Cutter with polycrystalline diamond layer and conic section profile

Polycrystalline diamond cutter (PDC) designs which substantially improve the penetration rate of fixed cutter drill bits while simultaneously reducing the wear on the bit during drilling operations are disclosed. The designs are based upon the observation that: 1) the wear pattern of a PDC is roughly a conic section and is parallel to bit rotation, and 2) the cutting surface is perpendicular to the rotation of the bit. The inventive PDC designs provide cutting action both perpendicular and parallel to the direction of bit rotation.

I. BACKGROUND OF THE INVENTION 
A. Field of the Invention 
This invention relates to the design of cutters used in fixed cutter drill 
bits such as are used for drilling holes for blasting, and oil and gas 
exploration and production. In particular, this invention relates to 
cutters for use on rotary drag bits which are configured to maximize wear 
resistance and to enhance drill bit performance. 
B. The Background Art 
It is known in the prior art to construct drill bits for drilling holes in 
rock formations by affixing a plurality of discrete cutting elements made 
of a superhard material (typically diamond) to a substrate of some other 
material, such as tungsten carbide. In the past, chips of diamond set in 
the surface of a drill bit, as disclosed by Havlick (U.S. Pat. No. 
2,264,440) have been used. More recently it has become common for drill 
bits to include cutting elements which are composites of a substrate 
material (e.g. tungsten carbide) and a superhard material (e.g. 
polycrystalline diamond). The superhard material most often serves as a 
surface material, but may also be used in internal reinforcing structures. 
These composite cutting elements are usually in the form of either short, 
cylindrical "compacts" which are used primarily in rotary drag bits, or 
buttons or inserts which are used in rolling cone or percussion bits. 
The simplest form of compact is simply a short cylinder (typically with a 
diameter greater than its height) of substrate material with a uniform 
layer of superhard material on one face. This type of compact is described 
in the patents of Daniels (U.S. Pat. No. 4,156,329) and Bovenkerk (U.S. 
Pat. No. 4,268,276). The superhard material provides a wear resistant 
cutting edge. Buttons and inserts may also be constructed with a superhard 
surface over a substrate material (Waldenstrom, U.S. Pat. No. 5,335,738 
and Keshavan, U.S. Pat. No. 5,158,148). 
Prior art improvements to the basic compact design include modifications to 
the interface between the substrate and the superhard material. Many 
previous patents describe modifications to the interface geometry which 
improve the transfer of stresses between the different materials, e.g,. 
patterns of linear ridges as disclosed by Dennis (U.S. Pat. Nos. 4,592,433 
and 5,120,327), Aronssen (U.S. Pat. No. 4,764,434) and Hall (U.S. Pat. No. 
4,629,373), or ridges extending radially outward (Flood, U.S. Pat. No. 
5,486,137; Smith, U.S. Pat. No. 5,351,772; and Dennis, U.S. Pat. Nos. 
5,379,854 and 5,544,713). Hardy et al (U.S. Pat. No. 5,355,969) describe a 
cutter design having a concentric pattern of ridges at the interface. 
Matthias et al. (U.K Patent No. 2,290,328) disclose cutters having various 
patterns of ridges at the interface in the region of the cutting edge. 
Matthias (U.K. 2,290,327) discloses a cutter with a star-shaped pattern of 
ridges which extend into the substrate. 
Projections which extend from the substrate into the superhard surface 
(Waldenstrom, U.S. Pat. Nos. 5,217,081 and 5,335,738), Frushour (U.S. Pat. 
No. 5,564,511), Hardy (U.S. Pat. No. 5,355,969) or from the surface into 
the substrate (Griffin, U.K. Patent No. 2,290,326) have also been 
disclosed. These projections are generally rounded; however, Griffin (U.S. 
Pat. No. 5,469,927) has also disclosed a cutter with an array of 
star-shaped projections which extend into the cutting surface from the 
substrate. 
Other prior art compacts have a cutting surface of superhard material which 
is thicker at the center of the cutter so that it projects into the 
substrate (Olmstead, U.S. Pat. No. 5,472,376). Alternatively, the 
superhard material may be thickest about the circumference of the compact 
(Flood et al., U.S. Pat. No. 5,486,137), on opposite sides of the compact 
(Tibbitts, U.S. Pat. No. 5,435,403), or on one side only (Flood et al., 
U.S. Pat. No. 5,494,477). In the Flood and Tibbitts patents, the thickness 
of the superhard material increases linearly from the central region of 
the cutter to the outer edge. These modifications to the geometry of the 
superhard layer are intended to reduce residual stresses in the cutter and 
thus reduce wear. In addition, by increasing the thickness of the 
superhard layer at the circumference of the cutter, where the most wear 
occurs, the lifetime of the cutter is increased. Other approaches to 
increasing the strength of compacts are to use polycrystalline diamond in 
reinforcing rods (Tibbitts et al., U.S. Pat. No. 5,279,375) or as a 
cylindrical core (Bovenkerk, U.S. Pat. No. 4,268,276). According to these 
references, the use of polycrystalline diamond inside a cutting element 
serves to reduce residual stresses. 
A further prior art method for making the cutting action of a standard 
diamond table more effective is to use a "scribing" action. This can be 
accomplished by including pointed cutting elements on a drill bit along 
with cylindrical cutters, so that the pointed cutters cut grooves or kerfs 
into the rock surface so that it can be more easily cut by the blunter 
cylindrical cutters. This approach is described by Weaver (U.S. Pat. No. 
4,602,691). Another approach is to wire electric discharge machine a point 
(parallel to bit rotation) into a standard polycrystalline diamond cutter 
(PDC), thus combining the scribing and standard cutting action in a single 
cutter. However, this cutter design has no additional diamond to provide 
greater wear resistance to the point and, consequently, the point is worn 
down in the first few hours of drilling. A scribing effect has also been 
attributed to DBS's "claw" cutters, as described in Dennis (U.S. Pat. No. 
4,784,023). The "claw" cutter addresses the wear problem by providing 
additional diamond, but the parallel cutting action provided by the small 
diamond-filled grooves is minimal at best. 
Each of the above patents is hereby incorporated by reference in its 
entirety. 
II. BRIEF SUMMARY AND OBJECTS OF THE INVENTION 
The invention is a compact-type cutter for use in fixed-cutter rotary drag 
bits. An example of such a drag bit is depicted in FIG. 1. The cutter is a 
composite having a polycrystalline diamond cutting surface on a carbide 
substrate. The polycrystalline diamond forms a layer which covers one face 
of the cutter and extends into the central portion of the cutter as a 
ridge-like structure. On the side of the cutter, the ridge-like structure 
presents a parabolic region of polycrystalline diamond which extends 
downward from the face of the cutter. The parabolic region of diamond 
corresponds to the region of the cutter in which a wear scar (or wear 
flat) would be formed during the drilling operation. By constructing this 
region of the cutter with a harder material (i.e., diamond rather than 
carbide), wear on the cutter is reduced and cutting action and lifetime of 
the cutter are improved. An example of one embodiment of the inventive 
cutter design is shown in FIGS. 6a-6c. 
The actual shape of the wear scar formed on a cutter is determined by 
several factors. In the least complex case, the polycrystalline diamond 
cutter can be approximated by a cylinder. A cylinder may also be viewed as 
a cone whose vertex has been moved to infinity. Therefore, in the drilling 
process the wear flat may be approximated by some type of a conic section, 
either a portion of an ellipse, or a parabola. If the cutter was a true 
cylinder and was composed of a uniform material, then the wear scar would 
simply be a section of an ellipse whose general dimensions would be 
determined by the angle at which the cutter contacts the rock. To form the 
full ellipse, the cutter would have to be lengthened proportional to the 
rake angle or angle of contact with the workpiece. However, in reality, a 
polycrystalline diamond cutter is not a true cylinder but is slightly 
tapered (approximately 0.3% taper), such that if the cutter is lengthened 
toward infinity, it would eventually become a cone. Also, in current bit 
designs, cutting inserts have a back rake angle generally between 10 and 
30 degrees from perpendicular to the workpiece surface. This taper becomes 
more of a factor in the shape of the wear scar as the back rake angle 
becomes smaller (i.e. the diamond face of the cutter becomes almost 
perpendicular and the cutter side becomes almost parallel to the rock). 
Therefore, the larger the back rake angle, the more the wear scar will 
appear to be a segment of an ellipse. Similarly, the smaller the back rake 
angle, the more the wear scar will have a tendency to move toward a 
parabola because of the effect of cutter taper. In addition to cutter 
taper, another factor which effects the shape of the wear flat is that the 
cutter is composed of a leading edge diamond layer on a tungsten carbide 
substrate. The difference in abrasion resistance between the two materials 
distorts the shape of the wear flat. In general, because the diamond layer 
wears slower than the carbide substrate, the wear scar elongates faster 
than it widens making it look more parabolic. It should be noted that 
neither an ellipse or a parabola is the true wear scar shape, but are only 
approximations to the wear flat pattern observed on cutters from a used 
drill bit. For this reason, the use of a parabolic shape to describe the 
wear flat is considered only as an approximation to the best shape of this 
invention and a segment of an ellipse or a variety of other shapes may be 
employed. 
It is an object of the present invention to provide a cutter having an 
extended lifetime and increased abrasion and impact resistance for use in 
rotary drag bits. This is accomplished by using a diamond layer in the 
entire wear region to slow wear of the cutter. Longer cutter life means 
less frequent replacement of cutters and fewer bits overall will be 
required to drill holes, thereby saving time and money. 
It is a further object of the present invention to provide a cutter which 
provides cutting action both parallel and perpendicular to the direction 
of bit rotation. This is accomplished by providing a cutter with a layer 
of diamond on its face and extending down its side. This gives improved 
cutting action over the lifetime of the cutter. Moreover, if a sharp 
cutting edge is maintained, the weight on the drill bit does not need to 
be increased as much over the lifetime of the bit to produce constant 
pressure on the cutter surface, as in bits in which prior art cutters are 
used. 
Another object of the invention is to provide a cutter in which minimal 
diamond is used in the area of the cutter which is brazed to the drill bit 
on which it is to be used. This is accomplished by placing diamond only on 
the regions of the cutter which experience the most wear. This has the 
advantage of maximizing the braze alloy coverage, thus facilitating the 
formation of a strong attachment of the cutter to the drill bit. 
Another object of the invention is to provide a cutter which has a 
"scribing" action. This is accomplished by forming a diamond region in the 
area of the developing wear scar, which is narrower than the wear scar 
that would be predicted to develop on the cutter, so that the generally 
parabolic diamond region cuts into the surface being drilled, thus 
decreasing its strength so that it may be more easily cut by the face of 
the cutter. 
Another object of the invention is to provide a larger surface area for 
attachment of the diamond to the carbide in the cutter. The generally 
parabolic portion of the diamond layer provides said larger surface area. 
The larger surface area results in a better attachment between the diamond 
and carbide. 
Another object of the invention is to provide for greater heat transfer 
from the cutter. This is achieved by using a substantial amount of 
additional diamond, which is a much better thermal conductor than tungsten 
carbide (or other substrate materials), in the region of the cutter which 
contacts the material which is being cut. This has the benefit that the 
cutter does not overheat, thus reducing wear. 
Another object of the invention is to reduce the surface friction between 
the cutter and the rock by providing a diamond-rock contact surface rather 
than a higher friction carbide-rock contact surface. The higher friction 
produced with a carbide contact surface generates excessive heat which 
results in heat checking and subsequent failure of the carbide; these 
problems are reduced or eliminated by the use of a diamond contact 
surface. 
Another object of the invention is to provide a diamond-carbide interface 
which does not have stress risers. This is achieved with the use of the 
modified generally parabolic configuration described herein. The chance of 
cracks being formed at the diamond-carbide interface is thus reduced. 
These and other objects of this invention are intended to be covered by 
this disclosure and are readily apparent to individuals of ordinary skill 
in the art.

IV. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The inventive cutter is intended for use in cutting tools such as the 
rotary drag bit 1 shown in FIG. 1. Each cutter 2 is brazed into the drill 
bit such that the face 3 of the cutter is perpendicular to the rotation 
direction of the bit (as indicated by the arrow) When the drill bit is 
rotated, the leading edge 4 of each cutter contacting the rock surface 
performs a cutting action on the rock surface. In the bit depicted in FIG. 
1, numerous cutters are arranged on the bit such that there is enough 
overlap between the areas cut by the different cutters that the entire 
surface of the rock face is being acted upon. 
As the drilling process proceeds, the working edge of each cutter begins to 
wear and forms a wear scar 5 as depicted in FIG. 2. The cutter shown in 
FIG. 2 includes a polycrystalline diamond layer 6 and a substrate 7. In 
reality, a composite cutter of the type shown in FIG. 2 does not form a 
perfectly planar wear scar; instead, the substrate region will be slightly 
undercut below the polycrystalline diamond layer and the wear scar becomes 
slightly elongated and thus parabolic in shape rather than ellipsoid as 
would be the case if the wear scar was perfectly planar. 
During the drilling operation, the capacity of a cutter to dig into the 
rock surface is roughly proportional to the pressure at the cutting edge. 
Initially, the cutting edge has a small surface contact area and the 
pressure at the cutting edge is very high, resulting in an aggressive 
cutting action and, therefore, a high penetration rate. If the weight on 
the bit is kept constant as the cutting edge wears, the contact pressure 
begins to decrease proportional to the increase in surface contact area, 
and the cutting action is decreased. In order to maintain the penetration 
rate, the weight on the bit must be increased. Once the cutter has worn 
down to its midsection, the wear scar has developed to its maximum size, 
and it is typically not possible to increase the weight on the bit enough 
to provide a sufficient penetration rate, due to limitations of the drill 
string integrity or the structural integrity of the bit itself. Also, at 
this point it becomes difficult to retain the cutters in their pockets. 
The drill bit is now at the end of its useful life. 
In the present invention, the performance of a fixed cutter bit is improved 
by providing a cutter which maintains the smallest possible wear surface 
while simultaneously maintaining a sharp cutting edge against the rock 
surface during its lifetime. This is accomplished by maximizing the amount 
of a diamond at the developing wear scar in such a way as to provide 
cutting action both perpendicular and parallel to the direction of a bit 
rotation. 
Diamond is used on the area in which the wear scar will develop, rather 
than on the entire perimeter of the cutter, with the advantage that the 
areas of the cutter which are to be brazed to the bit body are tungsten 
carbide, which can be readily brazed to the bit body. Polycrystalline 
diamond is not wetted by braze, and therefore, additional diamond around 
the perimeter of the cutter reduces the wettable area of the cutter, 
resulting in a weaker bond of the PDC to the bit body. In the prior art 
cutters of Smith (U.S. Pat. No. 5,351,772) and Flood et al. (U.S. Pat. No. 
5,486,137), the diamond layer is thickest at the outer edge of the cutter 
and radially symmetrical, which means that diamond is present on the side 
of the cutter which is brazed to the bit body, thereby weakening the bond 
between cutter and bit body. In the inventive cutter, the side of the 
cutter which is brazed to the bit body is similar to standard flat 
interface PDC cutters, which can be bonded easily and securely to the bit 
body. 
In the inventive cutter, the layer of polycrystalline diamond (or other 
superhard material) approximates the area of the developing wear scar. The 
polycrystalline diamond extends into the interior of the compact as a 
ridge with a curved profile. Although one previous cutter design of which 
we are aware (Flood et al., U.S. Pat. No. 5,494,477) includes a diamond 
layer which is thicker on one side, the interface differs in that it 
slopes linearly downward from a line which is a chord of the circular 
compact. The interface is thus planar; when the wear scar has developed 
far enough to cut through the interface into the substrate, the cutting 
performance of the cutter deteriorates and the substrate material is worn 
away. In contrast, the ridge of polycrystalline diamond in the inventive 
cutter has a curved, rather than planar profile, and is not constrained to 
extend from the center of the cutter radially to the edge In the preferred 
embodiment of the present invention, the parabolic ridge extends beyond 
the center of the cutter, so that a parabolic diamond region remains even 
when the cutter has worn to the mid-point. 
Two examples of the inventive cutter design are now described. Further 
modifications may be made without departing from the essential nature of 
the invention and such modifications are considered to fall within the 
scope of this patent. 
EXAMPLE I 
Simple Parabolic Cutting Surface 
This embodiment of the invention is depicted in FIGS. 6a-6c. The cutter is 
essentially cylindrical in shape. The inventive cutter has a layer of 
polycrystalline diamond 6 on a substrate 7. The polycrystalline diamond 
layer serves as a cutting surface. The cutting surface includes a surface 
layer which covers on face of the cutter, and, contiguous with the surface 
layer, a ridge of polycrystalline diamond extending into the substrate. On 
the side of the cutter, the end of the ridge (seen in cross section) 
approximates the shape of the wear scar; however, the polycrystalline 
diamond is not simply a surface feature but extends well into the 
substrate. This design is based on the theory that the simplest way to 
prevent a large wear scar from developing is to place a substantial amount 
of the most wear resistant material (e.g. diamond) in the area of the 
cutter where the wear scar will develop. Since the wear scar is 
essentially parabolic in shape, the additional diamond wear is also 
parabolic in shape. Prior to calculating the size of the parabolic region, 
the maximum parabolic wear scar must be determined. As shown in FIG. 3, in 
side view (looking perpendicular to the longitudinal axis of the cutter 
and parallel to the wear scar), if the simplifying assumption is made that 
the wear scar is planar, wear scar 5 is defined by a right triangle. The 
basic shape of the maximum wear scar can be determined from the contact 
angle .theta. (which is the rake angle of the cutter mounted in the bit), 
the depth of the wear scar as measured along the outside diameter of the 
cutter, and the maximum width as measured at the top of the diamond layer. 
The maximum possible wear scar has a depth d which is the same as the 
height b of the cutter. The angle .theta. is defined as the back rake 
angle. x is the surface length of the wear scar. a, b, and x define a 
right triangle, so 
EQU a=b tan .theta. . 1 
and 
EQU x=b/cos .theta. . 2 
Thus a and x can be solved for in terms of b and .theta.. FIG. 4 shows a 
top view of the cutter, including the wear scar. The half width of the 
wear scar on the top of diamond surface is: 
EQU y=(R.sup.2 -l.sup.2).sup.1/2 . 3 
where 
EQU l=R-a . 4 
with R being the radius of the cutter. Substituting . 1 and . 4 into 
. 3, the following is obtained: 
EQU y=(2R(b tan .theta.)-(b tan .theta.).sup.2).sup.1/2 . 5 
Thus, x and y can be solved from R, b, and .theta., which are known for a 
given cutter. FIG. 5 illustrates a typical parabola with x and y labeled. 
The area k and length of arc s can be calculated as follows: 
##EQU1## 
If it is assumed that an industry standard cutter having a height of b=8 mm 
and a diameter of 13 mm (so R=7.5 mm) is used, with a rake angle of 
20.degree., the maximum parabolic wear scar will have dimensions x=8.5 mm 
and y=5.92 mm. In the preferred embodiment of the invention, the parabolic 
region will be somewhat smaller than the maximum wear scar. 
As noted previously, the generally parabolic shaped region of diamond in 
the cutter is actually a ridge which extends from the perimeter to the 
interior of the cutter. The parabolic region visible on the surface of the 
cutter is one end of the ridge of superabrasive material, preferably 
having a parabolic cross section, which extends through the body of the 
cutter, as shown in FIGS. 6a-6c. The dimensions of the parabolic ridge 
through the cutter may be varied as needed. However, keeping the ridge 
within the limits of the maximum parabolic wear scar is thought to provide 
the best results. The apex of the ridge is defined by line 10 that at one 
end intersects the perimeter of the cutter at a point 11 at the 
diamond-carbide interface, above the base of the cutter, and at the other 
end intersects the diamond-carbide interface (point 12), either in the 
interior of the cutter (as shown), or on the perimeter of the cotter 
opposite point 11, or at some intermediate point. Line 10 makes an angle 
.phi. with the longitudinal axis of the cutter, as shown in FIGS. 6a-6c. 
In the preferred embodiment of the invention, angle .phi. will be 
substantially greater than the rake angle .theta., which is typically 
20.degree.. Angle .phi. is generally in the range of 10 to 80 degrees. In 
the preferred embodiment of the invention, .phi. will be between of 20 and 
70 degrees. It is most preferred that .phi. will be between 30 and 60 
degrees. If .phi. were the same as .theta., the diamond-carbide interface 
at the apex of the ridge would be substantially parallel to the direction 
of rotation of the bit, causing the interface to experience high shear 
loads which might delaminate the diamond region from the carbide. It is 
believed that .phi. should be two to three times rake angle .theta.. At a 
minimum, angle .phi. should be chosen such that the diamond ridge extends 
to the middle of diamond table. This allows for some of the parabolic 
ridge to still be present when the cutter is worn to the mid-point. 
The parabolic diamond region on the side of the cutter is parallel to the 
direction of rotation of the bit, and the first order effect is to provide 
greater wear resistance. A variation of this idea is to make the parabolic 
diamond ridge narrower than the generally parabolic wear scar that will be 
produced during drilling. As this cutter wears, the difference in abrasion 
resistance between the diamond and the carbide substrate will cause the 
diamond ridge to project a small distance above the carbide. The diamond 
above the carbide should provide a decreased contact area with the rock 
surface thereby increasing the aggressivity (ratio of normal to axial 
load) at a given depth of cut. The narrowing of the diamond parabola is 
achieved by making the value of y smaller than the calculated maximum wear 
scar halfwidth. 
Another important feature of the simple parabolic design is the 
continuously curved surface of the parabola. The curved surface provides 
increased surface area for diamond-to-carbide attachment. In addition, 
there are no sharp corners to act as stress risers to initiate cracks. 
It can be seen in FIGS. 6a-6c that the thickness of the parabolic section 
varies across the cutter. The parabola is thickest at one side of the 
cutter, and gradually becomes thinner toward the center of the cutter. 
This feature has two benefits: first, the extra diamond is only used where 
needed, and secondly, only carbide is exposed on the side of the cutter 
which will be bonded to the drill bit. Concentrating the diamond where it 
is needed reduces the overall cost of the cutter. From the perspective of 
cutter attachment, removing the diamond from the bond area permits maximum 
braze alloy coverage and thus a stronger bond to the bit. 
Although the cutter design shown here includes a ridge with a parabolic 
cross section, the invention is not limited thereto. Some other cross 
section which approximates the shape of the wear scar could be used as 
well. Although less preferred, cross sections which do not closely 
approximate the shape of the wear scar may also be used. Examples of 
various cross sections which may be used are presented in FIGS. 7a through 
7e. It will be appreciated that cross sectional shapes intermediate 
between those shown in FIG. 7 may also be used. Although the interface 
between the surface layer and the substrate are shown as being smooth, it 
would also be possible to include various mechanical modifications of the 
surface (e.g. as ridges, undulations, or dimples or chemical modifications 
to enhance the adhesion and transfer of stresses between the surface and 
the substrate. 
The cylindrical cutter 2 is constructed with a polycrystalline diamond 
cutting surface 6 on a tungsten carbide substrate 7. Alternatively, 
materials such as cubic boron nitride or other superabrasive materials 
could be used in place of polycrystalline diamond as the cutting surface. 
Materials such as boron tetracarbide, tantalum carbide, vanadium carbide, 
niobium carbide, halfnium carbide, or zirconium carbide could be used in 
place of tungsten carbide as the substrate. Superabrasive materials and 
substrate materials suitable for use in cutters are known in the prior 
art. 
The inventive cutters have a diamond layer 6 formed under high temperature 
and pressure conditions to a cemented carbide substrate 7 (such as 
cemented tungsten carbide) containing a metal binder or catalyst such as 
cobalt. The substrate 7 may be brazed or otherwise joined to an attachment 
member such as a stud or to a cylindrical backing element to enhance its 
affixation to the bit face. The cutting element may be mounted to a drill 
bit either by press-fitting or otherwise locking the stud into a 
receptacle on a steel-body drag bit, or by brazing the cutter substrate 
(with or without cylindrical backing) directly into a preformed pocket, 
socket or other receptacle on the face of a bit body, as on a matrix-type 
bit. 
A PDC is preferably fabricated by placing a disk-shaped cemented carbide 
substrate with a groove already formed in it into a container or cartridge 
with a layer of diamond crystals or grains loaded into the cartridge 
adjacent one face of the substrate. A number of such cartridges are 
typically loaded into an ultra-high pressure press. The substrates and 
adjacent diamond crystal layers are then compressed under ultra-high 
temperature and pressure conditions. The ultra-high pressure and 
temperature conditions cause the metal binder from the substrate body to 
become liquid and sweep from the region behind the substrate face next to 
the diamond layer through the diamond grains and act as a reactive liquid 
phase to promote a sintering of the diamond grains to form the 
polycrystalline diamond structure. As a result, the diamond grains become 
mutually bonded to form a diamond table over the substrate face, which 
diamond table is also bonded to the substrate face. 
The residual stresses mentioned previously, result when the diamond and 
carbide are bonded at high temperatures and pressures. The cause of the 
residual stress is the mismatch between the properties of the diamond and 
the substrate material; in particular, the respective thermal expansion 
coefficients are different and so are the respective elastic moduli. 
Alternatively, the diamond layer may be formed as above, but separately 
from the substrate, and may subsequently be bonded to the substrate 
material by brazing with a tungsten or titanium-based braze. Yet another 
alternative method is to deposit the diamond layer on the substrate by 
chemical vapor deposition (CVD) processing. 
The metal binder may remain in the diamond layer within the pores existing 
between the diamond grains or may be removed and optionally replaced by 
another material, as known in the art, to form a so-called thermally 
stable diamond. The binder is removed by leaching or the diamond table is 
formed with silicon, a material having a coefficient of thermal expansion 
similar to that of diamond. Variations of this general process exist in 
the art, but this detail is provided so that the reader will understand 
the concept of sintering a diamond layer onto a substrate in order to form 
a PDC cutter. 
In the case of the present invention, the desired parabolic ridge shape is 
achieved by using a tungsten carbide substrate which has a trough into 
which the ridge will extend formed into it by the manufacturer. 
Alternatively, the trough may be cut into the carbide substrate by either 
grinding or electric discharge machining. The diamond powder fills the 
trough and is sintered into place during the high pressure and high 
temperature cycle. An example of such a substrate is shown in FIG. 8. 
EXAMPLE II 
Modified Parabolic Cutting Surface 
A modification of the simple parabolic design is illustrated below in FIGS. 
9a through 9c. The primary focus of this design is to provide as much 
curved surface as possible to avoid stress concentrations that could cause 
cracking. This design performs the same functions of the first design. It 
also illustrates that the curvature of the diamond table may be varied as 
needed to counter residual stresses which may tear the parabolic region 
apart. The simple parabolic design has sharp corners 20 (shown in FIG. 6c) 
where the ridge sides intersect the diamond-carbide interface. This design 
smoothes that area, and cannot be modeled as a simple parabola. The 
cross-section shapes shown in FIGS. 7a through 7e can also be modified by 
the addition of a curved interface region, as shown in FIGS. 7f through 
7j. 
The same method is used for manufacturing the modified parabolic cutter as 
is used for manufacturing the simple parabolic cutter. 
As shown in FIGS. 6b and 9b, in the presently preferred embodiment of the 
invention, the apex of the ridge of polycrystalline diamond is defined by 
a line 10. In an alternative embodiment of the invention, the apex of the 
ridge may be defined by a curve rather than a line. Said curve may be 
concave upward (i.e., toward the PDC surface of the cutter)or downward, 
and the curve may have undulations, as well; it is only necessary that the 
apex of the ridge runs generally upward from point 11 to point 12, as 
shown in FIG. 6. 
The described embodiments are to be considered in all respects only as 
illustrative and not restrictive. Although the embodiments shown here 
include a ridge of polycrystalline diamond which has an essentially 
parabolic cross section, the invention is not limited thereto. The cross 
section could have any shape which approximates the shape of the wear 
scar. The scope of the invention is, therefore, indicated by the appended 
claims rather than by the foregoing description. All changes which come 
within the meaning and range of equivalency of the claims are to be 
embraced within their scope.