Figure 3) at the interior opening 18.

In a conventional manner, the drill bit 10 is connected to the drill string at connector end 14, the drill string mating against a mating surface 20 of the main body structure 12 to define a rigid, essentially fluid tight, connection. The drill string and attached rotary bit 10 are rotated about the primary rotational axis 17 in the primary rotational direction indicated by the arrows 19 (Figure 2, 5 and 6). Drilling fluid is pumped down the drill string through the drilling fluid passageway 16 exiting the nozzle 11 into the interior opening 18 of the main body structure 12. The drilling fluid has a viscosity for purpose of lubricating the cutting action of two cutter bodies 22, to be described in more detail hereinafter.The drilling fluid also carries cuttings or particulate matter sheared from the side wall and bottom of the well bore 13 back to the surface of a drilling rig (not shown) in continuous fluid circulation, as is well known in the art.

The main body structure 12 has relatively connected thereto, within the interior opening 18, two of the cutter bodies 22, though additional cutter bodies may be added. Each cutter body 22 includes a plurality of cutting elements or teeth 24 (Figures 8-11) formed on flutes 25 aligned in helical rows extending along the length of the cutter body 22. The helical rows of flutes 25 are so aligned along the length of the cutter body 22 as to impart a secondary rotational direction, indicated by arrow 21 in Figure 6, to the cutter body 22 as the cutter body 22 follows the primary rotation imparted by the rotating drill string and the main body structure 12. As a result of the configuration and orientation of the cutter body 12 and cutting elements 24, abrasive cutting or shearing at the side wall and bottom of the well bore 13 occurs. The well bore 13 is also gauged by the primary and secondary rotation of the cutter bodies 22 (Figure 3).

The main body 12 includes the threaded connector 14. The threaded connector 14 is of generally frustoconical-shape about the axis 17 of the drill bit 10 and drill string. The drilling fluid passageway 16 is formed centrally of and along the length of the threaded connector end 14, providing a passageway from the hollow interior of the pipe forming drill string to the interior opening 18 in the main body structure 12, terminating in the nozzle 11. The mating surface 20 is integrally formed with the connector 14, extending a relatively short distance away from the connector 14 in a plane perpendicular to the axis 17. The mating surface is to fully abut against and circumferentially seal with the drill string in a fluid tight connected configuration (not specifically shown).

A chamfered surface 27 extends from the mating surface 20, defining an edge with the mating surface 20. The cylindrical portion 15, defined by four integral elongate support fingers 28a, b, c and d (Figure 2), extend downwardly from the termination of the chamfered surface 27. The fingers 28a, b, c and d are parallel to the axis 17. The cylindrical portion 15 has a number of stabilizer ribs 30 (Figure 1) formed on an outer surface, which ribs 30 are abrasive material of known composition, to help gauge the well bore 13.

Between two of the fingers 28a and 28b is positioned one of the cutter bodies 22 while between the other two fingers 28c and 28d is positioned the second cutter body 22. A pair of arcuate cutter body openings 32 (Figures 1 and 2) are defined between the fingers 28a and 28b and between the fingers 28c and 28d. A drilling fluid outlet 34, also of arcuate shape, is formed in the main body structure 12 between each of the fingers 28b and 28c and between the fingers 28d and 28a. The drilling fluid outlets 34 are relatively smaller than the cutter body openings 32.

A return fluid flow path for drilling fluid ejected through the nozzle 11 is defined by elongate junk slots or grooves 35 and 36 formed in the main body structure 12 between each pair of adjacent fingers 28a, b, c and d. Shorter junk slots 36 provide for fluid communication between the cutter body opening 32 and the return flow path for drilling fluid between adjacent fingers 28a and 28b and between 28c and 28d respectively. Longer junk slots 35 provide for fluid communication between fluid outlets 34 and the return flow path for drilling fluid between adjacent fingers 28b and 28c and between 28d and 28a respectively. The return flow path for drilling fluid is defined by the annulus between the side wall of the well bore 13 and the outer surface of the drill string.

Each of the fingers 28 is seen to be of generally triangular transverse cross-section (Figures 2 and 5). Each finger 28 has a bore 37 formed therethrough (Figure 5) for receipt of a cutter body shaft 39, each shaft 39 rotatably supports one cutter body 22, as will be hereinafter described. Extending longitudinally substantially the length of the fingers 28b and 28d is an elongated generally cylindrical hollow opening defining a lubricant resevoir 40, forming part of the lubrication system to be described hereinafter (Figures 4 and 7).

At a bottom end of each finger 28 a cutting edge or gauge cutter 46 is formed. A chamfered surface extends inwardly from the exterior or periphery of the main body cylindrical portion 15 to define the cutting edge 46. The cutting edge or gauge cutter 46 helps gauge the well bore 13. With respect to gauging the bore, the rotary bit 10 of the present invention does not rely totally upon the cutting action of the cutting elements of a heel row of a conventional non-offset cone type rotary bit. Rather, the cutting elements 24, along with the edge 46, gauge the bore 13.

As seen in Figure 2, the main body structure 12, and specifically the integral fingers 28 thereof, establish four corners of a square at a bottom of the bit 10. A pair of end support points or positions 48 (Figure 4 and 5) exist at each end of each of the cutter bodies 22, located where each of the cutter body shafts 39 is secured to the fingers 28 by a weld or other means. The end support positions 48 for each cutter body 22 are equi-distant from the axis 17. Furthermore, a longitudinal rotational axis 50 exends longitudinally of each cutter body 22, defining an axis of revolution, and is in a plane perpendicular to axis 17. For purposes of this description, the orientation of the cutter bodies 22, as above described, is defined as fully offset with respect to the axis 17.

Each of the cutter bodies 22 is of the same generally ellipsoid shape, ends 54 of each cutter body 22 being truncated. The orientation of the flutes 25 from one cutter to the next cutter bodies 22 does differ. Each cutter body 22 has a bore 51 formed longitudinally along the rotational axis 50. The bore 51 receives the cutter body shaft 39. An outer surface 53 (Figure 7) of each ellipsoid cutter body 22 is slightly raised at a midportion 55 along the length between the two ends 54. The midportion 55 of the cutter body 22 is the location where the majority of the cutting or abrading of the earth formation takes place.

The cutting elements or teeth 24 and flutes 25 are formed on the outer surface 53 of the cutter body 22. As shown, the raised flutes are of generally rectangular cross section and project above the surface 53 to support the cutting elements 24. The flutes 25 are arranged in parallel rows. The abrasive cutting elements 24 contact and cut the earth formation in the shearing or abrading circular motion path defined by the primary rotation of the drill string and bit 10 along arrow 19 about axis 17 and in a secondary rotary or circular motion by reason of orientation of the flutes 25 of the cutting elements 24 along arrow 21 about axis 50.

Each row of cutting elements 24 is oriented with respect to an imaginary line 52 (Figure 6) perpendicular to both ends 54. The line 52 is coincident with any given flute 25 at an end of the flute 25 which precedes the cutter body 22 in the direction of primary rotation, indicated by arrows 19. The given flute 25 makes an angle with the line 52, which angle is continuously changing along the length of the flute 25. As the bit 10 rotates in the primary direction, the flute is a finite, yet ever increasing, perpendicular distance from the line 52. The flutes function as would threads of very large pitch to rotate the cutter body 22 in the secondary rotational direction.

The abrasion cutting elements 24 are preferebly of natural or synthetic diamond material type. Diamond material cutting elements are highly abrasive and highly resistive to wear in a shear or abrasion cutting mode.

One example of a well known synthetic diamond material cutting element is disclosed in U.S. Pat. No.

4,156,329. Synthetic cutting elements are commercially available from General Electric Company under the trademark STRATAPAX. The cutting elements 24 can also be tugsten carbide inserts or metallic teeth integrally formed on the cutter body 22 and hardened by various metalurgical techniques which are well known.

As seen in Figure 8, the cutting elements 24a are diamond, natural or synthetic, impregnated into the flutes 25a, which flutes are integrally formed on the cutter body 22a. This type of cutting element is best used for cutting a hard formation or a broken formation. Each cutter body 22, whatever cutting element 24 is used, has a different number of flutes 25 to eliminate tracking. The number of rown of flutes 25 on a given cutter body 22 will always differ from the number of rows of the other cutter body 22, the end result being no possibility of a path or track developing.

In Figure 9 the cutter body 22b has cutting elements 24b of natural diamond, polycrystaline diamond stud or carbonade diamond. Whatever material used, is cast into the flutes 25b. This cutting element embodiment is used for long life in cutting hard formations.

In Figure 10 the cutter body 22c has cutting elements 24c made of polycrystaline laminated disk. The elements 24c are of truncated disk shape. Either silver brasing into the flutes 25c or erosion resistant matrix casting is used to attach them. This embodiment is useful in cutting tar sands and other rock formations with rubberry or slick properties, where other cutting elements might slide over the surface of the formation.

In Figure 11 the cutter body 22d has cutting elements 24d of either aluminium oxide, tungsten carbide or cubic boron nitride (borizon). These elements are useful with soft non-abrasive formations like soft shale, limestone and coal.

As seen in Figure 7, the flutes 25 of cutting elements 24 of the cutter body 22 extend helically along the length of cutter body 22 between the ends 54. On a given cutter body 22, the flutes 25 of cutting elements 24 are essentially parallel to each other. On the other of the cutter bodies 22, the orientation of the rows 25 is a mirror image of that of the first cutter body 22.

Therefore, one cutter body 22 rotates about its rotational axis 50 in a given direction, while the other cutter body 22 rotates in the oppositie direction about its rotational axis 50, all as indicated by the arrows 21 in Figures 2 and 3. This orientation helps prevent the secondary rotational motion of the cutter bodies 22 imparted by the flutes 25 of cutting elements 24 from tracking and allows for increased or enhanced shearing action as each cutter body 22 follows the other in the primary rotational direction indicated by the arrow 19.

At each of the ends 54 of the cutter bodies 22, a counter sunk bore 57 is formed. One end 54 receives a thrust bearing 59 as seen in Figure 5. The other end 54 receives, in the counter sunk bore 57, a seal pack 56 and thrust washer 58. The thrust bearing 59 has a rubber seal around the circumference thereof and is mounted at the end 54 to trail the cutter body 22 in the primary direction of rotation. The thrust bearing 59, seal pack 56, washer 58, cutter body shaft 39, and roller bearings 60, form a bearing system or assembly, to be described later, for both primary rotational movement of the cutter body 22 against the thrust baring 59 as well as secondary rotational movement around the cutter body shaft 39 (Figure 7).

A space exists between the cutter body shaft 39 and the surface of the bore 51 of the cutter body 22 into which space the roller bearings 60 are placed. The roller bearings 60 are seen to be elongated rods of the needle type, though a journal type roller bearing could also be employed. The roller bearings 60 extend substantially the length of the cutter body 22 between the counter sunk holes 57.

The thrust bearings 59, seal pack 56 and washer 58 seal the ends 54 of the cutter bodies 22, at the counter bore 57, against loss of lubricant. The thrust bearings 59, seal pack 56 and washer 58 also hold the roller bearings 60 in place with respect to the cutter body 22.

The lubrication system for the cutter bodies 22 is best seen in Figures 4 and 7. The lubricant reservoir 40 is connected by a relatively smaller diameter lubricant passageway 41 within the cutter body shaft 39, which shaft 39 is positioned within the bore 37 and fixedly connected to the fingers 28. A filling hole 43 for each lubricant reservoir 40 is formed in the main body 12 through the mating surface 20 and chamfered surface 27. Suitable closure means, such as a threaded cap or fitting 44 are used to threadably connect to the reservoir at the hole 43 to close the holes 43. The fitting 44 has a screwdriver slot for removal of the fittings. The fittings 44 each have a bore 47 formed therethrough allowing drilling fluid into the reservior 40. A cylindrical floating piston 49 is slideable in and moves along the fluid reservoir 40.A bottom surface of the piston 49 applies a static pressure to the lubricant proportional to the weight of drilling fluid or mud on the piston 49 (Figure 7). A rubber seal around the circumference of the piston 49 prevents mud from contaminating the lubricant.

The end of each cutter body shaft 39 is fixedly connected at support points 48 within the bore 37 of the fingers 28, to the main body structure 12. Adjacent the support points 48 at the shaft end near the thrust bearings 59 (Figure 4) a lubricant entry groove 63 is formed in the shaft 39, which groove 63 is in fluid communication with the lubricant reservoir 40 through the lubricant passageway 41. An entry port 65 formed in the shaft 39 allows fluid from the lubricant passageway 41 to pass into a central longitudinal feed reservoir 66 formed along a portion of the length of each cutter body shaft 39. Each longitudinal feed reservoir 66 maintains a full level of lubricant as a result of the pressure maintained by the piston 49 on lubricant held in the lubricant reservoir 40, as previously discussed. A transverse exit feed port 67 projects from the longitudinal feed reservoir 66 of the cutter body shaft 39 to the surface of the shaft 39 at a point generally central with respect to the position of the roller bearings 60. It is through this second feed port 67 that the roller bearings 60, thrust bearing 59, and thrust washer 58 are lubricated.

In summary of the operation of the rotary drag bit 10, it is seen that a primary or circumferential rotational direction about axis 17 is imparted by the rotating drill string and bit 10 to move the fully offset cutter bodies 22 along their rotational axes 50 as indicated by arrow 19. A first abrasive or abrading mode of operation is applied along the length of each cutter body 22 by the flutes 25 of cutting elements 24 formed on the outside surface 53 of each of the cutter bodies 22 longitudinally intersecting the earth formation. The fully offset position, wherein the rotational axes 50 of the cutter bodies 22 are perpendicular to the longitudinal axis 17 of the drill string and bit 10 and each support point 48 of a given cutter body 22 is equidistant from the bit axis 17, is employed.

A second abrasive mode of operation helps gauge the bore 13 as seen in Figure 3. As a result of the spiral or threaded orientation of the flutes 25 of cutting elements 24, secondary or radial rotation referenced to axis 17, induced by the primary rotation of each cutter body 22, and about the cutter body 22 rotational axis 50 takes place. As the cutter bodies 22 rotate, the cutting elements are periodically moved into and out of contact with the earth formation. As the cutter bodies 22 rotate under the secondary rotational motion, the flutes 25 laterally intersect the earth formation. Not only is the bore 13 gauged in the second abrasive mode, but additional material is abraded from the side and bottom of the bore as each element 24 passes through its complete rotation around the rotational axis 50.

The flutes 25 on the two cutter bodies 22 are mirror images of one another so that each cutter body 22 rotates about axis 50 toward the other cutter body 22.

The nozzle 11 cleans the elements 24 as they pass beneath the nozzle and before re-contact is made with the earth formation.

The drilling fluids exit the bottom of the bore 13 through the junk slots 35 and 36, carrying cuttings from the bore 13. The fluid is returned to the surface for recycling and reuse.

In a first alternative embodiment seen in Figure 12, like parts being given prime suffixes, three cutter bodes 22' are seen. In all other respects, the structure of Figure 12 is as previously described.

A second alternative embodiment, seen in Figures 13 and 14 with like parts being given double prime suffixes, is used as a core bit 70. A body 72 of the core bit 70 is of elongated tubular construction with an internally threaded end 73 connected to a drill string (not shown). At a cutting end 74 of the core bit 70 are mounted four cutter bodies 22".

The cutting end 74 of the body 72 widens outwardly to rotably connect to the cutter bodies 22" in a manner as previously described. A core 75 has a diameter defined by the distances between the innermost cutting of the cutter bodies 22". An alternative cutter body 76 is seen to have a arcuate recess 77 around the periphery of the midportion 55".

Patent Application No8822257 filed contemporaneously herewith also divided from Application No. 8527556 describes the same subject matter but has different claims.

Claims (8)

1. A drag type rotary drill bit having abrasive cutting elements for drilling a bore into an earth formation by shearing cutting action on the earth formation, comprising means for periodically moving each of said abrasive cutting elements about said rotational axis and into and out of contact with said earth formation during primary rotation of said drill bit in contact with the earth formation, said rotational axis being offset from and perpendicular to said longitudinal axis, said periodic moving means further imparting both a radial component of abrading movement in the direction of the primary rotation along the rotational axis and a circumferential component of abrading movement about said rotational axis to each cutting element in contact with the earth formation, the circumferential component being greater than the radial component.

2. A drill bit as claimed in Claim 1 wherein said abrasion cutting elements are mounted on at least one cutter body rotatively connected to a main body structure of said drill bit, said cutter body rotated by said drill bit rotation.

3. A drill bit as claimed in Claim 2 wherein said cutter body has helical flutes formed on an exterior surface thereof and along the length of said cutter bodies, said flutes inducing a rotation of the cutter body about a rotational axis.

4. A drill bit as claimed in Claim 3 wherein there are two cutter bodies and the flutes of each of said cutter bodies helical in directions opposite to the other of said flutes.

5. A drill bit as claimed in Claim 3 wherein there are two cutter bodies, each of said cutter bodies having a different number of flutes.

6. A drill bit as claimed in Claim 2 wherein the rotational axis of each of said cutter bodies is perpendicular to a radial line extending from a longitudinal axis of the rotary drill bit and has a rotational end support at each end on the rotational axis of each of said cutter bodies which end supports are each equi-distant from said longitudinal drill bit axis.

7. A drill bit as claimed in Claim 2 wherein said cutter bodies are of ellipsoid shape having truncated ends.

8. A drag type rotary drill bit having abrasive cutting elements for drilling an earth formation by shearing cutting action on the earth formation, comprising at least one cutter body rotatably mounted on a main body structure of said drill bit, said cutter body having helical flutes extending along the length of said cutter body, said flutes having abrasive cutting elements operatively associated therewith, said cutter bodies being rotated during a primary rotation of said drill bit into contact with the earth formation, said flutes contacting said earth formation to move said cutter body about a rotational axis of said cutter body offset from a perpendicular to a longitudinal axis of said drill bit, said primary rotation and said movement about said rotational axis imparting both a radial and circumferential component of abrading movement to each cutting element while in contact with the earth formation, the circumferential component being greater than the radial component.