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CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/051,280, filed Jun. 30, 1997. 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0002]    The present invention relates to earth boring drill bits, such as percussion bits, having large inserts extending from certain portions of the bit face.  
         BACKGROUND OF THE INVENTION  
         [0003]    Referring initially to FIG. 1, a prior art percussion drill bit  10  is shown having a bit head  12  that includes a bit face  14  and a multitude of inserts  20  for impacting and fracturing the earthen formation (not shown). Inserts  20  were typically disposed on various portions of the bit face  14 . For example, inserts  20  are shown disposed on the central portion  19  of the bit face  14  in the proximity of the central axis  13  of the bit  10 , and other inserts  20  are disposed in numerous circumferential rows  70  on the bit face  14 , such as a first row  72 , second row  74 , third row  76  and gage row  78 . The term “gage row” as used herein refers to the row  70  extending around, or adjacent, the periphery, or edge,  15  of the bit face  14 . All of the inserts  20  on the bit face  14  of the prior art hammer bit  10  had substantially the same geometric shape and size, such inserts  20  being referred to herein as “small” inserts  22 . Typically, such inserts  22  had a diameter of 0.75 inches or smaller. The bit face  14  also included one or more fluid flow openings  16  and flow channels  18  for allowing the flow of circulation fluid (not shown) from within the bit  10  to the exterior  44  of the bit  10 .  
           [0004]    Different places on the bit head may see different conditions during drilling yet the same inserts typically are used at all places of the bit head of the prior art. A need exists for a drill bit with different inserts at different places on the bit head to better match the varying conditions or applications of different places on the bit head.  
         SUMMARY OF THE INVENTION  
         [0005]    In one aspect of the present invention, a percussion drill bit for percussive drilling in a formation is provided that comprises a bit head for percussive impact against the formation with at least a first plurality of first inserts and a second plurality of second inserts extending from the bit head. Each of the first inserts have a first base portion mounted to the bit head and a first exposed portion extending from the bit head with the first exposed portion having a first profile. Each of the second inserts have a second base portion mounted to the bit head and a second exposed portion extending from the bit head with each of the second exposed portions having a second profile that is appreciably different from the first profile of the first exposed portion. At least some of the second exposed portions enhanced with a superhard material.  
           [0006]    In other aspects of the present invention, the second inserts may also vary from the first inserts by radius of curvature of the exposed portions and/or by diameter of the base portion.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings wherein:  
         [0008]    [0008]FIG. 1 is a front view of a percussion drill bit of the prior art.  
         [0009]    [0009]FIG. 2 is a front view of a percussion drill bit having large inserts on the gage row made in accordance with the present invention.  
         [0010]    [0010]FIG. 3 is a front view of another embodiment of the present invention having a large insert across the bit face, except for the inserts on the central portion of the bit.  
         [0011]    [0011]FIG. 4 is a partial profile view of the prior art percussion drill bit of FIG. 1.  
         [0012]    [0012]FIG. 5 is a partial profile view of the percussion drill bit of FIG. 2.  
         [0013]    [0013]FIG. 6 is a partial profile view of the percussion drill bit of FIG. 3.  
         [0014]    [0014]FIG. 7 is a partial cross-sectional view taken through line  7 - 7  of FIG. 1.  
         [0015]    [0015]FIG. 8 is a partial cross-sectional view taken through line  8 - 8  of FIG. 2.  
         [0016]    [0016]FIG. 9 is an isolated view of an insert of the prior art drill bit of FIG. 1 and the earthen formation impact crater created thereby.  
         [0017]    [0017]FIG. 10 is an isolated view of a large insert of the drill bit of FIG. 3 and the earthen formation impact crater created thereby.  
         [0018]    [0018]FIG. 11 is an isolated view of an insert having an enhanced surface of a drill bit made in accordance with the prior art.  
         [0019]    [0019]FIG. 12 is an isolated view of an insert having an enhanced surface of a drill bit made in accordance with the present invention.  
         [0020]    [0020]FIG. 13 is a cross sectional view of a portion of the insert of FIG. 11 showing the various layers of the enhanced surface and the edge, or joint area, formed around the periphery of the enhanced surface.  
         [0021]    [0021]FIG. 14 is a cross sectional view of a portion of the insert of FIG. 12 showing the various layers of the enhanced surface and the edge, or joint area, formed around the periphery of the enhanced surface.  
         [0022]    [0022]FIG. 15 is an isolated view of a large insert of the drill bit of FIG. 3 disposed in the earthen formation.  
         [0023]    [0023]FIG. 16 is an isolated view an insert of the prior art drill bit of FIG. 1 disposed in the earthen formation at the same depth as the insert of FIG. 15.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Presently preferred embodiments of the invention are shown in the above-identified figures and described in detail below. In illustrating and describing the presently preferred embodiments, like or identical reference numerals are used to identify common or similar elements. The figures are not necessarily to scale and certain features and certain views of the figures may be shown exaggerated in scale or schematic form in the interest of clarity and conciseness.  
         [0025]    The percussion bit  11  of the present invention, as shown, for example, in FIGS. 2 and 3, also has a bit head  12 , a bit face  14  and a multitude of inserts  20 . It should be understood that while the present invention is shown and described herein with respect to percussion bits, which are useful with percussion drilling assemblies, such as those shown and described in U. S. Pat. No. 5,322,136 to Bui et al., U.S. Pat. No. 4,932,483 to Rear and U.S. Pat. No. 4,819,793 to Fuller, the invention is not limited to percussion bits and may be used with any other type of earth boring drill bit having cutting elements for impacting, fracturing or crushing an earthen formation. The inserts  20  of the bit  11  are shown disposed on various portions of the bit face  14 . Inserts  20  are disposed on the central portion  19  of the bit face  14  in the proximity of the central axis  13  of the bit  11 , and other inserts  20  are disposed on numerous circumferential rows  70 , such as a first row  72 , second row  74 , third row  76  and gage row  78 . It should be understood that the present invention is not limited to having inserts  20  disposed in these particular locations on the bit face  14 , or in the quantities shown.  
         [0026]    Still referring to FIGS. 2 and 3, the bit  11  of the present invention includes small inserts  22  and “large” inserts  32 . The large inserts  32  have a larger geometric size, a larger radius of curvature  36  (FIG. 10) and larger contact surface  38  (FIG. 10), as compared to the geometric size, radius of curvature  26  and contact surface  28  of the small inserts  22  (FIG. 9). The “contact surface” is that portion of the insert face surface  39  (FIGS. 9, 10) that engages the formation  120 . Generally, the larger the insert face surface  39 , the larger the “contact surface”.  
         [0027]    In FIG. 2, for example, the inserts  20  on the gage row  78  are large inserts  32 , while all other inserts  20  shown on the bit face  14  are small inserts  22 . In FIG. 3, all of the inserts  20  on the bit face  14  are large inserts  32 , except the inserts  20  disposed on the central portion  19  of the bit face  14  proximate to the central axis  13  and the inserts  20  on row  72 , which are small inserts  22 . The present invention is, however, not limited to the particular combinations of large and small inserts  32 ,  22  shown in FIGS. 2 and 3, but encompasses any configuration of inserts  20  that includes large and small inserts  32 ,  22  capable of providing one or more of the aspects, or benefits, of the invention described herein.  
         [0028]    Now referring to FIGS. 5 and 6, the inserts  20  are preferably embedded, or emplaced, in cavities  50  in the bit head  12 . The inserts  20  may possess any among a variety of geometric shapes, such as, for example, semi-round top, chisel and conical shaped inserts, as are or become known in the art. Further, any among a variety of types of inserts  20  that are or become known in the art may be used as small and large inserts  22 ,  32 , such as, for example, tungsten carbide inserts, tungsten carbide inserts having a super-abrasive surface, such as polycrystalline diamond (“PCD”) or cubic boron nitride (“PCBN”), and inserts constructed of a matrix of tungsten carbide and other material. The bit  11  of the present invention, as shown, for example, in FIGS. 5 and 6, preferably includes small inserts  22  having a diameter of 0.75 inches or smaller, and large inserts  32  having a diameter of over 0.75 inches, such as 22 millimeters. However, the present invention is not limited to the use of inserts  22 ,  32  of those sizes, but encompasses any suitable type of inserts  22 ,  32 , of any suitable sizes so long as the large inserts  32  are larger than the small inserts  22 , and the bit  11  is capable of providing one or more of the aspects, or benefits, of the invention described herein.  
         [0029]    [0029]FIG. 5 further illustrates a typical bottom hole pattern  150  of the earthen formation  120  formed by bit  11 . The bottom hole pattern  150  is shown generally divided into segments  160 ,  170  and  180 , which differ with respect to the loading conditions on the inserts  20  of the bit face  14 . Segment  160  represents the portion of the bottom hole pattern  150  most radially inboard relative, or proximate, to the central axis  13  of the bit  11 . This segment  160  corresponds with, or is engaged by, the inserts  20  disposed on the central portion  19  of the bit face  14 . These inserts  20  will be referred to as inserts  162 . Segment  180  represents the bottom hole pattern  150  most radially outboard relative to, or farthest from, the central axis  13  of the bit  11 . This section  180  corresponds with, or is engaged by, the inserts  20  on the gage row  78  (inserts  182 ). Segment  170  represents the portion of the bottom hole pattern  150  disposed between segments  160  and  180 , and corresponds with, or is engaged by, the inserts  20  disposed on the bit face  14  between the gage row  78  and the central portion  19 , and will be referred to as inserts  172 .  
         [0030]    Still referring to FIG. 5, it is known that when the earthen formation  120  includes substantial amounts of rock, the compressive strength of the formation  120  across the bottom hole pattern  150  increases substantially from segment  160  to segment  180  due to the confining pressure and the overburden pressure. Segment  180  thus generally possesses the highest compressive strength followed by segment  170 , which is followed by segment  160 , which has the lowest compressive strength. This places increasing load requirements on the inserts  162 ,  172  and  182  for fracturing or crushing the formation  120 . Thus, the bit  11  requires less load directed to inserts  162  to fracture or crush the formation  120  than to inserts  172 , and much less load than needs to be directed to inserts  182 , due to the gradient in compressive strength of the formation  120  from segments  160 - 180 . Uniform distribution of the load across the entire bit face  14 ; such as with the prior art bit  10  of FIG. 4, results in inefficient drilling.  
         [0031]    In accordance with the present invention, it has been discovered that the use of large inserts  32  on certain areas of the bit face  14 , as shown, for example in FIGS. 2 and 3, will optimize bit performance in view of the gradient in compressive strength of the earthen formation  120  (FIG. 5) across the bottom hole pattern  150 . In FIG. 5, the gage row inserts  182  are large inserts  32 . The large contact surfaces  38  of the large inserts  32  enables the distribution of sufficient increased load to segment  180  to overcome its higher strength, thus increasing drilling efficiency. The durability and survivability of the inserts  182  is preserved, or enhanced, because of the increased physical size, or robustness, and the larger radius of curvature  36  (FIG. 10) of the large inserts  32 . The forces upon the large inserts  32  of bit  11  from their interaction with the earthen formation  120  will be imparted across the larger, or broader, contact surface  38  of the insert  32  as compared to the contact surface  28  of the small inserts  22  (FIG. 9). As a result, the inserts  32  will be less susceptible to damage from interaction with the formation  120  and more durable than the inserts  22 .  
         [0032]    Referring to FIG. 6, large inserts  32  are shown as inserts  172  on rows  74  and  76  in addition to large inserts  32  on the gage row  78  (inserts  182 ). The benefits described above with respect to FIG. 5 will apply to this configuration, but to a lesser magnitude with respect to inserts  172  on rows  74  and  76  because the gradually increasing compression strength and reaction forces of segment  170  (not shown) are not as great as those of segment  180  (not shown), causing less increased load demand. In contrast, the use of small inserts  22  as inserts  162  and inserts  172  on row  72  provides sufficient load and penetration to fracture or crush the corresponding formation  120  and efficiently drill the bore hole (not shown), whereas the use of large inserts (not shown) at those locations may lead to inefficient drilling.  
         [0033]    Now referring to FIGS. 11 and 12, inserts  20  may be used that include an enhanced surface  100 , which is known to generally increase insert longevity and improving bit performance. For example, tungsten carbide inserts having a PCD surface  104 , such as those disclosed in U. S. Pat. No. 4,694,918 to Hall and U.S. Pat. No. 4,811,801 to Salesky et al., which are hereby incorporated by reference herein in their entireties, may be used. When inserts  20  are used having an enhanced surface  100 , the surface  100  is subject to similar loading conditions as discussed above. The use of large inserts  32  having an enhanced surface  100  in accordance with the present invention provides additional benefits to those described above.  
         [0034]    Referring to FIGS. 13 and 14, the enhanced surface  100  may include one, or numerous, layers  101  of enhanced material disposed upon the insert face surface  39 . An edge, or joint area,  190  is formed around the periphery of the enhanced surface  100  where the surface  100  begins, or blends into the insert substrate material, such as tungsten carbide,  86 . The edge, or joint area,  190 , is subject to cracking, flaking and breakage when contacted with the earthen formation, which can lead to breakage and failure of the enhanced surface  100 . In accordance with the present invention, the edge, or joint area,  190 , of the enhanced surface  100  is protected from contact with the earthen formation  120  as the insert  32  impacts, or interacts with, the formation  120 . As shown in FIGS. 15 and 16, the distance  222  between the enhanced surface edge, or joint area,  190  of large insert  32  and the earthen formation  120  is greater than the distance  220  between the enhanced surface edge, or joint area,  190  of small insert  22  and the formation  120  at uniform depths of penetration  224 , decreasing the susceptibility of the enhanced surface edge, or joint area,  190  of the larger inserts  32  to contact with the formation  120 .  
         [0035]    Referring back to FIGS. 11 and 12, the enhanced surface  100  of the large inserts  32  is larger and has a larger contact surface  107 , as compared to the size and contact surface  109  of the enhanced surface  100  of a small insert  22 . The forces on the enhanced surface  100  of the large inserts  32  of bit  11  from interaction with the earthen formation are imparted across the larger, or broader, contact surface  107 . As a result, the enhanced surface  100  of inserts  32  are less susceptible to damage from interaction with the formation, and more durable than the enhanced surface  100  of inserts  22 .  
         [0036]    Still referring to FIGS. 11 and 12, in accordance with the present invention, a preferred method to increase the size of the contact surface  107  of the enhanced surface  100  of insert  32  is by increasing the radius of curvature  106  of the enhanced surface  100 , which is done by increasing the radius of curvature  36  of the insert  32 . An increase in the radius of curvature  106  of the enhanced surface  100 , such as PCD surface  104 , reduces the highly concentrated contact stresses on the enhanced surface  100  caused by interaction with the earthen formation. These contact stresses cause micro-chipping, spalling and fracture of the enhanced surface  100 , which are major failure modes of inserts  20  having an enhances surface  100 , such as a PCD surface  104 . Thus, the enhanced surface  100  of inserts  32  will have reduced susceptibility to micro-chipping, spalling, and fracturing, preserving the integrity of the enhanced surface  100  and increasing its longevity.  
         [0037]    Now referring again to FIGS. 13 and 14, during the manufacturing process of an insert  20  having a PCD surface  104 , residual stress is generated in the PCD surface  104  and the tungsten carbide substrate  86  because of the mismatch of their differing thermal expansion coefficients. Such residual stress weakens the enhanced surface  104  and the tungsten carbide substrate  86  and increases the insert&#39;s  20  susceptibility to breakage and failure. The magnitude of this residual stress, however, is proportional to the ratio of the thickness  210  of the PCD surface  104  to the radius  200  (FIG. 14) of the substrate  86 . In accordance with the present invention, the large insert  32  with a PCD surface  104  having a thickness  210  is designed with a larger substrate radius  200 , as compared to the substrate radius  201  of a small insert  22  having a PCD surface  104  with a similar thickness  210 , reducing the amount of residual stress.  
         [0038]    Referring to FIGS. 11 and 12, another potential benefit from the invention is by reducing insert  20  failure due to irregular side impact loading on the inserts  20 . Such loading can cause shear failure in the carbide substrate  86 , which is known to be weaker under shear than under compression stresses. A large diameter insert  32  will better withstand irregular side impact loading, thus reducing shear stress on the insert  20 . In another aspect of the invention, large inserts  32  are also better able to withstand impact loading from lateral movement, or vibration of the bit  11 , as compared to small inserts  22 .  
         [0039]    In a further aspect of the invention, FIGS. 9 and 10 illustrate the general impact patterns in the earthen formation  120  caused by a prior art bit  10  and a bit  11  of the present invention, respectively. As shown in FIG. 9, insert  22  of the prior art bit  10  has a radius of curvature  26  and contact surface  28  that generally create an impact crater  116  in the earthen formation  120  upon contact. As the impact crater  116  is formed by the insert  22 , a pronounced ledge  117  is generally created around the crater  116 , serving as a barrier for the insert  22  to overcome as it rotates or indexes in the bore hole (not shown). The frictional engagement of the insert  22  and the ledge  117  imparts forces on the insert  22 , which causes higher torque on the bit  10 , increasing the bit&#39;s energy requirements and wear to the insert  22 , while decreasing the bit&#39;s rate of penetration, or drilling. For percussion bits  10  used with certain types of percussion assemblies (not shown), such as, for example, those shown and described in U.S. Pat. No. 5,322,136 to Bui et al., excessive torque on the inserts  22 , or bit  10 , can cause the percussion assembly to stall, or become inoperable.  
         [0040]    Now referring to FIG. 10, the contact surface  38  of the large inserts  32  of bit  11  is more gradually sloping as compared to the contact surface  28  of the small inserts  22  (FIG. 9). The large inserts  32  generally penetrate the earthen formation  120  less axially, or shallower, in the formation  120 , as compared to the small inserts  22  (FIG. 9). A shallow crater  116  with gradually sloping walls and a small, or no, ledge  117  is created. As a result, the insert  32  advances across the formation  120  with less resistance and reduced torque on the bit  11 .  
         [0041]    In another aspect of the invention, the large inserts  32  of the bit  11  may be formed with a length  34  that is greater than the length  24  of the small inserts  22 , as shown, for example, in FIGS. 7 and 8. In turn, the inserts  32  can be configured such that the (longer) large inserts  32  extend farther away from the face  14  of the bit  11  than the small inserts  22 . For example, large inserts  32  can be embedded in the head  12  of bit  11  at a depth  57  that allows the inserts  32  to extend farther from the bit face  14  than small inserts  22  embedded at a depth  56  in the head  12  of bit  10  or  11 . As a result, the bit face  14  of bit  11  has a larger bit standoff  33  from formation (not shown), as compared to the standoff  23  of the prior art bit  10 . The larger bit standoff  33  provides more open space volume  42  between inserts  20 , and between the bit face  14  and the earthen formation (not shown) during drilling operations. This increased open space volume  42  allows an increased flow of circulating fluid across the bit face  14 , enhancing the fluid&#39;s ability to clean the bit face  14 , move cuttings up the bore hole (not shown) and cool the inserts  20 , improving operational efficiency and bit longevity. Further, the increased flow of circulating fluid will reduce the velocity of the fluid across the face  14  of the bit  11  and around the inserts  20 , reducing erosion to the bit face  14 , bit head  12  and inserts  20 , thus improving bit longevity.  
         [0042]    It is generally known in the art that the bit head of a drill bit, such as a percussion bit, is subject to internal cracking from structural fatigue during normal operations. Referring again to FIGS. 7 and 8, when inserts  20  are disposed in the bit head  12  in cavities  50 , the bit head  12  is susceptible to the formation of internal fatigue cracks (not shown) proximate to the cavities  50 . In particular, it has been discovered that fatigue cracks tend to form in the bit head  12  at cavity base corners  58 . Fatigue cracks also form at cavity side corners  60 , which are located adjacent to a side corner, or change in shape,  61  of the corresponding insert  20 , such as where the taper begins on an embedded tapered insert. The corners  58 ,  60  are highly susceptible locations for the formation, or initiation, of fatigue cracks. After such fatigue cracks form, they tend to migrate, or increase in size, along a path of least resistance through the bit head  12  during the continued use of the bit.  
         [0043]    Still referring to FIGS. 7 and 8, catastrophic internal fatigue cracking can occur when inserts  20  are disposed in adjacent cavities at substantially uniform depths  56  in adjacent cavities  50 , such as shown in the prior art bit  10  of FIG. 7. The term “catastrophic internal fatigue cracking” as used herein refers to breakage, or significant fracture, of the bit head  12 , or loosening, or loss, of inserts  20 , which can lead to premature bit failure. The term “adjacent cavities” refers to two or more cavities  50 , whereby one cavity  50  is outward of and proximate to another cavity  50 . The term “outward” as used herein means away from the central axis  13  of the bit  10  (FIGS. 1, 5) on the bit head  12 , or face  14 . As shown in FIG. 7, the adjacent cavities  50  of the prior art bit  10  are separated from one another by a short distance  64 , or small section  65 , of the bit head  12 . Further, the adjacent corners  58  of cavities  50  have base planes  62  that intersect between the cavities  50  in bit section  65 . As a result, fatigue cracks initiating at adjacent corners  58  have a close path of least resistance extending between adjacent cavities  50  and are susceptible to joinder with one another or with the adjacent cavity  50 , which can lead to catastrophic internal fatigue cracking. The same problems exist for fatigue cracks initiating at adjacent side corners  60  of adjacent cavities  50  in prior art bit  10 .  
         [0044]    It has been discovered that the use of small and large inserts  22 ,  32 , disposed in adjacent cavities  50  of bit  11 , as shown, for example, in FIG. 8, will reduce the bit&#39;s susceptibility to, or will delay, catastrophic internal fatigue cracking as described above. In accordance with the present invention, the base planes  62  of adjacent cavities  50  carrying large and small inserts  32 ,  22  do not intersect in the bit section  65  between the cavities  50 . Further, the adjacent base corners  58  of adjacent cavities  50  are separated by a distance  66  that is greater than the distance  64  of the adjacent base corners  58  of adjacent cavities  50  of the typical prior art bit  10  (FIG. 7). As a result, a close path of least resistance for cracks forming at corners  58  in bit  11 , as in the prior art bit  10  (FIG. 7), is not created. Thus, the possibility of joinder of fatigue cracks forming at adjacent corners  58  and the likelihood of catastrophic internal fatigue cracking thereabouts is reduced, increasing bit integrity and longevity. The same effect will occur with respect to cracks forming at adjacent side corners  60  of adjacent cavities  50  of bit  11 . While this aspect of the present invention applies to adjacent insert cavities  50  that carry large and small inserts  32 ,  22  anywhere on the bit  11 , it is particularly significant with respect to adjacent cavities  50  located on the gage and third rows  78 ,  76  because the inserts  20 , bit head  12  and cavities  50  at the gage row  78  are subject to heightened stress and fatigue and are thus more susceptible to fatigue cracking than other areas of the bit  11 .  
         [0045]    Each of the foregoing aspects of the invention may be used alone or in combination with other such aspects. The embodiments described herein are exemplary only and are not limiting of the invention, and modifications thereof can be made by one skilled in the art without departing from the spirit or teachings of this invention. Many variations and modifications of the embodiments described herein are thus possible and within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described herein.

Summary:
A percussion drill bit is provided that has at least two different pluralities of inserts extending from the bit head to better match the conditions seen on the bit head during drilling. The preferred embodiment has a plurality of large inserts with a polycrystalline diamond layer located at least in the gage row and a plurality of smaller inserts located at least in the inner rows.