Abstract:
The invention provides an improved drill bit and a method for designing thereof. The drill bit includes a bit body, a journal depending from the bit body, and a disc rotatably mounted on the journal. The disc of the drill bit has PDC cutting elements disposed on it. Also provided is an improved cutting structure for the discs of the drill bit. The cutting structure includes a portion that is comprised from PDC.

Description:
BACKGROUND OF INVENTION  
       [0001]     Disc drill bits are one type of drill bit used in earth drilling applications, particularly in petroleum or mining operations. In such operations, the cost of drilling is significantly affected by the rate the disc drill bit penetrates the various types of subterranean formations. That rate is referred to as rate of penetration (“ROP”), and is typically measured in feet or inches per hour. As a result, there is a continual effort to optimize the design of disc drill bits to more rapidly drill specific formations and reduce these drilling costs.  
         [0002]     Disc drill bits are characterized by having disc-shaped cutter heads rotatably mounted on journals of a bit body. Each disc has an arrangement of cutting elements attached to the outer profile of the disc. Disc drill bits can have three discs, two discs, or even one disc. An example of a three disc drill bit  101 , shown in  FIG. 1A , is disclosed in U.S. Pat. No. 5,064,007 issued to Kaalstad (“the &#39;007 Patent”), and. incorporated herein by reference in its entirety. Disc drill bit  101  includes a bit body  103  and three discs  105  rotatably mounted on journals (not shown) of bit body  103 . Discs  105  are positioned to drill a generally circular borehole  151  in the earth formation being penetrated. Inserts  107  are arranged on the outside radius of discs  105  such that inserts  107  are the main elements cutting borehole  151 . Furthermore, disc drill bit  101  includes a threaded pin member  109  to connect with a threaded box member  111 . This connection enables disc drill bit  101  to be threadably attached to a drill string  113 .  
         [0003]     In this patent, inserts  107  on discs  105  are conically shaped and used to primarily generate failures by crushing the earth formation to cut out wellbore  151 . During drilling, a force (referred to as weight on bit (“WOB”)) is applied to disc drill bit  101  to push it into the earth formation. The WOB is translated through inserts  107  to generate compressive failures in the earth formation. In addition, as drill string  113  is rotated in one direction, as indicated by arrow  131 , bit body  103  rotates in the same direction  133  as drill string  113 , which causes discs  105  to rotate in an opposite direction  135 .  
         [0004]     Referring now to  FIG. 1B , another type of disc drill bit, as disclosed in U.S. Pat. No. 5,147,000 also issued to Kaalstad (“the &#39;000 Patent”) incorporated herein by reference in its entirety, is shown. The &#39;000 Patent discloses a similar three disc drill bit to that of the &#39;007 Patent, but instead shows another arrangement of the inserts on the discs of the disc drill bit. In  FIG. 1B , inserts  123  are disposed on the face of discs  125 , instead of on the outside radius. The primary function of inserts  123  is to cut out the borehole by generating compressive failures from WOB. After inserts  123  generate the primary compressive failures, they then perform a secondary function of excavating the compressively failed earth. The conical shape and location of inserts  123  on disc drill bit  121  are effective for generating compressive failures, but are inadequate in shape and location to excavate the earth formation also. When used to excavate the earth formation from the compressive failures, inserts  123  wear and delaminate very quickly.  
         [0005]     Although disc bits have been used successfully in the prior art, further improvements in the drilling performance may be obtained by improved cutting configurations.  
       SUMMARY OF THE INVENTION  
       [0006]     In one aspect, the present invention relates to a drill bit. The drill bit includes a bit body and a journal depending from the bit body. The drill bit further includes a disc rotatably mounted on the journal and PDC cutting elements disposed on the disc.  
         [0007]     In another aspect, the present invention relates to a cutting structure to be used with a disc drill bit. The cutting structure includes a shearing portion arranged in a shearing configuration, wherein the shearing portion comprises PDC. The cutting structure further includes a compressive portion arranged in a compressive configuration. The shearing portion and the compressive portion of the cutting structure are formed into a single body.  
         [0008]     In another aspect, the present invention relates to a method of designing a drill bit, wherein the drill bit includes a bit body, a journal depending from the bit body, a disc rotatably mounted to the bit body, first radial row of cutting elements, and second radial of row cutting elements. The method includes identifying a relative velocity of the drill bit, and determining a compressive configuration of the first radial row of cutting elements based on the relative velocity. The method further includes determining a shearing configuration of the second radial row cutting elements based on the relative velocity of the drill bit. Then, the first radial row cutting elements are arranged on the disc of the drill bit based on the compressive configuration, and the second radial row cutting elements are arranged on the disc of the drill bit based on the shearing configuration.  
         [0009]     Other aspects and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0010]      FIG. 1A  shows an isometric view of a prior art three disc drill bit.  
         [0011]      FIG. 1B  shows a bottom view of a prior art three disc drill bit.  
         [0012]      FIG. 2A  shows an isometric view of a disc drill bit in accordance with an embodiment of the present invention.  
         [0013]      FIG. 2B  shows an isometric view of the bottom of the disc drill bit of  FIG. 2A .  
         [0014]      FIG. 3A  shows a schematic view of a prior art disc drill bit.  
         [0015]      FIG. 3B  shows a schematic view of a prior art disc drill bit.  
         [0016]      FIG. 4  shows an isometric view of a prior art PDC bit.  
         [0017]      FIG. 5  shows a bottom view of a disc drill bit in accordance with an embodiment of the present invention.  
         [0018]      FIG. 6  shows a bottom view of the disc drill bit of  FIG. 5 .  
         [0019]      FIG. 7  shows an isometric view of a cutting structure in accordance with an embodiment of the present invention.  
         [0020]      FIG. 8A  shows a bottom view of a disc drill bit in accordance with an embodiment of the present invention.  
         [0021]      FIG. 8B  shows a bottom view of the disc drill bit of  FIG. 8A .  
         [0022]      FIG. 9A  shows an isometric view of a disc drill bit in accordance with an embodiment of the present invention.  
         [0023]      FIG. 9B  shows an isometric view of the disc drill bit of  FIG. 9A .  
         [0024]      FIG. 9C  shows an isometric view of the disc drill bit of  FIGS. 9A and 9B .  
         [0025]      FIG. 10A  shows an isometric view of a disc drill bit in accordance with an embodiment of the present invention.  
         [0026]      FIG. 10B  shows an isometric view of the disc drill bit of  FIG. 10A . 
     
    
     DETAILED DESCRIPTION  
       [0027]     As used herein, “compressive configuration” refers to a cutting element that primarily generates failures by crushing the earth formation when drilling.  
         [0028]     As used herein, “shearing configuration,” refers to a cutting element that primarily generates failures by shearing the earth formation when drilling.  
         [0029]     In one or more embodiments, the present invention relates to a disc drill bit having at least one disc and at least one cutting element disposed on the disc to be oriented in a either a compressive configuration or a shearing configuration. More particularly, the cutting element oriented in either configuration can be made of polycrystalline diamond compact (“PDC”). The compact is a polycrystalline mass of diamonds that are bonded together to form an integral, tough, high-strength mass. An example of a PDC cutter for drilling earth formation is disclosed in U.S. Pat. No. 5,505,273, and is incorporated herein by reference in its entirety.  
         [0030]     Referring now to  FIG. 2A , a disc drill bit  201  in accordance with an embodiment of the present invention is shown. Disc drill bit  201  includes a bit body  203  having one or more journals (not shown), on which one or more discs  205  are rotatably mounted. Referring now to  FIG. 2B , an enlarged view of disc drill bit  201  is shown. Disposed on at least one of discs  205  of disc drill bit  201  are a first radial row  207  of cutting elements and a second radial row  209  of cutting elements. First radial row  207  of cutting elements are located closer to an axis of rotation  202  of disc drill bit  201  than second radial row  209  of cutting elements. Thus, extending radially out from axis of rotation  202 , first radial row  207  of cutting elements come before second radial row  207  of cutting elements. First radial row  207  of cutting elements and second radial row  209  of cutting elements act together to drill a borehole with a radius at which second radial row  209  of cutting elements extend from the axis of rotation of the disc drill bit. First radial row  207  of cutting elements penetrate into the earth formation to form the bottom of the borehole, and second radial row  209  of cutting elements shear away the earth formation to form the full diameter of the borehole. In this particular embodiment, each cutting element of second radial row  209  is configured into a single cutting structure  211  with a corresponding cutting element of first radial row  207 .  FIG. 7  shows a similar cutting structure to that of cutting structure  211 . Cutting elements of first radial row  207  are arranged about the outside radius of discs  205  such that cutting elements of first radial row  207  are in a compressive configuration. Also, cutting elements of second radial row  209  are disposed on the face of discs  205  such that cutting elements of second radial row  209  are in a shearing configuration.  
         [0031]     In some embodiments, cutting elements of the first radial row are oriented in the compressive configuration may be comprised of tungsten carbide, PDC, or other superhard materials, and may be diamond coated. Cutting elements of the first radial row are designed to compress and penetrate the earth formation, and may be of conical or chisel shape. The second radial row cutting elements have PDC as the cutting faces, which contact the earth formation to cut out the borehole. Also, cutting elements of the second radial row are oriented to shear across the earth formation.  
         [0032]     Because the cutting elements of the first radial row on the discs of the disc drill bit are in a compressive configuration, the cutting elements primarily generate failures by crushing the earth formation when drilling. Additionally, because the cutting elements of the first radial row are more suited to compressively load the earth formation, significant shearing of the earth formation by the cutting elements of the first radial row should be avoided. Too much shearing may prematurely wear and delaminate the cutting elements of the first radial row. To arrange the cutting elements of the first radial row in a compressive configuration, the cutting elements should be oriented on the disc drill bit to have little or no relative velocity at the point of contact with respect to borehole. If the cutting elements of the first radial row have no relative velocity with the point of contact of the borehole, the cutting elements will generate compression upon the earth formation with minimal shearing occurring across the borehole.  
         [0033]     Referring now to  FIG. 8A , a relative velocity  855  of cutting elements of first radial row  207  and the components making up relative velocity  855  with respect to the borehole, is shown. Relative velocity  855  at the point of contact of cutting elements of first radial row  207  is made from two corresponding velocities. The first contributing velocity is bit body velocity  851 , the velocity of the cutting element of first radial row  207  from the rotation of the bit body. Bit body velocity  851  is the product of rotational speed of the bit body, ω bit , and distance of the cutting element of the first radial row from the axis of rotation of the bit body, R bit . The second contributing velocity is disc velocity  853 , the velocity of the cutting element of first radial row  207  from the rotation of the discs. Disc velocity  853  is the product of rotational speed of the of the disc, ω disc , and distance of the cutting element of the first radial row from the axis of rotation of the disc, R disc . Relative velocity  855 , V first radial row , is the sum of bit body velocity  851  and disc velocity  853 , and is shown below: 
 
 V   firstradialrow =(ω bit   ×R   bit )+(ω disc   ×R   disc )   [Eq. 1]
 
         [0034]     When the bit body is in one direction of rotation, the disc is put into an opposite direction of rotation. If such values are inserted into the formula then, either the value ω disc  or the value ω bit  would be negative. As cutting elements of first radial row  207  on the disc then passes through a contact point  871  with the borehole, the two corresponding velocity components, bit body velocity  851  and disc velocity  853 , can be of equal magnitude and cancel out one another, resulting in a relative velocity of zero for V first radial row . With little or no relative velocity then, the cutting elements of first radial row  207  located at contact point  871  would therefore generate almost entirely compressive loading upon the earth formation with minimal shearing occurring across the borehole. Thus, the cutting elements of the first radial row should be designed to contact and compress the borehole most at contact point  871 . When the cutting elements of the first radial row can no longer maintain little or no relative velocity, they should disengage with the earth formation to minimize shearing action. With the determination of the direction of the relative velocity, the compressive configuration can be optimized to improve the compressive action of the cutting elements of the first radial row.  
         [0035]     In contrast to cutting elements of first radial row  207 , cutting elements of second radial row  209  are oriented to use the relative velocity to improve their shearing cutting efficiency. Referring still to  FIG. 8A , a relative velocity  855  of cutting elements of second radial row  209  is made up of the same two corresponding velocities, bit body velocity  851  and disc velocity  853 , as discussed above. Because cutting elements of first radial row  207  and cutting elements of second radial row  209  are located closely together, relative velocity  855  of cutting elements of first radial row  207  and cutting elements of second radial row  209  at points  871  and  873  are similar. Cutting efficiency of cutting elements of second radial row  209  improves if the shear cutting action occurs in the direction of relative velocity  855 . Contact point  873  shows relative velocity  855  of cutting elements of second radial row  209 . When cutting elements of second radial row  209  are oriented to shear in the direction of relative velocity  855 , as shown, the shearing cutting efficiency is improved. With the determination of the direction of the relative velocity, the shearing configuration can be optimized to improve the shearing action of the cutting elements of the second radial row.  
         [0036]     Referring now to  FIG. 8B , another view of the embodiment of the present invention of  FIG. 8A  is shown.  FIG. 8B  depicts two zones  891 ,  893  of the cutting action from the disc drill bit. Compressive zone  891  is the zone which allows first radial row  207  of cutting elements to most effectively generate compressive failures. Contact point  871 , which minimizes relative velocity of first radial row  207  of cutting elements, is located in the compressive zone  891 . Shearing zone  893  is the zone which allows second radial row  209  of cutting elements to most efficiently generate shearing failures. Contact point  873 , which has a high relative velocity for shearing of second radial row  209  of cutting elements, is located in shearing zone  893 .  
         [0037]     In one or more embodiments of the present invention, the discs in the disc drill bit may be positively or negatively offset from the bit body. Referring now to  FIGS. 3A &amp; 3B , examples of negative and positive offset in a prior art disc drill bit  301  are shown. Disc drill bit  301  includes a bit body  303  having a journal (not shown), on which a disc  305  is rotatably mounted. Inserts  307  are arranged on the outside radius of disc  305 . Disc drill bit  301  further includes a center axis  311  of rotation of bit body  303  offset from an axis  313  of rotation of disc  305 . Bit body  303  rotates in one direction, as indicated in the figures, causing disc  305  to rotate in an opposite direction when cutting a borehole  331 . Referring specifically to  FIG. 3A , axis  313  of rotation of disc  305  is offset laterally backwards in relation to the clockwise rotation of bit body  303 , showing disc drill bit  301  as negatively offset. Referring specifically to  FIG. 3B , axis  313  of rotation of disc  305  is offset laterally forwards in relation to the clockwise rotation of bit body  303 , showing disc drill bit  301  as positively offset.  
         [0038]     The positive and negative offset of the discs ensures that only an appropriate portion of the PDC cutting elements and inserts are cutting the borehole at any given time. If -the entire surface of the disc was effectively drilling the borehole, the discs and drill would be prone to stalling in rotation. The offset arrangement of the discs assures that only a selected portion of the disc is cutting. Also, offsets force the discs to shear while penetrating the earth formation. The present invention is particularly well adapted to be used with negative offset.  
         [0039]     Referring now to  FIG. 5 , another disc drill bit  501  in accordance with an embodiment of the present invention is shown. Disc drill bit  501  includes a bit body  503  having one or more journals (not shown), on which one or more discs  505  are rotatably mounted. Disposed on at least one of discs  505  of disc drill bit  501  are first radial row  507  of cutting elements and second radial row  509  of cutting elements. In this embodiment, cutting elements of second radial row  509  are not configured into individual cutting structures with cutting elements of first radial row  507  and are instead maintained as separate bodies. Cutting elements of first radial row  507  are arranged about the outside radius of discs  505  in a compressive configuration. Cutting elements of second radial row  509  are disposed on the face of disc  505  in a shearing configuration. As shown in  FIG. 5 , first radial row  507  of cutting elements form a row arranged radially outboard (away from the center of the disc) of the radial position of a row formed by second radial row  509  of cutting elements.  
         [0040]     Disc drill bit  501  further includes a webbing  511  disposed on discs  505 . Webbing  511  is arranged on the outside radius of discs  505  and is adjacent to first radial row cutting  507  of cutting elements. Optionally, webbing  511  can be an integral part of discs  505 , as shown in  FIG. 5 , wherein webbing  511  is manufactured into discs  505 . However, webbing  511  can also be an overlay that is placed on discs  505  after they have been manufactured. Furthermore, discs  505  could be manufactured, webbing  511  then placed on discs  505  adjacent to first radial row  507  of cutting elements, and webbing  511  then brazed onto discs  505  if necessary.  
         [0041]     When drilling earth formations, webbing  511  can provide structural support for first radial row  507  of cutting elements to help prevent overloading. The compressive forces distributed on the cutting elements of first radial row  507  could be translated to webbing  511  for support. The height of webbing  511  can be adjusted such that the depth of cut of the cutting elements of first radial row  507  is limited. Having a low webbing height would allow the cutting elements of first radial row  507  to take a deeper cut when drilling into the earth formation, as compared to the depth of cut a high webbing height would allow. The adjustable webbing height further prevents overloading of the first radial row  509  of cutting elements.  
         [0042]     Furthermore,  FIG. 5  shows PDC cutting elements  551  located on the bottom of bit body  503  of disc drill bit  501 . Referring now to  FIG. 6 , an enlarged view of the arrangement of PDC cutting elements  551  is shown. As discs  505  of disc drill bit  501  cut out a borehole in the earth formation, a bottom uncut portion may form at the bottom of the borehole that is not covered by the cutting surface of discs  505 . Bottom uncut portion  171  is shown in  FIG. 1 . As disc drill bit  501  drills into the earth formation, PDC cutting elements  551  may be used to cut out the bottom of the borehole.  FIG. 6  also shows a nozzle  553 , which is located on the bottom of bit body  503 . Nozzle  553  provides circulation of drilling fluid under pressure to disc drill bit  501  to flush out drilled earth and cuttings in the borehole and cool the discs during drilling.  
         [0043]     Embodiments of the present invention do not have to include the features of the webbing arranged on the discs and the single cutting structure formed from the first and second radial row cutting elements. Embodiments are shown with the webbing alone, and embodiments are shown with the single cutting structure alone. However, other embodiments can be created to incorporate both the webbing and the single cutting structure or exclude both the webbing and the single cutting structure. Those having ordinary skill in the art will appreciate that the present invention is not limited to embodiments which incorporate the webbing and the single cutting structure.  
         [0044]     Further, those having ordinary skill in the art will appreciate that the present invention is not limited to embodiments which incorporate only two rows of cutting elements. Other embodiments may be designed which have more than two rows of cutting elements. Referring now to  FIG. 9A , another disc drill bit  901  in accordance with an embodiment of the present invention is shown. Disc drill bit  901  includes a bit body  903  having one or more journals (not shown), on which one or more discs  905  are rotatably mounted. Disposed on at least one of discs  905  of disc drill bit  901  are first radial row  907  of cutting elements, second radial row  909  of cutting elements, and third radial row  911  of cutting elements. Cutting elements of first radial row  907  are located closest to the axis of rotation of disc drill bit  901 , followed by the cutting elements of second radial row  909 , and then the cutting elements of third radial row  911 . The cutting elements of first radial row  907 , second radial row  909 , and third radial row  911  act together to drill a borehole with a radius at which the cutting elements of third radial row  911  extend from the axis of rotation of the disc drill bit. Cutting elements of first radial row  907  penetrate into the earth formation to form the bottom of the borehole, the cutting elements of second radial row  909  shear the earth formation to form the sides of the borehole, and the cutting elements of third radial row  911  ream and polish the earth formation to form the full diameter of the borehole. Cutting elements of third radial row  911  enlarge the borehole to a radius at which the third radial row  911  of cutting elements extend from the axis of rotation of disc drill bit  901 .  
         [0045]     Referring still to  FIG. 9A , first radial row  907  of cutting elements are arranged about the outside radius of discs  905  such that its cutting elements are in a compressive configuration. Second radial row  909  of cutting elements are disposed on the face of discs  905  such that its cutting elements are in a shearing configuration. The third radial row  911  of cutting elements are also disposed on the face of discs  905  of disc drill bit  901 , but second radial row  909  of cutting elements are radially outboard (away from the center of the disc) of the radial position of third radial row  911  of cutting elements.  
         [0046]     In some embodiments, the cutting elements of the first radial row are oriented in the compressive configuration and may be comprise tungsten carbide, PDC, or other superhard materials, and may be diamond coated. The cutting elements of the first radial row cutting elements are designed to compress and penetrate the earth formation, and may be of conical or chisel shape. Preferably, the cutting elements of the second radial row have PDC as the cutting faces, which contact the earth formation to cut out the borehole. The cutting elements of the second radial row are oriented to shear across the earth formation. Similarly, the cutting elements of the third radial row have cutting faces which are comprised of PDC. The cutting elements of the third radial row shear across the earth formation to enlarge the borehole to full diameter.  
         [0047]     In one or more embodiments of the present invention, to assist in the shearing action, the cutting elements of the second and third radial rows may be oriented with a negative or positive rake angle. Referring now to  FIG. 4 , an example of negative rake angle is shown in a prior art PDC cutter  401 . PDC cutter  401  has a PDC cutter disc  403  rearwardly tilted in relation to the earth formation being drilled. A specific angle “A” refers to the negative rake angle the PDC cutter is oriented. Preferably, a rake angle from about 5 to 30 degrees of rake angle orientation is used. Similarly, a positive rake angle would refer to the PDC cutter disc forwardly tilted in relation to the earth formation being drilled. An effective rake angle would prevent delamination of the PDC cutting element.  FIGS. 9B and 9C  show an embodiment incorporating the use of one rake angle orientation, and  FIGS. 10A and 10B  show another embodiment incorporating the use of two rake angle orientations.  
         [0048]     In  FIG. 9B , the cutting elements of second radial row  909  and third radial row  911  are oriented with a positive rake angle to allow the sides of the cutting elements to perform the cutting action. As shown in  FIG. 9C , when the cutting elements are moving in the direction  951 , the sides (cylindrical edge) of the cutting elements shear across the borehole to generate failures in the earth formation. Therefore, the sides of the cutting elements are loaded with the predominant cutting forces. The shearing sides of the cutting elements are shown in zones  991  and  993 .  
         [0049]     In  FIG. 10A , the cutting elements of third radial row  1011  are oriented with a positive rake angle to allow the sides of the cutting elements to perform the shearing cutting action. However, the cutting elements of second radial row  1009  are oriented in a negative rake angle to instead the faces of the cutting elements to perform the shearing cutting action. Thus, with a negative rake angle, the faces of the cutting elements are loaded with the predominant cutting forces. Referring now to  FIG. 10B , another view of the embodiment in  FIG. 10A  is shown. When the cutting elements are moving in the direction  1051  to maximize shearing, the cutting elements in zone  1093  are oriented in a positive rake angle to allow the sides of the cutting elements to shear across the borehole to generate failures in the earth formation, while the cutting elements in zone  1091  are oriented in a negative rake angle to allow the faces of the cutting elements to shear across the borehole. Both rake angle orientations can be used for the cutting elements of embodiments of the present invention. The rake angle orientation may be varied from disc to disc of the disc drill bit, or from radial row to radial row, or even from cutting element to cutting element. The rake angle orientation is not intended to be a limitation of the present invention.  
         [0050]     Those having ordinary skill in the art will appreciate that other embodiments of the present invention may be designed with arrangements other than three discs rotatably mounted on the bit body. Other embodiments may be designed to incorporate only two discs, or even one disc. Also, embodiments may be designed to incorporate more than three discs. The number of discs on the disc drill bit is not intended to be a limitation of the present invention.  
         [0051]     As seen in roller cone drill bits, two cone drill bits can provide a higher ROP than three cone drill bits for medium to hard earth formation drilling. This concept can also be applied to disc drill bits. Compared with three disc drill bits, two disc drill bits can provide a higher indent force. The “indent force” is the force distributed through each cutting element upon the earth formation. Because two disc drill bits can have a fewer amount of total cutting elements disposed on the discs than three disc drill bits, with the same WOB, two disc drill bits can then provide a higher indent force. With a higher indent force, two disc drill bits can then provide a higher ROP. Two disc drill bits can also allow larger cutting elements to be used on the discs, and provide more flexibility in the placement of the nozzles. Further, the discs on two disc drill bits can be offset a larger distance than the discs of three disc drill bits. In the event a two disc drill bit is designed, an angle from about 165 to 180 degrees is preferred to separate the discs on the disc drill bit.  
         [0052]     Additionally, those having ordinary skill in the art that other embodiments of the present invention may be designed which incorporates discs of different sizes to be disposed on the disc drill bit. Embodiments may be designed to incorporate discs to be rotatably mounted to the disc drill bit, in which the discs vary in size or thickness in relation to each other. The size of the discs is not intended to be a limitation of the present invention.  
         [0053]     Referring now to  FIG. 7 , a cutting structure  701  in accordance with another embodiment of the present invention is shown. Cutting structure  701  includes a compressive portion  705  and a shearing portion  703  formed into a single body. Shearing portion  703  of cutting structure  701  is comprised of PDC. Cutting structure  701  may be placed on a disc of a disc drill bit by being brazed onto the disc, or cutting structure  701  may be integrally formed into the discs when manufactured. Cutting structure  701  is then disposed on the disc such that shearing portion  703  is arranged in a shearing configuration to generate failures by shearing the earth formation when drilling and compressive portion  705  is arranged in a compressive configuration to generate failures by crushing the earth formation when drilling.  
         [0054]     In the embodiments shown, compressive portion  705  of cutting structure  701  may be comprised of tungsten carbide, PDC, or other superhard materials, and may be diamond coated. Compressive portion  705 , which may be of a conical or chisel shape, is designed to compress and penetrate the earth formation. Shearing portion  703  of cutting structure  701  has PDC as the cutting face which contacts the earth formation to cut out the borehole. Shearing portion  703  is designed to shear across the earth formation.  
         [0055]     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.