Kerf-cutting apparatus for increased drilling rates

An improved earth drilling bit and method that cuts at least one annular kerf ahead of at least one rolling cutter and provides for rock chip removal and bit cooling thereby increasing drilling rate and bit performance. One embodiment of the invention includes a bit body having a lower end forming an annular kerf cutter for cutting an outer annular kerf, an inner drill member positioned concentrically within the bit body having a lower end forming an annular kerf cutter for cutting an inner annular kerf, and at least one rolling cutter mounted to the bit body and extending from the interior of the outer kerf cutter to the longitudinal axis of the drill bit. Other embodiments include a chipway port defined by the drill bit for channeling rock chips away from beneath the bit; baffles for directing and accelerating drilling fluid flow; cutting edges having connecting webbs providing egress for rock chip and drilling fluid; and slots extending along the bit body for reducing surge and swab pressure.

FIELD OF THE INVENTION 
The present invention relates generally to bits used in drilling earth 
formations. More specifically, the present invention concerns an improved 
apparatus and method for increased drilling rates by cutting concentric 
annular kerfs ahead of primary drilling means. 
BACKGROUND OF THE INVENTION 
Modern drilling operations used to create boreholes in the earth for the 
production of oil, gas and geothermal energy typically employ rotary 
drilling techniques. In rotary drilling, a borehole is created by rotating 
a tubular drill string having a drill bit secured to its lower end. As 
drilling proceeds, additional tubular segments are added to the drill 
string to deepen the hole. While drilling, a pressurized fluid is 
continually injected into the drilling string. This fluid passes into the 
borehole through one or more nozzles in the drill bit and returns to the 
surface through the annular channel between the drill string and the walls 
of the borehole. The drilling fluid carries the rock cuttings out of the 
borehole and also serves to cool and lubricate the drill bit. 
One basic type of rotary rock drill is a drag bit. Some drag bits have 
steel or hard faced edges, but primarily they have a main body into the 
outer surface of which are embedded extremely hard cutting elements. These 
cutting elements are typically made of natural or synthetic diamonds. As 
the drag bit is rotated, the cutting elements scrape against the bottom 
and sides of the borehole to cut away rock. 
Another basic type of rotary rock drill uses roller cone cutters mounted on 
the body of the drill bit so as to rotate as the drill bit is rotated. The 
angles of the cones and bearing pins on which they are mounted are aligned 
so that the cones essentially roll on the bottom of the hole with 
controlled slippage. One type of roller cone cutter is an integral body of 
hardened steel with teeth formed on its periphery. Another type has a 
steel body with a plurality of tungsten carbide or similar inserts of high 
hardness that protrude from the surface of the body somewhat like teeth. 
As the roller cone cutters roll on the bottom of the hole being drilled, 
the teeth or carbide inserts apply a high compressive load to the rock and 
fracture it. The cutting action of roller cone cutters is typically by a 
combination of crushing, chipping and scraping. The cuttings from a roller 
cone cutter are typically a mixture of moderately large chips and fine 
particles. 
When drilling rock with a roller cone cutter, the fracture effect of 
loading on the teeth of the rock bed is limited due to the rock matrix 
surrounding the borehole. Failure of rock is prevented in a large degree 
by the restraint to movement offered by the surrounding rock. Thus, it 
appears in usual drilling operations that small cracks are created in the 
rock which return to the surface of the bottom of the wellbore creating 
chips instead of propagating deep into the rock itself. Thus, the bit 
tooth of the usual rock bit presses on the rock surface tending to create 
small cracks which propagate downward, but by virtue of the resistance to 
fracture offered by the surrounding rock matrix, a crack follows the path 
of least resistance and emerges at the surface on the bottom of the 
wellbore, thus creating the small chips. 
U.S. Pat. No. 3,055,443 to Edwards disclosed a combination drag bit and 
roller cone cutter which removes the lateral restraint on a core to be 
drilled. The drag bit component cuts a single annular kerf forming a core 
which is received within a hollow body member and drilled by multicone 
rolling cutters arranged within the hollow body member. Windows are 
provided in the bit body adjacent to the multicone cutters to provide an 
egress for chips formed by the destruction of the core. This bit design 
causes rapid failure of the drag cutters, however, since virtually all the 
drilling fluid escapes through the windows and results in insufficient 
fluid flow to cool the drag bit component. 
U.S. Pat. No. 4,892,159 to Holster describes a kerf-cutting bit wherein 
resistance of the rock to fracture is removed or reduced by employing a 
drill bit which destroys the rock rapidly and efficiently. The drill bit 
of Holster cuts multiple annular kerfs which result in more rapid drilling 
rates than those achieved by cutting a singular annular kerf. 
The present invention is an improvement over that described in U.S. Pat. 
No. 4,892,159 and U.S. Pat. No. 3,055,443. The improvements of the present 
invention relate to how and where rolling cutters of the drill bit are 
attached to the bit body; the use of baffles and internal flow passages to 
improve the egress of rock cuttings as they are generated at the bottom of 
the wellbore by the drilling action of the rolling cutter; and the use of 
connecting webbs between individual kerf cutting elements to provide 
convective cooling of the cutting edges and to assist in rock chip removal 
from within the kerf cutters. 
SUMMARY OF THE INVENTION 
In one embodiment of the invention, a drill bit comprises a bit body with a 
lower end forming an outer kerf cutter; at least one inner drill member 
positioned concentrically within the bit body with a lower end forming an 
inner kerf cutter; and at least one rolling cutter attached by mounting 
means to the bit body in a manner to permit rotation relative to the 
mounting means wherein the rolling cutter(s) extend from the interior of 
the outer kerf cutter to the longitudinal axis of the drill bit. 
In another embodiment, a drill bit comprises an outer kerf cutter, at least 
one inner kerf cutter, at least one rolling cutter, and at least one 
chipway port formed by the bit body and inner drill member extending from 
within the inner drill member and through the inner drill member and bit 
body to an outer surface of the bit body for channeling rock chips away 
from beneath the bit. 
In yet another embodiment of the invention, the drill bit comprises an 
outer kerf cutter, at least one inner kerf cutter, at least one rolling 
cutter, and a plurality of baffles extending between the inner and outer 
kerf cutters adjacent to the rolling cutters for directing drilling mud 
discharged from a conduit over the rolling cutters. 
In still another embodiment, the drill bit comprises an outer kerf cutter, 
at least one inner kerf cutter, and at least one rolling cutter wherein 
cutting edges of the outer and inner kerf cutters protrude from a 
connecting webb forming spaced between the protruding cutting edges to 
provide an egress for small rock chips and drilling fluid from beneath the 
bit and to provide convective cooling of the cutting edges of the kerf 
cutters. 
In another embodiment, the bit body defines at least one longitudinally 
elongated slot forming a passage extending generally along the bit body 
for reducing pressure while the drill bit is moving in a wellbore. 
In other embodiments of the invention, multiple annular kerfs may be cut by 
use of more than one inner drill members or only a single kerf may be cut 
by omitting the inner drill member. 
A further aspect of the present invention is a method of drilling a 
wellbore in an earthen formation comprising cutting at least one annular 
kerf into the formation; grinding material from within the annular kerf by 
a rolling cutter means; delivering drilling fluid to the bottom of the 
wellbore; removing rock chips generated by the cutting by flow of the 
drilling fluid through a chipway port; and removing rock chips generated 
by grinding and cooling kerf cutting edges by flow of the drilling fluid 
around webbs located between the kerf cutting edges. 
The above inventive embodiments may be employed separately or in 
combination.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Five specific improvements are provided by this invention over the prior 
art kerf-cutting bits of Holster (U.S. Pat. No. 4,892,159) and Edwards 
(U.S. Pat. No. 3,055,443). A first improvement relates to the reduction in 
the number of rolling cutters and the location and method of attachment to 
the drill bit body. Second, third and fourth improvements relate to 
increasing bit capability in removing drilled rock chips at a faster rate 
and increasing the cooling provided to the cutting elements mounted on the 
kerf cutters. A fifth improvement provides a method to reduce the surge 
and swab pressures associated with running bits having kerf cutters into 
and out of a wellbore. 
FIGS. 1, 2, and 3 illustrate a drill bit 10 incorporating a preferred 
embodiment of the present invention. Bit 10 includes a bit body 12 
provided on its upper end with connecting means 14 in the form of the 
usual pin for the attachment to the lower end of a hollow drill string. 
Any suitable connecting means may be employed in this invention, however. 
Bit body 12 is provided on its lower end with an outer kerf cutter 16 in 
the form of a kerf cutting skirt having a generally saw-tooth 
configuration, a plurality of face plates 18 comprising diamonds, 
including either natural or synthetic [such as polycrystalline diamond 
compact material (PDC)], attached to cutting faces of cutter 16, and 
defining outlet passages 20 between adjacent skirt teeth. The outer kerf 
cutter skirt 16 may be molded or machined as an integral part of bit body 
12 or it may be in the form of a cylindrical ring attached to bit body 12 
by welding or threading. Bit body 12 is further provided with a plurality 
of spaced apart junk slots or grooves 22 extending longitudinally from 
cutter 16 toward the upper end of bit body 12. The combination of outlet 
passages 20 and spaced apart grooves 22 aids removal of cuttings and 
drilling fluid from the kerfs below the bit 10 and cools the cutters 18 on 
the outer kerf cutting skirt 16. Bit body 12 may be still further provided 
with gauge wear pads 25 in the conventional manner for slowing the rate of 
wear on a bit body made of steel or other suitable hard material. Gauge 
wear pads 25 may comprise tungsten carbide buttons and may be press-fit 
into pre-drilled holes on the surface of bit body 12 between grooves 22 so 
that pads 25 are flush with the surface of bit body 12. 
Drill bit 10 also includes an inner drill member 24 positioned 
concentrically within bit body 12. Inner drill member 24 is connected at 
its upper end to bit body 12. Connection to bit body 12 may be in any 
manner including welding, threading, molding, or machining the bit body 
and inner drill member as one piece. Inner drill member 24 is provided on 
its lower end with an inner kerf cutter 26 in the form of a kerf cutting 
skirt having a generally saw-tooth configuration, a plurality of face 
plates 19 comprising diamonds, including either natural or synthetic, 
attached to cutting faces of cutter 26, and defining outlet passages 21 
between adjacent skirt teeth. 
As shown in FIG. 2, drill bit 10 further includes two spaced-apart rolling 
cutters 28 in the form of roller cone cutters attached by mounting means 
29, such as journal segments, to bit body 12 in a manner to permit 
rotation relative to mounting means 29. Other conventional bearings such 
as floating sleeve friction or roller bearings may be used in place of 
journal bearings. Rolling cutters 28 are provided with cutting teeth 30 
having cutting edges, said cutting teeth 30 comprising tungsten carbide or 
other suitable material. Rolling cutters are positioned so that lowermost 
cutting edges of cutting teeth 30 are above the teeth of outer and inner 
kerf cutters 16 and 26. The spacing of cutting teeth 30 on rolling cutters 
28 may be varied in the conventional manner to minimize tracking and 
maximize cutting efficiency by assuring cutting over the full face of 
rolling cutters 28. The angle between the journal axis of each rolling 
cutter 28 and a radial line perpendicular to the bit longitudinal axis at 
the point of attachment of each rolling cutter 28 may also be varied to 
minimize tracking and maximize cutting efficiency. The external contour of 
rolling cutters 28 may also be varied to accommodate the angles of 
attachment to allow for rotation of rolling cutters 28. As an alternative, 
rolling cutters may have spaced apart cutting discs rather than individual 
teeth. 
In one preferred embodiment of the invention, shown in FIGS. 1, 2, and 3, 
rolling cutters 28 extend from the inner surface of the outer kerf cutter 
to the longitudinal axis of the drill bit. When inner drill member 24 is 
present, rolling cutters 28 penetrate through a space defined by inner 
drill member 24. This embodiment represents one improvement over the prior 
art. The inventive attachment increases the bit loading that can be 
applied during drilling without causing premature structural failure. This 
improvement is achieved by reducing the total number of rolling cutters, 
increasing the size of the bearings, pins, and/or journals onto which they 
are affixed, and improving the location of the attachment point of the 
rolling cutters. Specifically, the rolling cutters are preferably in the 
form of rolling cones, and the rolling cones are attached to journal 
bearings which are cantilevered from locations just inside outer kerf 
cutter 16. The inner and outer rolling cutters of the prior art now 
effectively become unitized into one piece. In a preferred embodiment, two 
unitized pieces are present, each passing through openings 27 cut in inner 
drill member 24. In the most preferred case, two openings 27 exist, each 
located opposite the other, or 180.degree. around the circumference of the 
inner kerf cutting skirt from the other. The opening clearances are more 
easily visualized by examining FIG. 4 which shows a side view of inner 
drill member 24 with opening 27 cut through inner drill member 24 and the 
respective rolling cutter through opening 27. Outer kerf cutter 16 has 
been removed to enhance visualization of the opening in inner drill member 
24 and the resulting clearance between the rolling cutter 28 and inner 
drill member 24. Openings 27 are sized to be large enough to allow egress 
of rock chips generated in the annular space between the inner and outer 
kerf cutters 26 and 16, not shown. 
Rock chips and cuttings are removed from between and beneath rolling 
cutters 28 and outer and inner kerf cutters 16 and 26 by drilling fluid 
delivered through bit 10 by means of a drilling fluid conduit 36, shown in 
FIG. 2, which connects to the hollow drill string, now shown. Drilling 
fluid is delivered separately to jet nozzles by fluid passageways 38 and 
40, as shown in FIG. 5. Passageways 38 discharge drilling fluid through 
jet nozzles 42 located between each of rolling cutters 28 and the inner 
and outer kerf cutting skirts 26 and 16; passageways 40 discharge drilling 
fluid through jet nozzles 44 located inside the inner kerf cutter 26. 
In another preferred embodiment, drill bit 10 includes chipway ports, 31, 
shown in FIG. 2, formed by bit body 12 and inner drill member 24 extending 
from within inner drill member 24 through inner drill member 24 and bit 
body 12 to an outer surface of bit body 12 for channeling rock chips away 
from beneath the drill bit 10. This embodiment represents a second 
improvement to kerf-cutting bits of the prior art. Chipway ports 31 
connect the space within inner drill member 24 to the exterior of bit 10 
for the purpose of assisting in removal of the rock chips. These chipway 
ports 31 effectively trail or follow rolling cutters 28 as bit 10 is 
rotated about the bottom of a hole and provide a path for rapid egress of 
chips generated beneath the drill bit. 
For the bit shown in FIGS. 1 through 5, the drilling fluid carries cuttings 
and rock chips from the regions around and beneath the kerf cutters and 
rolling cutters through ports 31 extending through bit 10, through outlet 
passages 20 and 21, and around bit 10 through grooves 22. 
In another preferred embodiment, drill bit 10 includes a plurality of 
baffles 35 shown in FIGS. 3 and 4 which may be formed by bit body 12 and 
which extend between outer and inner kerf cutters 16 and 26 (now shown) 
for directing and accelerating drilling mud discharged from fluid 
passageways 38 over the plurality of rolling cutters 28. This third 
improvement over the prior art may be added to expedite chip removal. 
Baffles 35 can be added to the bottom of bit body 10, between the annular 
cutters 16 and 26 and immediately adjacent to either side of each of the 
rolling cutters 28. For the two-cone bit shown in FIGS. 1, 2, and 3, four 
baffles 35 are present. FIG. 4 shows the profile of a pair of baffles 35 
adjacent to rolling cone cutter 28. Baffles 35 serve to direct and 
accelerate the drilling mud being discharged from jet nozzles 42 located 
between the kerf cutters 16 and 26 over rolling cutters 28 at the point 
where the rock chips are being created, as they are being created. This 
high velocity flow which is parallel to the bottom of the hole can more 
effectively entrain rock chips than if the flow were not otherwise 
accelerated to a high velocity. The high velocity drilling fluid flow and 
the freshly cut rock chips then flow through openings 27 in the inner 
drill member 24 toward the interior of the bit and the chipway ports 31. 
The drilling fluid flow and entrained rock chips may then be egressed 
through chipway ports 31 previously described. 
In another embodiment of the present invention shown in FIGS. 6 and 7, 
cutting edges of the outer and inner kerf cutters 16 and 26 protrude from 
connecting webb 70 with spaces 20 formed between the protruding cutting 
edges. This embodiment represents a fourth improvement to prior art. Since 
the webbs between each cutting edge or face plate of the saw-tooth annular 
kerf cutters 16 and 26 extend into the annular kerfs cut in the rock 
passage of large rock chips generated by the rolling cutters is blocked, 
forcing the larger chips to egress out the chipway ports 31. However, the 
clearances, spaces 20, on either side of webbs 70 between the webb 
extensions and rock kerfs provide a path for drilling fluid and small rock 
chips to pass under the annular kerf cutter skirts and thereby provide 
convective cooling to the cutter face plates on the annular kerf cutters 
as depicted in FIGS. 6 and 7. In this manner, convective cooling can also 
be applied to the annular kerf cutter of the single kerf cutter bit of 
prior art (Edwards U.S. Pat. No. 3,055,443) and overcomes the heating 
problems encountered in application of that prior art. 
FIGS. 6 and 7 illustrate how cooling of outer and inner kerf cutters 16 and 
26 and chip removal from beneath the outer kerf cutter 16 are 
accomplished. Drilling mud, which is discharged from nozzles 42 (not 
shown) above and between kerf cutters 16 and 26, flows downward on the 
inside of the outer kerf cutter 16 in the space between earth material 49 
and webb 70. Upon reaching space 20, the mud flow turns and passes in an 
upward direction through groove(s) 22 located on the outer side of outer 
kerf cutter 16. The flow path is bounded by webb 70, the inner wall 68, 
the bottom 62, and the outer wall 66 of the outer annular kerf. The curved 
lines with arrows shown in both FIGS. 6 and 7 represent mud flow 
streamlines and serve to illustrate how this flow pattern provides cooling 
to the cutter face plates 18. 
A similar webbed pattern is provided on the lower end of inner kerf cutter 
26 to provide cooling to the plurality of face plates 19 attached to the 
cutting faces of the inner kerf cutter 26. 
Optionally, the relative vertical location of the rolling cutters could be 
adjusted downward so that the lowermost cutting teeth on the rolling 
cutters are sufficiently below the highest portion of the webbed structure 
between adjacent kerf cutter face plates so as to allow both increased 
drilling fluid flow for cooling and egress of larger rock chips generated 
by the rolling cutters. An additional option with this improvement to the 
single kerf cutter bit of prior art would be to place the rolling cutters 
that are interior to the single kerf cutter such that the point of tooth 
contact 64 with the rock is well below the uppermost portion of each of 
the webbs between adjacent kerf cutter face plates, thereby further 
increasing the portion of the space 20 that allows rock chip passage. 
Removal or elimination of the internal chipway ports would then force all 
rock chips and drilling fluid flow to exit from below the drill bit 
through the various spaces 20 under the annular kerf cutter 16 and up 
grooves 22. 
Drill bit 10 may include a plurality of longitudinally elongated slots 37, 
shown in FIGS. 8 and 9, forming channels or passages extending generally 
longitudinally along the bit body for enhanced removal of rock chips and 
for reducing surge or swab pressure while drill bit 10 is moved in and out 
of a wellbore. The slot does not extend to the edges of the outer kerf 
cutter as do grooves 22 and it may penetrate through the bit body into the 
space defined between the bit body and inner drill member depending on how 
drill bit 10 is constructed. While drilling, the presence of slots 37 will 
negate the effect of baffles 35, however. Therefore, if this option is 
used, use of baffles 35 is unnecessary. This fifth improved feature can be 
added as an option to the kerf cutting bit described herein. In certain 
drilling situations, the density, the viscosity and the gel-strength of 
the drilling fluid may be sufficiently high so as to create large surge 
pressures and swab pressures as the drill bit is lowered into the hole or 
raised from the bottom of the hole, respectively. By adding preferably at 
least two slots 37 connecting the space bounded by kerf cutters 16 and 26 
to the shank or upper portion of the bit, the magnitude of the surge and 
swab pressure will be reduced while running the bit to the bottom of a 
wellbore or extracting it to the surface. 
In another embodiment of the present invention shown in FIG. 10, kerf 
cutter 16 or 26 may form a kerf-cutting skirt having a generally saw-tooth 
configuration provided with abrasive resistant means 50 embedded on 
cutting faces of the cutter. Abrasive resistant means 50 may comprise 
diamonds, including either natural or synthetic [such as thermally stable 
polycrystalline diamond material (PDC)], diamond-tungsten carbide matrix, 
carbides such as, tungsten carbide, boron carbide or silicon carbide, or 
any other suitable hard material. 
In another embodiment shown in FIG. 11, the kerf cutters 16 or 26 may 
comprise a plurality of studs 52 protruding from the lower end of bit body 
12 or inner drill member 24 and may be provided with abrasive resistant 
means on cutting faces of studs 52. Again, abrasive resistant means may 
comprise diamonds, including either natural or synthetic, diamond-tungsten 
carbide matrix, carbides such as, tungsten carbide, boron carbide or 
silicon carbide, or any other suitable hard material. Face plates 23 
comprising diamonds, including either natural or synthetic, may also be 
attached to cutting faces of studs 52 as shown in FIG. 11 and may be 
constructed from PDC disks. This embodiment of kerf cutter 16 or 26 may be 
constructed by press fitting studs 52 into holes pre-drilled in the lower 
end of bit body 12 or inner drill member 24. 
In still another embodiment of the invention shown in FIG. 12, a plurality 
of inner drill members provided on lower ends with kerf cutters 26 and 26' 
are positioned concentrically one within the other and within the bit body 
which is provided on its lower end with kerf cutter 16. Each of the 
plurality of inner drill members is connected at its upper end to bit body 
12. Rolling cutters 28 are attachedly arranged to bit body 12 and are 
positioned so that lowermost cutting edges are above the cutting edges of 
the kerf cutters of the bit body and inner drill members. Upon rotation of 
the bit, annular cutters 26, 26', and 16 cut concentric annular kerfs 
ahead of rolling cutters. Rolling cutters 28 remove material between 
concentric annular kerfs and material surrounded by and within the 
innermost annular kerf. Cuttings and rock chips are removed from between 
and beneath annular cutters 26, 26', and 16 and rolling cutters by 
drilling fluid discharged from jet nozzles 42 and 42' located between 
rolling cutters 28 and jet nozzle 44 centrally located within kerf cutter 
26. There may be one, two, or three rolling cutters 28 and any number of 
inner kerf cutters (26 and 26'), to include 0, 1, and 2. For the purposes 
of example, two rolling cutters and two inner kerf cutters are shown here. 
In yet another embodiment of the invention shown in FIG. 13, the inner 
drill member is absent and a single kerf is cut by the outer kerf cutter 
16. This embodiment is useful for drilling small holes requiring small bit 
sizes. For small bit sizes, it may not be possible to fit more than one 
kerf cutter on the bit, and the use of only one kerf cutter is therefore 
within the scope of this invention. Although two or more kerfs are 
preferred, one kerf will improve the rate of penetration above bits 
cutting no kerfs. In small bit sizes, it is also anticipated that only one 
rolling cutter may be used. This embodiment will provide a large journal 
bearing and greater overall structural strength and load capacity for 
small diameter drill bits such as diameters of six inches or less. 
FIGS. 14 and 15 illustrate the bottom of a borehole 47 in which outer kerf 
cutter 16 has cut an outer annular kerf 46 and inner kerf cutter 26 has 
cut an inner annular kerf 48 positioned concentrically within the outer 
annular kerf 46 upon rotation of bit 10, shown in FIGS. 1 through 5. Since 
the lowermost cutting edges of rolling cutters 28 are positioned above the 
teeth of outer and inner kerf cutters 16 an 26, the annular kerfs 46 and 
48 are cut into the earth material 49 ahead of rolling cutters 28 thereby 
removing lateral restraint from material 49 between and within the outer 
and inner kerf cutters. Rolling cutters 28 fracture and remove material 
from between and within annular kerfs 46 and 48 rapidly and efficiently by 
crushing, chipping, grinding, and scraping action of the cutting teeth 30. 
Another aspect of the present invention is a method of drilling a wellbore 
in an earthen formation comprising cutting at least one annular kerf into 
the formation by a drill bit having a kerf cutting means with cutting 
edges positioned on the lower end thereof; grinding material from within 
the annular kerf by a rolling cutting means positioned within the drill 
bit; delivering drilling fluid to the lower end of the drill bit; removing 
rock chips generated by the grinding by flow of drilling fluid through a 
chipway port extending from within the kerf cutting means to an outside 
surface of the drill bit; and removing rock chips generated by the kerf 
cutting means and cooling the cutting edges of the kerf cutting means by 
flow of drilling fluid through outlet passages wherein said passages are 
defined between adjacent protruding cutting edges of said kerf cutting 
means. 
It is to be understood that any combination of the embodiments of the 
invention including rolling cutter and kerf cutter variations described in 
the above embodiments are included in the present invention. 
In order to illustrate the benefits of a multi-kerf cutting bit, laboratory 
drilling experiments were conducted using pre-kerfed rock and an oil field 
type bit, as discussed in Example I. 
Also a kerf-cutting drill bit incorporating some of the embodiments of the 
present invention was tested, as discussed in Example II, to prove that 
the bit can indeed achieve increased drilling rates over conventional oil 
field bits designed for the same rock type. 
EXAMPLE I 
Slabs of Carthage Marble were prepared by sawing 36 in. long by 15.5 in. 
diameter cores into six slabs each. Using a drill press with diamond core 
saws, some of these slabs were cut to have a single annular kerf, some 
slabs were cut to have multiple annular kerfs, and other slabs were left 
uncut. The slabs were then stacked and cemented together to form 36 in. 
long test samples. The assembled test samples were then jacketed with 
rubber and sealed by placing metal plates at each end. The top plate had 
an opening to allow a bit to pass through and contact the rock. These top 
and bottom plates were held in contact with the rock by threaded steel 
rods that extended axially along the perimeter of the samples and loaded 
in tension, thereby compressing the individual pre-kerfed slabs together 
tightly. The rubber sleeve was tightly wrapped around the entire sample to 
seal out confining fluid. Cutting a single kerf ahead of the primary rock 
cutting tool increased drilling rate by 63%, whereas cutting two 
concentric kerfs increased drilling rate by more than a factor of 4. Depth 
of kerf appears to be important when single kerfs are present, but much 
less significant when two or more annular kerfs have been cut. It was also 
found that the benefits are most apparent when the roller cone bit cutting 
structure is well matched to the kind of rock being drilled. 
EXAMPLE II 
A 9 7/8" diameter bit with two kerf cutters, two rolling cone cutters that 
are attached on journal bearings extending from the interior of the outer 
annular cutter, each passing through openings cut in the inner drill 
member, and four baffles located in the annular space between the two 
annular cutters, and with two "chip way" ports extending from the space 
inside the inner drill member to the exterior of the bit at its shank was 
tested. Two rock types were drilled with this bit over a wide range of bit 
loadings. The "weight-on-bit" (forces applied normal to the rock face) 
varied from 15,000 lb to 50,000 lb. Bit rotational speeds varied from 40 
to 120 revolutions per minute. The drilling rates obtained were compared 
with those obtained from similar experiments with conventional rolling 
cone bits with equivalent cutting structures on their rolling cones (IADC 
537 bits). The results are compared in FIGS. 16 and 17. FIG. 16 shows the 
measured drill rates or rates of penetration (ROP) for the bit with 
annular kerf cutters and a conventional IADC 537 bit loaded over a similar 
range of weights and rotational speeds while drilling Mancos shale rock. 
The resulting ROP's are plotted as functions of the bit loading parameter 
WOB .sqroot.N/d which allows for normalization and compression of the 
data. Note that at an equivalent loading parameter value of 30,000, the 
bit with the two annular kerf cutters, two rolling cones openings, baffles 
and egress ports drills about four (4) times as fast as the conventional 
rolling cone bit (IADC 537). FIG. 17 shows a similar comparison of the 
same two bits while drilling carthage marble (a medium hard limestone). At 
equivalent bit loading parameter values of 50,000, the bit with the 
features described herein drilled approximately 2 times as fast as the 
conventional IADC 537 bit. 
The preferred embodiments of the present invention have been described 
above. It should be understood that the foregoing description is intended 
only to illustrate certain preferred embodiments of the invention and is 
not intended to define the invention in any way. Other embodiments of the 
invention can be employed without departing from the full scope of the 
invention as set forth in the appended claims.