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CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation of application Ser. No. 09/104,620, filed Jun. 25, 1998, pending. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to rotary bits for drilling subterranean formations and, more specifically, to superabrasive cutters suitable for use on such bits, particularly of the so-called fixed cutter or “drag” bit variety.  
           [0004]    2. State of the Art  
           [0005]    Fixed-cutter, or drag, bits have been employed in subterranean drilling for many decades, and various sizes, shapes and patterns of natural and synthetic diamonds have been used on drag bit crowns as cutting elements. Polycrystalline diamond compact (PDC) cutters comprised of a diamond table formed under ultra-high temperature, ultra-high pressure conditions onto a substrate, typically of cemented tungsten carbide (WC), were introduced into the market about twenty-five years ago. PDC cutters, with their diamond tables providing a relatively large, two-dimensional cutting face (usually of circular, semi-circular or tombstone shape, although other configurations are known) have provided drag bit designers with a wide variety of potential cutter deployments and orientations, crown configurations, nozzle placements and other design alternatives not previously possible with the smaller natural diamond and polyhedral, unbacked synthetic diamonds previously employed in drag bits. The PDC cutters have, with various bit designs, achieved outstanding advances in drilling efficiency and rate of penetration (ROP) when employed in soft to medium hardness formations, and the larger cutting face dimensions and attendant greater extension or “exposure” above the bit crown have afforded the opportunity for greatly improved bit hydraulics for cutter lubrication and cooling and formation debris removal. The same type and magnitude of advances in drag bit design in terms of cutter robustness and longevity, particularly for drilling rock of medium to high compressive strength, have unfortunately, not been realized to a desired degree.  
           [0006]    State of the art substrate-supported PDC cutters have demonstrated a notable susceptibility to spalling and fracture of the PDC diamond layer or table when subjected to the severe downhole environment attendant to drilling rock formations of moderate to high compressive strength, on the order of nine to twelve kpsi and above, unconfined. Engagement of such formations by the PDC cutters occurs under high weight on bit (WOB) required to drill such formations and high impact loads from torque oscillations. These conditions are aggravated by the periodic high loading and unloading of the cutting elements as the bit impacts against the unforgiving surface of the formation due to drill string flex, bounce and oscillation, bit whirl and wobble, and varying WOB. High compressive strength rock, or softer formations containing stringers of a different, higher compressive strength, thus may produce severe damage to, if not catastrophic failure of, the PDC diamond tables. Furthermore, bits are subjected to severe vibration and shock loads induced by movement during drilling between rock of different compressive strengths, for example, when the bit abruptly encounters a moderately hard strata after drilling through soft rock.  
           [0007]    Severe damage to even a single cutter on a PDC cutter-laden bit crown can drastically reduce efficiency of the bit. If there is more than one cutter at the radial location of a failed cutter, failure of one may soon cause the others to be overstressed and to fail in a “domino” effect. As even relatively minor damage may quickly accelerate the degradation of the PDC cutters, many drilling operators lack confidence in PDC cutter drag bits for hard and stringer-laden formations.  
           [0008]    It has been recognized in the art that the sharp, typically 90° edge of an unworn, conventional PDC cutter element is usually susceptible to damage during its initial engagement with a hard formation, particularly if that engagement includes even a relatively minor impact. It has also been recognized that pre-beveling or pre-chamfering of the PDC diamond table cutting edge provides some degree of protection against cutter damage during initial engagement with the formation, the PDC cutters being demonstrably less susceptible to damage after a wear flat has begun to form on the diamond table and substrate.  
           [0009]    U.S. Pat. Nos. Re 32,036, 4,109,737, 4,987,800, and 5,016,718 disclose and illustrate bevelled or chamfered PDC cutting elements as well as alternative modifications such as rounded (radiused) edges and perforated edges which fracture into a chamfer-like configuration. U.S. Pat. No. 5,437,343, assigned to the assignee of the present application and incorporated herein by this reference, discloses and illustrates a multiple-chamfer PDC diamond table edge configuration which under some conditions exhibits even greater resistance to impact-induced cutter damage. U.S. Pat. No. 5,706,906, assigned to the assignee of the present application and incorporated herein by this reference, discloses and illustrates PDC cutters employing a relatively thick diamond table and a very large chamfer, or so-called “rake land”, at the diamond table periphery.  
           [0010]    However, even with the PDC cutting element edge configuration modifications employed in the art, cutter damage remains an all-too-frequent occurrence when drilling formations of moderate to high compressive strengths and stringer-laden formations.  
           [0011]    Another approach to enhancing the robustness of PDC cutters has been the use of variously-configured boundaries or “interfaces” between the diamond table and the supporting substrate. Some of these interface configurations are intended to enhance the bond between the diamond table and the substrate, while others are intended to modify the types, concentrations and locations of stresses (compressive, tensile) resident in the diamond tables and substrates after the cutter is formed in an ultra-high pressure, ultra-high temperature process, as is known in the art. Still other interface configurations are dictated by other objectives, such as particularly desired cutting face topographies. Additional interface configurations are employed in so-called cutter “inserts” used on the rotatable cones of rock bits. Examples of a variety of interface configurations may be found, by way of example only, in U.S. Pat. Nos. 4,109,737, 4,858,707, 5,351,772, 5,460,233, 5,484,330, 5,486,137, 5,494,477, 5,499,688, 5,544,713, 5,605,199, 5,657,449, 5,706,906 and 5,711,702.  
           [0012]    While cutting faces have been designed with features to accommodate and direct forces imposed on PDC cutters, see, for example, above-referenced U.S. Pat. No. 5,706,906, state-of-the-art PDC cutters have, to date, failed to adequately accommodate such forces at the diamond table-to-substrate interface, resulting in a susceptibility to spalling and fracture in that area. While the magnitude and direction of such forces might, at first impression, seem to be predictable and easily accommodated, based upon cutter back rake and WOB, such is not the case, due to the variables encountered during a drilling operation, previously noted herein. Therefore, it would be desirable to provide a PDC cutter having a diamond table/substrate end face interface able to accommodate the wide swings in both magnitude and direction of forces encountered by PDC cutters during actual drilling operations, particularly in drilling formations of medium-to-high compressive strength rock, or containing stringers of such rock, while at the same time providing a superior mechanical connection between the diamond and substrate and sufficient diamond volume across the cutting face for drilling an extended borehole interval.  
         BRIEF SUMMARY OF THE INVENTION  
         [0013]    The present invention addresses the requirements stated above, and includes PDC cutters having an enhanced diamond table-to-substrate interface, as well as drill bits so equipped.  
           [0014]    The cutters of the present invention, while having demonstrated utility in the context of PDC cutters, encompass any cutters employing superabrasive material of other types, such as thermally stable PDC material and cubic boron nitride compacts. The inventive cutters may be said to comprise, in broad terms, cutters having a superabrasive table formed on and mounted to a supporting substrate. Again, while a cemented WC substrate may be usually employed, substrates employing other materials in addition to, or in lieu of, WC may be employed in the invention.  
           [0015]    The inventive cutter comprises a table comprising a volume of superabrasive material and exhibiting a two-dimensional, circular cutting face mounted to an end face of a cylindrical substrate. An interface between the end face of the substrate and the volume of superabrasive material includes at least one annular surface of substrate material which is defined, in cross-section taken across and parallel to the longitudinal axis of the cutter, by an arc. The annular surface is preferably a spherical, or spheroidal, surface of revolution about the longitudinal axis of the cutter, or a portion of a toroid transverse to and centered on the longitudinal axis. If a spherical surface of revolution is employed, the center point thereof lies coincident with the longitudinal axis or centerline of the cutter. The surface of revolution may or may not extend at its outer periphery to the side of the substrate and is bounded at its inner periphery by another surface of revolution. The center of the substrate end face lying within the annular surface of revolution may exhibit a variety of topographic configurations. The superabrasive table formed over the substrate end face conforms thereto along the interface, while the exterior surface of the table may be provided with features such as chamfers as are conventional and known in the art.  
           [0016]    The annular surface of the substrate end face, by virtue of its arcuate cross-sectional configuration, provides an interface designed to address multi-directional resultant loading of the cutting edge at the periphery of the cutting face of the superabrasive table. In general, resultant loads at the cutting edge are directed at an angle with respect to the longitudinal axis or centerline of the cutter which varies between about 20° and about 70°. The arcuate surface is designed so that a normal vector to the substrate material will lie parallel to, and opposing, the force vector loading the cutting edge of the cutter. Stated another way, since the angle of cutting edge loading varies widely, the arcuate surface presents a range of normal vectors to the resultant force vector loading the cutting edge so that at least one of the normal vectors will, at any given time and under any anticipated resultant loading angle, be parallel and in opposition to the loading. Thus, at the area of greatest stress experienced at the interface, the superabrasive material and adjacent substrate material will be in compression, and the interface surface will lie substantially transverse to the force vector, beneficially dispersing the associated stresses and avoiding any shear stresses. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a side elevation of a first embodiment of a superabrasive cutter according to the present invention;  
         [0018]    [0018]FIG. 2 is a side elevation of a second embodiment of a superabrasive cutter according to the present invention;  
         [0019]    [0019]FIG. 3A is a side half-sectional elevation of a supporting substrate having utility in a third embodiment of a superabrasive cutter according to the present invention, FIG. 3B is a side elevation of the substrate of FIG. 3A, FIG. 3C is a top elevation of the substrate of FIG. 3A, and FIG. 3D is an enlarged cross-sectional detail of area D in FIG. 3A;  
         [0020]    [0020]FIGS. 4 through 16 depict, in side sectional elevation, additional embodiments of substrates having utility with superabrasive cutters according to the present invention; and  
         [0021]    [0021]FIG. 17 is a side perspective view of a rotary drag bit equipped with cutters according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0022]    Referring to FIG. 1 of the drawings, a first embodiment  10  of the inventive cutter will be described. Cutter  10  includes a substrate  12  having an end face  14  on which a superabrasive table, such as a polycrystalline diamond compact (PDC) table  16 , is formed. Substrate  12  is shown in side elevation with table  16  thereon shown as transparent (rather than in cross-section, with hatching) for clarity in explaining the structure and advantages of the invention in detail, although those of ordinary skill in the art will appreciate that the superabrasive material, such as a PDC, is opaque.  
         [0023]    Substrate  12  is substantially cylindrical in shape, of a constant radius about centerline or longitudinal axis L. End face  14  of substrate  12  includes annular surface  20  comprising a spherical surface of revolution of radius R 1  having an inner circular periphery  22  and an outer circular periphery  24 , the center point of the sphere being located at  26 , coincident with centerline or longitudinal axis L. The inner periphery  22  abuts a flat annular surface  28  extending transverse to centerline or longitudinal axis L, while the concave center  30  of substrate end face  14  comprises another spherical surface of revolution of radius R 2  about center point  32 , again coincident with centerline or longitudinal axis L. Superabrasive table  16  overlies end face  14  and is contiguous therewith, extending to side wall  34  of substrate  12  and defining a linear exterior boundary  36  therewith. Cylindrical side wall  38  of table  16 , of the same radius as substrate  12 , lies above boundary  36  and extends to inwardly-tapering frustoconical sidewall  40 , which terminates at cutting edge  42  at the periphery of cutting face  44 . As shown, cutting edge  42  is chamfered at  46  as known in the art, although this is not a requirement of the invention. Typically, however, a nominal 0.010 inch (about 0.25 mm) depth, 45° angle chamfer may be employed. Larger or smaller chamfers may also have utility, depending upon the relative hardness of the formation or formations to be drilled and the need to employ chamfer surfaces of a given cutter or cutters to enhance bit stability as well as cut the formation. Cutter  10  is shown in FIG. 1 oriented with respect to a formation  50 , as it would be conventionally oriented on the face  52  of bit  54  (both shown in broken lines for clarity) during drilling, with cutting face  44  oriented generally transverse to the direction of cutter travel as the bit rotates and the cutter traverses a shallow, helical path as the bit drills ahead into the formation. Also as is conventional, cutter  10  is oriented so that the cutting face  44  exhibits a negative back rake toward formation  50 , leaning backward with respect to the direction of cutter travel from a line perpendicular to the path P of cutter travel through the formation  50 .  
         [0024]    As cutter  10  travels ahead and engages the formation to a depth of cut (DOC) dependent upon WOB and formation characteristics, cutter  10  is loaded at cutting edge  42  by a resultant force F R , which is dependent upon WOB and torque applied to the drill bit, the latter being a function of bit rotational speed, DOC and formation hardness. As previously mentioned, instantaneous WOB, rotational speed and DOC may fluctuate widely, resulting not only in substantial changes in magnitude of F R , but also in the angle α thereof, relative to longitudinal cutter axis L. As noted above, under most drilling conditions and even under the widest variation in drilling parameters and cutter back rakes, angle α varies in a range between an α 1  of about 20° and an α 2  of about 70°. As can readily be seen in FIG. 1, annular surface  20 , comprising the aforementioned spherical surface of revolution, lies in an area where forces acting on the cutter  10  are greatest and presents a surface orientation facing F R  so that normal vectors to annular surface  20  are oriented over a range V N1  through V N2 , within which range there is at least one normal vector V NP , which is parallel to and coincident with, or only minutely offset from, F R  at any given instant in time. This load-accommodating topography of annular surface  20  thus distributes F R  in an area of substrate end face  14  substantially perpendicular to F R . It is also notable that the area of end face  14  lying within annular surface  20  is configured with annular surface  28  and concave center  30  to provide a substantial superabrasive material depth for table  16  and also an effective mechanical interlock along the interface between table  16  and substrate  12 . Moreover, the presence of annular surface  20 , dictating an increasing depth of superabrasive material as the table  16  approaches its periphery, generates a beneficial residual (from fabrication) compressive stress concentration in the area of the table periphery where cutter loading is greatest and provides a large volume of superabrasive material in the area of contact with the formation to minimize cutter wear.  
         [0025]    Referring to FIG. 2, another embodiment  110  of the cutter of the invention will be described. Features of cutter  10  also incorporated in cutter  110  are identified by the same reference numerals for clarity. Cutter  110  includes a substrate  112  having an end face  114  on which a superabrasive table, such as a polycrystalline diamond compact (PDC) table  116 , is formed. Substrate  112  is shown in side elevation with table  116  thereon shown as transparent (rather than in cross-section, with hatching) for clarity in explaining the structure and advantages of the invention in detail, although those of ordinary skill in the art will appreciate that the superabrasive material, such as a PDC, is opaque.  
         [0026]    Substrate  112  is substantially cylindrical in shape, of a constant radius about longitudinal axis or centerline L. End face  114  of substrate  112  includes annular surface  120  comprising a spherical surface of revolution of radius R 3  having an inner circular periphery  122  and an outer circular periphery  124 , the center point of the sphere being located at  126 , coincident with longitudinal axis or centerline L. The inner periphery  122  abuts another annular surface  128  comprising a spherical surface of revolution of radius R 4 , the center point of the sphere being located at  130 , coincident with longitudinal axis or centerline L. The inner periphery  132  of annular surface  128  abuts yet another arcuate, spherical surface of revolution  134 , of radius R 5  about center point  136 , coincident with longitudinal axis or centerline L. It should be noted that the uppermost portion of spherical surface of revolution  134  is at the same elevation as inner periphery  122  of annular surface  120 , although this is not a requirement of the invention.  
         [0027]    Superabrasive table  116  overlies end face  114  and is contiguous therewith, extending to side wall  34  of substrate  112  and defining a linear exterior boundary  36  therewith. Inwardly-tapering frustoconical side wall  40  of table  116  commences adjacent boundary  36  and is of the same radius as substrate  112 , extending above boundary  36  to cutting edge  42  at the periphery of cutting face  44 . As shown, cutting edge  42  is chamfered at  46  as known in the art, although this is not a requirement of the invention.  
         [0028]    As with cutter  10 , it will be readily appreciated that annular surface  120  of end face  114  of substrate  112  of cutter  110  will provide a range of normal vectors sufficient to accommodate the range of orientations of resultant force loads acting on cutter  110  proximate cutting edge  42  during a drilling operation and distribute them over an area of end face  14  lying substantially transverse to the loads. Again as with cutter  10 , it will be appreciated that a substantial depth of superabrasive material is retained for table  116 , and that a mechanically effective, symmetrical interlocking arrangement is provided at the interface between table  116  and substrate  112 .  
         [0029]    [0029]FIG. 3A shows yet another substrate end face configuration for a cutter according to the present invention in cross-section, while FIG. 3B shows substrate  212  in side elevation and FIG. 3C is a top elevation of end face  214 . As with the other embodiments, substrate  212  is substantially cylindrical and includes a number of contiguous, annular surfaces surrounding a circular central surface on end face  214 . From the side exterior of substrate  212  inwardly, an annular lip or shoulder  240  extends inwardly from side wall  234 , meeting annular surface  242 , which comprises a spherical surface of revolution. Annular, arcuate surface  244  lies inwardly of annular surface  242 , within which lies arcuate surface  246 , within which lies a central surface of revolution  248 . Surfaces  242 ,  244  and  246  are substantially coincident at their mutual boundaries, while the transition between lip  240  and annular surface  242  comprises a small, but measurable, radius  250  (see enlarged detail in FIG. 3D). Similarly, the transition between surface  246  and central surface of revolution  248  comprises a small, but measurable, radius  252 .  
         [0030]    [0030]FIGS. 4 through 16 illustrate a number of other substrate end face configurations according to the invention, it being understood that superabrasive tables such as PDC tables, when formed thereon, will provide cutters according to the invention.  
         [0031]    [0031]FIG. 4 depicts a side sectional elevation of a substantially cylindrical substrate  312  having an end face  314  comprising a plurality of mutually adjacent spherical surfaces of revolution  320 ,  322 ,  324 ,  326  and  328 , the center points of which all lie coincident with the centerline or longitudinal axis L of the substrate  312 . In this and subsequent figures, extensions of the actual end face spherical surfaces of revolution in the plane of the paper have been shown in broken lines for a better appreciation of the spherical nature thereof.  
         [0032]    [0032]FIG. 5 depicts a side sectional elevation of a substantially cylindrical substrate  412  having an end face  414  comprising a single, outer, spherical, annular surface of revolution  420  surrounding an upward-facing conical surface of revolution  422 , the center points of both surfaces of revolution lying on the centerline or longitudinal axis L of the substrate  412 .  
         [0033]    [0033]FIG. 6 depicts a side sectional elevation of a substantially cylindrical substrate  412   a  having an end face  414   a  comprising a single, outer, spherical, annular surface of revolution  420  surrounding an upward-facing frustoconical surface of revolution  424 , which in turn surrounds a convex, spherical surface of revolution  426 . All three surfaces of revolution have center points coincident with the centerline or longitudinal axis L of substrate  412   a.    
         [0034]    [0034]FIG. 7 depicts a side sectional elevation of a substantially cylindrical substrate  412   b  having an end face  414   b  comprising a single, outer, spherical, annular surface of revolution  420  surrounding an upward-facing frustoconical surface of revolution  424 , which in turn surrounds a central, circular surface  428 . Both surfaces of revolution have center points coincident with the centerline or longitudinal axis L of substrate  412   b.    
         [0035]    [0035]FIG. 8 depicts a side sectional elevation of a substantially cylindrical substrate  412   c  having an end face  414   c  comprising a single, outer, spherical, annular surface of revolution  420  surrounding a plurality of concentric annular grooves  430  having ridges  432  therebetween, the end face features being centered about centerline or longitudinal axis L.  
         [0036]    [0036]FIG. 9 depicts a side sectional elevation of a substantially cylindrical substrate  512  having an end face  514  comprising a central hemispherical surface  522  contiguous with and surrounded by a concave annular surface  520  comprised of a portion of a toroid of circular cross-section centered about the centerline or longitudinal axis L of substrate  512 .  
         [0037]    [0037]FIG. 10 depicts a side sectional elevation of a substantially cylindrical substrate  512   a  similar to substrate  512 , having an end face  514   a  comprising a central hemispherical surface  522  contiguous with and surrounded by an annular surface  520  comprised of a portion of a toroid of circular cross-section. Hemispherical surface  522 , however, is intersected by a smaller, spherical surface of revolution  524  defining a central recess or concavity therein.  
         [0038]    Other combinations of substrates exhibiting end faces comprised of various combinations of spherical, toroidal and linear surfaces of revolution are depicted in FIGS. 11 through 15. As with the preceding FIGS. 4 through 10, spherical surfaces of revolution and toroids, parts of which comprise substrate surfaces, have been shown, in part in most instances, in broken lines for clarity, as have center points of certain features.  
         [0039]    Spherical surfaces of revolution have been designated with an “S”, toroids with a “T”, and linear surfaces of revolution with an “LS”.  
         [0040]    It will also be understood that spherical surfaces of revolution may be replaced, as noted above, by spheroidal surfaces of revolution, as depicted in FIG. 16 showing a substrate  612  having ellipsoidal surface of revolution E on its end face  614 . Other non-linear, or arcuate, surfaces of revolution may also be employed, as desired, in a similar or transverse orientation to that shown in FIG. 16.  
         [0041]    [0041]FIG. 17 depicts a rotary drag bit equipped with cutters C in accordance with the present invention.  
         [0042]    It will be understood that the reference to “annular” surfaces herein is not limited to surfaces defining a complete annulus or ring. For example, a partial annulus in the area of the substrate end face oriented to accommodate resultant loading on the cutting edge is contemplated as included in the present invention. Similarly, a discontinuous or segmented annular surface is likewise included. Moreover, an “arcuate” surface topography includes surfaces which curve on a constant radius, such as spherical surfaces of revolution and toroids of circular cross-section as well as spheroidal surfaces as those which include components from, for example, two distinct radii about center points, and further include surfaces which are non-linear but curve on varying or continuously or intermittently variable radii.  
         [0043]    While the present invention has been disclosed in terms of certain exemplary embodiments, those of ordinary skill in the art will understand and appreciate that it is not so limited. Many additions, deletions and modifications to the invention as disclosed herein may be effected, as well as combinations of features from the various disclosed embodiments, without departing from the scope of the invention as defined by the claims.

Summary:
A cutter for drilling subterranean formations including a superabrasive table formed on an end face of a supporting substrate, there being an interface between the table and the end face defined by at least one annular surface centered about the centerline of the cutter in a location adjacent the side periphery of the substrate, the annular surface having an arcuate topography of an orientation and radial width sufficient to accommodate resultant loading of the cutting edge of the cutter throughout a variety of angles with vectors normal to the surface at a variety of angles such that at least one normal vector is aligned substantially parallel to the resultant loading on the cutting edge.