Patent Publication Number: US-7594554-B2

Title: Cutting element insert for backup cutters in rotary drill bits, rotary drill bits so equipped, and methods of manufacture therefor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Patent Application Ser. No. 60/775,866, filed Feb. 23, 2006, the disclosure of which is incorporated herein in its entirety by this reference. 

   FIELD OF THE INVENTION 
   The present invention relates generally to fixed-cutter rotary drill bits having a bit body and, more specifically, to retention of backup cutting elements within a bit body of a rotary drill bit for drilling subterranean formations. 
   BACKGROUND OF THE INVENTION 
   Rotary drill bits are commonly used for drilling bore holes or wells in earth formations. One type of rotary drill bit is the fixed-cutter bit (often referred to as a “drag” bit), which typically includes a plurality of cutting elements secured to a face region of a bit body. Generally, the cutting elements of a fixed-cutter type drill bit have either a disk shape or, in some instances, a more elongated, substantially cylindrical shape. A cutting surface comprising a hard, super-abrasive material, such as mutually bound particles of polycrystalline diamond forming a so-called “diamond table,” may be provided on a substantially circular end surface of a substrate of each cutting element. Such cutting elements are often referred to as “polycrystalline diamond compact” (PDC) cutting elements or cutters. Typically, the PDC cutting elements are fabricated separately from the bit body and secured within pockets formed in the outer surface of the bit body. A bonding material such as an adhesive or, more typically, a braze alloy may be used to secure the cutting elements to the bit body. 
   The bit body of a rotary drill bit typically is secured to a hardened steel shank having an American Petroleum Institute (API) thread connection for attaching the drill bit to a drill string. The drill string includes tubular pipe and equipment segments coupled end-to-end between the drill bit and other drilling equipment at the surface. Equipment such as a rotary table or top drive may be used for rotating the drill string and the drill bit within the bore hole. Alternatively, the shank of the drill bit may be coupled directly to the drive shaft of a down-hole motor, which then may be used to rotate the drill bit. 
   Referring to  FIG. 1 , a conventional fixed-cutter rotary drill bit  10  includes a bit body  12  that has generally radially projecting and longitudinally extending wings or blades  14 , which are separated by junk slots  16  extending from channels on the face  20  of the bit body. A plurality of PDC cutters  18  are provided on the blades  14  extending over face  20  of the bit body  12 . The face  20  of the bit body  12  includes the surfaces of the blades  14  that are configured to engage the formation being drilled, as well as the exterior surfaces of the bit body  12  within the channels and junk slots  16 . The plurality of PDC cutters  18  may be provided along each of the blades  14  within pockets  22  formed in rotationally leading edges thereof, and the PDC cutters  18  may be supported from behind by buttresses  24 , which may be integrally formed with the bit body  12 . 
   The drill bit  10  may further include an API threaded connection portion  30  for attaching the drill bit  10  to a drill string (not shown). Furthermore, a longitudinal bore (not shown) extends longitudinally through at least a portion of the bit body  12 , and internal fluid passageways (not shown) provide fluid communication between the longitudinal bore and nozzles  32  provided at the face  20  of the bit body  12  and opening onto the channels leading to junk slots  16 . 
   During drilling operations, the drill bit  10  is positioned at the bottom of a well bore hole and rotated while drilling fluid is pumped through the longitudinal bore, the internal fluid passageways, and the nozzles  32  to the face  20  of the bit body  12 . As the drill bit  10  is rotated, the PDC cutters  18  scrape across and shear away the underlying earth formation. The formation cuttings mix with and are suspended within the drilling fluid and pass through the junk slots  16  and up through an annular space between the wall of the bore hole and the outer surface of the drill string to the surface of the earth formation. 
   The bit body  12  of a fixed-cutter rotary drill bit  10  may be formed from steel. Such steel bit bodies are typically fabricated by machining a steel blank (using conventional machining processes including, for example, turning, milling, and drilling) to form the blades  14 , junk slots  16 , pockets  22 , buttresses  24 , internal longitudinal bore and fluid passageways (not shown), and other features of the drill bit  10 . 
     FIG. 2  is an enlarged perspective view of a blade  14  showing a plurality of PDC cutters  18  mounted thereon in pockets  22  and supported from behind by buttresses  24 . As seen therein, the PDC cutters  18  may include a polycrystalline diamond compact table  36  formed on a substantially planar end surface of a cylindrical substrate  38 , the latter being formed of a hard metallic material such as tungsten carbide. Generally, the PDC cutters  18  are secured by their substrates  38  within the pockets  22  by brazing, welding, or adhering using a high-strength adhesive. 
   In order to enhance the cutting action of the drill bit  10  and/r to prevent wear of drill bit  10 , it may be desirable to provide additional “backup” cutters  18 ′ on one or more blades  14  rotationally behind at least some of the primary PDC cutters  18 . 
   Provision of such backup cutters  18 ′ in a drill bit  10  that includes a steel bit body  12  may be difficult due to the difficulty of machining pockets  22 ′ for the backup cutters  18 ′ using conventional machining equipment (such as, for example, a multiple-axis milling machine) and techniques due to interference between the machining equipment or the cutting element thereof and other features of the drill bit  10  such as, for example, adjacent blades  14 . Stated another way, interference between the machining equipment and the drill bit  10  may preclude positioning of the machining equipment and, in particular, the cutting element thereof, in a manner that allows machining of the pockets  22 ′ for the backup cutters  18 ′. Furthermore, it may be difficult to machine the pockets  22 ′ for backup cutters  18 ′ without machining other areas of the drill bit  10  that are not intended to be machined. 
   U.S. Pat. No. 7,070,011 to Sherwood, Jr., et al. discloses steel body rotary drill bits having primary cutting elements that are disposed in cutter pocket recesses that are partially defined by cutter support elements. The support elements are affixed to the steel body during fabrication of the drill bits. At least a portion of the body of each cutting element is secured to a surface of the steel bit body, and at least another portion of the body of each cutting element matingly engages a surface of one of the support elements. U.S. Pat. No. 7,070,011 does not describe, teach, or suggest, however, using the support elements disclosed therein to secure backup cutters to a rotary drill bit having a steel body. 
   Therefore, there is a need in the art for methods that facilitate placement of backup cutters on rotary drill bits, and for rotary drill bits including backup cutters. 
   BRIEF SUMMARY OF THE INVENTION 
   In some embodiments, the present invention includes cutter inserts for fixed-cutter rotary drill bits. The cutter inserts have a cutter insert body including at least one surface defining a cutter recess in the cutter insert body. The cutter recess may be configured to receive at least a portion of a backup cutting element therein. 
   In additional embodiments, the present invention includes fixed-cutter rotary drill bits for drilling subterranean formations. At least one cutter insert for retaining a backup cutter may be removably affixed to the face of a bit body rotationally behind a cutter pocket for a primary cutter. The cutter insert may include at least one surface defining a cutter recess therein that is configured to receive at least a portion of the backup cutter (such as, for example, a PDC cutting element) therein. The back rake and exposure of the backup cutter may be easily adjusted by appropriately configuring the position and orientation of the cutter recess. 
   In yet additional embodiments, the present invention includes methods of manufacturing fixed-cutter rotary drill bits. The methods may include providing a bit body (which may have a plurality of blades) having a face configured to engage a subterranean formation during drilling. At least one cutter insert recess is formed in the face of the bit body (e.g., on the face of a blade) rotationally behind a cutter pocket that is configured to receive a primary cutting element therein. One or more cutter inserts may be provided that include a cutter insert body having at least one surface defining a cutter recess therein that is configured to receive at least a portion of a backup cutting element therein. The one or more cutter inserts each may be secured at least partially within a cutter insert recess on the face of the bit body, and at least one backup cutter may be secured at least partially within the cutter recesses of the one or more cutter inserts. The one or more cutter inserts may be removably secured in the cutter insert recesses to facilitate removal and repair of components of the cutter insert/backup cutting element assemblies. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, various features and advantages of this invention may be more readily ascertained from the following description of the invention when read in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a perspective view of an example fixed-cutter rotary drill bit; 
       FIG. 2  is an enlarged view of a blade of a fixed-cutter rotary drill bit like that shown in  FIG. 1  illustrating a backup cutter mounted on the blade in accordance with an embodiment of the present invention; 
       FIG. 3A  is a perspective view of an embodiment of a cutter insert that may be used to mount a backup cutter on the blade of a rotary drill bit; 
       FIG. 3B  is another perspective view of the cutter insert shown in  FIG. 3A ; 
       FIG. 3C  is a top view of the cutter insert shown in  FIG. 3A ; 
       FIG. 3D  is a side view of the cutter insert shown in  FIG. 3C ; 
       FIG. 4  is a perspective view of another embodiment of a cutter insert that may be used to mount a backup cutter on the blade of a rotary drill bit; 
       FIGS. 5A-5F  illustrate a method of mounting a backup cutter on the blade of a rotary drill bit using an insert like that shown in  FIGS. 3A-3D ; 
       FIG. 6  is an end view of a fixed-cutter rotary drill bit illustrating various locations on blades of a drill bit at which backup cutting cutters may be mounted using cutter inserts like those shown in  FIGS. 3A-3D ; and 
       FIG. 7  illustrates the cutter profile of the fixed-cutter rotary drill bit shown in  FIG. 6 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The illustrations presented herein are, in some instances, not actual views of any particular cutting element insert, cutting element, or drill bit, but are merely idealized representations which are employed to describe the present invention. Additionally, elements common between figures may retain the same numerical designation. 
   A cutter insert  50  that may be used to secure a backup cutter  18 ′ on the face  20  of a rotary drill bit  10  ( FIG. 1 ) (which may have a steel bit body) is shown in  FIGS. 3A-3D . Referring to  FIG. 3A , the cutter insert  50  may include a generally cylindrical cutter insert body  52  having a cutter recess  54  provided at one end thereof. The cutter insert body  52  may comprise, for example, a steel alloy, tungsten carbide, or any other sufficiently hard and wear-resistant material. As another example, the cutter insert body  52  may comprise a particle-matrix composite material such as, for example, a material comprising tungsten carbide particles cemented together by a metal matrix material, such as, for example, cobalt or a cobalt-based alloy. 
   Referring to  FIGS. 3B and 3C , the cutter insert body  52  may have a generally cylindrical lateral side surface  60  and two opposing substantially planar and parallel end surfaces  61 A, only one of which is visible in  FIGS. 3A ,  3 B, and  3 C. In some embodiments, the cutter insert body  52  also may have one or more additional substantially planar flat surfaces  61 B, which may be co-planar and disposed in a plane oriented at an acute angle of less than ninety degrees (90°) relative to the parallel end surfaces  61 A. The cutter recess  54  may have a size and shape configured to receive a backup cutter  18 ′ ( FIG. 2 ) at least partially therein. In some embodiments, the cutter recess  54  may have a size and shape configured to receive a backup cutter  18 ′ substantially entirely therein, such that the backup cutter  18 ′ does not project laterally outward from the cutter insert body  52  beyond the generally cylindrical lateral side surface  60 , as shown in  FIG. 3D . In other words, the backup cutter  18 ′ may only project vertically outwardly beyond the upper end surface  61 A, as shown in  FIG. 3D . 
   By way of example and not limitation, the cutter recess  54  ( FIG. 3C ) may have a shape corresponding to a partial right-ended cylinder, and may be defined by a generally cylindrical, arcuate surface  56  of the cutter insert body  52  and a generally planar surface  58  of the cutter insert body  52 , at which the arcuate surface  56  terminates. In other words, the arcuate surface  56  intersects the planar surface  58  and may extend substantially transverse therefrom. In some embodiments, the generally planar surface  58  also may be disposed in a plane oriented at an acute angle  67  of less than ninety degrees (90°) relative to one or both of the end surfaces  61 A, and/or at an acute angle  68  of less than ninety degrees (90°) relative to the longitudinal axis A L  of the cutter insert body  52 . The cutter recess  54  may intersect one or more of the flat surfaces  61 B, the lateral side surface  60 , and an end surface  61 A, as shown in  FIG. 3B . Furthermore, the cutter recess  54  may extend along a recess axis A R  that is oriented at an acute angle  69  of less than ninety degrees (90°) relative to the longitudinal axis A L  of the cutter insert body  52 . As used herein, the term “recess axis” means any axis about which the shape of the cutter recess  54  is substantially symmetric. In some embodiments, the recess axis may be co-incident with a drilling or machining axis used to form the cutter recess  54 . In such a configuration, the recess axis A R  also may be oriented at an acute angle of less than ninety degrees (90°) relative to the lateral side surface  60 . A backup cutter  18 ′ such as, for example, a PDC backup cutter  18 ′ may be at least partially received within the cutter recess  54  of the cutter insert body  52 , as shown in  FIG. 3D . 
   As shown in  FIG. 3D , the cutter recess  54  ( FIG. 3C ) may be selectively oriented within the cutter insert body  52  such that the cutting face  19  of a backup cutter  18 ′ positioned therein is oriented at a selected angle  66  with respect to a side surface  60  of the cutter insert body  52 , which is parallel to a longitudinal axis A L  of the cutter insert body  52 . The selected angle  66  may be used to define a selected back rake angle of the backup cutter  18 ′ when the cutter insert  50  is secured to the face  20  of a drill bit  10  ( FIG. 1 ), and the backup cutter  18 ′ is positioned within the cutter recess  54  of the cutter insert  50 . Similarly, the depth D of cutter recess  54  in cutter insert body  52  may be adjusted to provide a selected exposure for a backup cutter  18 ′ when the cutter insert  50  is secured to the face  20  of a drill bit  10  ( FIG. 1 ), and the backup cutter  18 ′ is positioned within the cutter recess  54  of the cutter insert  50 . 
   As shown in  FIG. 3D , the cutter insert body  52  may be configured such that the backup cutter  18 ′ may be predominantly or primarily secured to the cutter insert body  52  within the cutter recess  54 . In other words, the backup cutter  18 ′ is not significantly directly bonded to any surface of the blade  14  of the bit body on which the backup cutter  18 ′ is mounted. 
   Optionally, the cutter insert  50  may include one or more alignment features configured to facilitate providing the cutter insert  50  at a selected orientation, in terms of side rake, within a bit body  12  of a drill bit  10  ( FIG. 1 ). By way of example and not limitation, the cutter insert  50  may include a rotational alignment pin  62 , which may be partially inserted into a corresponding hole  64  formed in a surface  60  of the cutter insert body  52 . The rotational alignment pin  62  may be used to provide the cutter insert  50  at a selected rotational orientation within a bit body  12  of a drill bit  10 , thereby defining a selected side rake angle of the backup cutter  18 ′ when the cutter insert  50  is secured to a blade  14  over the face  20  of a drill bit  10  ( FIG. 1 ), and the backup cutter  18 ′ is positioned within the cutter recess  54  of the cutter insert  50 . 
   An additional embodiment of the cutter insert  50  is shown in  FIG. 4 . As seen therein, the cutter insert body  52  may include a generally planar alignment surface or flat  71 , which may be formed in and intersect the lateral side surface  60  of the cutter insert body  52 . The alignment surface or flat  71  may be used to provide the cutter insert  50  at a selected rotational orientation within a bit body  12  of a drill bit  10  ( FIG. 1 ), in lieu of the rotational alignment pin  62  shown in  FIGS. 3A-3D . In still further embodiments, the cutter insert  50  may include an alignment mark such as a line or groove defined in a surface of the cutter insert body  52 , which may be aligned with a corresponding mark (or other feature) provided on the bit body  12  of a drill bit  10  ( FIG. 1 ). 
   A method of securing a backup cutter  18 ′ to the face  20  of a rotary drill bit  10  like that shown in  FIG. 1  using the cutter insert  50  shown in  FIGS. 3A-3D  will now be described with reference to  FIGS. 5A-5F . 
     FIG. 5A  is an enlarged partial perspective view of a blade  14  of a fixed-cutter rotary drill bit  10  like that shown in  FIG. 1 , which may have a steel bit body  12 . The blade  14  is shown in  FIG. 5A  after cutter pockets  22  have been formed therein, but prior to securing primary PDC cutters  18  ( FIG. 2 ) therein. The cutter pockets  22  have been formed adjacent to a leading surface  26  of the blade  14 . The leading surface  26  generally refers to the side of the blade  14  that is rotationally forward or leading in relation to the direction of rotation of the bit body  12  during drilling. As seen in  FIG. 5A , a cutter insert recess  70  may be provided in the face  20  of the blade  14  at a desired location thereon at which it is desired to provide a backup cutter  18 ′. For example, a cutter insert recess  70  may be provided in the face  20  of the blade  14  rotationally behind a cutter pocket  22  on that same blade that is configured to receive a primary PDC cutter  18  therein. The cutter insert recess  70  may have a size and shape configured to receive a cutter insert  50  ( FIGS. 3A-3D ) therein. By way of example and not limitation, the cutter insert recess  70  may be generally cylindrical and may have a diameter D that is larger than a diameter of a generally cylindrical cutter insert body  52  of a cutter insert  50  by a few to several hundredths of an inch to allow the cutter insert  50  to be welded or brazed within the cutter insert recess  70  of the blade  14 . In other embodiments, the cutter insert recess  70  may have a diameter D that is larger than a diameter of the generally cylindrical cutter insert body  52  of the cutter insert  50  by a few to several thousandths of an inch to allow the cutter insert  50  to be press-fit or shrink-fit into the cutter insert recess  70  of the blade  14 . 
   By way of example and not limitation, the cutter insert recess  70  may be formed in the blade  14  by drilling the cutter insert recess  70  into the blade  14  using, for example, a conventional drilling or milling machine equipped with a flat-bottomed cylindrical cutting element. 
   Referring to  FIG. 5B , the cutter insert recess  70  may be formed by drilling into a blade  14  substantially along a drilling axis  80 . By way of example and not limitation, the drilling axis  80  may be oriented generally perpendicular to a plane  82  that is substantially tangent to the face  20  of the blade  14  at the location at which the cutter insert recess  70  is to be formed. By allowing the cutter insert recess  70  to be drilled in such a manner, conventional drilling equipment and techniques maybe used to drill the cutter insert recess  70  without encountering the interference problems previously described herein that can arise when attempting to machine a cutter pocket  22 ′ located and configured to receive a backup cutter  18 ′ ( FIG. 2 ) directly into a blade  14  of a drill bit  10 . 
   Referring to  FIG. 5C , an alignment pin recess  72  that extends from the cutter insert recess  70  and is configured to receive the alignment pin  62  of the cutter insert  50  ( FIGS. 3A-3D ) therein may be provided in the blade  14 . By way of example and not limitation, the alignment pin recess  72  may be formed in the blade  14  by milling the alignment pin recess  72  in the blade  14  using, for example, a conventional milling machine equipped with a round-bottomed cylindrical cutting element. 
   As previously described, the alignment pin recess  72  may be provided at a selected position about the circumferential edge  74  of the cutter insert recess  70  such that the cutter insert  50  is positioned at a selected rotational orientation within the cutter insert recess  70  when the cutter insert  50  is inserted into the cutter insert recess  70  and the alignment pin  62  is disposed within the alignment pin recess  72 . In this manner, the side rake angle of a backup cutter  18 ′ positioned within a cutter insert  50  disposed in the cutter insert recess  70  may be selectively defined. 
   Referring to  FIG. 5D , after forming the cutter insert recess  70  and the alignment pin recess  72  in the blade  14 , a cutter insert  50  may be aligned with and inserted into the cutter insert recess  70  such that the alignment pin  62  is disposed in the alignment pin recess  72 . 
   After inserting the cutter insert  50  into the cutter insert recess  70 , the cutter insert  50  may be secured within the cutter insert recess  70  (if the cutter insert  50  has not been press-fit or shrink-fit into the cutter insert recess  70 , or if that additional means for securing the cutter insert  50  within the cutter insert recess  70  is desired in addition to a press-fit or shrink-fit). As previously described, a brazing material or an adhesive material optionally may be provided at the interface between the cutter insert  50  and the surrounding surfaces of the blade  14  within the cutter insert recess  70 . In such a configuration, cutter insert  50  may be relatively easily removed, if damaged, for replacement. 
   Referring to  FIG. 5E , in addition to, or as an alternative to, the previously described means for securing the cutter insert  50  within the cutter insert recess  70 , a weld bead  78  may be provided along at least a portion of an interface between the cutter insert  50  and the region of the blade  14  adjacent the cutter insert recess  70 . Again, such a method of securing the cutter insert  50  within the cutter insert recess  70  facilitates removal and repair of the cutter insert  50 . Moreover, a hard-facing material (not shown in  FIG. 5E ) may be selectively applied over and around selected areas of the cutter insert  50 , the weld bead  78 , and the surrounding regions of the blade  14  as necessary or desired. Such hard-facing materials are well known in the art and often include hard particles (such as, for example, particles of tungsten carbide material) dispersed throughout a metal alloy matrix material, and may be used to prevent wear of the underlying structures. Notably, no additional heat cycles of the bit body are required when fabricating a drill bit incorporating the present invention. 
   After the cutter insert  50  has been inserted into and secured within the cutter insert recess  70 , a backup cutter  18 ′ may be inserted into and secured within the cutter recess  54  ( FIG. 3C ) of the cutter insert  50 , as shown in  FIG. 5F . By way of example and not limitation, the backup cutter  18 ′ may be secured within the cutter recess  54  using a brazing material or an adhesive material in a similar manner to that described previously herein for securing the cutter insert  50  within the cutter insert recess  70 . In additional methods, a backup cutter  18 ′ may be inserted into and secured within a cutter recess  54  of a cutter insert  50  prior to inserting and/or securing the cutter insert  50  within a cutter insert recess  70 , although thermal constraints may dictate that this approach not be taken due, for example, to the potential for damage to the diamond table of a backup cutter  18 ′ if welding is to be used to secure a cutter insert  50  to a blade  14 . 
   While the backup cutter  18 ′ has been described and illustrated herein as comprising a PDC cutter, in additional embodiments the backup cutter  18 ′ may comprise any type or configuration of superabrasive or other cutter known in the art, such as, for example, a stud that comprises a hard material such as tungsten carbide, but does not include a diamond table thereon. Furthermore, while only a single cutter insert  50  and a single backup cutter  18 ′ have been described herein and illustrated in the figures thus far, a steel-bodied drill bit  10  may be provided with a plurality of cutter inserts  50  and a plurality of backup cutters  18 ′. 
   An end view of a fixed-cutter rotary drill bit  90  of the present invention is shown in  FIG. 6 . As seen therein, a plurality of backup cutters  18 ′ may be secured to at least one blade  14  of the drill bit  90  using cutter inserts  50  (not shown in  FIG. 6 ), as previously described herein. Each backup cutter  18 ′ may include a PDC backup cutter  18 ′. In some embodiments, each backup cutter  18 ′ may have a diameter that is smaller than a diameter of a primary cutter  18 . For example, each backup cutter  18 ′ may have a diameter that is smaller than a diameter of a primary cutter  18  that is disposed on the same blade  14  and rotationally forward therefrom. In additional embodiments, each backup cutter  18 ′ may have a diameter that is equal to or larger than a diameter of a primary cutter  18  that is disposed on the same blade  14  and rotationally forward therefrom. Furthermore, at least one backup cutter  18 ′ may be provided on each primary blade  14  of the drill bit  90  (as used herein, the term “primary blade” means a blade  14  that extends substantially to the center of a drill bit), as shown in  FIG. 6 . In additional embodiments, one or more backup cutters  18 ′ may be provided on each blade  14  of the drill bit  90 . 
   As seen in  FIG. 6 , at least one blade  14  of the drill bit  90  may include one or more bearing structures  94  in lieu of backup cutters  18 ′. Each bearing structure  94  may include a stud or pad comprising a hard material such as, for example, tungsten carbide. Each bearing structure  94  may have at least one bearing surface  95  configured to engage a surface of a subterranean formation during drilling. Furthermore, each bearing structure  94  may include a diamond table covering at least a portion of the bearing surface  95  thereof, diamond impregnated material, or any other structure or material comprising one or more diamonds configured to impart wear-resistance to the bearing structure  94 . 
     FIG. 7  illustrates what is known in the art as the “cutter profile” of the drill bit  90  shown in  FIG. 6 , and shows a cross-section of one blade  14 . Each of the overlapping circles represents the position that would be occupied on the blade  14  by the primary cutters  18  and the backup cutters  18 ′ if each of the primary cutters  18  and backup cutters  18 ′ were rotated circumferentially about the longitudinal axis of the drill bit  90  to a position on the blade  14  shown in  FIG. 7 . In some embodiments, the backup cutters  18 ′ may be provided substantially along the shoulder region, which is indicated in  FIG. 7  generally at  96 , of each blade  14  on which the respective backup cutters  18 ′ are mounted. In additional embodiments, at least one backup cutter  18 ′ may be provided within a cone region (indicated generally at  98 ), within a nose region (indicated generally at  100 ), or within a gage region (indicated generally at  102 ) of each blade  14  on which the respective backup cutters  18 ′ are mounted. In some embodiments, a backup cutter  18 ′ may be configured to be relatively underexposed relative to a primary cutting element  18 . In other words, a backup cutter  18 ′ may extend outward from the face  20  of a blade  14  by a distance that is less than a distance by which a primary cutter  18  positioned rotationally forward therefrom on the same blade extends outward from the face  20  of the blade  14 . In additional embodiments, a backup cutter  18 ′ may be configured to be relatively overexposed relative to a primary cutting element  18 , or to have a substantially equal exposure relative to a primary cutting element  18 . In other words, a backup cutter  18 ′ may extend outward from the face  20  of a blade  14  by a distance that is equal to, or greater than, a distance by which a primary cutter  18  positioned rotationally forward therefrom on the same blade extends outward from the face  20  of the blade  14 . 
   A rotary drill bit having a steel body and six blades was fabricated according to the present invention. Between two and three backup cutters were secured to the face of the drill bit on each of the blades in a shoulder region thereof using cutter inserts in a manner substantially similar to that previously described in relation to the cutter insert  50  and backup cutter  18 ′ with reference to  FIGS. 5A-5F . The drill bit was then used in four test runs, two of which were conducted in each of two different subterranean formations. The test runs included both drilling a well bore hole, and reaming out a previously drilled well bore hole. The operating parameters for the four test runs were carried out at maximum weights-on-bit (WOB) ranging from about 15,000 to about 30,000 pounds, maximum torques of between about 3,600 and about 7,500 foot-pounds, and maximum rates-of-penetration (ROP) of between about 100 and about 250 feet per hour. 
   After conducting the test runs, the backup cutters were inspected, both visually and with the aid of a magnetic particle inspection (MPI) process, to determine whether the backup cutters and cutter inserts experienced unacceptable levels of wear. The backup cutters and cutter inserts did not appear to exhibit unacceptable levels of wear. In view of the above, the present invention may facilitate the use of backup cutters on rotary drill bits that have a steel bit body, which may facilitate the manufacture of steel-bodied rotary drill bits that exhibit improved durability and/or stability. 
   As discussed above, the present invention has utility in relation to rotary drill bits having bit bodies comprising steel. Recently, new methods of forming rotary drill bits having bit bodies comprising particle-matrix composite materials have been developed in an effort to improve the performance and durability of earth-boring rotary drill bits. Such methods are disclosed in pending U.S. patent application Ser. No. 11/271,153, filed Nov. 10, 2005 and pending U.S. patent application Ser. No. 11/272,439, also filed Nov. 10,2005, the disclosure of each of which application is incorporated herein in its entirety by this reference. 
   In contrast to conventional infiltration methods (in which hard particles (e.g., tungsten carbide) are infiltrated by a molten liquid metal matrix material (e.g., a copper based alloy) within a refractory mold, these new methods generally involve pressing a powder mixture to form a green powder compact, and sintering the green powder compact to form a bit body. The green powder compact may be machined as necessary or desired prior to sintering using conventional machining techniques like those used to form steel bit bodies. Furthermore, additional machining processes may be performed after sintering the green powder compact to a partially sintered brown state, and/or after sintering the green powder compact to a desired final density. For example, it may be desired to machine pockets  22 ′ for backup cutters  18 ′ ( FIG. 2 ) on one or more blades  14  of a bit formed by such a process while the bit body is in the green, brown, or fully sintered state. However, as with steel-bodied drill bits, interference problems may prevent the formation of the desired pockets  22 ′. Therefore, embodiments of the present invention also may be used to secure backup cutters  18 ′ to the face of a drill bit having a bit body comprising a particle-matrix composite material. 
   By way of example and not limitation, a cutter insert recess  70  like that shown in  FIGS. 5A and 5B  may be provided in the face  20  of a blade  14  comprising a particle-matrix composite material in the green, brown, or fully sintered state. Optionally, an alignment pin recess  72  like that shown in  FIG. 5C  also may be provided in the blade  14 . A cutter insert  50  like that shown in  FIGS. 3A-3D  then may be secured within the cutter insert recess  22 , as shown in  FIG. 5D . In some embodiments, the cutter insert  50  may be secured to the blade  14  by co-sintering the blade  14  (or bit body) and the cutter insert  50 . In such embodiments, the cutter insert  50  may comprise a green or brown structure when the cutter insert  50  is positioned within the cutter insert recess  22 , and the cutter insert  50  may be sintered to a desired final density as the cutter insert  50  is co-sintered with the blade  14  and secured thereto. In other embodiments, the cutter insert  50  may have a desired final density prior to positioning the cutter insert  50  within the cutter insert recess  70 . In yet other embodiments, the cutter insert  50  may not be inserted into the cutter insert recess  70  until after the blade  14  and the bit body have been sintered to a desired final density. A backup cutter  18 ′ then may be inserted into and secured within the cutter recess  54  of the cutter insert  50 , as previously described in relation to  FIG. 5F . 
   While the present invention has been described herein with respect to certain preferred embodiments, those of ordinary skill in the art will recognize and appreciate that it is not so limited. Rather, many additions, deletions and modifications to the preferred embodiments may be made without departing from the scope of the invention as hereinafter claimed. In addition, features from one embodiment may be combined with features of another embodiment while still being encompassed within the scope of the invention as contemplated by the inventors. Further, the invention has utility with different and various bit profiles as well as cutter types and configurations.