Drill bit having combined positive and negative or neutral rake cutters

A drill bit, and a cutter for a drill bit, are provided, wherein the cutter will encounter the formation with cutting surfaces of differing rake angles to optimize cutting efficiency. In most circumstances, the cooperating cutters will have differing, positive, and negative or neutral rakes. Cutters of differing rakes may be cooperatively paired on a drill bit such that the portion of a formation which is affected by the action of one cutter may be similarly affected by the operation of the other cutter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to drill bits and drill bit cutter 
arrangement primarily for use in plastic formations, and more particularly 
relates to a bit that includes cooperative combinations of positive and 
neutral or negative rake cutters. 
2. State of the Art 
Conventional rotary drill bits typically employ hardened cutters formed of 
materials such as polycrystalline diamond compacts (PDC's), boron nitride, 
or tungsten carbide and disposed on the bit face in order to produce 
shearing forces in the formation to be cut. Ordinarily, these cutters are 
angularly positioned on the face of the drill bit according to the 
formation material that they are designed to cut. 
For example, positive rake or "front raked" cutters have an angle of 
inclination in the direction of bit rotation of greater than 90.degree.. 
In other words, positive rake cutters lean forward, or in the direction of 
bit rotation, and the included angle between the cutter face and the 
formation in front of it is greater than 90.degree.. These positive rake 
cutters tend to "dig in" to the formation material, as they do not put 
additional compressional stresses into the formation, which would give it 
a higher effective strength. The rotation and weight on the drill bit 
encourages these positive rake cutters to cut into the formation to their 
fully exposed depth, which could risk stalling of the bit. However, the 
hardness of the formation material may resist full depth penetration by 
the positive rake cutter. Thus, in relatively hard material the positive 
rake cutters will typically not invade the formation material to their 
full depth, although the possibility of stalling the drill bit may still 
be a consideration. 
On the other hand, a drill bit having positive rake cutters that is used in 
a formation having a greater plasticity will likely result in full depth 
entry of the positive cutters and will correspondingly result in high 
torque which may stall the bit. Accordingly, drill bits designed primarily 
for use in formations of greater plasticity typically employ cutters 
having a negative rake. 
The face of a negative rake or "back raked" cutter has an angle of 
inclination or included angle relative to the formation, that is less than 
90.degree., or opposite to that of a positive rake cutter. In use, the 
negative rake cutter has a tendency to "ride" along the surface of the 
formation giving it a higher effective strength and more "plasticity," 
resisting entry into the formation and making only a shallow cut as a 
result of the weight on the bit. It can be seen that while negative rake 
cutters advantageously resist stalling of the bit in plastic formations 
because of lower aggressiveness, the linear rate of cut for a bit having 
negative rake cutters is typically substantially less than the linear rate 
of cut for a bit having positive rake cutters. 
It is known in the art from U.S. Pat. No. 4,554,986 to utilize positive 
rake cutters disposed on a radially-oriented ridge on a bit face, trailing 
and separated from a leading radially-oriented ridge, the former being 
devoid of cutters but having wear elements embedded therein. The leading 
ridge limits the depth of penetration of the positive rake cutters on the 
trailing edge. 
It is also known in the art from U.S. Pat. No. 4,981,184 to utilize 
ridge-mounted positive rake cutters disposed on a bit face in trailing 
relationship to ridge-mounted, dome-shaped "cutter inserts" which 
purportedly deform and stress the formation being drilled to its elastic 
limit, following which the positive rake cutters clip off the deformed 
formation. Each positive rake cutter is preceded by a dome-shaped cutter 
insert. 
The cutter penetration limitation approach as described in the '986 patent 
does not take advantage of the cutting characteristics of positive rake 
cutters. The '184 patent, on the other hand, seeks to employ a "twin 
blade" approach similar to that utilized with facial razors, and is 
admirable in theory. However, variations in formation characteristics, 
pressures, drilling fluid weights and compositions during actual drilling 
all serve to preclude the realization of an actual drill bit performing in 
the manner described. 
SUMMARY OF THE INVENTION 
In contrast to the prior art, the present invention provides a new drill 
bit which utilizes combinations of positive and neutral or negative rake 
cutters, the differing cutter types being cooperatively arranged to 
improve formation cutting and to avoid "digging in" and stalling of the 
bit under a variety of diverse real world drilling conditions. 
In one exemplary embodiment of the present invention a drill bit is adapted 
for rotatably cutting a borehole. The drill bit includes a bit body having 
an exterior face adapted for substantial contact with the formation at the 
bottom of the borehole. In one exemplary embodiment of the invention, a 
first plurality of cutters is distributed across the face of the bit. Each 
of these cutters follows a preselected helical path into the formation 
during the cutting of the formation borehole. Each of the cutters has a 
cutting surface formed thereon and angularly positioned relative to the 
preselected helical cutting path at an angle of greater than 90.degree., 
i.e., effective, "positive rake." In this exemplary embodiment, a second 
plurality of cutters is also distributed across the face of the bit. Each 
of the cutters of this second plurality of cutters, again, follows a 
preselected helical path into the formation during the cutting of the 
formation borehole. Each of the second plurality of cutters has a cutting 
surface formed thereon and angularly positioned relative to the 
preselected helical path at an angle of 90.degree. or less, i.e., an 
effective "neutral rake" or "negative rake." In a particularly preferred 
embodiment, each of the first plurality of cutters is cooperatively 
associated with at least one of the second plurality of cutters. This may 
serve both to limit the cutting depth of the first plurality of cutters, 
and to enhance the cooperative cutting by both sets of cutters. It is 
contemplated that a positive rake cutter may lead or follow its 
cooperating neutral or negative rake cutter in the direction of bit 
rotation, or be radially adjacent thereto. 
In a further exemplary embodiment of the invention, the bit will include 
cutters which have first and second cutting surfaces formed thereon which 
are disposed at differing cooperating rakes. For example, the first 
cutting surface may be angularly positioned relative to the preselected 
helical cutting path at a positive rake and the second cutting surface may 
be angularly positioned relative to the preselected helical cutting path 
at a neutral or negative rake. Additionally, one of these cutting 
surfaces, such as the negatively raked surface, may be disposed at an 
angle, commonly termed the degree of "side rake," relative to the face of 
the other cutting surface; or one or both surfaces may be positioned at a 
side rake angle relative to a radius of the bit. 
Additionally, bits in accordance with the present invention may include 
cutting surfaces having differing degrees of a similar rake (i.e., for 
example, differing degrees of positive rake) and may be cooperatively 
paired to function as a unit. For example, such cutting surfaces may be 
placed in such proximity and in such relation (such as side rake of one or 
more cutting surfaces) that the portion of a formation affected by one 
cutting surface encounters the other cooperating cutting surface.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
Referring to FIG. 1 of the drawings, it should be noted that, while the 
angle of inclination of a cutting surface relative to the formation 18 is 
determinative of whether a particular cutter is classified as positive or 
negative rake cutters, the contact between the formation 18 and a cutter 
does not occur on a horizontal path. Rather, since a drill bit is rotating 
and moving downward into the formation as the borehole is cut, the cutting 
path followed by an individual cutter on the surface of the bit follows a 
helical path, as conceptually shown with respect to bit 10 depicted in 
FIG. 1. Bit 10 is illustrated having a longitudinal axis or centerline 24 
that coincides with and extends into the longitudinal axis of a borehole 
26. For illustrative purposes, bit 10 is shown having a single cutter 28 
affixed on the exterior surface of the drill bit 10. It should be 
understood that a bit typically employs numerous cutters, but for the 
purposes of illustrating the helical path followed by an individual cutter 
on bit 10, as well as the effective rake angle of an individual cutter, 
only a single cutter 28 has been illustrated. The helical cutting path 
traveled by the cutter 28 is illustrated by solid line 30 extending the 
borehole 26 into formation 18. 
The lone cutter 28 may have what would appear to be a negative rake angle 
relative to the horizontal surface 19' of the formation 18. The angle 8 
formed between the horizontal and the planar cutting surface 29 of the 
cutter 28 is less than 90.degree.. However, since bit 10 produces a 
downward linear motion as it drills the borehole 26, the effective path 
followed by the cutter 28 is generally downward at an angle of inclination 
related to the drilling rate of bit 10. 
For example, a bit 10 having a cutter 28 rotating in a radius of six 
inches, at a drilling rate of ten feet per minute, and a rotational speed 
of 50 revolutions per minute results in the helical path 30 having an 
angle of inclination relative to horizontal of approximately 4.degree.. 
Accordingly, if the cutting surface 29 of cutter 28 has an apparent angle 
of inclination relative to horizontal of approximately 86.degree. 
(4.degree. negative rake, relative to horizontal), then the cutting 
surface 29 has an effective angle of inclination, or effective rake, of 
precisely 90.degree. and will be neither negatively nor positively raked. 
Such a rake angle may be termed a "neutral" rake or rake angle. 
It should be recognized that the radial position of the cutter 28 is 
determinative as to the effective rake angle. For example, if the cutter 
28 is positioned on the surface of the drill bit 10 at a radial distance 
of only three inches from the center, then its path has an angle of 
inclination relative to the horizontal of approximately 7.degree.. The 
closer a cutter is positioned to the bit center, the greater the angle of 
inclination relative to the horizontal for a given rotational speed and 
given actual rake, and the greater the apparent negative rake of the 
cutter must be to obtain an effective negative rake angle. 
In order to properly locate and orient cutter 28 and cutting surface 29 to 
have an effective positive, neutral or negative rake, it is desirable to 
estimate performance characteristics of the drill bit 10, as well as to 
determine the radial position of the cutter 28. For example, assuming that 
the cutter 28 is radially located six inches from the bit centerline and 
cutting surface 29 is inclined at an angle of 88.degree. (2.degree. 
negative rake relative to horizontal) and the drill bit 10 is being 
designed to achieve the drilling rate and rotational speed characteristics 
discussed immediately above, such that the helical path is inclined at an 
angle of 4.degree., then the effective rake angle of the cutting surface 
29 is 92.degree. (88.degree.+4.degree.=92.degree.=2.degree. positive 
rake). Thus, while the apparent angle of inclination or rake angle of the 
cutting surface 29 appears to be negative, the effective rake angle is 
actually positive. Such a design methodology would, of course, be 
performed for each cutter on a drill bit. It should be noted that not all 
boreholes have a vertical longitudinal axis. Therefore, it is appropriate 
to refer to the apparent angle of inclination as the angle formed between 
the planar cutting surface and a plane perpendicular to the longitudinal 
axis 24 of the bit. The "effective rake angle," on the other hand, refers 
to the effective angle of inclination when the rotational speed and rate 
of penetration of bit 10 are taken into account. Accordingly, with the 
"effective rake angle" the angles of inclination of the cutting surface of 
drill bit cutters described hereinafter are measured and characterized as 
positive, negative or neutral relative to the intended helical cutting 
path 30 and not relative to horizontal (unless otherwise noted). 
Referring now particularly to FIG. 2, therein is depicted a side elevation 
of a portion of a drill bit 10 with a positive rake cutter 12 and a 
negative rake cutter 14 affixed thereto. As noted above with respect to 
FIG. 1, the terms "positive" and "negative" rake are employed with 
reference to the effective angle between the cutting surface and the 
formation. The cutters 12 and 14 are secured in the bit body 16 in a 
conventional manner, such as by being furnaced therewith in the body of a 
metal matrix type bit, attached to a bit body via studs, or brazed or 
otherwise attached to the bit body 16. It should be understood that the 
present invention is applicable to any type of drill bit body, including 
matrix, steel and combinations thereof, the latter including without 
limitation the use of a solid metal (such as steel) core with matrix 
blades, or a matrix core with hardfaced, solid metal blades. Stated 
another way, the present invention is not limited to any particular type 
of bit design or materials. In FIG. 1, the positive rake cutter 12 and the 
negative rake cutter 14 are illustrated removing formation material 18 in 
response to movement of the bit body 16 (and therefore cutters 12, 14), in 
a direction as indicated by arrow 19. The formation material 18 is in a 
plastic stress state and may be thought of as a flowing type material. 
Cutters 12, 14 each preferably includes a generally planar cutting surface 
20, 22. These cutting surfaces 20, 22 can be any of a variety of shapes 
known in the art. For the illustrated example, they may be considered as 
being of a conventional circular or disc shape. Cutting surfaces 20, 22 
are preferably formed of a hard material, such as diamond or tungsten 
carbide, to resist wearing of the cutting surfaces caused by severe 
contact with the formation 18. In a particularly preferred embodiment, 
these cutting faces will each be formed of a diamond table, such as a 
single synthetic polycrystalline diamond PDC layer (including thermally 
stable PDC), a mosaic surface composed of a group of PDC's, or even a 
diamond film deposited by chemical vapor deposition techniques known in 
the art. 
The angle of inclination of the cutting surfaces 20, 22 relative to the 
formation 18 is defined as positive or negative according to whether the 
angle formed therebetween is greater than or less than 90.degree., 
respectively, relative to the direction of cutter travel. For example, the 
cutting surface 20 of positive rake cutter 12 is illustrated having an 
angle of inclination or included angle .alpha. relative to the formation 
of greater than 90.degree.. That is to say, the bit face end or edge of 
planar cutting surface 20 leans away from the formation 18 with the 
leading edge of the cutting surface 20 contacting the formation 18. This 
positive rake of the cutting surface 20 encourages the cutter 12 to "dig 
in" to the formation 18 until the bit body 16 contacts the formation 18. 
In contra-distinction thereto, the negative rake angle of cutting surface 
22 of cutter 14 has an angle of inclination or included angle .beta. 
relative to the formation that is less than 90.degree. relative to the 
formation 18. The lower circumferential cutting edge of the cutting 
surface 22 engaging formation 18 trails the remaining portion of the 
cutting surface 22, such that the cutter 14 has a tendency to ride along 
the surface of the formation 18, making only a shallow cut therein. The 
cutting action caused by the cutter 14 is induced primarily by the weight 
on bit 10. Cutting surface 22 may also be oriented substantially 
perpendicularly to formation 18, thus being at a "neutral" rake, or at 
0.degree. backrake. In such an instance, cutting surface 22 will engage 
the formation 18 in a cutting capacity but will also ride on the formation 
as is the case negative rake cutters. It is believed that enhanced side 
rake of such a cutter will increase its cutting action by promoting 
clearance of formation cuttings from the cutter face. 
The combined use of positive and negative or neutral rake cutters has a 
balancing effect that results in the positive rake cutter producing a 
shallower cut than it would otherwise do absent the negative or neutral 
rake cutter 14. Similarly, the negative or neutral rake cutter 14 produces 
a deeper cut than it would otherwise do absent the positive rake cutter 
12. For example, while the positive rake cutter 12 encourages the drill 
bit 10 to be pulled into the formation 18, the negative or neutral rake 
cutter 14 urges the drill bit 10 to ride along the surface. Therefore, the 
combined effect of the positive and negative or neutral rake cutters 12, 
14 is to allow a bit 10 to produce cuts at a depth somewhere between the 
full and minimal depth cuts which could be otherwise urged by the positive 
and negative rake cutters individually. It should be noted that the rake 
of positive rake cutter 12 may be more radical or significant in the 
present invention than might be expected or even possible without the 
cooperative arrangement of cutters 12 and 14, in order to aggressively 
initiate the cut into formation 18, rather than "riding" or "skating" 
thereon, and to cut without stalling, even in softer formations. 
FIGS. 3A-B illustrates a top view (looking through the drill bit at the 
formation) of two pairs of positive and negative rake cutters 12, 14 
cooperatively positioned to cut plastic formation material. Referring 
first to FIG. 3A, the pair of cutters 31 is depicted having a direction of 
travel as indicated by the arrow 32, such that the longitudinal axes 33, 
34 of the cutters 12, 14 are generally parallel therewith. The cutter 12 
includes its generally circular cutting surface 20 arranged at a positive 
rake. The plane of the cutting surface 20 is generally perpendicular with 
the direction of travel, indicated by arrow 32. More precisely, a tangent 
line at the top or bottom portion of the circular cutting surface 20 lies 
within the cutting plane 20 and is perpendicular to the longitudinal axis 
33. 
The negative rake cutter 14 is adjacent the positive rake cutter 12 with 
its cutting surface 22 defining a plane which is angularly disposed 
relative to the axis 34 of cutter 14, and to the direction of rotation 32; 
i.e., the cutting face is "side raked." In the depicted pair 31, the 
trailing edge of cutting face 22 is adjacent cutting face 20; thereby 
leading toward cutter 12. Preferably, at least a portion of the 
intersection of the cutting planes 20 and 22 occurs along the cutting 
surface 20. In this manner, plastic formation material 18 first engages 
the lower cutting surface of negative rake cutter 14 and is moved in a 
direction generally toward positive rake cutter 12. Thereafter, the 
cutting surface 20 of cutter 12 shearingly removes the formation material 
18 that the cutter 14 has directed to it. Thus, the cutters 12, 14 
cooperatively interact with one another to remove formation material. 
In FIG. 3B, second pair 31' of cutters 12' and 14' differs from pair 31 in 
that negative rake cutter 14' is arranged such that cutting face 22' of 
negative rake cutter 14 is still at a side rake relative to the direction 
of rotation, but is perpendicular to the body of cutter 14' (rather than 
at an angle as with cutter 14 of pair 31). 
FIGS. 4A and B illustrate an embodiment of a combination cutter 36 having 
both positive and negative rake cutting surfaces 38, 40 disposed thereon. 
The direction of travel of the combination cutter 36 is generally 
indicated by arrow 37. Combination cutter 36 is of a generally cubic 
configuration with the cutting surfaces 38, 40 formed thereon. Combination 
cutter 36 can be divided into two functional halves along a longitudinal 
centerline 44 parallel to the direction of travel. The first half of the 
cutter 36 includes the negative rake cutting surface 38 slanted toward the 
positive rake cutting surface 40, similar to the negative rake cutter 14 
and cutting surface 22 of FIG. 3 relative to positive rake cutter 12 of 
that figure. 
The second portion of the cutter 36 includes the positive rake cutting 
surface 40 inclined toward the formation material 18 with the lower 
cutting edge being generally perpendicular to the direction of travel. The 
lower cutting edges of the cutting surfaces 38, 40 are generally adjacent 
one another and, preferably, they are immediately adjacent one another at 
their intersection with the longitudinal centerline 44 along a bottom 
surface 46 of the cutter 36. 
The negative rake cutting surface 38 is shown leading the positive rake 
cutting surface 40 in the direction of travel. Like the pairs of cutters 
31 and 31' in FIG. 3, the cutting surfaces 38, 40 of combination cutter 36 
are defined by planes that intersect, at least partially, along the 
cutting surface 40. In this manner, the negative rake cutting surface 38 
displaces a portion of the plastic formation material 18 and urges the 
displaced formation material 18 in a direction generally toward the 
positive rake cutting surface 40. 
Combination cutter 36 may be secured to a bit body in a conventional 
manner, such as, being formed in the metal matrix of the bit body, or by 
attachment thereto such as by studs integrally furnaced within the matrix 
of the bit body-steel body 16, or by other mechanical arrangements. 
Cutting surfaces 38, 40 can be any of a variety of shapes known in the 
art, but preferably are of a conventional rectangular cross section. 
Further, the cutting surfaces 38, 40 are preferably formed of diamond as 
described relative to cutters 12 and 14 of FIG. 1. 
Referring now to FIG. 5, a generally cylindrical cutter 50 having positive 
and negative cutting surfaces 52, 54 is illustrated from the perspective 
of one looking through the bit face into the formation. In this embodiment 
the cutting surfaces 52, 54 are not defined by a planar surface but rather 
are arcuately shaped, such as may be defined by a cylinder intersecting 
the cylindrical cutter 50 at a right angle or other angle relative to the 
direction of travel and at an angle relation to a horizontal line through 
cutter 50. Cutter 50 may be placed in the bit crown at any angle skewed 
with respect to an axis perpendicular to the bit profile, for example, 
such that the positive rake cutting surface 52 leads the negative rake 
cutting surface 54. The result of this is that when cutter 50 is moving in 
the direction of travel indicated by the arrow 56, the positive cutting 
surface 52 is separating a layer of formation material 18 and directing it 
generally toward the negatively raked cutting surface 54. It should be 
noted that cutter 50 may also be rotated about its longitudinal axis 58 as 
desired for appropriate orientation of cutter 50 with respect to the bit 
face. Once against, the cutting surfaces 52, 54 preferably are formed of a 
hardened material, such as diamond or tungsten carbide. 
Referring now to FIG. 6, therein is depicted another embodiment 60 of a 
combination cutter. Combination cutter 60 is substantially similar to the 
embodiment illustrated in FIG. 4, with the exception that the cutter 60 is 
formed from a cylindrical body, rather than a cubic body. Thus, 
combination cutter 60 has a pair of cutting surfaces that are generally 
half ovoid in cross section. The negative rake cutting surface 62 
preferably leads the positive rake cutting surface 64 in a direction of 
travel indicated by the arrow 66. In this manner, like combination cutter 
36 of FIG. 4, negative rake cutting surface 62 displaces a portion of the 
plastic formation material 18 and directs the displaced formation material 
18 in a direction generally toward positive rake cutting surface 64. 
Referring now to FIGS. 7-9, depicted therein is another exemplary 
embodiment of a cooperative cutter arrangement in accordance with the 
present invention. FIG. 7A depicts a combination cutter 70 which includes 
three proximately located and cooperatively associated cutting surfaces: 
two positive rake cutting surfaces 72, 74, disposed on opposing sides of a 
negative rake cutting surface 76. In this embodiment, each of the cutting 
faces 72, 74, 76 also include an identical side rake, along axis 78). As 
with previous embodiments, each cutting surface 72, 74, 76 is preferably 
formed of a hardened material such as diamond or tungsten carbide. As can 
be seen in FIGS. 7B and C, each cutter face extends the same distance 80 
from the surface 82 of bit body 16. Combination cutter arrangement 70 may 
be secured to a bit body in various manners, such as by being brazed on as 
a separate unit; formed in the metal matrix of a bit body; or by being 
attached by means of studs secured within the matrix or steel core of a 
bit body. 
FIGS. 8A-B depict a combination cutter 86 which is a variation of 
combination cutter 70 of FIGS. 7A-C and similar elements are numbered 
identically. Combination cutter 86 differs from combination cutter 70 in 
that a central portion 87 including negatively raked cutting face 88 
extends a greater distance 90 from the surface 82 of the bit body than do 
adjacent positively raked cutting faces 72 and 74. 
Similarly, FIGS. 9A-B depict a combination cutter 94 which is also a 
variation of combination cutter 70 of FIG. 8 wherein the central portion 
95 including negative rake cutting face 96 extends a lesser distance 98 
from surface 82 of the bit body than do cutting faces 72 and 74. 
Referring now to FIGS. 10-12, and first to FIGS. 10A-B, therein is depicted 
another alternative embodiment of combination cutter 100 constructed 
similarly to combination cutter 70 of FIG. 7. Combination cutter 100 
includes two negatively raked cutting surfaces 106, 108 disposed on either 
side of a positively rake cutting surface 110. In combination cutter 100, 
each of the cutting surfaces 106, 108, 110 extends a generally uniform 
distance from surface 112 of the bit body. 
FIGS. 11A-B depict an alternative embodiment of a cutter 102 which differs 
from cutter 100 in that a central portion 114, including positively raked 
cutting surface 115, extends a greater distance from surface 112 of the 
bit body than do flanking portions carrying cutting surfaces 106 and 108. 
Conversely, FIGS. 12A-B depict a cutter 104 wherein central portion 116 
carrying positively raked cutting face 118 extends a lesser distance from 
surface 112 of the bit body than do the outer flanking portions of cutter 
102 carrying negatively raked cutting surfaces 106 and 108. 
In the embodiments of FIGS. 10-12, the cutting faces do not include any 
side rake, but extend relatively along an axis 120 which is perpendicular 
to the direction of travel of the cutter 122. As will be readily 
appreciated by those skilled in the art, however, the combination cutters 
100, 102, and 104 of FIGS. 10-12 could include a side rake. 
Referring now to FIGS. 13-15, therein are depicted further alternative 
embodiments of combination cutters in accordance with the present 
invention. Referring first to FIGS. 13A-B, combination cutter 130 includes 
a central portion 132 carrying a leading negatively raked cutting face 
134, and two flanking portions indicated generally at 136 and 138, each of 
which carry positively raked cutting surfaces 140 and 142, respectively. 
Cutting face 140 and 142 are each side raked in opposing directions, 
outwardly from central negatively raked cutting face 134. 
Combination outter 146 depicted in FIG. 14, includes a similar 
construction, except that central portion 147 including negatively raked 
cutting face 148 extends a greater distance from the bit body thereby 
flanking portions 136 and 138 carrying positive rake cutting faces 140 and 
142, respectively. Conversely, combination cutter 150, as depicted in FIG. 
15, includes a central portion 152 carrying negatively rake cutting 
surface 154 which extends a lesser distance from the surface of the bit 
body than do flanking portions 136 and 138 including positively raked 
cutting faces 140 and 142. 
As to each of cutters 130, 146, and 150 of FIGS. 13-15, although positively 
rake cutting faces 140 and 142 are depicted as having similar side rakes 
in opposing directions, all of the cutting surfaces (both positive and 
negative) may include differing, or non-complimentary, side rakes. 
Further, as to each of the embodiments of FIGS. 7-15, as well as other 
embodiments depicted herein, the cutter combinations need not be formed in 
individual units or assemblies, but may be composed of individual cutters 
arranged on a bit to function cooperatively. For example, radially 
adjacent but discrete positive and negative (or neutral) rake cutters may 
be secured to the bit face, or the negative or neutral rake cutters may be 
placed in staggered but substantially overlapping relationship to the 
positive rake cutters. The primary concept underlying the combinations of 
varyingly raked cutters according to the present invention is that of 
cooperation between the differing rake cutting elements. In fact, groups 
of positive rake cutters may cooperate with groups of negatively-raked 
cutters. Thus, cutter cooperation may be on a "micro" level, with 
individual positive and negative cutter cooperation, or on a "macro" 
level, wherein groups of positive cutters cooperate with groups of 
negative or neutral rake cutters. 
FIG. 16 depicts a bottom view (looking upward from the formation) of a 10 
5/8" diameter rotary drill bit 200 of the general type disclosed and 
claimed in U.S. Pat. No. 4,883,132, assigned to the assignee of the 
present invention and incorporated herein by this reference. The prior art 
bit has, however, been modified in accordance with the present invention 
to include both positive and negative rake cutters on the blades 202 
thereof, such cutters being designated by the letters "P" and "N," 
respectively. Bit 200 includes seven positively raked, disc-shaped PDC 
cutters, at 10.degree. positive rake with respect to the longitudinal axis 
(looking perpendicularly into FIG. 16) of bit 200 (see FIG. 16A), and five 
negatively raked, disc-shaped PDC cutters, at (20.degree. negative rake 
with regard to the bit axis (see FIG. 16B). Other conventional, negative 
rake gage cutters G are also depicted in FIG. 16, but do not form a part 
of the present invention. 
It should be noted with respect to FIG. 16A that the positively raked 
cutter assemblies P are in the form of truncated cones, or of 
frustoconical shape, including the edge of diamond table 204, in 
supporting tungsten carbide substrate or backing 206, and tungsten carbide 
carrier element 208 furnaced into blade 202. The frustoconical shape of 
the cutter assembly provides access by cutting edge 210 of diamond table 
204 to formation 18, whereas a normal cylindrical or disc-shaped cutter 
assembly (as shown in broken lines) would in a positively raked 
orientation, ride on the formation 18 via backing 206 or carrier element 
208, blocking contact of cutting edge 208 with the formation 18. It is 
contemplated that at least part of the periphery of diamond table 204 may 
be chamfered or radiused, as known in the art, to enhance the durability 
and fracture resistance thereof. Of course, if half-round cutters would be 
employed, cutter assemblies P would comprise longitudinally-sectional 
truncated cones. If square or tombstone-shaped cutters were to be employed 
in positively-valued cutter assemblies P, an appropriately tapered shape 
would be employed to provide access by the cutting edges to the formation. 
FIG. 16B depicts a cross-section of a portion of a blade 202 carrying a 
negative take cutter N of conventional cylindrical configuration. 
It should be noted that the bit 200 depicted by FIG. 16 provides for full 
cutter coverage by positive rake cutters P. Stated another way, the 
rotational paths of the seven positive rake cutters P are substantially 
adjacent to ensure that substantially the entire formation 18 at the 
bottom of the borehole is engaged by the more aggressive positive rake 
cutters P to avoid the situation where the bit would be riding on a ring 
of formation material cut only by the less aggressive, negative rake 
cutters N. 
While the rake angles of the cutters P and N have been described in FIG. 16 
with respect to the bit axis, and not as effective rake angles, it should 
be noted that, given the bit diameter, a rotational speed of approximately 
80-120 revolutions per minute, and a maximum design rate of penetration of 
fifty feet per hour, all of the positively-raked cutters P will have an 
effective positive rake, while negatively-raked cutters N will possess 
effective negative rakes. 
Referring to FIGS. 17A, 17B and 17C of the drawings, yet another embodiment 
300 of the invention is depicted. Embodiment 300 includes positive rake 
concave cutter 302 in combination with negative rake concave cutter 304. 
While shown to extend substantially the same height above bit face 306, 
the cutter heights may differ as noted with respect to previous 
embodiments of the invention. Moreover, as shown in FIG. 17B, a view 
looking onto the bit face, negative rake cutter 304 may comprise a 
triangular or "plow" type cutter to direct the formation toward a positive 
rake cutter 302 on one or both sides of negative rake cutter 304. It is 
contemplated that such curved cutters may be formed of an array of PDC's 
or thermally stable PDC's, such as the MOSAIC.TM. type cutters 
manufactured by Eastman Christensen Company of Houston, Tex., and 
disclosed and claimed in U.S. Pat. No. 5,028,177. Alternatively, curved 
diamond cutters may ideally be formed of a diamond film, applied by 
chemical vapor deposition (CVD) techniques known in the art. It is also 
contemplated that a cutter (positive or negative rake) having a curved 
(concave) cutting surface may be combined with one having a substantially 
planar one. 
Many modifications and variations may be made in the techniques and 
structures described and illustrated herein without departing from the 
spirit and scope of the present invention. Accordingly, it should be 
readily understood that the embodiments described and illustrated herein 
are illustrative only and are not intended as limitations upon the scope 
of the present invention.