Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
The present invention relates generally to drill bits,
and more specifically relates to drill bits for earth
boring, which includes cutters comprising an array of
discrete cutting elements.
It is known in the art that certain earth formations
are more susc~ptible to being bored with bits having large
cutters thereon, usually so-called "plastic" or "gumbo~
formations, where small cutters get mud-bound with drilling
mud and the bit conseguently "balls up", slowing or
stopping forward progress of the well bore. Large unitary
cutters, large being referred to herein as those of 3/4"
diameter and above, are generally more expensive than their
smaller counterparts, and present problems of their own
when mounted on a bit face. Specifically, when
polycrystalline diamond compact (~PDC") cutters are brazed
or otherwise metallurgically bonded to a support or carrier
surface on a bit face, the differing coefficients of
thermal expansion between the PDC substrate material and
that of the support or carrier subject the PDC to a large,
permanPnt residual stress when the braze cools, thus
rendering the PDC more susceptible to fracture upon impact
with the formation and/or fracture at the braze or
metallurgical bond line. Moreover, as alluded to above,
PDC's must be bonded to the bit body or to a carrier, which
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itself is secured on the bit face after the furnacing of a
matrix-type bit, which usually comprises a matrix of
tungsten carbide powder bonded together by a copper-based
binder alloy. The method of producing such a bit is well
known in the art, and comprises manufacturing a mold or
"boat" of graphite, ceramic or other material which
possesses on its interior the characteristics of the bit
face to be produced, these characteristics being milled or
otherwise cut or molded therein; filling the mold with a
tungsten carbide or other suitable powder, placing beads of
a binder alloy in the mold as well as flux; and furnacing
the bit at a temperature high enough to infiltrate the
powder with the meltPd binder alloy.
If, as noted above, one wishes to use PDC cutters on
the bit, it is necessary to bond them to the bit ~ace after
furnacing, as the furnacing temperature, generally in
excess of 1070C, will thermally degrade PDC's into a
fragilè, brittle and/or relatively soft state, making them
useless as cutters. It is known to furnace natural
diamonds directly into a bit body, as natural diamonds have
a thermal stability suitable for such an operation.
Similarly, there ~xist on the market so-called "thermally
stable" polycrystalline diamond compact products ~"TSP's")
which can survive furnacing without significant
degradation. Two types of TSP's are on the market today,
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leached products, where most o~ the non-diamond material in
the compact has been removed, and unleached products, where
the non-diamond material in the compact possesses similar
thermal expansion characteristics to the diamond and does
not degrade the diamond at temperatures up to 1200C. In
either case, these TSP~s may be furnaced into the bit,
providing a cutter-laden bit in a single oparation.
Affixation of the TSP cutters to the bit face may be
enhanced by coating them with metal as is known in the art,
to provide a chemical (metallurgical) bond between the bit
matrix and cutter. One exemplary apparatus and method for
coating TSP elements is described in copending application
Serial No. 09S,054, filed September 15, 1987, in the names
of Sung and Chen. The specification of application Serial
No. 095,054 is incorporated herein by this reference.
In some soft, plastic formations, there are stringers
of harder, more abrasive rock, or a bit may have to drill
through both soft and hard, abrasive rock in close
succession without being pulled from the well ~ore. Bits
having several types of cutting elements for cutting
different types of formations are known; see for example,
V.S. Patent No. 4,512,426 to ~idegaray, assigned to Eastman
Christensen Company. Using TSP elements in conjunction
with PDC's is known. One such bit design uses PDC cutters
in combination with cutters comprising mosaic-like arrays
of small, triangular-faced polyhedral TSP's, each array
simulating a larger unitary cutter. Such bits are sold by
the Eastman Christensen Company of Salt Lake City, Utah,
U.S.A., as the MosaicTM series of bits. The type of
cutter utilized on the aforesaid bits is described in U.S.
Patent No. 4,726,718, assigned to Eastman Christensen
Company and the bonding of the TSP's into an array may be
enhanced by the coating process of the above-referenced
Sung and Chen application.
Planar TSP cutters up to at least 1.5 inches in
diameter are available from DeBeers under the trade-name
"Syndax 3." Such cutters are not readily bonded during
infiltration to matrix-type bits and substantial residual
stresses will result upon cooling the bit due to the
difference in thermal expansion of the TSP and the bit
matrix. Moreover, large single pieces provide less
geometric flexibility.
It has been proposed to fabricate very large TSP array
cutters, and even entire cutter blades extending from the
gage of the bit to the center of the bit face. See, for
example, copending U.S. Patent application Serial No.
07/204,683, filed on June 9, 1988, in the name of Mark L.
Jones, and assigned to Eastman Christensen Company. Such
TSP-array cutter bits would not only provide a large
cutting surface for plastic formations, but be
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abrasion-resistant so as to better surYive stringers, in
addition to being furnaceable into the bit.
Clearly, it is desirable to produce a bit having larye
cutting surfaces at reasonable cost and without the
aforementioned thermal stress problems. Merely enlarging
the array of small TSP elements, such as is suggested in
the Jones application, was believed to be a solution, the
theory being that a plurality of small TSP ~lements would
economically form a large, predominantly-diamond cutting
surface without being detrimentally affected by the thermal
stress associated with a large, unitary cutter. However,
it has been discovered that this thermal stress problem
pervades even a TSP array, in that bits, incorporating
large TSP arrays, have encountered delamination of the
entire layer of TSP elements, both before and during
drilling, due to the stress between the TSP elements and
the bit matrix. The coating method of the above-referenced
Sung and Chen application, while enhancing the diamond to
matrix bond, actually aygravates the stress problem due to
the strength of the diamond to matrix bond. In fact,
instances of diamond fracture instead of bond fracture have
been experienced under stress.
Stress between the TSP elements and the bit matrix is
believed to occur during cooling of the bit after furnacing
as a result of the different thermal expansion rates o the
TSP and the matrix. Stress cracks are generally parallel
to the TSP~matrix interface, and may later intersect with
cracks in the cutter surface caused by impact stresses
experienced during drilling, thereby resulting in premature
cutter loss from the bit.
Accordingly, there is a need for a cutter
configuration which can provide large cutting surfaces
without the sel~-destructive tendencies of the large
cutters and cutter arrays of the prior art.
SUMMARY OF THE INVENTION
In contrast to the prior art, the present invention
affords a simple but elegant means and method of providing
a large cutter of any configuration without a destructive
level of thermally-induced stress. The cutter of the
present invention comprises a substantially planar array of
small TSP elements bonded into a bit face matrix. The
matrix behind the array may be reinforced against impact,
such as by a steel blade, pins or other means, and the TSP
elements may be coated for bond-enhancement with the
matrix. The T5P element array i5 interrupted at intervals
by discontinuities where no TSP elements are located,
thereby forming sub-arrays. Preferably, the
discontinuities are linear, and most preferably, occur at
intervals of no more than substantially one inch (l"). The
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discontinuities may extend from the bit fac~ to the edge of
the array in contact with the formation, and in bits with
very deep cutting arrays, such as bladed bits, the
discontinuities may run in several directions to intersect
5 and thereby further segregate sub-arrays. Moreover, the
discontinuities may comprise matrix material or be formed
by offsetting portions of the array from other portions.
The discontinuous cutting element arrays of the
present invention provide lower residual stress in each
sub-array than in a large cutter without such
discontinuities, and the discontinuities also provide a
barrier to crack propagation across an entire array, so
that a crack or failure in a particular sub-array will not
cause catastrophic failure of the entire array, hut will be
locally contained.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be more readily appreciated
by one of ordinary skill in the art through a reading of
the following detailed description of the preferred
embodiments, taken in conjunction with the accompanying
drawings, wherein: -
FIG. 1 is a perspective view of a core bit utilizing
cutting arrays according to a first preferred embodiment of
the present invention.
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FIG. 2 is an enlarged perspective vie~ of a single
cutting array fro~ th~ bit of FIG. 1.
FIG. 3 is a partial side sectional elevation of the
array of FIG. 2.
FIG. 4 is an enlarged perspective view of a single
cutting array according to a second preferred embodiment of
the present invention, utilized on a drill bit.
FIG. 5 is an enlarged perspective view of a third
preferred embodiment of the present invention.
FIG. 6 is an enlarged perspective view of a fourth
preferred embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
Referring to FIGS. 1, 2 and 3, core bit 10 includes a
body section 12 having mounted on its face 14 cutting
arrays, indicated generally at 16, and gage pads, indicated
generally at 18. Cutting arrays 16 are each "blades" in
configuration, comprising a plurality of TSP elements, and
engage the earth formation as the drill bit rotates in
penetration of the earth. Gage pads 18 may serve a cutting
function, but normally would not unless extending radially
beyond those portions of cutter blades 16 which extend to
the gage of core bit 10.
Body 12 of bit 10 is preferably, at least in part, a
molded component fabricated through conventional metal
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infiltration technology, wherein body 12 comprises a
tungsten carbide matrix infiltrated with a copper-based
binder alloy when the bit mold is placed in a furnace and
heated to a temperature sufficient to melt the binder but
not the tungsten carbide, and below the thermal degradation
temperature o~ the cutting elements 20, which are
preferably TSP's.
In formation of the core bit 10 or a drill bit with
integral cutting arrays 16, the bit mold or "boat" is
carved, milled, or otherwise configured on its interior
with the exterior configuration of bit 10, including blades
16. The TSP elements 20 are then disposed in their
intended positions on the blades, and adhesively maintained
there to secure them in place until furnacing.
Alternatively, the TSP's may be affixed to a mesh, screen
or other support to maintain positioning and spacing, and
the mesh, screen or other support or the cutting elements
thereon secured to the mold area defining the front or
cutting face 22 of the cutting array. Tungsten carbide
powder is then placed in the mold, and vibrated to
uniformly compact it. Binder alloy is then placed in the
mold over the tungsten carbide, and flux above the binder.
Prior to placing the tungsten carbide powder in the mold, a
tubular bit blank 2g is suspended above the mold and
partially extended into the interior thereof. After
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loading the tungsten carbide powder and binder, the mold is
then placed in a furnace, and the binder alloy melted to
infiltrate the bit body tungsten carbide matrix. Upon
solidifying, the binder consolidates the matrix powder and
bonds the blank thereto. This bit blank is subsequently
interiorly threaded on the end extending out of the bit
body to form a bit shank 26, or may be welded to such a
threaded shank for connection to a coring tool. If a drill
bit is being made, the bit blank is exteriorly threaded or
may be welded to a threaded shank for connection to a drill
string or to the drive shaft of a downhole motor.
After the bit body 12 is furnaced and cooled, the
cutting elements 20 have been metallurgically secured into
cutting arrays 16 by the previously described means known
in the art. As in prior art bits, however, there is
residual thermal stress between the cutting elements 20 and
the matrix supporting the arrays 16. The present invention
comprises the incorporation of discontinuities 28 in the
cutting arrays 16, whereby residual thermal stresses are
minimized and localized.
In the embodiments of FIGS 1-3, discontinuities 28
comprise linear discontinuities of matrix material dividing
cutting arrays 16 into sub-arrays 30. Discontinuities 28
are oriented substantially parallel to the axis of the bit
10 and to the direction of travel of the bit 10 when it is
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in operation. In order to engage or sweep the formation
being cut by the arrays 16 from the inner gage 32 of the
arrays to the outer gage 3~, the discontinuities of each
blade may be radially offset ~rom those on the other blades
so that there is no rotational path swept only by matrix
material, which would obviously be detrimental to cutting
action and destructive to the arrays 16.
If it is desired to form an array 16 with
discontinui~ies but without gaps in the diamond cutting
face presented to the formation as the bit rotates, a
cutting array 116, shown in FIG. 4 of the drawings, may be
employed. In array 116, cutting elements 20 are again
grouped in sub-arrays 130, but the discontinuities 128 in
the array 116 are achieved by offsetting the sub-arrays 130
in the direction of rotation of the bit 10. The embodiment
of FIG. 9 thus interrupts residual thermal stress extending
across the cutting face 122 of the array 116 by placing
thermal stresses of each sub-array in different, offset
planes rather than by interrupting a single planar array of
cutting elements.
While the bit of FIGS. 1~3 utilizes triangular cutting
elements 20, and that of FIG. 4 employs square or
rectangular cutting elements 20, the shape and/or size of
,the elements 20 is not critical to and does not limit the
invention. For example, in FIG. 5 of the drawings, cutting
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elements 20 in array Z16 are of both shapes, and
discontinuities 22~ are oriented at an angle to the
direction of bit travel. Further, as the array 216 is
deeper or higher than that of the previously discussed
embodiments, discontinuities 228 are placed at two
differen~ angles so as to intersect and further subdivide
array 216 into sub-arrays 230. While discontinuities 228
are shown in FIG. 5 to intersect at a substantially right
angle, the invention is not so limited, and other
intersection angles have equal utility.
As shown in FIG. 6 of the drawings, intersecting
discontinuities 328 may be utilized in an array 316 so that
the array is divided horizontally and vertically instead of
at oblique angles as in array 216. In such an instance, it
would be desirable, as noted previously with respect to the
embodiment of FIGS. 1-3, to radially offset the vertical
discontinuities to achieve full cutting element coverage of
the face of the bit, and additionally to vertically offset
the horizontal discontinuities to avoid destruction of the
cutting arrays on the bit by presenting only matrix
material to the formation as the arrays wear and the
horizontal discontinuities are reached.
In both FIGS 5 and 6 the discontinuities are shown as
interruptions in the array of cutting elements 20 which are
filled with matrix material. However, the sub-array-offset
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type discontinuities depicted in FIG. 4 may be utilized in
lieu of, or even in addition to, t~le sub-array-interruption
type o discontinuity.
While it has not been established that a particular
discontinuity spacing is optimum, such being in large part
dependent upon the composition of the bit matrix and o~ the
cutting elements as well as the nature of the bond
therebetween, it is believed tha~ the discontinuities
should be placed at no more than substantially one inch
intervals in any one direction on the cutting face of the
array to prevent accumulation of large residual
thermally-induced stresses ~hich could augment impact
stresses encountered during drilling to promote bit
failure. In the unlikely event that the accumulated
residual stresses are sufficient to cause delamination of
elements 20 from the array under impact, the existence of
the discon~inuities will preclude the delamination and
failure of the sub-array from spreading to adjacent
sub-arrays.
The previously-disclosed embodiments of the invention
have been described and depicted in terms of perfectly
planar cutting arrays, but it should be understood and
appreciated that the term "planar" encompasses not only
both an array on a single plane and adjacent but offset
perfectly planar arrays, but also arrays, such as is
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depicted in FIG. 7 of the drawings, wherein cutting
el~ments 20 define an arcuate cutting surface 22. The
advantage of such an arcuate surface is to pro~ide
additional bonding capability between the bit matrix and
the elements 20 by allowing the matrix material as at 50 to
extend between adjacent elements 20. This provides not
only more opportunity for a strong metallurgical bond if
the elements are metal coated as is known in the art, but
also lends mechanical support.
While the drill bit and cutting array of the present
invention has been described in terms of preferred
embodiments, it will be understood that it is not so
limited. Those of ordinary skill in the art will
appreciate that many additions, deletions and modifications
to the preferred embodiments may be made without departing
from the spirit and scope of the claimed invention. For
example, the cutting array of the present invention may be
employed with a steel body bit, the array being pre-ormed
by hot pressing or inf,ltration techniques known in the
art. The preform is then post brazed or otherwise secured
2Q to the bit after the array is furnaced. Alternatively, the
cutting array might be formed on or bonded to a support
including one or more studs which are inserted in apertur~s
on the face of the bit, which technique also facilitates
replacement of worn or damaged cutting arrays, or tailoring
cutting element compositions to particular formations.
