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CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 14/180,121, filed on Feb. 13, 2014, which is a broadening reissue of U.S. application Ser. No. 12/796,560, filed on Jun. 8, 2010, issued as U.S. Pat. No. 8,113,303 on Feb. 14, 2012, which is a continuation of U.S. application Ser. No. 11/855,770, filed Sep. 14, 2007 issued as U.S. Pat. No. 7,757,785 on Jul. 20, 2010, which is a continuation of U.S. patent application Ser. No. 11/117,647, filed Apr. 28, 2005, now abandoned, which claims priority, pursuant to 35 U.S.C. §119(e), to U.S. Provisional Patent Application No. 60/648,863, filed Feb. 1, 2005, U.S. Provisional Patent Application No. 60/584,307 filed Jun. 30, 2004, and U.S. Provisional Patent Application No. 60/566,751 filed Apr. 30, 2004. These applications are incorporated herein by reference in their entireties. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The disclosure relates generally to modified cutters. 
         [0004]    2. Background Art 
         [0005]    Rotary drill bits with no moving elements on them are typically referred to as “drag” bits. Drag bits are often used to drill a variety of rock formations. Drag bits include those having cutters (sometimes referred to as cutter elements, cutting elements or inserts) attached to the bit body. For example, the cutters may be formed having a substrate or support stud made of cemented carbide, for example tungsten carbide, and an ultra hard cutting surface layer or “table” made of a polycrystalline diamond material or a polycrystalline boron nitride material deposited onto or otherwise bonded to the substrate at an interface surface. 
         [0006]    An example of a prior art drag bit having a plurality of cutters with ultra hard working surfaces is shown in  FIG. 1 . The drill bit  10  includes a bit body  12  and a plurality of blades  14  that are formed on the bit body  12 . The blades  14  are separated by channels or gaps  16  that enable drilling fluid to flow between and both clean and cool the blades  14  and cutters  18 . Cutters  18  are held in the blades  14  at predetermined angular orientations and radial locations to present working surfaces  20  with a desired back rake angle against a formation to be drilled. Typically, the working surfaces  20  are generally perpendicular to the axis  19  and side surface  21  of a cylindrical cutter  18 . Thus, the working surface  20  and the side surface  21  meet or intersect to form a circumferential cutting edge  22 . 
         [0007]    Nozzles  23  are typically formed in the drill bit body  12  and positioned in the gaps  16  so that fluid can be pumped to discharge drilling fluid in selected directions and at selected rates of flow between the cutting blades  14  for lubricating and cooling the drill bit  10 , the blades  14  and the cutters  18 . The drilling fluid also cleans and removes the cuttings as the drill bit rotates and penetrates the geological formation. The gaps  16 , which may be referred to as “fluid courses,” are positioned to provide additional flow channels for drilling fluid and to provide a passage for formation cuttings to travel past the drill bit  10  toward the surface of a wellbore (not shown). 
         [0008]    The drill bit  10  includes a shank  24  and a crown  26 . Shank  24  is typically formed of steel or a matrix material and includes a threaded pin  28  for attachment to a drill string. Crown  26  has a cutting face  30  and outer side surface  32 . The particular materials used to form drill bit bodies are selected to provide adequate toughness, while providing good resistance to abrasive and erosive wear. For example, in the case where an ultra hard cutter is to be used, the bit body  12  may be made from powdered tungsten carbide (WC) infiltrated with a binder alloy within a suitable mold form. In one manufacturing process the crown  26  includes a plurality of holes or pockets  34  that are sized and shaped to receive a corresponding plurality of cutters  18 . 
         [0009]    The combined plurality of surfaces  20  of the cutters  18  effectively forms the cutting face of the drill bit  10 . Once the crown  26  is formed, the cutters  18  are positioned in the pockets  34  and affixed by any suitable method, such as brazing, adhesive, mechanical means such as interference fit, or the like. The design depicted provides the pockets  34  inclined with respect to the surface of the crown  26 . The pockets  34  are inclined such that cutters  18  are oriented with the working face  20  at a desired rake angle in the direction of rotation of the bit  10 , so as to enhance cutting. It will be understood that in an alternative construction (not shown), the cutters can each be substantially perpendicular to the surface of the crown, while an ultra hard surface is affixed to a substrate at an angle on a cutter body or a stud so that a desired rake angle is achieved at the working surface. 
         [0010]    A typical cutter  18  is shown in  FIG. 2 . The typical cutter  18  has a cylindrical cemented carbide substrate body  38  having an end face or upper surface  54  referred to herein as the “interface surface”  54 . An ultra hard material layer (cutting layer)  44 , such as polycrystalline diamond or polycrystalline cubic boron nitride layer, forms the working surface  20  and the cutting edge  22 . A bottom surface  52  of the cutting layer  44  is bonded on to the upper surface  54  of the substrate  38 . The joining surfaces  52  and  54  are herein referred to as the interface  46 . The top exposed surface or working surface  20  of the cutting layer  44  is opposite the bottom surface  52 . The cutting layer  44  typically has a flat or planar working surface  20 , but may also have a curved exposed surface, that meets the side surface  21  at a cutting edge  22 . 
         [0011]    Cutters may be made, for example, according to the teachings of U.S. Pat. No. 3,745,623, whereby a relatively small volume of ultra hard particles such as diamond or cubic boron nitride is sintered as a thin layer onto a cemented tungsten carbide substrate. Flat top surface cutters as shown in  FIG. 2  are generally the most common and convenient to manufacture with an ultra hard layer according to known techniques. It has been found that cutter chipping, spalling and delamination are common failure modes for ultra hard flat top surface cutters. 
         [0012]    Generally speaking, the process for making a cutter  18  employs a body of cemented tungsten carbide as the substrate  38 , wherein the tungsten carbide particles are cemented together with cobalt. The carbide body is placed adjacent to a layer of ultra hard material particles such as diamond or cubic boron nitride particles and the combination is subjected to high temperature at a pressure where the ultra hard material particles are thermodynamically stable. This results in recrystallization and formation of a polycrystalline ultra hard material layer, such as a polycrystalline diamond or polycrystalline cubic boron nitride layer, directly onto the upper surface  54  of the cemented tungsten carbide substrate  38 . 
         [0013]    It has been found by applicants that many cutters develop cracking, spalling, chipping and partial fracturing of the ultra hard material cutting layer at a region of cutting layer subjected to the highest loading during drilling. This region is referred to herein as the “critical region”  56 . The critical region  56  encompasses the portion of the cutting layer  44  that makes contact with the earth formations during drilling. The critical region  56  is subjected to the generation of high magnitude stresses from dynamic normal loading, and shear loadings imposed on the ultra hard material layer  44  during drilling. Because the cutters are typically inserted into a drag bit at a rake angle, the critical region includes a portion of the ultra hard material layer near and including a portion of the layer&#39;s circumferential edge  22  that makes contact with the earth formations during drilling. 
         [0014]    The high magnitude stresses at the critical region  56  alone or in combination with other factors, such as residual thermal stresses, can result in the initiation and growth of cracks  58  across the ultra hard layer  44  of the cutter  18 . Cracks of sufficient length may cause the separation of a sufficiently large piece of ultra hard material, rendering the cutter  18  ineffective or resulting in the failure of the cutter  18 . When this happens, drilling operations may have to be ceased to allow for recovery of the drag bit and replacement of the ineffective or failed cutter. The high stresses, particularly shear stresses, can also result in delamination of the ultra hard layer  44  at the interface  46 . 
         [0015]    One type of ultra hard working surface  20  for fixed cutter drill bits is formed as described above with polycrystalline diamond on the substrate of tungsten carbide, typically known as a polycrystalline diamond compact (PDC), PDC cutters, PDC cutting elements, or PDC inserts. Drill bits made using such PDC cutters  18  are known generally as PDC bits. While the cutter or cutter insert  18  is typically formed using a cylindrical tungsten carbide “blank” or substrate  38  which is sufficiently long to act as a mounting stud  40 , the substrate  38  may also be an intermediate layer bonded at another interface to another metallic mounting stud  40 . 
         [0016]    The ultra hard working surface  20  is formed of the polycrystalline diamond material, in the form of a cutting layer  44  (sometimes referred to as a “table”) bonded to the substrate  38  at an interface  46 . The top of the ultra hard layer  44  provides a working surface  20  and the bottom of the ultra hard layer cutting layer  44  is affixed to the tungsten carbide substrate  38  at the interface  46 . The substrate  38  or stud  40  is brazed or otherwise bonded in a selected position on the crown of the drill bit body  12  ( FIG. 1 ). As discussed above with reference to  FIG. 1 , the PDC cutters  18  are typically held and brazed into pockets  34  formed in the drill bit body at predetermined positions for the purpose of receiving the cutters  18  and presenting them to the geological formation at a rake angle. 
         [0017]    In order for the body of a drill bit to be resistant to wear, hard and wear-resistant materials such as tungsten carbide are typically used to form the drill bit body for holding the PDC cutters. Such a drill bit body is very hard and difficult to machine. Therefore, the selected positions at which the PDC cutters  18  are to be affixed to the bit body  12  are typically formed during the bit body molding process to closely approximate the desired final shape. A common practice in molding the drill bit body is to include in the mold, at each of the to-be-formed PDC cutter mounting positions, a shaping element called a “displacement.” 
         [0018]    A displacement is generally a small cylinder, made from graphite or other heat resistant materials, which is affixed to the inside of the mold at each of the places where a PDC cutter is to be located on the finished drill bit. The displacement forms the shape of the cutter mounting positions during the bit body molding process. See, for example, U.S. Pat. No. 5,662,183 issued to Fang for a description of the infiltration molding process using displacements. 
         [0019]    It has been found by applicants that cutters with sharp cutting edges or small back rake angles provide a good drilling ROP, but are often subject to instability and are susceptible to chipping, cracking or partial fracturing when subjected to high forces normal to the working surface. For example, large forces can be generated when the cutter “digs” or “gouges” deep into the geological formation or when sudden changes in formation hardness produce sudden impact loads. Small back rake angles also have less delamination resistance when subjected to shear load. Cutters with large back rake angles are often subjected to heavy wear, abrasion and shear forces resulting in chipping, spalling, and delamination due to excessive downward force or weight on bit (WOB) required to obtain reasonable ROP. Thick ultra hard layers that might be good for abrasion wear are often susceptible to cracking, spalling, and delamination as a result of residual thermal stresses associated with forming thick ultra hard layers on the substrate. The susceptibility to such deterioration and failure mechanisms is accelerated when combined with excessive load stresses. 
         [0020]      FIG. 3  shows a prior art PDC cutter held at an angle in a drill bit  10  for cutting into a formation  45 . The cutter  18  includes a diamond material table  44  affixed to a tungsten carbide substrate  38  that is bonded into the pocket  34  formed in a drill bit blade  14 . The drill bit  10  (see  FIG. 1 ) will be rotated for cutting the inside surface of a cylindrical well bore. Generally speaking, the back rake angle “A” is used to describe the working angle of the working surface  20 , and it also corresponds generally to the magnitude of the attack angle “B” made between the working surface  20  and an imaginary tangent line at the point of contact with the well bore. It will be understood that the “point” of contact is actually an edge or region of contact that corresponds to critical region  56  (see  FIG. 2 ) of maximum stress on the cutter  18 . Typically, the geometry of the cutter  18  relative to the well bore is described in terms of the back rake angle “A.” 
         [0021]    Different types of bits are generally selected based on the nature of the geological formation to be drilled. Drag bits are typically selected for relatively soft formations such as sands, clays and some soft rock formations that are not excessively hard or excessively abrasive. However, selecting the best bit is not always straightforward because many formations have mixed characteristics (i.e., the geological formation may include both hard and soft zones), depending on the location and depth of the well bore. Changes in the geological formation can affect the desired type of a bit, the desired ROP of a bit, the desired rotation speed, and the desired downward force or WOB. Where a drill bit is operated outside the desired ranges of operation, the bit can be damaged or the life of the bit can be severely reduced. 
         [0022]    For example, a drill bit normally operated in one general type of formation may penetrate into a different formation too rapidly or too slowly subjecting it to too little load or too much load. For another example, a drill bit rotating and penetrating at a desired speed may encounter an unexpectedly hard formation material, possibly subjecting the bit to a “surprise” or sudden impact force. A formation material that is softer than expected may result in a high rate of rotation, a high ROP, or both, that can cause the cutters to shear too deeply or to gouge into the geological formation. 
         [0023]    This can place greater loading, excessive shear forces and added heat on the working surface of the cutters. Rotation speeds that are too high without sufficient WOB, for a particular drill bit design in a given formation, can also result in detrimental instability (bit whirling) and chattering because the drill bit cuts too deeply or intermittently bites into the geological formation. Cutter chipping, spalling, and delamination, in these and other situations, are common failure modes for ultra hard flat top surface cutters. 
         [0024]    Dome cutters have provided certain benefits against gouging and the resultant excessive impact loading and instability. This approach for reducing adverse effects of flat surface cutters is described in U.S. Pat. No. 5,332,051. An example of such a dome cutter in operation is depicted in  FIG. 4 . The prior art cutter  60  has a dome shaped top or working surface  62  that is formed with an ultra hard layer  64  bonded to a substrate  66 . The substrate  66  is bonded to a metallic stud  68 . The cutter  60  is held in a blade  70  of a drill bit  72  (shown in partial section) and engaged with a geological formation  74  (also shown in partial section) in a cutting operation. The dome shaped working surface  62  effectively modifies the rake angle A that would be produced by the orientation of the cutter  60 . 
         [0025]    Scoop cutters, as shown at  80  in  FIG. 5  (U.S. Pat. No. 6,550,556), have also provided some benefits against the adverse effects of impact loading. This type of prior art cutter  80  is made with a “scoop” or depression  90  formed in the top working surface  82  of an ultra hard layer  84 . The ultra hard layer  84  is bonded to a substrate  86  at an interface  88 . The depression  90  is formed in the critical region  56 . The upper surface  92  of the substrate  86  has a depression  94  corresponding to the depression  90 , such that the depression  90  does not make the ultra hard layer  84  too thin. The interface  88  may be referred to as a non-planar interface (NPI). 
         [0026]    What is still needed, however, are improved cutters for use in a variety of applications. 
       SUMMARY 
       [0027]    In one aspect, the present disclosure relates to a modified cutting element that includes a base portion, an ultrahard layer disposed on said base portion, and at least one modified region disposed adjacent to a cutting face of the cutter. 
         [0028]    In one aspect, the present disclosure relates to a drill bit that includes a bit body; and at least one cutter, the at least one cutter comprising a base portion, an ultrahard layer disposed on said base portion, and at least one modified region disposed adjacent to a cutting face of the cutter. 
         [0029]    Other aspects and advantages of the disclosure will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0030]      FIG. 1  is a perspective view of a prior art fixed cutter drill bit sometimes referred to as a “drag bit”; 
           [0031]      FIG. 2  is a perspective view of a prior art cutter or cutter insert with an ultra hard layer bonded to a substrate or stud; 
           [0032]      FIG. 3  is a partial section view of a prior art flat top cutter held in a blade of a drill bit engaged with a geological formation (shown in partial section) in a cutting operation; 
           [0033]      FIG. 4  is a schematic view of a prior art dome top cutter with an ultra hard layer bonded to a substrate that is bonded to a stud, where the cutter is held in a blade of a drill bit (shown in partial section) and engaged with a geological formation (also shown in partial section) in a cutting operation; 
           [0034]      FIG. 5  is a perspective view of a prior art scoop top cutter with an ultra hard layer bonded to a substrate at a non-planar interface (NPI); 
           [0035]      FIGS. 6A ,  6 B, and  6 C show a side, front, and perspective view of a cutter in accordance with an embodiment of the present invention; 
           [0036]      FIG. 7  shows a cutter in accordance with another embodiment of the present invention; and 
           [0037]      FIG. 8  shows a blade including cutters in accordance with an embodiment of the present invention. 
           [0038]      FIG. 9  shows a PDC bit including cutters formed in accordance with an embodiment of the present invention. 
           [0039]      FIGS. 10A ,  10 B, and  10 C are perspective and cross-sectional views of an ultra hard top layer having a varied geometry chamfer circumferentially around the cutting edge of the working surface of the ultra hard layer wherein the size of the chamfer is varied circumferentially around the cutting edge according to one embodiment; 
           [0040]      FIG. 11  is a graph showing the average chamfer size as varied with different cutting depths for a cutter having varied chamfer as compared to a cutter having fixed geometry chamfer. 
           [0041]      FIG. 12  shows an ultra hard layer according to one or more embodiments. 
           [0042]      FIG. 13  shows a cutter according to one or more embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0043]    The present disclosure relates to shaped cutters that provide advantages when compared to prior art cutters. In particular, embodiments of the present disclosure relate to cutters that have structural modifications to the cutting surface in order to improve cutter performance. As a result of the modifications, embodiments of the present disclosure may provide improved cooling, higher cutting efficiency, and longer lasting cutters when compared with prior art cutters. 
         [0044]    Embodiments of the present disclosure relate to cutters having a substrate or support stud, which in some embodiments may be made of cemented carbide, for example tungsten carbide, and an ultra hard cutting surface layer or “table” made of a polycrystalline diamond material or a polycrystalline boron nitride material deposited onto or otherwise bonded to the substrate at an interface surface. Also, in selected embodiments, the ultra-hard layer may comprise a “thermally stable” layer. One type of thermally stable layer that may be used in embodiments of the present disclosure is leached polycrystalline diamond. 
         [0045]    A typical polycrystalline diamond layer includes individual diamond “crystals” that are interconnected. The individual diamond crystals thus form a lattice structure. A metal catalyst, such as cobalt may be used to promote recrystallization of the diamond particles and formation of the lattice structure. Thus, cobalt particles are typically found within the interstitial spaces in the diamond lattice structure. Cobalt has a significantly different coefficient of thermal expansion as compared to diamond. Therefore, upon heating of a diamond table, the cobalt and the diamond lattice will expand at different rates, causing cracks to form in the lattice structure and resulting in deterioration of the diamond table. 
         [0046]    In order to obviate this problem, strong acids may be used to “leach” the cobalt from the diamond lattice structure. Examples of “leaching” processes can be found, for example in U.S. Pat. Nos. 4,288,248 and 4,104,344. Briefly, a hot strong acid, e.g., nitric acid, hydrofluoric acid, hydrochloric acid, or perchloric acid, or combinations of several strong acids may be used to treat the diamond table, removing at least a portion of the catalyst from the PDC layer. 
         [0047]    Removing the cobalt causes the diamond table to become more heat resistant, but also causes the diamond table to be more brittle. Accordingly, in certain cases, only a select portion (measured either in depth or width) of a diamond table is leached, in order to gain thermal stability without losing impact resistance. As used herein, thermally stable polycrystalline diamond compacts include both of the above (i.e., partially and completely leached) compounds. In one embodiment, only a portion of the polycrystalline diamond compact layer is leached. For example, a polycrystalline diamond compact layer having a thickness of 0.010 inches may be leached to a depth of 0.006 inches. In other embodiments, the entire polycrystalline diamond compact layer may be leached. A number of leaching depths may be used, depending on the particular application, for example, in one embodiment the leaching depth may be 0.05 mm. 
         [0048]      FIGS. 6A-6C  show multiple views of a cutter formed in accordance with an embodiment of the present invention. In  FIG. 6A , a cutter comprises a substrate or “base portion,”  600 , on which an ultrahard layer  602  is disposed. In this embodiment, the ultrahard layer  602  comprises a polycrystalline diamond layer. As explained above, when a polycrystalline diamond layer is used, the layer may further be partially or completely leached. A beveled edge  606  may be provided on at least one side of the ultrahard layer  602 , but more commonly, may be placed on at least two sides, so that the cutter may be removed and reoriented for use a second time. Further, at least one modified region  604  is formed on the ultrahard layer  602 .  FIGS. 6B and 6C  show that, in this embodiment, two modified regions  604  have been formed on the ultrahard layer  602 . In particular, in  FIG. 6C  the modified regions  604  comprise tapered portions that have been machined from the ultrahard layer  602 . 
         [0049]    The original height of the diamond table layer is shown as unmodified portion  608 , as the modified regions  604  are designed such that the unmodified portion  608  has a discrete width in this embodiment. In some instances the modified region or regions  604  may be formed when the cutter is actually being bonded together (i.e., a modified region is originally built into the ultrahard layer), but in other instances, the modified region may be formed after the formation of the ultrahard layer, by using electrical discharge machining, for example. In addition, in select embodiments, only portions of the modified surface may be leached. Those having ordinary skill in the art will recognize that masking agents may be used to prevent leaching in certain areas, to provide regions that are leached and legions that are unleached. 
         [0050]    Wire electrical discharge machining (EDM) is an electrical discharge machining process with a continuously moving conductive wire as tool electrode. The mechanism of metal removal in wire EDM involves the complex erosion effect of electric sparks generated by a pulsating direct current power supply between two closely spaced electrodes in dielectric liquid. The high energy density erodes material from both the wire and workpiece by local melting and vaporizing. Because the new wire keeps feeding to the machining area, the material is removed from the workpiece with the moving of wire electrode. Eventually, a cutting shape is formed on the workpiece by the programmed moving trajectory of wire electrode. 
         [0051]    As the term is used herein, a modified region constitutes at least one area, adjacent to the cutting face, that has a lower overall height than the cutting face itself. Cutters containing the modified region  604  have a number of advantages when compared to prior art planar cutters. For example, because the modified region is a depressed area adjacent to the cutting face, improved cooling (due to better fluid flow and/or air flow) around the cutting edge may be seen, which may help prevent failure due to thermal degradation. 
         [0052]    In the embodiment shown in  FIG. 6   c , the beveled edge  606  is formed such that when placed into a pocket, the beveled edge  606  will form the cutting face of the cutter. Those having ordinary skill in the art will appreciate that the size of the beveled edge may be modified depending on the application. For example, in selected applications, the size may range from five thousandths of an inch (0.005 inches) to about fifty thousandths of an inch (0.050 inches). In addition, the bevel may be located at other portions, or additional beveled regions may be provided. In selected embodiments, the modified region  604  is provided such that a self-sharpening effect occurs at the cutting face. That is, as portions of the cutter chip away, a fresh portion is exposed. Having this self-sharpening beveled edge  606  may provide higher cutting efficiency as compared to prior art cutters, as the beveled edge may initially fracture rock more efficiently than a typical planar contact. This feature may be particularly useful in higher hardness formations. Embodiments may also include cutters having shaped working surfaces with a varied geometry chamfer. Referring now to  FIG. 10A ,  FIG. 10A  shows an ultra hard top layer  800  for a cutter that has a shaped working surface  102  including a varied geometry chamfer  104  circumferentially around the cutting edge  106 . The bevel  104  is varied in size circumferentially around the cutting edge  106  according to one embodiment. The change in the size or the width of the bevel is demonstrated in the elevation section views of  FIGS. 10B and 10C  taken along section lines B-B and C-C of  FIG. 1  OA, respectively. In this embodiment, the width  108  in  FIG. 10B  is smaller than the width  110  in  FIG. 10C . The angle  112  of the bevel at section B-B,  FIG. 10B , is the same as angle  114  at section line C-C,  FIG. 10C ; however, in other embodiments, the angle of the bevel is varied circumferentially around the cutting edge. It will be understood that a varied geometry of a bevel could also be provided as a combination of varied size and varied angle. Additionally, in one or more embodiments, the bevel is formed so that its size increases away from the area of the cutter surface engaged with the geological formation. For example, referring to  FIG. 11 , the amount of the variable size bevel in contact with the formation increases with the depth of cut. Thus, when the cutter digs into the formation, a greater portion of the cutting edge has a larger bevel to give more protection against chipping and spalling. 
         [0053]    In  FIG. 7 , another embodiment of the present invention is shown. In  FIG. 7 , a cutter  700 , is shown having a base portion  702  and a ultrahard layer  704  disposed thereon. Further, a beveled edge  706  is provided at a cutting face of the insert. In this embodiment, a modified region  708  extends over substantially all of the cutter  700 . In this embodiment, the modified region  708  comprises a substantially continuous “saddle shaped” region. In this embodiment, if the modified region is formed after the deposition of an ultrahard layer, the modified region may be formed in a single manufacturing pass, whereas with the multiple modified regions in  FIGS. 6A ,  6 B, and  6 C, multiple manufacturing passes may be required. As can be seen from  FIG. 7 , the ultrahard material layer has an exposed upper surface  710  and a peripheral surface  712 , such that the upper surface intersects the peripheral surface along a peripheral edge  714 . As can be seen, the peripheral edge  714  continuously decreases in height and increases in height as measured from a first plane  716  perpendicular to a longitudinal axis  718 . The peripheral edge decreases from a maximum height  719  as measured from a plane  716  to a minimum height of  720  as measured from the same plane  716 . As second plane  722  along the longitudinal axis  718  intersects the peripheral edge at a first point  724  and a second point  726 . A third plane  728  along the longitudinal axis  718  insects the peripheral edge at a third point  730  and a fourth point  732 . As can be seen from  FIG. 7 , the peripheral edge has a first convex portion  740  extending from the first point  724  in a direction towards the third point  730 . In addition, a first concave portion  742  extends from the first convex portion  740  to the third point  730 . Similarly, a second concave portion extends from the third point in a direction towards the second point  726  and a second convex portion extends from the second concave portion to the second point  726 . Moreover, a third convex portion extends from the second point  726  in a direction towards the fourth point  732  and a third concave portion extends from the third convex portion to the fourth point  732 . In addition, a fourth concave point extends from the fourth point  732  in a direction towards the first point  724  and a fourth convex portion extends from the fourth concave portion to the first point  724 . 
         [0054]    After formation of the saddle-shaped cutter, mill tests were performed to determine the performance of the cutters. Test results showed that approximately a 20% increase in performance when compared to prior art cutters was seen when a polycrystalline diamond surface was used. In addition, when thermally stable polycrystalline diamond was used as the ultrahard layer, a performance jump of nearly 70% was seen as compared to unmodified thermally stable polycrystalline diamond cutters. As stated above, without being limited to any particular theory, that the improved performance may be due to a number of factors such as, improved cooling around the cutting face, higher cutting efficiency (due to the non-planar interaction at the cutting face), and the fact that a non-planar interface leads to less flaking of the thermally stable polycrystalline diamond. 
         [0055]    Cutters formed in accordance with embodiments of the present invention may be used either alone or in conjunction with standard cutters depending on the desired application. In addition, while reference has been made to specific manufacturing techniques, those of ordinary skill will recognize that any number of techniques may be used. 
         [0056]      FIG. 8  shows a view of cutters formed in accordance with embodiments of the present invention disposed on a blade of a PDC bit. In  FIG. 8 , modified cutters  804  are intermixed on a blade  800  with standard cutters  802 . Similarly,  FIG. 9  shows a PDC bit having modified cutters  904  disposed thereon. Referring to  FIG. 9 , the fixed-cutter bits (also called drag bits)  900  comprise a bit body  902  having a threaded connection at one end  903  and a cutting head  906  formed at the other end. The head  906  of the fixed-cutter bit  900  comprises a plurality of blades  908  arranged about the rotational axis of the bit and extending radially outward from the bit body  902 . Modified cutting elements  904  are embedded in the blades  908  to cut through earth formation as the bit is rotated on the earth formation. As discussed above, the modified cutting elements may be mixed with standard cutting elements  905 . 
         [0057]      FIG. 12  shows another embodiment of an ultra hard top layer  140  for a cutter with a shaped working surface  142  and having a varied geometry chamfer  144  circumferentially around a cutting edge  146  at the intersection of the shaped working surface  142  and a side surface  148 . The shaped working surface  142  includes one or more depressions  150   a ,  150   b , and  150   c  extending radially outwardly to the cutting edge  146 . While three depressions  150   a - c  are depicted uniformly spaced around the shaped working surface  142 , fewer or a greater number with uniform or non-uniform spacing may be formed without departing from certain aspects of the disclosure. For example, one or more depressions  150   a - c  can be formed as one or more planar surfaces or facets in a face  154 . 
         [0058]    Depending upon the embodiment, the face  154  may be a planar shaped surface, a dome shaped surface or a surface having another shape. The depressions  150   a - c  in this embodiment comprise planar surfaces or facets each at an obtuse angle relative to a central axis  152  of the cylindrical ultra hard top layer. The obtuse angle is different from the angle of other portions of the working surface, such that a relative depressed area defining the depressions  150   a - c  is formed the face  154 . Where the surrounding portions of the face  154  are planar and at a 90-degree angle with respect to the axis of the cutter, the obtuse angle is generally greater than 90 degrees with respect to the axis  152  of the cutter. However, according to alternative embodiments of the invention, the obtuse angle may be less than 90 degrees. It will also be understood that in other alternative embodiments, each of the depressions  150   a - c  can be multi-faceted or comprised of multiple planar surfaces. Alternatively, the depressions  150   a - c  can also be formed with simple curved surfaces that may be concave or convex or can be formed with a plurality of curved surfaces or with a smooth complex curve. 
         [0059]    The depressions  150   a - c  may be formed and shaped during the initial compaction of the ultra hard layer  140  or can be shaped after the ultra hard layer is formed, for example by Electro Discharge Machining (EDM) or by Electro Discharge Grinding (EDG). The ultra hard layer  140  may, for example, be formed as a polycrystalline diamond compact or a polycrystalline cubic boron nitride compact. Also, in selected embodiments, the ultra-hard layer may comprise a “thermally stable” layer. One type of thermally stable layer that may be used in embodiments may be a TSP element or partially or fully leached polycrystalline diamond. The depressions  150   a - c  extend generally at an angle relative to the face  154  outward to the edge of the cutter. It has been found that a varied chamfer  144  can be conveniently made with a fixed angle and fixed depth EDM or EDG device. For example, an EDM device will typically cut deepest into the edge  146  where the raise areas of face  154  extend to the edge  146  and will cut less deep where the depressions  150   a - c  extend to the edge  146 . The chamfer  144  is cut the least at the lowest edge point in each depression  150   a - c  and progressively deeper on either side of the lowest edge point. A varied width or size chamfer is conveniently formed circumferentially around the edge  146  of the ultra hard cutter layer  140 . Alternatively, variable or programmable angle and depth EDM or EGM can be used to form the variable geometry chamfer.  FIG. 13  shows a three-dimensional model of a cutter  160  having an ultra hard layer  162  with a shaped working surface  164 . The ultra hard layer  162  is bonded to a substrate  166  at a non-planar interface  168  according to one embodiment of the invention. 
         [0060]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.

Summary:
A cutter for a drag bit may include a substrate and an ultrahard layer on an end surface of the substrate. The ultrahard layer may include an exposed surface having at least three depressions extending from an interior of the exposed surface radially outward to a peripheral edge formed between the working surface and a side surface of the ultrahard layer, the at least three depressions separated from each other by at least three raised regions forming an apex of the exposed surface, the at least three raised regions connected to each other proximate the central axis and extending from proximate the central axis to the peripheral edge. Other working surfaces are also included.