Rotary drill bit having a non-rotating gauge section

A rotary drill bit comprises a bit body, a shank for connection to a drill string, a plurality of cutters mounted on the bit body, and a gauge structure which extends around the bit body and, in use, engages the surrounding formation forming the sides of the borehole being drilled. At least a section of the gauge structure is rotatably mounted on the bit body so that, in use, the gauge section may remain substantially non-rotating in engagement with the formation while the bit body rotates relative to it. The external surface of the non-rotating gauge section may be formed with longitudinal grooves to permit the flow of drilling fluid past the gauge section to the annulus. Alternatively the outer surface of the gauge section may be generally cylindrical, in which case internal passages are provided through the gauge section, and/or the bit body, for the flow of drilling fluid past the gauge section to the annulus.

BACKGROUND TO THE INVENTION 
The invention relates to rotary drill bits of the kind comprising a bit 
body, a shank for connection to a drill string, a plurality of cutters 
mounted on the bit body, and a gauge structure which extends around the 
bit body and, in use, engages the surrounding formation forming the sides 
of the borehole being drilled. 
The invention is particularly, but not exclusively, applicable to drag-type 
drill bits in which some or all of the cutters are preform (PDC) cutters 
each formed, at least in part, from polycrystalline diamond. One common 
form of cutter comprises a tablet, usually circular or part-circular, made 
up of a superhard table of polycrystalline diamond, providing the front 
cutting face of the element, bonded to a substrate which is usually of 
cemented tungsten carbide. The bit body may be machined from solid metal, 
usually steel, or may be moulded using a powder metallurgy process in 
which tungsten carbide powder is infiltrated with metal alloy binder in a 
furnace so as to form a hard matrix. 
The invention may also be applied to other types of drill bits, such as 
roller-cone drill bits. 
While drag-type PDC bits of the kind referred to have been very successful 
in drilling relatively soft formations, they have been less successful in 
drilling harder formations and soft formations which include harder 
occlusions or stringers. Although good rates of penetration are possible 
in harder formations, the PDC cutters may suffer accelerated wear and bit 
life can be too short to be commercially acceptable. 
Studies have suggested that the rapid wear of PDC bits in harder formations 
is due to chipping of the cutters as a result of impact loads caused by 
vibration, and that the most harmful vibrations can be attributed to a 
phenomenon called "bit whirl". Bit whirl arises when the instantaneous 
axis of rotation of the bit precesses around the central axis of the hole 
when the diameter of the hole becomes slightly larger than the diameter of 
the bit. Bit whirl may be initiated, for example, when the drill bit meets 
a harder occlusion or stringer in the formation which obtrudes into the 
borehole, at least initially, in only one area of the bottom or sides of 
the borehole. As each cutter strikes the occlusion or harder formation the 
bit will try to rotate about the cutter which is for the time being 
restrained by the harder formation, thus initiating bit whirl. 
When a bit begins to whirl some cutters can be moving sideways or backwards 
relative to the formation and may be moving at much greater velocity than 
if the bit were rotating truly. Once bit whirl has been initiated, it is 
difficult to stop since the forces resulting from the bit whirl, such as 
centrifugal forces, tend to reinforce the effect. 
One method which has been employed to overcome the bit whirl is to design 
the drill bit so that it has, when rotating, an inherent lateral imbalance 
force which is relatively constant in direction and magnitude. The gauge 
structure of the bit body then includes one or more low friction pads 
which are so located as to transmit this lateral imbalance force to the 
part of the formation which the pad is for the time being engaging. The 
low friction pad thus tends to slide over the surface of the formation 
which it engages, thereby reducing the tendency for bit whirl to be 
initiated. 
However, where harder occlusions or formations are encountered, as 
described above, the direction and/or amplitude of the out of balance 
force changes as the bit rotates, so that there is no stable out of 
balance force or direction. Under such conditions the anti-whirl 
characteristics of such a bit may be reduced or nullified. 
The frictional engagement of the gauge structure of a drill bit and the 
surrounding formation can contribute substantially to the drilling torque 
and can initiate bit whirl. It has therefore been considered desirable to 
reduce the diameter of the gauge section relative to the cutting structure 
to reduce this friction. However, this reduces the ability of the gauge to 
limit longitudinal and lateral movement. 
The present invention sets out to provide a new and improved form of drill 
bit in which the tendency for bit whirl to be initiated is reduced, 
without the problems referred to above. 
SUMMARY OF THE INVENTION 
According to the invention there is provided a rotary drill bit comprising 
a bit body, a shank for connection to a drill string, a plurality of 
cutters mounted on the bit body, and a gauge structure which extends 
around the bit body and, in use, engages the surrounding formation forming 
the sides of the borehole being drilled, at least a section of said gauge 
structure being rotatably mounted on the bit body whereby, in use, the 
gauge section may remain substantially non-rotating in engagement with the 
formation while the bit body rotates relative thereto. 
By providing a gauge section which can remain stationary the invention 
substantially reduces the frictional restraint to rotation of the bit 
body, thus reducing the tendency for bit whirl to be induced as a result 
of frictional engagement between the gauge section and the formation. At 
the same time, the gauge section may be of any axial length necessary to 
provide the necessary longitudinal stability of the drill bit. Also, the 
provision of a non-rotating gauge structure reduces the frictional 
restraint to rotation of the drill bit. Conventional bit gauges rub on the 
well bore and the resulting friction can be high, thereby reducing the 
torque available for drilling. 
Preferably the gauge section is formed at its outer periphery with means to 
engage the formation in a manner to restrain the gauge section against 
rotation relative to the formation. Said means may comprise elements 
projecting outwardly from the gauge section to dig into the surrounding 
formation. Preferably each such element is of small dimension in the 
peripheral direction, e.g. is knife-edged, to minimise the restraint 
provided by the element to longitudinal sliding movement of the gauge 
section along the borehole. It will be appreciated that the non-rotating 
gauge section must be free to move longitudinally of the borehole, both 
during drilling and when tripping the drill bit into and out of the 
borehole. 
Each such element may project from a socket in the gauge section, the 
element being movable inwardly and outwardly of the socket and means, such 
as spring means, being provided to urge the element outwardly. 
Preferably the outer surface of the gauge section is shaped so that only a 
minor proportion of said outer surface contacts the surrounding formation 
in use. For example, the gauge section may comprise a plurality of 
peripherally spaced axially extending projections separated by axially 
extending grooves. 
Alternatively, the outer surface of the gauge section may be a generally 
cylindrical surface which is substantially entirely in engagement with the 
surrounding formation, in which case the interior of the gauge section is 
formed with longitudinally extending passages to permit the flow of 
drilling fluid through the gauge section and along the annulus between the 
bit body and the formation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the drill bit comprises a bit body 10 having a shank 
11 for connection to a drill string. The end face 12 of the bit body is 
formed with a plurality of radially extending blades 13 and spaced apart 
along each blade is a series of preform cutters 14. 
Each cutter is in the form of a tablet of polycrystalline diamond bonded to 
a substrate of cemented tungsten carbide, in known manner. 
The bit body is formed with a central passage 15 from which subsidiary 
passages 16 lead to nozzles 17 in the end surface of the bit body. In use, 
drilling fluid is delivered under pressure through the passage 15 to the 
nozzles 17, for cooling and cleaning the cutters 14 and for returning the 
cuttings of formation to the surface upwardly through the annulus 18 
between the bit body and the surrounding formation 19. 
The bit body is provided with a gauge section which is spaced 
longitudinally above the end face 12 of the bit and which has an outer 
surface 21 which engages the surrounding formation forming the sides of 
the borehole. The gauge section serves primarily to stabilise the drill 
bit longitudinally within the borehole. 
Normally, the gauge section is an integral part of the bit body. In 
accordance with the present invention, however, the gauge structure 20 is 
rotatably mounted on a cylindrical portion 22 of the bit body by means of 
roller or other bearings, as indicated diagrammatically at 23. 
As best seen in FIG. 2, the gauge structure may comprise a plurality of 
peripherally spaced axially extending lobes 24 separated by axially 
extending grooves 25 which permit drilling fluid to pass upwardly through 
the borehole between the gauge structure and the surrounding formation. 
In use, the gauge structure 20 frictionally engages the sides of the 
borehole and is thereby restrained from rotating so that the bit body 
rotates relative to the gauge structure 20 by virtue of the bearings 23. 
Accordingly, in the gauge area, the only frictional restraint to rotation 
of the bit body is provided by the bearings 23 and there is therefore no 
tendency for bit whirl to be initiated as a result of irregular frictional 
restraint between the outer surface of the gauge structure and the 
surrounding formation, as is the case of prior art constructions. 
In order to improve the restraint against rotation of the gauge structure 
20, one or more of the lobes 24 of the structure may be provided with one 
or more outwardly projecting scribes 26 which are shaped to dig into the 
surface of the formation 19. Preferably the scribes extend longitudinally 
of the gauge section but are of narrow width in the peripheral direction 
so as to provide minimum restraint to longitudinal sliding movement of the 
gauge structure 20 along the borehole. 
FIG. 3 shows an alternative form of restraining element where the element 
27 is in the form of a longitudinally extending insert or blade which is 
mounted in a slot 28 in the lobe 24 so as to be slidable inwardly and 
outwardly. The insert 27 is urged outwardly by springs 29 located between 
the inner edge of the insert and the bottom of the slot, the springs 29 
serving to force the insert 27 outwardly at all times into engagement with 
the surface 19 of the formation so as to provide increased frictional 
restraint against rotation of the gauge structure. As in the previously 
described arrangement the upper and lower edges of the insert 27 are 
preferably knife-edged to provide minimum resistance to longitudinal 
movement of the gauge structure along the borehole. 
Instead of the gauge section being shaped as shown in FIG. 2 to provide 
external grooves 25 for the passage of drilling fluid along the annulus, 
the outer surface of the gauge section may be in the form of a 
substantially continuous cylinder so that it contacts the surrounding 
surface of the formation around the whole of its outer peripheral surface. 
In this case, the part of the gauge section between the bit body 22 and 
the surrounding formation 19 is formed with longitudinally extending 
through passages to enable the flow of drilling fluid through the gauge 
section, as indicated in broken lines at 9 in FIG. 1. 
The use of a gauge section having an outer continuous cylindrical surface, 
with through passages in the gauge section, provides substantial stability 
to the drill bit and may also be applicable to otherwise conventional 
drill bits where the gauge section is integral with the bit body, as well 
as to drill bits of the kind described above where the bit body can rotate 
relative to the gauge section. It is believed that the tendency for bit 
whirl to be initiated in an otherwise conventional drill bit may be 
reduced by providing the gauge section with a continuous outer cylindrical 
surface substantially all of which is in contact with the surrounding 
formation as the drill bit rotates. 
Although provision of a gauge section having an outer continuous 
cylindrical surface may help reduce bit vibration, enhance stability and 
prevent bit whirl, it may give rise to the problem that the bit will not 
fit down a slightly undersized bore hole. To eliminate this problem the 
cylindrical gauge may be applied to the pilot portion of a bi-centre bit 
having an eccentric lobe cutting the required hole size. Such arrangement 
is shown diagrammatically in FIG. 4. 
The arrangement of FIG. 4 is generally similar to the arrangement of FIG. 1 
and the same references are therefore used for corresponding elements of 
the two designs. However, the bit design of FIG. 4 is modified by the 
provision of an eccentric lobe 30 formed on the bit body 10 above the 
rotatably mounted gauge structure 20. In the arrangement of FIG. 4 the 
upper and lower surfaces of the gauge structure 20 are radial instead of 
being inclined as in the embodiment of FIG. 1. 
The eccentric lobe 30 has mounted thereon cutters 31 which may be of 
similar form to the cutters 14 and these cutters serve to slightly open 
out the bore hole above the gauge structure 20 so as to facilitate 
subsequent withdrawal and reentry of the bit into the bore hole. However, 
when the bit is drilling the outer periphery of the gauge structure 20 
will engage the surrounding formation and improve stability and inhibit 
bit whirl in the manner previously described. 
In the arrangement of FIG. 4 the gauge structure, having a substantially 
continuous cylindrical outer surface, is rotatable with respect to the bit 
body, but it would be appreciated that a similar problem could arise with 
a fixed gauge section having a substantially continuous outer cylindrical 
surface, and an eccentric cutting lobe above such a gauge section could be 
provided to alleviate the problem in similar manner to the arrangement of 
FIG. 4. 
As previously mentioned the invention is also applicable to roller-cone and 
other drill bits, in addition to drag-type drill bits of the kinds shown 
in FIGS. 1-4. The application of the invention to a roller-cone drill bit 
is shown diagrammatically in FIG. 5 which is a longitudinal half-section 
through the drill bit. 
The drill bit includes a body 32 formed with a downwardly extending 
peripheral skirt 33. Three cantilevered bearing spindles 34, only one of 
which is shown, are spaced equally apart around the internal periphery of 
the skirt 33, and each spindle extends inwardly and downwardly towards the 
central axis of the drill bit. A generally conical rolling cutter 35 is 
rotatably mounted upon each spindle 34 as hereinafter described. Attached 
to the rolling cutter 35 are cutting inserts 36 which engage the earth to 
effect a drilling action and cause rotation of the rolling cutter 35. 
Typically, each cutting insert 36 will be formed of hard, wear-resistant 
material. 
Internal passages (not shown) in the bit body, skirt 33 and spindle 34 are 
filled with lubricant and communicate with a reservoir 37. Pressure 
differentials between the lubricant and the external environment of the 
bit are equalised by the movement of a pressure balanced diaphragm 38. The 
lubricant helps reduce friction during bit operation and is retained 
within the cutter 35 by a dynamic seal 39. In order to provide a rotary 
bearing between the rolling cutter 35 and the spindle 35, a separate 
sliding bearing member 40 is mounted between the spindle 34 and a mating 
bearing cavity formed in the cutter 35. A retaining ring 41 is screwed 
into the interior of the cutter 35 and is received within an annular 
groove around the spindle 34 so as to retain the cutter on the spindle. 
The bit body 32 is provided with an annular gauge section 42 which is 
spaced longitudinally above the lower extremities of the cutters 35 and 
has an outer surface 43 which engages the surrounding formation forming 
the sides of the borehole. In accordance with the present invention, the 
gauge structure 42 encircles the bit body 32 and is rotatably mounted on 
the bit body by means of roller or other bearings, as indicated 
diagrammatically at 44. 
The gauge structure 42 may be of the same general configuration as shown in 
FIG. 2 or FIG. 3, i.e. it may comprise a plurality of peripherally spaced 
axially extending lobes separated by axially extending grooves which 
permit drilling fluid to pass upwardly between the gauge structure and the 
surrounding formation and then upwardly along the annulus between the 
drill string and the walls of the borehole. Alternatively, the outer 
surface of the gauge section 42 may be in the form of a substantially 
continuous cylinder so that it contacts the surrounding surface of the 
formation around substantially the whole of its outer peripheral surface. 
In this case there are provided, inwardly of the outer surface of the 
gauge, longitudinally extending through passages to enable the flow of the 
drilling fluid past the gauge section. Such through passages may be formed 
in the rotatable gauge section 42 itself or in the bit body inwardly of 
the rotatable section, or in both said parts. 
As in the previously described arrangements, in use the gauge section 42 
frictionally engages the sides of the borehole and is thereby restrained 
from rotating so that the bit body 33 rotates relative to the gauge 
section 42 by virtue of the bearings 44. The gauge section 42 therefore 
serves to stabilise the drill bit in the borehole without the drill bit 
becoming unbalanced as a result of irregular frictional restraint between 
the outer surface of the gauge structure and the surrounding formation. 
As in the previously described arrangements, the outer surface of the gauge 
section 42 may be provided with projecting scribes or spring-loaded blades 
of the kind illustrated in FIGS. 2 and 3. 
In the arrangements described above the whole of the gauge section 20 is 
rotatable relative to the bit body. However, the invention includes within 
its scope arrangements in which only a part of the gauge section is 
rotatable relative to the bit body, the gauge section including other 
parts which are integral with the bit body and rotate therewith.