Note: Descriptions are shown in the official language in which they were submitted.
CA 02565201 2006-11-10
75674-27D
DRL~,L BIT WITH CANTED GAGE II~SERT
BACKGROUND OF THE lItiTVENTION
The present invention relates generally to -diamond enhanced inserts for
use in drill bits and more particularly to diamond enhanced inserts for use in
the
; gage or near-ga,re rows of rolling cone bits. Still more particularly, the
present
invention relates to placement of a diamond coating on an insert and to
positioning the insert in a cone such that wear and breakage of the insert are
reduced and the life of the bit is enhanced.
An earth-boring drill bit is typically mounted on the louler end of a drill
string and is rotated by rotating the drill string at the surface or by
actuation of
downhole motors or turbines, or by both methods. With weight applied by the
drill
string, the rotating drill bit engages the earthen formation and proceeds to
form a
borehole along a predetermined path towa_Td a tarbet zone. The borehole
fosrrn.ed in
the drilling process will have a diameter generally equal to the diameter or
"gage" of
the drill bit_
A typical earth-boring bit includes one or more rotatable cutters that perform
their cutting function due to the rolling movement of the cutters acting
against the
formation material. The cutters roll and slide upon the bottom of the borehole
as the
bit is rotated, the cutters thereby engagina and disintegating the formation
material
in its path. The rotatable cutters may be described as generally conical in
shape and
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are therefore sometimes referred to as rolling cones. Such bits typically
include a bit
body with a plurality of journal segment legs. Each rolling cone is mounted on
a
bearing pin shaft that extends downwardly and inwardly from a journal segment
leg.
The borehole is formed as the gouging and scraping or crushing and chipping
action
of the rotary cones remove chips of formation material that are catried upward
and
out of the borehole by drilling fluid that is pumped downwardly through the
drill
pipe and out of the bit. The drilling fluid carries the chips and cuttings in
a slurry as
it flows up and out of the borehole. The earth disintegrating action of the
rolling
cone cutters is enhanced by providing the cutters with a plurality of cutter
elements.
The cost of drilling a borehole is proportional to the length of time it takes
to
drill to the desired depth and location. The time required to drill the well,
in turn, is
greatly affected by the number oftimes the drill bit must be changed in order
to
reach the targeted formation. This is the case because each time the bit is
changed,
the entire string of drill pipe, which may be miles long, must be retrieved
from the
borehole, section by section. Once the drill string has been retrieved and the
new bit
installed, the bit must be lowered to the bottom of the borehole on the drill
string,
which again must be constructed section by section. As is thus obvious, this
process, known as a "trip" of the drill string, requires considerable time,
effort and
expense. Accordingly, it is always desirable to employ drill bits that will
drill faster
and longer and are usable over a wider range of formation hardnesses.
The length of time that a drill bit may be employed before it must be
changed depends upon its rate of penetration ("ROP"), as well as its
durability or
ability to maintain an acceptable ROP. The form and positioning of the cutter
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elements on the cutters greatly impact bit durability and ROP and thus are
critical to
the success of a particular bit design.
Bit durability is, in part, measured by a bit's ability to "hold gage,"
meaning
its ability to maintain a full gage borehole diameter over the entire length
of the
borehole. Gage holding ability is particularly vital in directional drilling
applications. If gage is not maintained at a relatively constant dimension, it
becomes
more difficult, and thus more costly, to insert drilling assemblies into the
borehole
than if the borehole had a constant full gage diameter. For example, when a
new,
unworn bit is inserted into an undergage borehole, the new bit will be
required to
ream the undergage hole as it progresses toward the bottom of the borehole.
Thus,
by the time it reaches the bottom, the bit may have experienced a substantial
amount
of wear that it would not have experienced had the prior bit been able to
maintain
full gage. This unnecessary wear will shorten the bit life of the newly-
inserted bit,
thus prematurely requiring the time-consuming and expensive process of
removing
the drill string, replacing the worn bit, and reinstalling another new bit
downhole.
Cutter elements are generally of two types: inserts formed of a very hard
material, such as tungsten carbide, that are press fit into undersized
apertures in the
cone surface; or teeth that are milled, cast or otherwise integrally formed
from the
material of the rolling cone. Bits having tungsten carbide inserts are
typically
referred to as "TCI" bits, while those having teeth formed from the cone
material are
known as "milled tooth bits." In each case, the cutter elements on the
rotating
cutters functionally breakup the formation to form new borehole by a
combination
of gouging and scraping or chipping and crushing. While the present invention
has
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primary application in bits having inserts rather than milled teeth and the
following
disclosure is given in terms of inserts, it will be understood that the
concepts
disclosed herein can also be used advantageously in milled tooth bits.
In Figure 1 the positions of all of the cutter inserts from all three cones
are
shown rotated into a single plane. As shown in Figure 1, to assist in
maintaining the
gage of a borehole, conventional rolling cone bits typically employ a row of
heel
cutters 14 on the heel surface 16 of each rolling cone 12. The heel surface 16
is
generally frustoconical and is configured and positioned so as to generally
align with
the sidewall of the borehole as the bit rotates. The heel cutters 14 contact
the
borehole wall with a sliding motion and thus generally may be described as
scraping
or reaming the borehole sidewall. The heel cutters 14 function primarily to
maintain
a constant gage and secondarily to prevent the erosion and abrasion of the
heel
surface of the rolling cone.
In addition to heel row cutter elements, conventional bits typically include a
row of gage cutter elements 30 mounted in gage surface 31 and oriented and
sized in
such a manner so as to cut the corner of the borehole. For purposes of the
following
discussion, the gage row is defined as the first row of inserts from the bit
axis of a
multiple cone bit that cuts to full gage. This insert typically cuts both the
sidewall of
the borehole and a portion of the borehole floor. Cutting the corner of the
borehole
entails cutting both a portion of the borehole side wall and a portion of the
borehole
floor. It is also known to accomplish the corner cutting duty that is usually
performed by the gage cutters by dividing it between adjacent gage and nestled
gage
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cutters (not shown) such that the nestled gage cutters perform most of the
sidewall
cutting and the adjacent gage cutters cut the bottom portion of the corner.
Conventional bits also include a number of additional rows of cutter
elements 32 that are located on the main, generally conical surface of each
cone in
rows disposed radially inward from the gage row. These inner row cutter
elements
32 are sized and configured for cutting the bottom of the borehole and are
typically
described as inner row cutter elements.
In Figures 1, 3, 5 and 7, the positions of all of the cutter inserts from all
three
cones are shown rotated into a single plane. As can be seen, the cutter
elements in
the heel and gage rows typically share a common position across all three
cones,
while the cutter elements in the inner rows are radially spaced so as to cut
the
borehole floor in the desired manner. Excessive or disproportionate wear on
any of
the cutter elements can lead to an undergage borehole, decreased ROP, or
increased
loading on the other cutter elements on the bit, and may accelerate wear of
the cutter
bearing and ultimately lead to bit failure.
Relative to polycrystalline diamond, tungsten carbide inserts are very tough
and impact resistant, but are vulnerable to wear. Thus, it is known to apply a
cap
layer of polycrystalline diamond (PCD) to each insert. The PCD layer is
extremely
wear-resistant and thus improves the life of a tungsten carbide insert.
Conventional
processing techniques have, however, limited the use of PCD coatings to
axisymmetrical applications. For example, a common configuration for PCD
coated
inserts can be seen in Figures 1 and 2, wherein insert 30 comprises a domed
tungsten
carbide base or substrate 40 supporting a hemispherical PCD coating 42.
Inserts of
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this type are formed by forming a non-reactive container also known as a
"can",
corresponding to the external shape of the insert, positioning a desired
amount of
PCD powder in the can, placing the substrate in the can on top of the PCD
powder,
enclosing and sealing the can, and applying sufficient pressure and
temperature to
sinter the PCD and adhere it to the substrate. If required, the resulting
diamond or
substrate layers can be ground into a final shape following demolding.
The shape of PCD layers formed in this manner is based on consideration of
several factors. First, the difference in the coefficients of thermal
expansion of
diamond and tungsten carbide gives rise to differing rates of contraction as
the
sintered insert cools. This in turn causes residual stresses to exist in the
cooled insert
at the interface between the substrate and the diamond layer. If the diamond
layer is
too thick, these residual stresses can be sufficient to cause the diamond
layer to
break away from the substrate even before any load is applied. On the other
hand, if
the diamond layer is too thin, it may not withstand repetitive loading during
operation and may fail due to fatigue. The edge 61 of the diamond coating is a
particular source of stress risers and is particularly prone to failure.
For all of these reasons, PCD coated inserts have typically been
manufactured with a hemispherical top, commonly referred to as a "semi-round
top" or SRT. Referring again to Figure 2, the SRT 103 is aligned with the
longitudinal axis 41 of the substrate such that its center point lies
approximately
on axis 41. The inner surface of the diamond coating corresponds to the domed
shape of the substrate. Thus, the thickness of the diamond coating is greatest
on
the axis of the insert and decreases toward the edge of the coating layer.
While
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inserts in which the diamond coating is of uniform thickness are known, e.g.
U.S.
Patent No. 5,030,250, it is more common to form a diamond layer that decreases
in thickness as distance from the center point increases, resulting in the
crescent-
shaped cross-section shown in Figure 2. Nevertheless, it is contemplated that
diamond layer 42 can be other than crescent-shaped. For example, the thickest
portion of diamond layer 42 could comprise a region rather than a point. The
diamond layer typically tapers toward the outer diameter of the substrate (the
diamond edge 61). This tapering helps prevent cracks that have been known to
develop at the diamond edge when a substantially uniform diamond layer is
used.
Because of the interrelationship between the shape of each cone and the
shape of the borehole wall, cutter elements in the heel row and inner rows are
typically positioned such that the longitudinal axes of those cutter elements
are
more or less perpendicular to the segment of the borehole wall (or floor) that
is
engaged by that cutter element at the moment of engagement. In contrast,
cutter
elements in the gage row do not typically have such a perpendicular
orientation.
This is because in prior art bits, the gage row cutter elements are mounted so
that
their axes are substantially perpendicular to the cone axis 13. Mounted in
this
manner, each gage cutter element engages the gage curve 22 at a contact point
43
(Figure 2) that is close to the thin edge of the diamond coating on the
hemispherical top of each cutter element.
Still referring to Figure 2, the angle between the insert axis 41 and a radius
terminating at contact point 43 is hereinafter designated E. In prior art
bits, the
angle $ has typically been in the range of 54 to 75 , with E being greater
for
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harder formation types. For example, in a typical 12 1/o" rock bit, E may be
about
57
The prior art configuration described above is not satisfactory, however,
because contact point 43 is at the edge of diamond layer 4?, where the diamond
layer is relatively thin, and is subjected to particularly high stresses and
is
therefore especially vulnerable to cracking and breaking, which in turn leads
to
premature failure of the inserts in the gage row.
Accordingly, there remains a need in the art for a gage insert that is more
durable than those conventionally known and that will yield greater ROP's and
an
increase in footage drilled while maintaining a full gage borehole.
Preferably, the
gage insert would also be relatively simple to manufacture.
BRIEF SUMMARY OF THE INVENTION
In one aspect of the present invention, an earth-boring drill bit for drilling
a
borehole of a predetermined gage is provided that comprises a bit body having
a
bit axis and a plurality of rolling cone cutters, each rotatably mounted on
the bit
body about a respective cone axis and having a plurality of rows of cutting
inserts
thereon. One of the rows is a gage row with gage inserts located such that it
is the
first row of inserts from the bit axis that cuts the predetermined gage and
the
borehole corner substantially unassisted. The gage inserts have a generally
cylindrical base portion secured in the cone and defining an insert axis, and
a
cutting portion extending from the base portion. The cutting portion comprises
a
generally convex gage cutting surface with a center axis that is oblique to
the insert
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75674-27D
axis and at least a portion of the gage cutting surface is
enhanced with a super abrasive material.
In another aspect of the invention there is
provided a cutting insert for use in an earth boring drill
bit, comprising: (a) a generally cylindrical base portion
defining an insert axis; (b) a cutting portion extending
from the base portion comprising a generally convex gage
cutting surface with a center axis and enhanced with a super
abrasive material, the center axis of the gage cutting
surface canted with respect to the insert axis of the base
portion in a direction perpendicular to a direction of
rotation of a drill bit cutter.
In the present invention the axis of the gage
cutting surface of the gage insert is repositioned so that
it is more normal to the gage curve and less normal to the
cone axis. This decreases the angle alpha so that the
contact point on the gage insert is farther from the edge of
the diamond layer, thereby providing a thicker diamond layer
at the contact point and enhancing insert life and bit ROP.
BRIEF DESCRIPTION OF THE DRAWINGS
For a detailed description of a preferred
embodiment of the invention, reference will now be made to
the accompanying Figures, wherein:
Figure 1 is a side schematic view of one leg and
one rolling cone cutter of a rolling cone bit constructed
according to the prior art;
Figure 2 is an enlarged view of the gage insert of
Figure 1;
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75674-27D
Figure 3 is a side schematic view of one leg and
one rolling cone cutter of a rolling cone bit constructed in
accordance with a first embodiment of the present invention;
Figure 4 is an enlarged view of the gage insert of
Figure 3;
Figure 5 is a side schematic view of one leg and
one rolling cone cutter of a rolling cone bit constructed in
accordance with a second embodiment of the present
invention;
Figure 6 is an enlarged view of the gage insert of
Figure 5;
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Figure 7 is a side schematic view of one leg and one rolling cone cutter of a
rolling cone bit constructed in accordance with a alternative embodiment of
the
device of Figure 5;
Figure 8 is an enlarged view of the gage insert of Figure 7;
Figure 9 is a side schematic view of one leg and one rolling cone cutter of a
rolling cone bit constructed in accordance with a third embodiment of the
present
invention;
Figure 10 is an enlarged view of the gage insert of Figure 9;
Figures 11 and 12 are side views of a diamond enhanced insert, showing
one technique for constructing an insert having a canted diamond layer; and
Figures 13 and 14 are side views of alternative axisymmetric diamond
coated inserts that could be canted in accordance with the principles of the
present
invention.
In Figures 1, 3, 5, 7 and 9, the positions of all of the cutter inserts from
all
three cones are shown rotated into a single plane.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to Figures 3 and 4, according to a first embodiment of the
present invention, each gage cutter insert 30 is repositioned such that its
axis 41 is
no longer perpendicular to the cone axis 13. Instead, the axis 41 of each gage
cutter insert is rotated around the center of its hemispherical top such that
its base
is shifted toward the tip of the cone 12 and its axis 41 is more normal to
gage
curve 22. Rotation in this manner has the desired effect of moving contact
point
43 away from the edge 61 of diamond layer 42. Because the insert is rotated
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about the center of its hemispherical top, the gage curve 22 remains
tangential to
the surface of the insert and the cutting load is not altered.
Surface 31, which defines a land 35 around each insert, is reshaped so that
it remains perpendicular to axis 41. Modification of surface 31 in this manner
is
preferred because it provides better support for each cutter and because it is
generally easier to carry out the drilling and press-fitting manufacturing
steps
when the hole into which the insert is set is perpendicular to the land
surface.
Moreover, it allows all of the grip on base 40 to be maintained while also
allowing the extension portion of cutter element 30 to be unchanged.
According to one preferred embodiment, axis 41 is rotated until the angle
$ is between 0 and 50 , and more preferably is no more than 40 . It would be
preferable to reduce t to 0, if possible, but rotation of axis 41 is limited
by
geometry of the cone. That is, either the clearance between the bottom of an
insert in the gage row and an insert in the next, inner row becomes inadequate
to
retain the insert, or the holes for adjacent inserts run into each other.
Thus, it is
generally preferable to keep $ in the range of about 25 to 55 .
Referring now to Figures 5 and 6, according to a second embodiment of
the present invention, each gage cutter insert 30 is reconfigured such that
the
center point of its diamond insert layer 42 no longer coincides with axis 41.
Instead, diamond layer 42 and the axisymmetric SRT cutting surface defined
thereby are canted with respect to axis 41 such that the thickest portion of
diamond layer 42 is closer to the gage curve 22. Canting the SRT 103 in this
manner has the desired effect of moving contact point 43 away from the edge 61
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of diamond layer 42. It is preferred but not necessary that the thickest
portion of
diamond layer 42 be between axis 41 and contact point 43.
Cone surface 31 is reshaped so that each land 35 remains aligned with the
lower edge of the SRT. Thus, in this embodiment, surface 31 is no longer
perpendicular to axis 41. Modification of surface 31 in this manner allows the
amount of extension of insert 30 to remain unchanged. While the hole into
which
insert 30 is pressfit is no longer perpendicular to surface 31, this method
has the
advantage of maintaining a larger clearance between the base of each gage
insert
and the bases of adjacent inserts.
According to a preferred embodiment, the center point of the diamond layer
42 is shifted until the angle e(Figure 6), defined as the angle between axis
41 of
insert 30 and a radius through the thickest portion of diamond layer 42, is at
least 5 ,
and more preferably at least 10 . It is not typically possible to cant the SRT
by more
than about 45 . Canting the SRT results in $ being reduced by an amount
approximately equal to e, so that E preferably ranges from about 25 to about
55 .
When SRT 103, which extends outward from land 35, is canted, a wedge-
shaped portion 101 is defined between SRT 103 and the cylindrical portion of
base
40. Because both SRT 103 and the base portion 40 have circular cross-sections
with
substantially the same diameter, the outer surface of wedge-shaped portion 101
forms a transition between the surface of base 40 and the surface of SRT 103.
Referring now to Figures 7 and 8, an alternative embodiment of the insert
shown in Figures 5 and 6 again comprises an insert having a canted SRT. In
this
embodiment, however, the outer surface of base 40 is maintained as a right
cylinder
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and the geometry of the SRT is re-shaped so as to conform to the outer surface
of
base 40. Thus, the footprint of the diamond enhanced portion becomes an
ellipse,
rather than a circle, with its minor diameter equal to the diameter of base 40
and its
major diameter equal to the diameter of base 40 divided by the cosine of e and
cutting portion of insert 30 is no longer axisymmetric.
Referring now to Figures 9 and 10, according to a third embodiment of the
present invention, the concepts described with respect to Figures 3-8 above
are
combined. In this embodiment, the axis 41 of each gage cutter insert 30 is
rotated
around the center of its hemispherical top and each gage cutter insert 30 is
reconfigured such that the center point of its diamond insert layer 42 no
longer
coincides with axis 41. Together these modifications preferably result in a
reduction of $ to a range of about 15 to about 45 . For a typical 12 1/a"
rock bit,
>/ may be about 29 in this embodiment.
Referring now to Figures 11 and 12, one technique for creating an insert
having a canted diamond layer is to form an axisymmetric diamond-coated insert
70 having a cylindrical base 72. By cutting insert 70 on a plane 71 that forms
an
angle e with respect to a plane perpendicular to the axis of the insert 70, a
top
portion 74 is generated, as shown in Figure 11. When top portion 74 is rotated
180 and re-attached to base 72, it will be canted with respect to base 72 at
an
angle e that is equal to 2e.
Figures 13 and 14 illustrate a conical insert extension and a bullet-shaped
extension, respectively. Both of these axisymmetric shapes can be used in
inserts
having a diamond layer that is canted in accordance with the principles
disclosed
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herein. It will be recognized that the conical insert of Figure 13 is conical
only at
the lower portion of its extension, its tip being rounded to form a curved
cutting
surface.
It will be understood that the foregoing concepts have primary
applicability to diamond enhanced inserts in the gage row. Nevertheless, some
of
the principles disclosed herein can be applied to inserts in other rows, such
as a
nestled gage row, if the configuration of the cone and borehole wall would
otherwise cause each insert in that row to contact the wall at a point that is
close
to the edge of its diamond layer. For example, if desired, the canted SRT can
be
used on inserts occupying what is sometimes referred to as the nestled gage
row.
Likewise these concepts can be used to advantage in inserts having a non-
tapered
diamond layer of uniform thickness. Such inserts tend to be prone to cracking
near the edge of the diamond layer, so that moving the contact point away from
the diamond edge results in a longer-lived insert.
While various preferred embodiments of the invention have been shown
and described, modifications thereof can be made by one skilled in the art
without
departing from the spirit and teachings of the invention. The embodiments
described herein are exemplary only, and are not limiting. Many variations and
modifications of the invention and apparatus disclosed herein are possible and
are
within the scope of the invention. Accordingly, the scope of protection is not
limited by the description set out above, but is only limited by the claims
that
follow, that scope including all equivalents of the subject matter of the
claims.
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