Patent Publication Number: US-8522899-B2

Title: Wear resistant material at the shirttail edge and leading edge of a rotary cone drill bit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is related to co-pending U.S. patent application Ser. No. 12/896,484 filed Oct. 1, 2010 entitled “Wear Resistant Material for the Shirttail Outer Surface of a Rotary Cone Drill Bit”, the disclosure of which is hereby incorporated by reference to the maximum extent allowable by law. 
     BACKGROUND 
     1. Technical Field of the Invention 
     The present invention relates to earth boring bits, and more particularly to those having rotatable cutters, also known as rotary cone drill bits. 
     2. Description of Related Art 
     Reference is made to  FIGS. 1 and 2 , wherein  FIG. 1  illustrates an isometric view of a prior art rotary cone drill bit  10  and  FIG. 2  illustrates a cross-sectional view of a portion of the prior art rotary cone drill bit  10  of  FIG. 1 . A leg  12  depends from a body portion  14  of the drill bit  10 . The leg  12  includes a bearing shaft  16  which extends in a downward and radial inward direction. The bearing shaft  16  includes a cylindrical bearing surface  18 . A cutter cone  20  is mounted to the bearing shaft  16  and supported for rotation by the bearing surface  18 . In an alternative implementation, the cutter cone  20  is supported for rotation on the bearing shaft  16  by a set of roller bearings. The shape and configuration of the cone  20 , as well its rotatable attachment to the bearing shaft  16 , is well known in the art. In sealed bearing implementations, the bearing (journal or roller) between the cone  20  and bearing shaft  16  is lubricated by a lubricant (such as a grease) that fills regions adjacent to the bearing as well as other passages  21  in the rotary cone drill bit in a manner well known by those skilled in the art. This lubricant is retained within the rotary cone drill bit through the use of, for example, a resilient seal in the form of an o-ring  22  positioned in a seal gland  24  between the inner cylindrical surface  26  near the base of the cone  20  and the outer cylindrical surface  28  near the base of the bearing shaft  16 . 
     The outer surface  30  of the leg  12  terminates at a semicircular edge  32  proximal to the cone  20 . The region of the leg  12  associated with the surface  30  is known in the art as the “shirttail region,” and the edge  32  is known in the art as the “shirttail edge.” The shirttail edge  32  is provided where the terminal portion of the surface  30  transitions to an inside radial surface  34  oriented parallel to the base of the cone  20  (and perpendicular to the bearing shaft  16 ) and positioned at the base of the bearing shaft  16 . On a rotary cone drill bit  10 , one of the primary forms of bit failure can be traced back to shirttail wear. In one form of such shirttail wear, the shirttail edge  32  wears down, the radial extent of the inside radial surface  34  is decreased by this wear, and the resilient o-ring  22  seal in sealed bearings is exposed. If the bearing is instead an open (non-sealed or air) bearing, the wearing of the shirttail edge may expose the air bearing. 
     The prior art teaches two methods for delaying shirttail wear.  FIG. 3  illustrates a first method in which a layer of welded hardfacing material  40  is applied to the surface  30  extending along at least a portion of the shirttail edge  32 . The hardfacing material is typically a deposit of tungsten carbide hardmetal  40  applied to the surface  30 . The material is typically pelletized tungsten carbide carried in a nickel welding medium. This solution does not work well when the rotary cone drill bit is run in a highly abrasive rock formation because the hardfacing material  40  wears down too quickly. It is primarily the welding medium, typically nickel, which accounts for the relative poor performance of the weld on material.  FIG. 4  illustrates a second method in which tungsten carbide inserts  42  are press-fit into holes  44  formed in the surface  30  near the shirttail edge  32 . While these inserts  42  provide better abrasion resistance (in comparison to the use of hardfacing material), the inserts  42  do not provide protection for the shirttail edge  32 . The reason for this is that the holes  44  must be located at some appreciable distance from the shirttail edge  32  in order for the press-fit to function properly and peripherally retain the inserts  42 . For example, a separation d 1  of at least 0.125 inches is typically provided from the edge of the hole  44  to the shirttail edge  32 . Thus, the method of  FIG. 4  functions to primarily protect the shirttail region near to, but not exactly at, the shirttail edge  32 . Furthermore, in order to be suitably retained, the press-fit inserts  42  must typically have a thickness t (with a corresponding depth of the hole  44 ) such that a ratio of the thickness of the insert to a diameter d′ of the insert (where the inserts are round) or width w of the insert (with other shapes) exceeds about 0.5 (i.e., t/d′≧0.5; or t/w≧0.5). 
     A need accordingly exists in the art to provide an improved method of protecting the shirttail edge  32 . 
     With reference once again to  FIGS. 1 and 2 , the outer surface  30  of the leg  12  in the shirttail region laterally terminates at a leading shirttail edge  50  and a trailing shirttail edge  52 . The leading shirttail edge  50  is especially susceptible to wear during operation of the rotary cone drill bit  10 . The prior art again teaches two methods for delaying wear of the leading shirttail edge  50 .  FIG. 5  illustrates a first method in which a layer of welded hardfacing material  40  is applied to the surface  30  extending along at least a portion of the leading shirttail edge  50 . The hardfacing material is typically a deposit of tungsten carbide hardmetal  40 . The material is typically pelletized tungsten carbide carried in a nickel welding medium. This solution does not work well when the rotary cone drill bit is run in a highly abrasive rock formation because the hardfacing material  40  wears down too quickly. It is primarily the welding medium, typically nickel, which accounts for the relative poor performance of the weld on material.  FIG. 6  illustrates a second method in which tungsten carbide inserts  42  are press-fit into holes  44  formed in the surface  30  near the leading shirttail edge  50 . While these inserts  42  provide better abrasion resistance (in comparison to the use of hardfacing material), the inserts  42  do not provide protection for the leading shirttail edge  50 . The reason for this is that the holes  44  must be located at some appreciable distance from the leading shirttail edge  50  in order for the press-fit to function properly and peripherally retain the inserts  42 . For example, a separation d 2  of at least 0.125 inches is typically provided from the edge of the hole  44  to the leading edge  50 . Thus, the method of  FIG. 6  functions to primarily protect the shirttail region near to, but not exactly at, the leading shirttail edge  50 . Furthermore, in order to be suitably retained, the press-fit inserts  42  must typically have a thickness t (with a corresponding depth of the hole  44 ) such that a ratio of the thickness of the insert to a diameter d′ of the insert (where the inserts are round) or width w of the insert (with other shapes) exceeds about 0.5 (i.e., t/d′≧0.5; or t/w≧0.5). 
     Although not explicitly shown in  FIGS. 5 and 6 , the protection mechanisms shown could alternatively, or additionally, be provided on the leading side surface  54  of the leg  12 . This leading side surface  54  is adjacent the outer surface  30  of the leg  12  at the leading shirttail edge  50 . 
     A need thus exists in the art to provide an improved method of protecting the leading shirttail edge  50  and leading side surface  54  of the shirttail. 
     SUMMARY 
     In an embodiment, a rotary cone drill bit comprises: a body, a leg depending from the body, a bearing shaft extending from the leg and a cone mounted to the bearing shaft. The leg terminates at a shirttail edge adjacent a base of the cone. A bottom surface of a hard material plate having an edge is attaching to a substantially conforming surface of the leg in a position where the edge of the hard material plate defines at least a portion of the shirttail edge. 
     In an embodiment, a rotary cone drill bit comprises: a body, a leg depending from the body, a bearing shaft extending from the leg and a cone mounted to the bearing shaft. The leg includes a lateral leading edge. A bottom surface of a hard material plate having an edge is attached to a substantially conforming surface of the leg in a position where the edge of the hard material plate defines at least a portion of the lateral leading edge. 
     In an embodiment, a rotary cone drill bit comprises: a body, a leg depending from the body, a bearing shaft extending from the leg and a cone mounted to the bearing shaft. The leg includes a surface edge that is subject to wear during operation of the bit. A bottom surface of a hard material plate having an edge is attached to a substantially conforming surface of the leg in a position where the edge of the hard material plate defines at least a portion of the surface edge of the leg. 
     In any of the foregoing embodiments, the conforming surface to which attachment is made may comprise: a floor surface formed in or by an outer shirttail surface of the leg, a floor surface formed in or by a leading side surface of the leg, a floor surface of a slot formed in the outer shirttail surface of the leg, or a floor surface of a slot formed in the leading side surface of the leg. 
     In any of the foregoing embodiments, a material for attaching the hard material plate may comprise a flowable adhesive material interposed between the bottom surface of the hard material plate to the floor surface of the leg. That material may comprise, for example, a brazing material. 
     In any of the foregoing embodiments, the hard material plate may comprise polycrystalline diamond compact, or be made of a material such as solid tungsten carbide, or comprise a polycrystalline cubic boron nitride compact, or comprise an impregnated diamond segment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the invention will become clear in the description which follows of several non-limiting examples, with references to the attached drawings wherein: 
         FIG. 1  illustrates an isometric view of a prior art rotary cone drill bit; 
         FIG. 2  illustrates a cross-sectional view of a portion of a leg of the prior art rotary cone drill bit of  FIG. 1 ; 
         FIG. 3  illustrates application of a layer of hardfacing material extending along at least a portion of the shirttail edge; 
         FIG. 4  illustrates the use of tungsten carbide inserts near the shirttail edge; 
         FIG. 5  illustrates application of a layer of hardfacing material extending along at least a portion of the leading edge of the shirttail; 
         FIG. 6  illustrates the use of tungsten carbide inserts near the leading edge of the shirttail; 
         FIG. 7  illustrates an isometric view of a rotary cone drill bit including protection mechanisms for the shirttail edge and the leading edge of the shirttail; 
         FIG. 8  illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a shirttail edge protection mechanism; 
         FIGS. 9 and 10  illustrate cross-sectional views of a portion of a leg of a rotary cone drill bit which include embodiments of a mechanism for protecting the leading edge of the shirttail; 
         FIGS. 11 and 12  illustrate isometric views of a portion of the leg and including protection mechanisms for the shirttail edge; 
         FIG. 13  illustrates an isometric view of a rotary cone drill bit including protection mechanisms for the shirttail edge and the leading edge of the shirttail; 
         FIG. 14  illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a shirttail edge protection mechanism; 
         FIGS. 15 and 16  illustrate cross-sectional views of a portion of a leg of a rotary cone drill bit which include embodiments of a mechanism for protecting the leading edge of the shirttail; and 
         FIGS. 17 and 18  illustrate isometric views of a portion of the leg and including protection mechanisms for the shirttail edge. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Reference is now made to  FIG. 7  which illustrates an isometric view of a rotary cone drill bit  110  including protection mechanisms for the shirttail edge and the leading edge (also referred to as the lateral leading edge) of the shirttail. A leg  112  depends from a body portion  114  of the drill bit  110 . The leg  112  includes a bearing shaft (not shown, see  FIG. 8  reference  116 ) which extends in a downward and radial inward direction. A cutter cone  120  is mounted to the bearing shaft and supported thereon for rotation. The outer surface  130  of the leg  112  terminates at a semicircular edge  132  proximal to the cone  120 . The region of the leg  112  associated with the surface  130  is known in the art as the “shirttail region,” and the edge  132  is known in the art as the “shirttail edge.” The outer surface  130  of the leg  112  laterally terminates at a leading shirttail edge  150  and a trailing edge  152  of the shirttail. The lateral leading edge  150  and lateral trailing edge  152  of the shirttail comprise extensions of the shirttail edge  132  extending along the length of the leg  112 . The shirttail region further includes a leading side surface  154  which is adjacent the outer surface  130  of the leg  112  at the leading shirttail edge  150 . Although illustrated for example as including a sealed bearing system, it will be understood that the present invention is applicable to both sealed and non-sealed (air) bearing bits. 
     To protect the shirttail edge  132 , a slot  134  is provided in the outer surface  130  of the leg  112  extending inwardly from the shirttail edge  132 , and a hard plate insert  136  is adhered to a floor surface within the slot  134 . See, also,  FIG. 8 . To protect the lateral leading edge  150  of the shirttail, a slot  138  is provided in the outer surface  130  of the leg  112  extending inwardly from the leading edge  150  of the shirttail, and a hard plate insert  140  is adhered to a floor surface within the slot  138 . See, also,  FIG. 9 . To protect the leading side surface  154 , a slot  142  is provided in the leading side surface  154  of the leg  112  extending outwardly from the leading edge  150  of the shirttail, and a hard plate insert  144  is adhered to a floor surface within the slot  142 . See, also,  FIG. 10 . The slots may be milled or cast into the outer surface  130  and/or leading side surface  154  of the leg  112  at desired positions, specifically positions on the leg which are susceptible to wear during operation of the bit, and may have flat or curved floor surface geometries as desired and which conform with the bottom surfaces of the inserts  136 ,  140  and  144 . 
     Although all three protection mechanisms are illustrated in  FIG. 7 , it will be understood that any one or more of the illustrated protection mechanisms may be selected for use on the rotary cone drill bit  110 . Although  FIG. 7  illustrates the use of several hard plate inserts  136  along the shirttail edge  132 , it will be understood that one slot  134  could instead be provided extending along all or a portion of the shirttail edge  132 , with a single hard plate insert  136  adhered within the slot  134 . Although  FIG. 7  illustrates the use of several hard plate inserts  140  along the leading edge  150  of the shirttail, it will be understood that one slot  138  could instead be provided extending along all or a portion of the leading edge  150  of the shirttail, with a single hard plate insert  140  adhered within the slot  138 . Although  FIG. 7  illustrates the use of several hard plate inserts  144  along the leading edge  150  of the shirttail, it will be understood that one slot  142  could instead be provided extending along all or a portion of the leading edge  150  of the shirttail, with a single hard plate insert  144  adhered within the slot  142 . In each of the foregoing implementations, the portion of the edge (shirttail edge  132  and/or leading shirttail edge  150 ) selected to receive protection would be that portion of the edge which is most susceptible to wear during operation of the rotary cone drill bit  110 . 
     Reference is now made to  FIG. 8  which illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a shirttail edge protection mechanism. In this embodiment, the slot  134  is provided in the outer surface  130  of the leg  112  extending inwardly from the shirttail edge  132 . The slot  134  may be milled or cast into the outer surface  130  of the leg  112 . The slot  134  is defined by a floor surface  160 , a rear wall  162  and two side walls  164  (see, also,  FIG. 7 ). The floor surface  160  may, for example, be a substantially flat surface, or alternatively a curved surface. The hard plate insert  136  is adhered within the slot  134 . In a preferred embodiment, a bottom surface of hard plate insert is adhered to a substantially conforming floor surface  160  of the slot  134 . The bottom surface of the insert may, for example, comprise a flat surface which conforms with the flat floor surface, or have a curved surface conforming to the curved floor surface. The means for adhering the bottom surface to the floor surface may, for example, comprise any suitable adhering material which is interposed between the substantially conforming (for example, parallel) surfaces including adhesive material flowable between the substantially conforming surfaces by capillary action such as a brazing material, solder, adhesives, resins, and the like (see, for example, U.S. Patent Application Publication No. 2009/0038442, the disclosure of which is hereby incorporated by reference). Because of drawing scale, the adhesive material is not explicitly shown in  FIG. 8 , but it will be understood that the adhesive material is present between the conforming bottom surface and floor surface. The adhesive material preferably has a substantially uniform thickness between the conforming bottom surface and floor surface. The hard plate insert  136  has a thickness such that when adhered within the slot  134 , a top surface  166  of the plate insert  136  is substantially flush with, or slightly exposed beyond, or slightly recessed below, the outer surface  130  of the leg  112 . Furthermore, the hard plate insert  136  is sized such that an edge  168  of the plate insert opposite the rear wall  162  of the slot  134  defines (or is coincident with or nearly coincident with) the shirttail edge  132 . The hard plate insert  136  is made of a material or combination of materials which are more abrasion resistant than the material used to make the leg and shirttail of the bit. In a preferred implementation, the hard plate insert is made of a material such as solid tungsten carbide, polycrystalline diamond compact (PDC), polycrystalline cubic boron nitride compactimpregnated diamond segment, and the like. These materials are superior to the traditional weld on tungsten carbide hardfacing known in the prior art because they are denser and are not as susceptible to abrasion and erosion. 
     The shirttail edge  132  is provided where the terminal portion of the surface  130  transitions to an inside radial surface  192  oriented parallel to the base of the cone  120  (perpendicular to the bearing shaft  116 ) and positioned at the base of the bearing shaft  116 . The hard plate inserts  136  function to protect against wearing of the shirttail edge  132  and erosion of the inside radial surface  192 . Although a sealed bearing system is illustrated, it will be understood that edge protection in accordance with the present invention is applicable to both sealed and non-sealed (air) bearing bits. 
     The hard plate inserts  136  have a thickness t and width w (wherein the width is measured in a direction perpendicular to the shirttail edge  132 ). The hard plate inserts  136  are thin inserts. In this case, a ratio of the thickness t of the insert to a width w of the insert is less than 0.5 (i.e., t/w&lt;0.5). More particularly, the ratio of the thickness t of the insert to the width w of the insert is substantially less than 0.5 (i.e., t/w&lt;&lt;0.5). Even more particularly, the ratio of the thickness t of the insert to the width w of the insert is less than 0.2 (i.e., t/w&lt;0.2), and may even be less than 0.1 (i.e., t/w&lt;0.1). This is permitted because the hard plate inserts  136  are retained by adhesion to their bottom surface and not their peripheral edge (as is the case with the press-fit inserts used in the prior art (see,  FIG. 4 )). 
       FIGS. 11 and 12  illustrate isometric views of a portion of the leg  130  and including protection mechanisms for the shirttail edge  132  as shown in  FIG. 8 . The cone  120  has been omitted from  FIGS. 11 and 12  to show how the hard plate inserts  136  are positioned at the shirttail edge  132 .  FIGS. 11 and 12  further show how the slots  134  are provided in the outer surface  130  of the leg  112  extending inwardly from the shirttail edge  132 . A portion  190  of the shirttail material remains at the floor surface  160  of each slot  134  (adjacent the inside radial surface  192 ), and it is at the floor surface  160  where adhesion (for example, through brazing) is made to the hard plate insert  136 . In this way, the adhesive material is not externally exposed and subject to possible wear. The insert may have a thickness in the range of 0.050 to 0.500 inches. 
     As shown in  FIG. 12 , where the protection is desired to extend in a continuous manner along an extended length of the shirttail edge  132 , the slot likewise extends in a continuous manner along that extended length of the edge  132 . In one embodiment, the floor surface may curve with the radius of the bit, and thus the bottom surface of the one or more included inserts will have a conforming curve. In another embodiment, the slot is formed to include a plurality of substantially flat and adjacent floor surfaces  160 , and a hard plate insert  136  with a conforming flat bottom surface is provided for each flat surface and inserts are arranged in a tiled edge-to-edge configuration (see, dotted line reference  137  indicating adjacent tile edges). 
     Reference is now made to  FIG. 9  which illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a protection mechanism for the lateral leading edge of the shirttail. In this embodiment, the slot  138  is provided in the outer surface  130  of the leg  112  extending inwardly from the lateral leading edge  150  of the shirttail. The slot  138  may be milled or cast into the outer surface  130  of the leg  112 . The slot  138  is defined by a floor surface  170 , a rear wall  172  and two side walls  174  (see, also,  FIG. 7 ). The hard plate insert  140  is adhered within the slot  138 . In a preferred embodiment, a bottom surface of hard plate insert is adhered to the floor surface  170  of the slot  138 . The means for adhering the bottom surface to the floor surface may, for example, comprise any suitable adhering material which is interposed between the substantially conforming (for example, parallel) surfaces including adhesive material flowable between the substantially conforming surfaces by capillary action such as a brazing material, solder, adhesives, resins, and the like (see, for example, U.S. Patent Application Publication No. 2009/0038442, the disclosure of which is hereby incorporated by reference). Because of drawing scale, the adhesive material is not explicitly shown in  FIG. 9 , but it will be understood that the adhesive material is present between the bottom surface and the floor surface. The adhesive material preferably has a substantially uniform thickness between the conforming bottom surface and floor surface. The hard plate insert  140  has a thickness such that when adhered within the slot  138 , a top surface  176  of the plate insert  140  is substantially flush with, or slightly exposed beyond, or slightly recessed below, the outer surface  130  of the leg  112 . Furthermore, the hard plate insert  140  is sized such that an edge  178  of the plate insert opposite the rear wall  172  of the slot  138  defines (or is coincident with or nearly coincident with) the leading edge  150  (leading surface  154 ) of the shirttail. The hard plate insert  140  is made of a material or combination of materials which are more abrasion resistant than the material used to make the leg and shirttail of the bit. In a preferred implementation, the hard plate insert is made of a material such as tungsten carbide, polycrystalline diamond compact (PDC), polycrystalline cubic boron nitride compactimpregnated diamond segment, and the like. These materials are superior to the traditional weld on tungsten carbide hardfacing known in the prior art because they are denser and are not as susceptible to abrasion and erosion. Again, the adhesive material is this implementation is not externally exposed and subject to possible wear. 
     The hard plate inserts  140  have a thickness t and width w (wherein the width is measured in a direction perpendicular to the leading edge  150 ). The hard plate inserts  140  are thin inserts. In this case, a ratio of the thickness t of the insert to a width w of the insert is less than 0.5 (i.e., t/w&lt;0.5). More particularly, the ratio of the thickness t of the insert to the width w of the insert is substantially less than 0.5 (i.e., t/w&lt;&lt;0.5). Even more particularly, the ratio of the thickness t of the insert to the width w of the insert is less than 0.2 (i.e., t/w&lt;0.2), and may even be less than 0.1 (i.e., t/w&lt;0.1). This is permitted because the hard plate inserts  140  are retained by adhesion to their bottom surface and not their peripheral edge (as is the case with the press-fit inserts used in the prior art (see,  FIG. 4 ). 
     Reference is now made to  FIG. 10  which illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a protection mechanism for the lateral leading edge of the shirttail. In this embodiment, the slot  142  is provided in the leading side surface  154  of the leg  112  extending outwardly from the lateral leading edge  150  of the shirttail. The slot  142  may be milled or cast into the leading side surface  154  of the leg  112 . The slot  142  is defined by a floor surface  180 , a rear wall  182  and two side walls  184  (see, also,  FIG. 7 ). The hard plate insert  144  is adhered within the slot  142 . In a preferred embodiment, a bottom surface of hard plate insert is adhered to the floor surface  180  of the slot  142 . The means for adhering the bottom surface to the floor surface may, for example, comprise any suitable adhering material which is interposed between the substantially conforming (for example, parallel) surfaces including adhesive material flowable between the substantially conforming surfaces by capillary action such as a brazing material, solder, adhesives, resins, and the like (see, for example, U.S. Patent Application Publication No. 2009/0038442, the disclosure of which is hereby incorporated by reference). The adhesive material preferably has a substantially uniform thickness between the conforming bottom surface and floor surface. Because of drawing scale, the adhesive material is not explicitly shown in  FIG. 10 , but it will be understood that the adhesive material is present between the bottom surface and the floor surface. The hard plate insert  144  has a thickness such that when adhered within the slot  142 , a top surface  186  of the plate insert  144  is substantially flush with, or slightly exposed beyond, or slightly recessed below, the leading side surface  154  of the leg  112 . Furthermore, the hard plate insert  144  is sized such that edge  188  of the plate insert opposite the rear wall  172  of the slot  138  defines (or is coincident with or nearly coincident with) the leading edge  150  (outer surface  130 ) of the shirttail. The hard plate insert  140  is made of a material or combination of materials which are more abrasion resistant than the material used to make the leg and shirttail of the bit. In a preferred implementation, the hard plate insert is made of a material such as tungsten carbide, polycrystalline diamond compact (PDC), polycrystalline cubic boron nitride compact impregnated diamond segment, and the like. These materials are superior to the traditional weld on tungsten carbide hardfacing known in the prior art because they are denser and are not as susceptible to abrasion and erosion. Again, the adhesive material is this implementation is not externally exposed and subject to possible wear. 
     The hard plate inserts  144  have a thickness t and width w (wherein the width is measured in a direction perpendicular to the leading edge  150 ). The hard plate inserts  144  are thin inserts. In this case, a ratio of the thickness t of the insert to a width w of the insert to is less than 0.5 (i.e., t/w&lt;0.5). More particularly, the ratio of the thickness t of the insert to the width w of the insert is substantially less than 0.5 (i.e., t/w&lt;&lt;0.5). Even more particularly, the ratio of the thickness t of the insert to the width w of the insert is less than 0.2 (i.e., t/w&lt;0.2), and may even be less than 0.1 (i.e., t/w&lt;0.1). This is permitted because the hard plate inserts  144  are retained by adhesion to their bottom surface and not their peripheral edge (as is the case with the press-fit inserts used in the prior art (see,  FIG. 4 ). 
     It will be noted that the slots and plate inserts may be of any selected geometry thus allowing for the application of protection to complex surfaces of the bit. Tiling of the inserts edge-to-edge permits the application of protection to be extended continuously over a complex curved surface. Alternatively, a single insert with a complex curved bottom surface could be provided. 
     The illustration of protection being applied using slots and plate inserts at the shirttail edge and/or leading shirttail edge is by way of example only, it being understood that the protection mechanisms described can be applied to any edges of the bit that are susceptible to wear. 
     Reference is now made to  FIG. 13  which illustrates an isometric view of a rotary cone drill bit  210  including protection mechanisms for the shirttail edge and the leading edge of the shirttail. A leg  212  depends from a body portion  214  of the drill bit  210 . The leg  212  includes a bearing shaft (not shown, see  FIG. 14  reference  216 ) which extends in a downward and radial inward direction. A cutter cone  220  is mounted to the bearing shaft and supported thereon for rotation. The outer surface  230  of the leg  212  terminates at a semicircular edge  232  proximal to the cone  220 . The region of the leg  212  associated with the surface  230  is known in the art as the “shirttail region,” and the edge  232  is known in the art as the “shirttail edge.” The outer surface  230  of the leg  212  laterally terminates at a leading shirttail edge  250  and a trailing edge  252  of the shirttail. The leading edge  250  and a trailing edge  252  of the shirttail comprise extensions of the shirttail edge  232  extending along the length of the leg  212 . The shirttail region further includes a leading side surface  254  which is adjacent the outer surface  230  of the leg  212  at the leading shirttail edge  250 . Although illustrated for example as including a sealed bearing system, it will be understood that the present invention is applicable to both sealed and non-sealed (air) bearing bits. 
     To protect the shirttail edge  232 , a hard plate  236  is adhered to a floor surface  231  provided in or by the curved outer surface  230  of the leg  212  extending inwardly from the shirttail edge  232 . See, also,  FIG. 14 . To protect the leading edge  250  of the shirttail, a hard plate  240  is adhered to a floor surface  231  provided in or by the curved outer surface  230  of the leg  212  extending inwardly from the leading edge  250  of the shirttail. See, also,  FIG. 15 . To protect the leading side surface  254 , a hard plate  244  is adhered to a floor surface  231  provided in or by the leading side surface  254  of the leg  212  extending outwardly from the leading edge  250  of the shirttail. See, also,  FIG. 16 . Although all three protection mechanisms are illustrated in  FIG. 13 , it will be understood that any one or more of the protection mechanisms may be selected for use on the rotary cone drill bit  210 . The floor surfaces  231  are preferably machined or cast into the outer surfaces of the shirttail region along the edge  232  and edge  250 , and may have flat or curved surface geometries as desired and which conform with the bottom surfaces of the plates  236 ,  240  and  244 . 
     Although  FIG. 13  primarily illustrates the use of polygonal plates, it will be understood that the plates can have any desired shape (including circular shapes, oval shapes, and the like). Furthermore, as shown in  FIG. 13 , the plates can be of different sizes, perhaps with size selection depending on placement position. 
     Although  FIG. 13  illustrates the use of several hard plates  236  along the shirttail edge  232 , it will be understood that one plate  236  could instead be provided extending along all or a portion of the shirttail edge  232 . Although  FIG. 13  illustrates the use of several hard plates  240  along the leading edge  250  of the shirttail, it will be understood that one plate  240  could instead be provided extending along all or a portion of the leading edge  250  of the shirttail. Although  FIG. 13  illustrates the use of several hard plates  244  along the leading edge  250  of the shirttail, it will be understood that one plate  244  could instead be provided extending along all or a portion of the leading edge  250  of the shirttail. In each of the foregoing implementations, the portion of the edge (shirttail edge  232  and/or leading shirttail edge  250 ) selected to receive protection would be that portion of the edge which is most susceptible to wear during operation of the rotary cone drill bit  210 . 
     Reference is now made to  FIG. 14  which illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a shirttail edge protection mechanism. In this embodiment, the bottom surface  260  of the hard plate  236  is adhered to a substantially conforming floor surface  231  provided in or by the curved outer surface  230  of the leg  212  and extending inwardly from the shirttail edge  232 . The plate  236  is further defined by a rear edge  262  and two side edges  264  (see, also,  FIG. 13 ). The means for adhering the bottom surface to the floor surface may, for example, comprise any suitable adhering material which is interposed between the substantially conforming (for example, parallel) surfaces including adhesive material flowable between the substantially parallel surfaces by capillary action such as a brazing material, solder, adhesives, resins, and the like (see, for example, U.S. Patent Application Publication No. 2009/0038442, the disclosure of which is hereby incorporated by reference). Because of drawing scale, the adhesive material is not explicitly shown in  FIG. 14 , but it will be understood that the adhesive material is present between the bottom surface and the outer surface. The adhesive material preferably has a substantially uniform thickness between the conforming bottom surface and floor surface. The hard plate  236  is sized such that its front edge  268  defines (or is coincident with or is nearly coincident with) the shirttail edge  232 . The thickness of the plate  236  may range from 0.050 to 0.500 inches. The hard plate  236  is made of a material or combination of materials which are more abrasion resistant than the material used to make the leg and shirttail of the bit. In a preferred implementation, the hard plate is made of a material such as tungsten carbide, PDC, polycrystalline cubic boron nitride compactimpregnated diamond segment, and the like. These materials are superior to the traditional weld on tungsten carbide hardfacing known in the prior art because they are denser and are not as susceptible to abrasion and erosion. The conforming surfaces where adhesion takes place may curve, for example, with the radius of the bit, or have any selected curved configuration. 
     The shirttail edge  232  is provided where the terminal portion of the surface  230  transitions to an inside radial surface  292  oriented parallel to the base of the cone  220  (perpendicular to the bearing shaft  216 ) and positioned at the base of the bearing shaft  216 . The hard plates  236  function to protect against wearing of the shirttail edge  232  and erosion of the inside radial surface  292 . Although a sealed bearing system is illustrated, it will be understood that edge protection in accordance with the present invention is applicable to both sealed and non-sealed (air) bearing bits. 
     The hard plates  236  have a thickness t and width w (wherein the width is measured in a direction perpendicular to the shirttail edge  232 ). The hard plates  236  are thin inserts. In this case, a ratio of the thickness t of the plate to a width w of the plate is less than 0.5 (i.e., t/w&lt;0.5). More particularly, the ratio of the thickness t of the plate to the width w of the plate is substantially less than 0.5 (i.e., t/w&lt;&lt;0.5). Even more particularly, the ratio of the thickness t of the plate to the width w of the plate is less than 0.2 (i.e., t/w&lt;0.2), and may even be less than 0.1 (i.e., t/w&lt;0.1). 
       FIGS. 17 and 18  illustrate isometric views of a portion of the leg  230  and including protection mechanisms for the shirttail edge  232  as shown in  FIG. 13 . The cone  220  has been omitted from  FIGS. 17 and 18  to show how the hard plates  236  are positioned at the shirttail edge.  232 . It is at the floor surface  231  formed in or by outer surface  230  where adhesion (for example, through brazing) is made to the conforming bottom surface  260  of each hard plate  236 . In this way, the adhesive material is not externally exposed and subject to possible wear. 
     As shown in  FIG. 18 , protection is desired to extend in a continuous manner along an extended length of the shirttail edge  232 . In one embodiment, the floor surface may curve with the radius of the bit, and thus the bottom surface of the one or more included plates will have a conforming curve. In another embodiment, the floor surface is formed to include a plurality of substantially flat and adjacent floor surfaces  260 , and a hard plate  236  with a conforming flat bottom surface is provided for each flat surface and plates are arranged in a tiled edge-to-edge configuration (see, dotted line reference  237  indicating adjacent tile edges). 
     Reference is now made to  FIG. 15  which illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a protection mechanism for the leading edge of the shirttail. In this embodiment, the bottom surface  270  of the hard plate  240  is adhered to a substantially conforming floor surface  231  formed in or by the outer surface  230  of the leg  212  and extending inwardly from the leading shirttail edge  250 . The plate  240  is further defined by a rear edge  272  and two side edges  274  (see, also,  FIG. 13 ). The means for adhering the bottom surface to the floor surface may, for example, comprise any suitable adhering material which is interposed between the substantially conforming (for example, parallel) surfaces including adhesive material flowable between the substantially conforming surfaces by capillary action such as a brazing material, solder, adhesives, resins, and the like (see, for example, U.S. Patent Application Publication No. 2009/0038442, the disclosure of which is hereby incorporated by reference). Because of drawing scale, the adhesive material is not explicitly shown in  FIG. 15 , but it will be understood that the adhesive material is present between the bottom surface and the flattened surface. The adhesive material preferably has a substantially uniform thickness between the conforming bottom surface and floor surface. The hard plate  240  is sized such that its front edge  278  defines (or is coincident with, or is nearly coincident with) the leading edge  250  of the shirttail. The thickness of the plate  240  may range from 0.050 to 0.500 inches. The hard plate  240  is made of a material or combination of materials which are more abrasion resistant than the material used to make the leg and shirttail of the bit. In a preferred implementation, the hard plate is made of a material such as tungsten carbide, PDC, polycrystalline cubic boron nitride compactimpregnated diamond segment, and the like. These materials are superior to the traditional weld on tungsten carbide hardfacing known in the prior art because they are denser and are not as susceptible to abrasion and erosion. Again, the adhesive material is this implementation is not externally exposed and subject to possible wear. The conforming surfaces where adhesion takes place may curve, for example, with the radius of the bit, or have any selected curved configuration. 
     The hard plates  240  have a thickness t and width w (wherein the width is measured in a direction perpendicular to the leading edge  250 ). The hard plates  240  are thin inserts. In this case, a ratio of the thickness t of the plate to a width w of the plate is less than 0.5 (i.e., t/w&lt;0.5). More particularly, the ratio of the thickness t of the plate to the width w of the plate is substantially less than 0.5 (i.e., t/w&lt;&lt;0.5). Even more particularly, the ratio of the thickness t of the plate to the width w of the plate is less than 0.2 (i.e., t/w&lt;0.2), and may even be less than 0.1 (i.e., t/w&lt;0.1). 
     Reference is now made to  FIG. 16  which illustrates a cross-sectional view of a portion of a leg of a rotary cone drill bit which includes an embodiment of a protection mechanism for the leading edge of the shirttail. In this embodiment, the bottom surface  280  of the hard plate  244  is adhered to a substantially conforming floor surface  231  formed in or by the leading surface  254  of the leg  212  and extending outwardly from the leading shirttail edge  250 . The plate  244  is further defined by a rear edge  282  and two side edges  284  (see, also,  FIG. 13 ). The means for adhering the bottom surface to the floor surface may, for example, comprise any suitable adhering material which is interposed between the substantially conforming (for example, parallel) surfaces including adhesive material flowable between the substantially conforming surfaces by capillary action such as a brazing material, solder, adhesives, resins, and the like (see, for example, U.S. Patent Application Publication No. 2009/0038442, the disclosure of which is hereby incorporated by reference). Because of drawing scale, the adhesive material is not explicitly shown in  FIG. 16 , but it will be understood that the adhesive material is present between the bottom surface and the flattened surface on the leading side surface. The adhesive material preferably has a substantially uniform thickness between the conforming bottom surface and floor surface. The hard plate  244  is sized such that its front edge  288  defines (or is coincident with, or is nearly coincident with) the leading edge  250  of the shirttail. The thickness of the plate  244  may range from 0.050 to 0.500 inches. The hard plate  244  is made of a material or combination of materials which are more abrasion resistant than the material used to make the leg and shirttail of the bit. In a preferred implementation, the hard plate is made of a material such as tungsten carbide, PDC, polycrystalline cubic boron nitride compactimpregnated diamond segment, and the like. These materials are superior to the traditional weld on tungsten carbide hardfacing known in the prior art because they are denser and are not as susceptible to abrasion and erosion. Again, the adhesive material is this implementation is not externally exposed and subject to possible wear. The conforming surfaces where adhesion takes place may curve, for example, with the radius of the bit, or have any selected curved configuration. 
     The hard plates  244  have a thickness t and width w (wherein the width is measured in a direction perpendicular to the leading edge  250 ). The hard plates  244  are thin inserts. In this case, a ratio of the thickness t of the plate to a width w of the plate is less than 0.5 (i.e., t/w&lt;0.5). More particularly, the ratio of the thickness t of the plate to the width w of the plate is substantially less than 0.5 (i.e., t/w&lt;&lt;0.5). Even more particularly, the ratio of the thickness t of the plate to the width w of the plate is less than 0.2 (i.e., t/w&lt;0.2), and may even be less than 0.1 (i.e., t/w&lt;0.1). 
     It will be noted that the hard plates may be of any selected geometry thus allowing for the application of protection to complex surfaces of the bit. 
     The illustration of protection being applied using plates at the shirttail edge and/or leading shirttail edge is by way of example only, it being understood that the protection mechanisms described can be applied to any edge of the bit that are susceptible to wear. 
     Although preferred embodiments of the method and apparatus have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the spirit of the invention as set forth and defined by the following claims.