Abstract:
An earth-boring bit has a bit body, at least one cantilevered bearing shaft depending inwardly and downwardly from the bit body, and a cutter mounted for rotation on the bearing shaft. The cutter includes a plurality of teeth that are covered with a hardfacing layer. At least some of the teeth have a leading side that has a streamlined contour. The streamlined contour is generally conical in some of the embodiments. In others, the streamlined contour is defined by a corner between diverging inner and outer sections of the leading side.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to earth-boring drill bits and particularly to improved cutting structures for such bits. 
   BACKGROUND OF THE INVENTION 
   In drilling bore holes in earthen formations by the rotary method, rock bits fitted with one, two, or three rolling cutters are employed. The bit is secured to the lower end of a drill string that is rotated from the surface, or the bit is rotated by downhole motors or turbines. The cutters or cones mounted on the bit roll and slide upon the bottom of the bore hole as the bit is rotated, thereby engaging and disengaging the formation material to be removed. The rolling cutters are provided with cutting elements that are forced to penetrate and gouge the bottom of the borehole by weight of the drill string. The cuttings from the bottom of the borehole are washed away by drilling fluid that is pumped down from the surface through the hollow drill string. 
   The earliest rolling cutter, earth boring bits had teeth machined integrally from steel, earth disintegrating cutters. These bits, typically known as “steel tooth” or “milled tooth” bits, are used for penetrating the relatively soft geological formations of the earth. The strength and fracture toughness of steel teeth enables the aggressive gouging and scraping action that is advantageous for rapid penetration of soft formations with low compressive strengths. However the same cutting structure that drills sand formations fast, slows down considerably when it encounters shales. This is due in part to the shale sticking to the bit when it cannot be readily removed by the drilling fluid because of the chisel shape of the teeth and their location on the bit. 
   It has been common in the arts since at least the 1930s to provide a layer of wear-resistance metallurgical material called “hardfacing” over those portions of the steel teeth exposed to the severest wear. The hardfacing typically consists of extremely hard particles, such as sintered, cast, or macrocrystalline tungsten carbide dispersed in a steel matrix. Such hardfacing materials are applied by welding a metallic matrix to the surface to be hardfaced and applying the hard particles to the matrix to form a uniform dispersion of hard particle in the matrix. 
   Typical milled tooth bits have their teeth milled such that the inner and outer ends and leading and trailing flanks are fairly wide flat surfaces. The flat wide surfaces normal to the direction of rotation increase the tendency for the bit to ball up when sliding in shales. Typical hardfacing deposits are welded over a steel tooth that have a shape similar to the shape of the underlying tooth. 
   BRIEF SUMMARY OF THE INVENTION 
   An earth-boring bit has a bit body and at least one cantilevered bearing shaft depending inwardly and downwardly from the bit body. A cutter is mounted for rotation on each bearing shaft wherein each cutter includes a plurality of hardfaced teeth. At least some of the teeth have a leading side that has a streamlined contour. The leading side has an advance portion that leads inner and outer portions of the leading side. The advance portion has a narrow width compared to the base of the tooth. 
   In one embodiment, the streamlined contour is defined by making at least the leading portion of the tooth conical. The apex is rounded, and the trailing flank may be either conical or conventional in shape. Heel row teeth can be streamlined with a conical leading and inner side. The outer or gage side may remain flat. 
   In another embodiment, the streamlined contour is defined by providing the leading side with a leading edge. The leading edge is formed by the corner junction of inner and outer diverging sides, which may be flat. Preferably, the included angle of the corner junction is at least 90 degrees. 
   Also, at least one inner row may have teeth that incline in opposite directions. Each inclined tooth has a central axis that is inclined relative to an axis of rotation of the cone. Preferably, the inclined teeth alternate with each other, with half of the teeth inclining inward and the other half inclining outward. 
   The teeth of the various embodiments have a crest and a base. The crest may be rounded, as in the case of an apex of a conical contour, or it may be flat. Preferably, the crest is narrow compared to the base, having a width that is less than one-third the width of the base. 
   In manufacturing, tooth-stubs are machined on the cutter in the desired streamlined configuration. The tooth-stubs have a hardfacing on their surfaces that is a composition of carbide particles dispersed in a metallic matrix. Each tooth-stub and the hardfacing define one of the cutting elements of the cutter. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of an earth-boring bit of the steel tooth type constructed in accordance with this invention. 
       FIG. 2  is an enlarged perspective view of a heel row tooth of the earth-boring bit shown in FIG.  1 . 
       FIG. 3  is a cross sectional view, taken along the line  3 — 3  of  FIG. 2 , of the heel row tooth illustrated in FIG.  2 . 
       FIG. 4  is an enlarged perspective view of an inner row tooth of the earth-boring bit shown in FIG.  1 . 
       FIG. 5  is a cross sectional view, taken along the line  5 — 5  of  FIG. 4 , of the inner row tooth illustrated in FIG.  4 . 
       FIG. 6  is a front elevational view of an alternate embodiment of a tooth for the earth-boring bit shown in  FIG. 1 , the tooth being a three-sided pyramid in configuration. 
       FIG. 7  is a top plan view of the tooth of FIG.  6 . 
       FIG. 8  is a front elevational view of another alternate embodiment of a tooth for the earth boring bit of  FIG. 1 , the tooth being a four-sided pyramid in configuration. 
       FIG. 9  is a top plan view of the tooth of FIG.  8 . 
       FIG. 10  is a front elevation view of another alternate embodiment of a tooth for the earth boring bit of  FIG. 1 , the tooth having a leading side that is conical. 
       FIG. 11  is a top plan view of the tooth of FIG.  10 . 
       FIG. 12  is a schematic view of an alternate embodiment of an inner row of teeth for the earth boring bit of FIG.  1 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , an earth-boring bit  11  according to the present invention is illustrated. Bit  11  includes a bit body  13  having threads  15  at its upper extent for connecting bit  11  into a drill string (not shown). Each leg of bit  11  is provided with a lubricant compensator  17 . At least one nozzle  19  is provided in bit body  13  for directing pressurized drilling fluid from within the drill string to cool and lubricate bit  11  during drilling operations. At least one cutter  21  is rotatably secured to a leg of bit body  13 . Typically, each bit  11  has three cutters  21 , two of which are shown in FIG.  1  and another that is obscured from view in FIG.  1 . 
   Each cutter  21  has a shell surface including a gage surface  25 . Heel row teeth  29  are the outermost teeth and are located at the junction of the conical surface of cutter  21  and gage surface  25 . As shown in  FIGS. 2 and 3 , each heel row tooth  29  has an underlying support member  31 , or tooth-stub, that is machined from the conical surface of cutter  21 . A layer of hardfacing material  33  is welded over tooth-stub  31 . Hardfacing  33  typically consists of extremely hard particles, such as sintered, cast, or macrocrystalline tungsten carbide, dispersed in a steel matrix. Hardfacing materials  33  are typically applied by welding a metallic matrix to the surface to be hardfaced and applying the hard particles to the matrix to form a uniform dispersion of hard particle in the matrix. Each heel row tooth-stub  31  has an outer end  35  that is substantially flat and flush with gage surface  25 . Hardfacing  33  is applied to outer end  35  so that gage surface  25  is substantially continuous up the outer end of heel row tooth  29 , as illustrated in FIG.  1 . 
   In the embodiment shown in  FIGS. 2 and 3 , at least the leading portion of each heel row tooth  29  is shaped to be streamlined. The term “streamline” herein means a contour of a tooth constructed so as to offer minimum resistance to material flow. The leading side of the tooth is designed to provide less resistance than in the prior art to the flow of sticky shale and mud around the tooth as the tooth rotates and slides through the shale. The leading side is configured so that the flow vectors of the shale and mud do not make sharp turns as they pass the tooth. Generally that means that there will be little, if any, portion of the leading side that is flat and normal to the direction of rotation of the cutter. Preferably, all surfaces having any significant width on the leading side are at least 45° from a position facing into the direction of rotation. 
   In the embodiment of  FIGS. 2 and 3 , heel row tooth  29  is generally conical except for the flat outer end  35 . Rather than being elongated, the crest or apex  36  is rounded and dome-shaped. The leading and trailing flanks and the inner end, referenced as inner portion  37 , are rounded into the shape of a cone. Inner portion  37  forms a heel row tooth  29  that is thus partially conical in shape. The width or diameter of apex  36  is measured at the point of curvature from the sloping sides. The width or diameter of the base of tooth  29  is measured at the point where tooth  29  joins the supporting metal of cone  21 , and it is measured from outer end  35  to the inner portion  37 . The width of apex  36  is preferably less than one-third the width of the base. 
   The underlying support metal or tooth-stub  31  is formed in this partially conical shape. Hardfacing  33  is applied over tooth-stub  31 , typically, in a generally uniform thickness. The leading side of conical inner portion  37  has no flat areas that might impede the flow of viscous shale and drilling mud. 
   Referring again to  FIG. 1 , a plurality of inner row teeth  39  are formed on each cutter  21  radially inward from heel row teeth  29  up to the apex of cutter  21 . One of cutters  21  typically has a spear point (not shown) on its apex, another an inner row of teeth  39  (not shown) near its apex, and the third has a conical apex free of inner row teeth  39 . Each cutter  21  will have one or more rows of inner row teeth  39 . 
   Referring to  FIGS. 4 and 5 , at least some of the inner row teeth  39  have an underlying support metal or tooth-stub  41  that has a leading side with a streamlined configuration. Tooth-stub  41  is machined from the metal of cutters  21  and may have different shapes. In this embodiment, tooth-stub  41  is conical with a rounded apex  43 . The width of apex  43  is less than one-third the width of the base of tooth-stub  41 . A uniform hardfacing layer  45  is applied over tooth-stub  41 . The exterior of inner row tooth  39 , being conical, does not have any flat areas normal to the direction of rotation. 
   Referring to  FIG. 6 , tooth  47  is another embodiment of an inner row tooth. Tooth  47  has a configuration of a three-sided pyramid. Tooth  47  has a base  48  that is triangular, as shown in FIG.  7 . Three sides  49 ,  51  and  53 , each being triangular, lead to an apex  55 . Although apex  55  is shown as sharp, it could be truncated and rounded. If truncated or rounded, preferably the width of apex  55  will be less than one-third the width of base  48  of tooth  47 . Sides  49  and  51  form the leading side of tooth  47 , while side  53  trails, considering the direction of rotation or sliding indicated by the arrow. Sides  49 ,  51  are outer and inner portions, respectively, of the leading side. Sides  49 ,  51  intersect each other at an advance portion, the advance portion being a portion of tooth  47  that leads the remaining portions of tooth  47 . This advance portion comprises a leading edge or corner  57  defined by the intersection of outer and inner sides  49 ,  51 . Corner  57  is fairly sharp, thus has a width much smaller than the width of tooth  47 . Outer and inner sides  49 ,  51  are shown to be flat, but they could be curved, either concave or convex. The included angle  59  of corner junction  57  is preferably less than 90°, and in this embodiment it is 60°. Consequently, outer and inner sides  49 ,  51  are oriented 60° from the direction of rotation. Tooth  47  is hardfaced as in the other embodiments. 
   Referring to  FIGS. 8 and 9 , tooth  61  is another embodiment of an inner row tooth that has the shape of a pyramid. Tooth  61  has a rectangular base  62  and four sides  63 ,  65 ,  67  and  69 . Sides  63 ,  65  are on the leading side of tooth  61  considering the direction of rotation. Sides  67 ,  69  are on the trailing sides. Sides  63 ,  65 ,  67 ,  69  join each other at an apex  70 . Apex  70  could be rounded or truncated rather than sharp as shown. Also, its width will be less than one-third the width of base  62  if truncated or rounded. 
   Sides  63 ,  65  are the inner and outer portions, respectively, of the leading side of tooth  61 . Sides  63 ,  65  join each other at a corner junction  71 . Corner junction  71  is the advance portion of tooth  61  because it leads all the remaining portions. Corner  71  is defined by the intersection of the diverging inner and outer sides  63 ,  65 . In this embodiment the included angle  73  of corner junction  71  is 90°. Consequently, each inner and outer side  63 ,  65  is oriented 45° relative to the direction of rotation. Outer and inner sides  63 ,  65 , although shown to be flat, could be concave or convex to some extent. The width of corner  71  is very small compared to the width of base  62  from corner to the other corner. 
   In the embodiment of  FIGS. 10 and 11 , tooth  75  has a leading side  77  that is conical and a trailing side  79  that is a generally flat flank. The conical leading side  77  joins an outer side  81  and an inner side  83 , both of which are flat and parallel to the direction of rotation. The conical contour of leading side  77  is truncated, defining a flat crest  85 . Crest  85  preferably has a width that is less than one-third the width of the base of tooth  77 . The advance portion of leading side  77  is a center line  87  of conical leading side  77  that extends from the base to crest  85 . Preferably, leading side  77  extends a full 180° to junctions  89  with sides  81  and  83 . The angle  91  between advance center line  87  and each junction line  89  is 45°. Tooth  75  is also hardfaced in the same manner as the other embodiments. 
     FIG. 12  illustrates an inward inclined tooth  93  that is in an alternate embodiment row to one of the inner rows shown in FIG.  1 . Inward inclined tooth  93  has a central axis  95  that extends from its base to its apex. Axis  95  is located equidistant between an inner side  94  and outer side  96  of tooth  93 . Axis  95  is inclined or skewed relative to an axis of rotation rather than being in a plane perpendicular as in the prior art. Axis  95  inclines inward, and the row contains a number of similar inward inclined teeth  93 . 
   The same row contains a number of outward inclined teeth  97 . Each outward inclined tooth  97  has a central axis  99  that inclines also, but in an opposite direction from axis  95 . Each axis  99  is located equidistant between the inner and outer sides of outward inclined tooth  97 . The amount of inclination relative to a line that is perpendicular to the rotational axis may vary. 
   Preferably, each inward inclined tooth  93  alternates with one of the outward inclined teeth  97 . This results in a clearance between teeth  93 ,  97  that is parallel to the direction of rotation to facilitate the flow of sticky shales through teeth  93 ,  97  of the row. Teeth  93 ,  97  are shown schematically, and could be conventional. Alternately, they could have streamlined contours, similar to any of the embodiments above. Although teeth  93 ,  97  are shown schematically to have a base and a crest that are about the same width, the crest could be much smaller than the width of the base. As in the other embodiments, the crest could have a width less than one-third the width of the base of each tooth  93  and  97 . 
   The invention has significant advantages. Streamlined teeth as described facilitate better cuttings removal while maintaining an aggressive cutting structure. The particular shape for the teeth can vary depending on each drilling application. Not all of the inner teeth need to be the same shape. The shape of the heel row teeth can differ as well. Shapes other than conical or pyramidal are feasible. 
   While the invention has been shown in only a few of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.