Inclined chisel inserts for rock bits

An inclined chisel crested insert is disclosed for use on the gage row of a cone for a rotary cone rock bit. The insert has a different cone angle on opposite sides of the crown of the insert. An elongated conically shaped gage cutting surface of the insert provides point or line contact with a borehole wall as opposed to a full surface contact with the wall as is common with state of the art flat sided gage row inserts. This inclined chisel insert also has advantages over the symmetrical chisel type gage insert in that it is designed to provide increased crest length while providing the desired gage surface angle. The conically shaped gage row inserts with offset chisel crest are less prone to frictional heating due to the point or line contact design. As a result the elongated conical gage cutting surface of the chisel crest insert minimizes gage insert wear and subsequent breakage by eliminating high cycle thermal fatigue.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to rotary cone rock bits having hard metal cutter 
inserts strategically positioned within the rotary cones of the rock bit. 
More particularly, this invention relates to inclined chisel inserts used 
particularly in a gage row of a rotary cone for a rock bit. 
2. Description of the Prior Art 
There are a number of prior art patents that disclose inserts that have 
certain non-symmetric features. For example, U.S. Pat. No. 3,442,342 
discloses a rotary cone rock bit having tungsten carbide chisel inserts in 
a gage row of each of the three cones. After the bit is assembled, the 
sides of the gage row inserts are ground flat to the precise gage diameter 
of the hole to be drilled. The gage row inserts are intentionally 
installed so that the rock bit, when all three cones are in position, is 
overgage. The gage row inserts then have to be ground to provide a flat 
surface so that the diameter of the bit is correct. The patent goes on to 
teach that if there were no flats on the gage row inserts and the convex 
surface were simply tangent to a side wall of a borehole, there would be 
nothing but point contact and the borehole would quickly become undergage 
as the contact points of the inserts wore away. 
It has been determined, however, that gage chisel type inserts having flat 
spots ground therein provide a relatively large contact area against the 
borehole sides. Each of the inserts then can be susceptible to heat 
checking, resulting in premature wear and/or insert breakage. Insert heat 
checking can be defined as high cycle thermal fatigue due to intermittent 
frictional heat generated by borehole wall to gage insert contact and 
subsequent cooling by drilling fluid per each revolution. Certain 
formations such as shales can generate inordinate amounts of frictional 
heat at the borehole wall/gage insert interface. If the cobalt contents of 
the tungsten carbide alloy inserts is reduced or the tungsten carbide 
grain size is adjusted to reduce the tendency to heat check (independent 
of geometry change), then typically, the fracture toughness of the insert 
is reduced and the design is more susceptible to pure mechanical fatigue 
failure. 
U.S. Pat. No. 4,058,177 describes a non-symmetric gage row insert which 
provide a large wall contacting surface supposedly decreasing the wear on 
the gage insert because of the larger contact area and increasing the 
ability of the earth boring apparatus to maintain a full gage hole. The 
insert has a shape prior to assembly onto the rock bit apparatus that 
includes a base integrally joined to a non-symmetric head. The base is 
mounted within the cone and the head projects from the rock bit cone and 
includes an extended gage cutting surface that is flat. The gage cutting 
surface contacts the wall of the hole with the majority of the length of 
its extended surface. 
This patent, like the foregoing patent, provides a gage row insert with a 
large flat surface that parallels the borehole wall and thus is subject to 
the same insert degradation as the foregoing patent. 
Another U.S. Pat. No. 4,108,260, describes specially shaped non-symmetrical 
inserts to be used in rotary cone rock bits. The insert is generally 
chisel-shaped with flanks converging to a crest. The flanks are 
non-symmetrical with respect to each other, the leading flank is 
scoop-shaped and the trailing flank is rounded outwardly. This insert is 
designed for increased penetration in a rock formation. The insert is not, 
however, designed specifically for a gage row of a rock bit to maintain 
the gage of the bit as it is used in a borehole. 
Still another prior art U.S. Pat. No. 4,334,586, describes inserts for 
drilling bits. The insert cutting elements comprise non-symmetrical 
inserts placed in at least one circumferential row in a roller cone in 
alternating alignment. This non-symmetrical type insert is cone-shaped 
with the apex of the insert rounded and off-center. Each insert in the 
circumferential row is alternated so that its apex is not aligned with its 
neighboring insert, every other insert being so arranged in rows on a 
rotary cone of a rock bit. 
This non-symmetrical insert, like the foregoing insert, is not designed to 
be placed in a gage row of a cone to provide maximum gage protection 
during bit operation in a borehole. 
The foregoing prior art patents are disadvantaged, especially those patents 
that teach a flattened area to be positioned adjacent a gage row of a 
rotary cone. The large area flat surface paralleling the wall of a 
borehole makes the gage row inserts susceptible to heat checking thereby 
prematurely wearing the insert and, in many cases, causing the insert to 
fracture through thermal fatigue failure. When this occurs the rock bit 
quickly goes undergage, creating all kinds of problems for subsequent new 
bits that are placed back into the borehole for further penetration of a 
formation. If a dull bit is undergage when removed or "tripped" from the 
borehole, a following new full gage bit will immediately pinch, forcing 
the cones inwardly towards each other and rendering the bit useless 
thereafter. The remedy is a costly reaming operation to bring the borehole 
back to gage. 
Symmetrical chisel type inserts are sometimes used on gage and they do 
provide a conical rather than flat gage cutting surface adjacent to the 
borehole wall. However, the cutting surface of these inserts often does 
not closely parallel the borehole wall, therefore allowing the bit to go 
undergage much earlier. When the cone angle of a standard chisel insert is 
increased to improve the gage surface angle (or the angle between the side 
of the cone and the borehole wall), the extension of the insert becomes 
limited because the crest length decreases as the insert extension 
increases. Therefore, a special non-symmetrical insert is designed to 
provide increased crest length while providing the desired gage surface 
angle, thus providing maximum gage-keeping capability while minimizing 
wear on the special non-symmetric inserts as taught in the present 
invention. It has been found that conical-shaped gage cutting surfaces 
provide a more desirable line or point contact rather than a full surface, 
large area contact like a gage chisel insert having a flat side as 
indicated in the foregoing prior art. The conically shaped gage cutting 
surface reduces the possibility of heat checking that can lead to 
catastrophic failure of the insert. In other words, it is desirable to 
have a design balance between the thermal fatigue associated with heat 
checking and the mechanical fatigue associated with insert shape and 
respective strength. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a gage row insert for rotating 
cones of a rotary cone rock bit which balances maximum gage-keeping 
capabilities with minimum wear on the gage row inserts. 
More specifically it is an object of this invention to provide 
non-symmetrical chisel type gage row inserts wherein the gage cutting 
surface, being rounded, more closely parallels the wall of the borehole 
which will keep the bit in gage after some wear of the gage row inserts 
has occurred. 
A hard metal gage row insert for a roller cone rock bit is disclosed which 
consists of a generally cylindrical base portion at one end of the insert. 
The base portion of the insert is inserted into an insert hole formed by 
the cone, the insert forming an elongated crest portion at an opposite 
cutting end of the insert. The insert has to different conical surfaces on 
opposite sides of the elongated chisel crest. A first elongated conical 
surface is a gage cutting surface adapted to be in contact with a borehole 
wall formed in a formation by the rock bit. A second conical surface on an 
opposite end of the elongated chisel crest serves to support the chisel 
crest. The conical surface of the elongated gage cutting side of the 
insert is oriented with respect to the borehole wall such that the 
elongated conical surface makes, substantially, an initial point or line 
contact with the borehole wall prior to any wear of the insert during rock 
bit operation. The angle between the elongated conical gage cutting 
surface and the borehole wall may be between zero degrees and twenty-five 
degrees. The preferred angle between the conical gage cutting surface and 
the borehole wall is about at the midpoint between these two angles. 
An advantage, then, of the present invention over the prior art is the 
elongated conical gage cutting surface adjacent the borehole wall. 
Moreover, the inwardly facing, non-gage cutting, conical surface, adjacent 
the crest of the insert has a different conical surface than the conical 
surface of the elongated gage side, thereby allowing the insert to have a 
longer crest length. The non-symmetrical crested gage insert provides a 
more aggressive and less fragile looking insert as well as better bottom 
hole coverage. 
The above noted objects and advantages of the present invention will be 
more fully understood upon a study of the following description in 
conjunction with the detailed drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS AND BEST MODE FOR CARRYING OUT THE 
INVENTION 
FIG. 1 illustrates a prior art gage row chisel insert. The insert consists 
of a crest 5, a conical back surface 6, flat sides 4 and flat cutting 
surface 3. The prior art insert, before use, has a crest length 8. As the 
insert is worn during operation of the roller cone rock bit in a borehole, 
the flattened cutting surface 3, is progressively worn along dotted 
surfaces "b", "c" and "d". Surface "a" is the original flattened cutting 
surface prior to rock bit use. As is readily apparent, the crest length 8 
becomes narrower as the bit is worn down towards surface "d" resulting in 
a crest length 9 which is relatively small and fragile. When the prior art 
insert reaches this worn condition the shortened crest length easily 
breaks off resulting in catastrophic failure of the insert. 
Moreover, when tungsten carbide pieces mix in with the cuttings from the 
borehole bottom, the entire bit is in jeopardy oftentimes resulting in 
more broken inserts, or worse yet, loss of a cone on the bottom of the 
borehole. "Fishing" operations (retrieval of bit parts from the borehole 
bottom) are expensive and result in non-productive downtime for the rig 
operators. 
FIG. 2 shows the cutting face 3 of the prior art insert before use. Surface 
3, is relatively large and is oriented parallel or adjacent to a borehole 
wall during operation of the rock bit in a borehole. The insert is subject 
to frictional heat build up since there is a large surface area in contact 
with the borehole wall. As the insert wears during use the surface "a" 
becomes larger as it approaches condition "b", "c" and "d". This 
enlargement of the already enlarged cutting surface results in even 
greater frictional heat build up which, of course, accelerates failure of 
the inserts through thermal fatigue. 
FIG. 3 is an oblique section taken through FIG. 1 to show the sharp-angled 
corner 1 which transitions from the cutting face 3 to the insert sides 4 
on either side of the crested ridge 5. The sharp corner 1 is present 
through all stages of wear of "b", "c" and "d" and results in chipping and 
cracking along this vulnerable edge during the working of the rock bit in 
a borehole. The included angle between cutting face 3 and insert sides 4 
is about 110 degrees, resulting the sharp corner 1. 
FIG. 4 illustrates the broad contacting surface 3, and the sharp-angled 
corners 1, which intersects into the side flats 4 of the prior art insert 
2. 
The prior art FIGS. 1a through 1d illustrate the cutting surface 3 as it 
transitions through the various stages of wear. For example, in FIG. 1b 
the area "b" is widened with respect to the new surface "a" of FIG. 1a. In 
addition, the surface begins to heat check at location 7 near the center 
of worn surface "b". FIG. 1c shows a progression of wear "c" with the 
wider surface area and pronounced heat checking 7. Finally, the prior art 
FIG. 1d shows an extremely worn surface "d" that is thoroughly heat 
checked. The crest 5 is shortened and in danger of breaking off as is 
illustrated in the prior FIG. 1. 
The foregoing prior art gage row insert illustrated in FIGS. 1 through 4 
and FIGS. 1a through 1d clearly illustrate the degradation of these full 
contact inserts. The pronounced heat checking caused by the frictional 
heating of the enlarged areas 1a through 1d against the borehole wall is a 
major contributor to the early failure of rock bits incorporating these 
types of gage row inserts. Attempts to correct the heat checking through 
adjustments in tungsten carbide particle grain size or cobalt content, can 
create inserts that also have low fracture toughness values leading to 
increased mechanical fatigue failures. 
The perspective view of FIG. 5 illustrates a 3-cone rock bit. The rock bit 
generally designated as 10 consists of a bit body 12 having a pin end 14 
at one end and a cutting end generally designated as 16 at the other end. 
A rotary cone 18 is rotatively connected to a thrust bearing journal which 
is cantilevered inwardly from a rock bit leg 15 (not shown). The cone 18 
has, for example, a multiplicity of tungsten carbide cutter inserts 20 
interference fitted into holes drilled in the surface of the cone 18 (not 
shown). A series of gage row inserts 22 are pressed into holes drilled 
into an annular surface formed by the cone. The gage row inserts 22 
contact the borehole wall and ultimately determine the diameter of the 
borehole. A series of flush type button inserts 21, for example, may be 
pressed into the base of the cone. These inserts reinforce the gage row of 
the cone and serve to prevent degradation of the cone while it works in 
the borehole. 
Nozzle 17 provided in the bit body 12 directs hydraulic fluid toward the 
borehole bottom and serves to sweep detritous from the borehole and to 
clean and cool each of the cutter cones 18. In sealed bearing rock bits a 
lubrication chamber 19 is formed in each leg and serves to supply 
lubricant to the bearing surfaces formed between a journal and the cone 18 
(not shown). 
Turning now to FIG. 6, a partially cutaway rock bit leg 15 supports a cone 
18 which is rotatively secured to a journal bearing cantilevered from the 
leg 15. The gage row inserts of the present invention, generally 
designated as 22, are pressed into the gage row of the cone 18 with a 
cutting surface 42 facing towards the borehole or gage curve 26. The base 
40 of insert 22 is typically interference fitted within a hole drilled 
into the gage row of cone 18. The extended portion of the insert 22 is 
inclined or non-symmetrical and comprises an elongated conical cutting 
surface 42, a crest 44 and a conical back surface 45. The sides 43 of the 
insert are substantially flat and terminate at crested surface 44 of the 
insert 22. The conical cutting surface 42 is longer than the back conical 
surface 45. The angle with respect to a centerline of the insert is 
greater along the conical cutting surface 42 (hence longer) than the angle 
of back conical surface 45. The cutting surface 42 intersects a "gage 
curve" 26, and determines the diameter of a hole the rotary cone cutter 
cuts. 
A gage curve is a tool that rock bit engineers use to determine that the 
bit design in question will cut a specified hole diameter. A gage curve is 
defined as follows: 
For a bit of a given diameter, journal angle and journal offset, all the 
points that will cut the correct size hole projected into a plane through 
the journal centerline and parallel to the bit center. The foregoing 
definition is complicated by the fact that most rock bits utilize rotating 
cones that are offset from a true radial line emanating from the 
centerline of the rock bit. This parameter coupled with an oblique angle 
of the journal as is cantilevered off of the rock bit legs necessitates 
the use of the foregoing formulation to determine exactly where the gage 
row inserts will contact the borehole. Hence, the angle formed between the 
elongated cutting surface 42 of the insert 22 and the gage curve 26 should 
be an angle indicated as 28 that is between 0 degrees and 25 degrees. More 
specifically, this angle is optimized near the midpoint between these two 
angles. 
To put it another way, FIG. 7 illustrates a single cone shown in phantom as 
it is viewed when looking up a borehole at the bit. As stated before the 
gage row containing the gage row inserts 22 of the present invention 
establishes the diameter of the borehole 36. The cutting surface 42 of 
insert 22 contacts the borehole wall 37 at point "a" and the angle 30 
between the borehole wall 37 add elongated cutting surface 42 is between 0 
degrees and 25 degrees. The preferred angle being near the midpoint. This 
angulation (0.degree. to 25.degree.) between the gage row cutting surface 
42 and the borehole wall has been determined to provide the best angle of 
the point contact of cutting surface 42 with the borehole wall 37. 
By providing essentially a point contact "a" on an elongated rounded 
conical surface 42, the wear of the insert is minimized since surface 42 
is not flat. Even if elongated conical surface 43 is in full contact with 
a borehole wall (0 degree angulation between surface 42 of insert 22 and 
borehole wall 37) a line contact only would occur between the two 
surfaces, thereby greatly reducing the area of contact and the inherent 
frictional heat generation problems that result therefrom (not shown). To 
further clarify this aspect of th preferred embodiment, reference is now 
made to FIGS. 8, 9, 10 and 11, as well as FIGS. 8a through 8d. 
Referring now to FIG. 8 an insert of the preferred embodiment is shown and 
designated generally as 22. Insert 22 consists of base portion 40, the 
cutting end of the insert comprising an elongated conical cutting surface 
42, side surfaces 43 and conical back surface 45. The insert projection 
terminates at a rounded crest or crown portion 44. The elongated conical 
cutting surface 42 makes an initial contact with a borehole wall 37 (FIG. 
7) at surface "a" and as the insert works in the borehole it is worn 
through dotted surfaces "b", "c" and "d". As the insert wears from surface 
"a" through surface "d", the crest length 46 is reduced to crest length 
47. (Crest length 47, however, is much longer than the crest length of a 
standard symmetrical chisel insert with the same conical gage cutting 
surface. This is due to the fact that the insert 22 is non-symmetrical, 
the shortened conical backface 45 permitting the crest length 44 to be 
essentially longer in length.) Consequently, when the insert reaches the 
worn position "d" there is sufficient crest length 47 to adequately 
support the insert even though it is at an advanced state of wear. 
Referring now to FIG. 9, the insert is rotated 90.degree. so that we are 
now looking at the elongated cutting surface 42. In this view it is 
readily apparent that surfaces from "a" through "d" are much smaller in 
area than those surfaces depicted in the prior art FIGS. 1 through 4. 
Consequently, even though the insert wears, the worn surface area is 
smaller (more like a line contact) than the surface area of the prior art 
insert; hence, heat checking and fracturing of the insert is much more 
minimized. In addition, the corners 48 created between the worn surface 
and the conical surface 42 are much less severe. 
Referring now to FIG. 10, it can be seen through this oblique section taken 
through FIG. 8 that the corners 48 are very gentle and less severe than 
corners 1 of FIGS. 1 through 4. The included angle "f", for example, 
formed between progressively worn surfaces "b", "c" and "d" and elongated 
conical surface 42 is about 145 degrees. The included angle may be between 
114 degrees and 170 degrees. The included angle G of the prior art insert 
shown in FIG. 3, for example, has an included angle of about 110 degrees 
and is much more vulnerable to chipping and cracking as a result as 
heretofore described. Consequently, it is quite apparent that there is 
very little chance of the insert chipping or failing along this 
intersection between worn surfaces "b" through "d" and the elongated 
conical, or rounded surface 42 of insert 22. 
FIGS. 8a, 8b, 8c and 8d depict the insert through various stages of wear. 
FIG. 8a illustrates the elongated conical surface 42 of insert 22 with the 
initial point "a" in contact with a borehole wall 37 (FIG. 7) FIG. 8b 
shows the insert with a little bit of wear "b" that is devoid of sharp, 
angular corners typical of the prior art of FIGS. 1 through 4. FIG. 8c 
shows worn surface"c" which is still small in area. Since surface "c" is 
small in area it is not as subject to heat degradation as the prior art 
inserts. Finally, FIG. 8d shows an insert that is considerably worn yet, 
surface "d" is much smaller in area than surface "d" of FIG. 1d; hence, 
while the surface is worn the integrity of the insert of the instant 
invention is maintained because very little of the insert is worn away due 
to the line contact nature of the cutting surface 42. The gentle or less 
severe corners 48, also serve to maintain the integrity of the insert as 
it wears from surface "a" to surface "d" virtually eliminating 
catastrophic failures of the gage row inserts 22 as they are working in a 
borehole. 
The gage row inserts 22 may be of the enhanced type wherein the 
non-symmetrical insert is crowned with a layer of diamond (not shown). 
Such enhanced inserts are the subject of U.S. Pat. No. 4,604,106 entitled 
Composite Polycrystalline Diamond Compact assigned to the same assignee as 
the present invention. 
Moreover, the conically shaped non-symmetrical gage surface illustrated in 
FIG. 8 of the preferred embodiment is uniquely suited to the foregoing 
invention point or line contact with a borehole wall). It is well known by 
the diamond cutting insert manufacturers that full contact with a gage 
surface will create heat that is detrimental to a diamond cutting surface. 
The use of a diamond coated gage row insert of the present invention, 
wherein point contact conical gage surfaces are employed, virtually 
assures maintainance of the full gage dimater of the borehole since 
diamond surfaces do not wear or disintegrate when heat generation is 
controlled. These enhanced diamond layered inserts may be obtained from 
Megadiamond of Provo, Ut., a subsidiary of Smith International, Inc. 
The preferred embodiment (FIG. 8) of gage row insert 22, while at first 
glance does not appear to be much different than the prior art inserts, is 
surprizingly different in performance. The affect of the elongated conical 
surface 42 as it works in a borehole and the angle at which surface 42 
contacts the borehole wall is dramatically different than the inserts of 
the prior art. Thus, the insert of the instant invention is far superior 
to that illustrated in the prior art. Furthermore, the present invention 
teaches away from the principals set forth in the prior art. 
The principles taught in this invention may be utilized in borehole cutting 
tools other than rotary cone rock bits. For example, insert 22 may be 
employed in a drag bit or hole opener commonly employed in the petroleum 
industry. 
It will of course be realized that various modifications can be made in the 
design and operation of the present invention without departing from the 
spirit thereof. Thus, while the principal preferred construction and mode 
of operation of the invention have been explained in what is now 
considered to represent its best embodiments, which have been illustrated 
and described, it should be understood that within the scope of the 
appended claims, the invention may be practiced otherwise than as 
specifically illustrated and described.