Patent Publication Number: US-6209668-B1

Title: Earth-boring bit with improved cutting structure

Description:
RELATED APPLICATION 
     This application is a continuation-in-part of application Ser. No. 08/689,404, filed Aug. 6, 1996, now U.S. Pat. No. 5,819,861, which is a continuation-in-part of application Ser. No. 08/373,149, filed Jan. 17, 1995, now U.S. Pat. No. 5,542,485, Aug. 6, 1996, which is a continuation-in-part of application Ser. No. 08/293,228, filed Aug. 19, 1994, now U.S. Pat. No. 5,479,997, Jan. 2, 1996, which is a continuation of application Ser. No. 08/089,318, filed Jul. 8, 1993, now U.S. Pat. No. 5,351,768, Oct. 4, 1994. 
    
    
     TECHNICAL FIELD 
     The present invention generally relates to earth-boring drill bits. More particularly, the present invention relates to improved cutting structures or geometries for earth-boring drill bits. 
     BACKGROUND ART 
     The success of rotary drilling enabled the discovery of deep oil and gas reservoirs. The rotary rock bit was an important invention that made the success of rotary drilling possible. Only soft earthen formations could be penetrated commercially with the earlier drag bit, but the two-cone rock bit, invented by Howard R. Hughes, U.S. Pat. No. 930,759, drilled the caprock at the Spindletop field, near Beaumont, Tex. with relative ease. That venerable invention within the first decade of this century could drill a scant fraction of the depth and speed of the modern rotary rock bit. The original Hughes bit drilled for hours, the modern bit drills for days. Modern bits sometimes drill for thousands of feet instead of merely a few feet. Many advances have contributed to the impressive improvements in rotary rock bits. 
     In drilling boreholes in earthen formations by the rotary method, rotary rock bits having one, two, or three rolling cutters rotatably mounted thereon are employed. The bit is secured to the lower end of a drillstring that is rotated from the surface or by downhole motors or turbines. The cutters mounted on the bit roll and slide upon the bottom of the borehole as the drillstring is rotated, thereby engaging and disintegrating the formation material to be removed. The roller cutters are provided with teeth that are forced to penetrate and gouge the bottom of the borehole by weight from the drillstring. 
     The cuttings from the bottom and sides of the borehole are washed away by drilling fluid that is pumped down from the surface through the hollow rotating drillstring, and are carried in suspension in the drilling fluid to the surface. The form and location of the teeth or inserts upon the cutters have been found to be extremely important to the successful operation of the bit. Certain aspects of the design of the cutters become particularly important if the bit is to penetrate deep into a formation to effectively strain and induce failure in the formation material. 
     The current trend in rolling cutter earth-boring bit design is toward coarser, more aggressive cutting structures or geometries with widely spaced teeth or inserts. These widely spaced teeth prevent balling and increase bit speed through relatively soft, low compressive strength formation materials such as shales and siltstones. However, large spacing of heel teeth or inserts permits the development of large “rock ribs,” which originate in the corner and extend up the wall of the borehole. In softer, low compressive strength formations, these rock ribs form less frequently and do not pose a serious threat to bit performance because they are disintegrated easily by the deep, aggressive cutting action of even the widely spaced teeth or inserts. 
     In hard, high compressive strength, tough, and abrasive formation materials, such as limestones, dolomites and sandstones, the formation of rock ribs can affect bit performance seriously, because the rock ribs are not destroyed easily by conventional cutter action due to their inherent toughness and high strength. Because of the strength of these materials, tooth or insert penetration is reduced, and the rock ribs are not as easily disintegrated as in the softer formation materials. Rock ribs formed in high compressive strength, abrasive formation materials can become quite large, causing the cutter to ride up on the ribs and robbing the teeth or inserts of the unit load necessary to accomplish effective penetration and crushing of formation material. 
     Maintenance of the gage or diameter of the borehole and reduction of cutter shell erosion in hard, tough, and abrasive formations is more critical with the widely spaced tooth type of cutting structure, because fewer teeth or inserts are in contact with the borehole bottom and sidewall, and more of the less abrasion-resistant cutter shell surface can come into contact with the borehole bottom and sidewall. Rock ribs can contact and erode the cutter shell surface around and in between heel and gage inserts, sometimes enough to cause insert loss. Additionally, wear may progress into the shirttails of the bit, which protect the bearing seals, leading to decreased bearing life. 
     Provision of cutters with more closely spaced teeth or inserts reduces the size of rock ribs in hard, tough, and abrasive formations, but leads to balling, or clogging of cutting structure, in the softer formation materials. Furthermore, the presence of a multiplicity of closely spaced teeth or inserts reduces the unit load on each individual tooth and slows the rate of penetration of the softer formations. 
     As heel inserts wear, they become blunted and more of the cutter shell surface is exposed to erosion. Extensive cutter shell erosion leads to a condition called “rounded gage.” In the rounded gage condition, both the heel inserts and the cutter shell surface wear to conform generally to the contours of the corner of the borehole, and the gage inserts are forced to bear the entire burden of maintaining a minimum borehole diameter or gage. Both of these occurrences generate undesirable increase in lateral forces on the cutter, which lower penetration rates and accelerate wear on the cutter bearing and subsequent bit failure. 
     One way to minimize cutter shell erosion is to provide small, flat-topped compacts in the heel surface of the cutter alternately positioned between heel inserts, as disclosed in U.S. Pat. No. 3,952,815, Apr. 27, 1976, to Dysart. However, such flat-topped inserts do not inhibit the formation of rock ribs. The flat-topped inserts also permit the gage inserts to bear an undesirable proportion of the burden of maintaining minimum gage diameter. 
     U.S. Pat. No. 2,804,242, Aug. 27, 1957, to Spengler, discloses gage shaving teeth alternately positioned between heel teeth, the shaving teeth having outer shaving surfaces in the same plane as the outer edges of the heel teeth to shave the sidewall of the borehole during drilling operation. The shaving teeth are preferably one-half the height of the heel teeth, and thus function essentially as part of the primary heel cutting structure. In the rounded condition, the shaving teeth conform to the corner of the borehole, reducing the unit load on the heel teeth and their ability to penetrate and disintegrate formation material. The shaving teeth disclosed by Spengler are generally fragile and thus subject to accelerated wear and rapid rounding, exerting the undesirable increased lateral forces on the cutter discussed above. 
     A need exists, therefore, for an earth-boring bit having an improved ability to maintain an efficient cutting geometry as the bit encounters both hard, high-strength, tough and abrasive formation materials and soft, low-strength formation materials and as the bit wears during drilling operation. 
     DISCLOSURE OF INVENTION 
     A principal object of the present invention is to provide an earth-boring bit having an improved ability to maintain an efficient cutting geometry or structure as the earth-boring bit alternately encounters hard and soft formation materials and as the bit wears during drilling operation in borehole. 
     This and other objects of the present invention are achieved by providing an earth-boring bit having a bit body and at least one cutter rotatably secured to the bit body. The cutter has a cutter shell surface including a gage surface and a heel surface. A plurality of cutting elements inserts are arranged in generally circumferential rows on the cutter. At least one scraper cutting element is secured at least partially to the heel surface of the cutter. The scraper cutting element includes an outermost surface, generally aligned with the gage surface of the cutter, that defines a plow edge or point for shearing engagement with the sidewall of the borehole while redirecting cuttings up the borehole. 
     According to the preferred embodiment of the present invention, an outermost surface of the chisel-shaped insert is generally aligned with and projects beyond the gage surface. Alternatively, the outermost surface is relieved between about three and 15 degrees from the borehole wall. 
     Other objects, features, and advantages of the present invention will be apparent with reference to the figures and detailed description of the preferred embodiment, which follow. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a perspective view of an earth-boring bit according to the present invention. 
     FIGS. 2A through 2C are fragmentary, longitudinal section views showing progressive wear of a prior-art earth-boring bit. 
     FIGS. 3A through 3C are fragmentary, longitudinal section views of the progressive wear of an earth-boring bit according to the present invention. 
     FIG. 4 is an enlarged view of a scraper cutting element in contact with the sidewall of the borehole. 
     FIGS. 5A and 5B are plan and side elevation views, respectively, of the preferred scraper cutting element of FIG.  4 . 
     FIG. 6 is a fragmentary section view of a portion of the earth-boring bit according to the present invention in operation in a borehole. 
     FIG. 7 is a perspective view of an earth-boring bit according to the present invention. 
     FIG. 8 is a fragmentary section view of the earth-boring bit of FIG. 7, depicting the relationship of the cutting elements of the cutters of the bit on the bottom of the borehole. 
     FIG. 9 is a fragmentary section view of an earth-boring bit according to the present invention embodying a variation of the invention illustrated in FIGS. 7 and 8. 
     FIG. 10 is a fragmentary section view of a milled- or steel-tooth bit according to the preferred embodiment of the present invention. 
     FIG. 11 is a plan view of a cutting element according to the preferred embodiment of the present invention. 
     FIG. 12 is an elevation view of the cutting element of FIG.  11 . 
     FIG. 13 is a fragmentary view, partially in section, of the cutting element of FIGS. 11 and 12 in drilling operation. 
     FIG. 14 is a plan view of a cutting element according to the preferred embodiment of the present invention. 
     FIG. 15 is an elevation view of the cutting element of FIG.  14 . 
     FIGS. 16A and 16B are fragmentary, longitudinal section views of a typical cutting element and an improved cutting element according to the invention, respectively. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, an earth-boring bit  11  according to the present invention is illustrated. Bit  11  includes a bit body  13 , which is threaded at its upper extent  15  for connection into a drillstring. Each leg of bit  11  is provided with a lubricant compensator  17 , a preferred embodiment of which is disclosed in U.S. Pat. No. 4,276,946, Jul. 7, 1981, to Millsapps. At least one nozzle  19  is provided in bit body  13  to spray drilling fluid from within the drillstring to cool and lubricate bit  11  during drilling operation. Three cutters  21 ,  23 ,  25  are rotatably secured to each leg of bit body  13 . Each cutter  21 ,  23 ,  25  has a cutter shell surface including a gage surface  31  and a heel surface  41 . 
     A plurality of cutting elements, in the form of hard metal inserts, are arranged in generally circumferential rows on each cutter. Each cutter  21 ,  23 ,  25  has a gage surface  31  with a row of gage elements  33  thereon. A heel surface  41  intersects each gage surface  31  and has at least one row of heel cutting elements  43  thereon. 
     At least one scraper element  51  is secured to the cutter shell surface at the intersection of or generally circular juncture between gage and heel surfaces  31 ,  41  and generally intermediate a pair of heel cutting elements  43 . Preferably, a scraper cutting element  51  is located between each heel cutting element  43 , in an alternating arrangement. As is more clearly illustrated in FIGS. 4-5B, scraper element  51  comprises a generally cylindrical body  53 , which is adapted to be received in an aperture in the cutter shell surface at the intersection of gage and heel surfaces  31 ,  41 . Preferably, scraper element  51  is secured within the aperture by an interference fit. Extending upwardly from generally cylindrical body  53  are a pair of element surfaces  55 ,  57 , which converge to define a cutting edge  59 . Preferably, cutting edge  59  is oriented circumferentially, i.e., normal to the axis of rotation of each cutter  21 ,  23 ,  25 . 
     As is more clearly depicted in FIGS. 3A-3C, scraper cutting element is secured to the cutter shell surface such that one of scraper surfaces  55 ,  57  defines a gage element surface that extends generally parallel to the sidewall ( 205  in FIG. 3A) of the borehole. Another of scraper element surfaces  55 ,  57  defines a heel element surface. 
     As depicted in FIG. 4, scraper cutting element  51  is oriented such that gage scraper surface  57  is generally aligned with and projects beyond gage surface  31 . It is contemplated that surface  57  may be relieved away from the sidewall of the borehole a clearance angle α between three and 15 degrees. Relieving surface  57  decreases engagement between scraper cutting element  51  and the sidewall of the borehole, which may reduce the ability of scraper  51  to protect gage surface  31  against abrasive wear. However, it is believed that the reduction in frictional engagement between scraper  51  and the sidewall more than compensates for the reduction in abrasion resistance. 
     FIGS. 2A-2B are fragmentary, longitudinal section views of the cutting geometry of a prior-art earth-boring bit, showing progressive wear from a new condition to the “rounded gage” condition. The reference numerals in FIGS. 2A-2C that begin with the numeral  1  point out structure that is analogous to that illustrated in earth-boring bit  11  according to the present invention depicted in FIG. 1, e.g., heel tooth or cutting element  143  in FIG. 2A is analogous to heel cutting element  43  depicted in FIG. 1, heel surface  141  in FIG. 2A is analogous to heel surface  41  depicted in FIG. 1, etc. 
     FIG. 2A depicts a prior-art earth-boring bit in a borehole. FIG. 2A depicts the prior-art earth-boring bit in a new or unworn condition, in which the intersection between gage and heel surfaces  131 ,  141  is prominent and does not contact sidewall  205  of borehole. The majority of the teeth or cutting elements engage the bottom  201  of the borehole. Heel teeth or elements  143  engage corner  203  of the borehole, which is generally defined at the intersection of sidewall  205  and bottom  201  of borehole. Gage element  133  does not yet engage sidewall  205  of the borehole to trim the sidewall and maintain the minimum gage diameter of the borehole. 
     FIG. 2B depicts the prior-art earth-boring bit of FIG. 2A in a moderately worn condition. In the moderately worn condition, the outer end of heel tooth or element  143  is abrasively worn, as is the intersection of gage and heel surfaces  131 ,  141 . Abrasive erosion of heel tooth or element  143  and gage and heel surfaces  131 ,  141  of cutter shell causes the earth-boring bit to conform with corner  203  and sidewall  205  of the borehole. Thus, gage element  133  cuts into sidewall  205  of the borehole to maintain gage diameter in the absence of heel inserts&#39;  143  ability to do so. Sidewall of borehole  205  is in constant conforming contact with the cutter shell surface, generally at what remains of the intersection between gage and heel surfaces  131 ,  141 . These two conditions cause the cutters of the prior-art earth-boring bit to be increasingly laterally loaded, which accelerates bearing wear and subsequent bit failure. 
     FIG. 2C illustrates the prior-art earth-boring bit of FIGS. 2A and 2B in a severely worn, or rounded gage, condition. In this rounded gage condition, the outer end of heel tooth or element  143  is severely worn, as is the cutter shell surface generally in the area of the intersection of gage and heel surfaces  131 ,  141 . Moreover, because severely worn heel tooth or element  143  is now incapable of cutting and trimming sidewall of  205  of the wellbore to gage diameter, gage element  133  excessively penetrates sidewall  205  of the borehole and bears the bulk of the burden in maintaining gage, a condition for which gage element  133  is not optimally designed, thus resulting in inefficient gage cutting and lower rates of penetration. Thus, the conformity of the cutter shell surface with corner  203  and sidewall  205  of the borehole, along with excessive penetration of sidewall  205  of the borehole by gage element  133 , are exaggerated over that shown in the moderately worn condition of FIG.  2 B. Likewise, the excessive lateral loads and inefficient gage cutting also are exaggerated. Furthermore, excessive erosion of the cutter shell surface may result in loss of either gage element  133  or heel element  143 , clearly resulting in a reduction of cutting efficiency. 
     FIGS. 3A-3C are fragmentary, longitudinal section views of earth-boring bit  11  according to the present invention as it progressively wears in a borehole. FIG. 3A illustrates earth-boring bit  11  in a new or unworn condition, wherein the majority of the teeth or elements engage bottom  201  of the borehole. Heel elements or teeth  43  engage corner  203  of the borehole. As more clearly illustrated in FIG. 4, one of scraper element surfaces  57  defines a gage element surface  57  that extends generally parallel to sidewall  205  of the borehole. Another of scraper element surfaces  55 ,  57  defines a heel element surface  55  that defines a negative rake angle β with respect to sidewall  205  of the borehole. 
     Scraper element  51  is constructed of a material having greater wear-resistance than at least gage and heel surfaces  31 ,  41  of the cutter shell surface. Thus, the gage element surface of scraper element  51  protects gage surface  31  from severe abrasive erosion resulting from contact with sidewall  205  of the borehole. Likewise, the heel element surface of scraper element  51  protects heel surface  41  from abrasive erosion resulting from contact with corner  203  of the borehole. Scraper element  51  also inhibits formation of rock ribs between adjacent heel cutting elements  43 . Cutting edge  59  creates a secondary corner  207  and kerfs nascent rock ribs, disintegrating them before they can detract from efficient drilling. 
     FIG. 3B depicts earth-boring bit  11  in a moderately worn condition in which the outer end of heel tooth or element  43  is worn. However, scraper element  51  has prevented a great deal of the cutter shell erosion at the intersection of gage and heel surfaces  31 ,  41 , and still functions to form a secondary corner, thereby maintaining a clearance between gage element  33  and sidewall  205  of the borehole, and avoiding conformity. Thus, the presence of scraper element  51  promotes cutting efficiency and deters rapid abrasive erosion of the cutter shell surface. 
     FIG. 3C illustrates earth-boring bit  11  according to the present invention in a severely worn condition in which the outer end of heel tooth or element  43  is severely worn and the cutter shell surface is only moderately eroded. By preventing excessive cutter erosion, conformity of the cutter shell surface with sidewall  205  of the borehole is greatly reduced, along with the attendant increased lateral loads on cutters  21 ,  23 ,  25  and inefficient cutting by gage element  33 . Only in this most severely worn condition, where heel elements  43  are extremely worn, do gage elements  33  actively cut sidewall  205  of borehole. 
     FIGS. 5A and 5B are enlarged elevation and plan views of a preferred scraper element  51  according to the present invention. Scraper element  51  is formed of a hard metal such as cemented tungsten carbide or similar material having high hardness and abrasion-resistance. As stated before, upon installation of scraper element  51  by interference fit in an aperture generally at the intersection of gage and heel surfaces  31 ,  41 , one of scraper element surfaces  55 ,  57  will define a gage element surface, and the other of scraper element surfaces  55 ,  57  will define a heel element surface. The gage element and heel element surfaces  55 ,  57  converge at a right angle to define a circumferentially oriented cutting edge  59  for engagement with sidewall  205  of the borehole. Preferably, the radius or width of cutting edge  59  is less than or equal to the depth of penetration of cutting edge  59  into formation material of the borehole as bit  11  wears or rock ribs form. 
     Efficient cutting by scraper element  51  requires maintenance of a sharp cutting edge  59 . Accordingly, one of scraper element surfaces  55 ,  57  preferably is formed of a more wear-resistant material than the other of surfaces  55 ,  57 . The differential rates of wear of surfaces  55 ,  57  results in a self-sharpening scraper element  51  that is capable of maintaining a sharp cutting edge  59  over the drilling life of earth-boring bit  11 . The more wear-resistant of scraper elements surfaces  55 ,  57  may be formed of a different grade or composition of hard metal than the other, or could be formed of an entirely different material such as polycrystalline diamond or the like, the remainder of the element being a conventional hard metal. In any case, scraper element  51  should be formed of a material having a greater wear-resistance than the material of the cutter shell surface, which is usually steel, so that scraper element  51  can effectively prevent erosion of the cutter shell surface at the intersection of gage and heel surfaces  31 ,  41 . 
     In addition to, and perhaps more important than its protective function, scraper element  51  serves as a secondary cutting structure. The cutting structure is described as “secondary” to distinguish it from primary cutting structure such as heel elements  43 , which have the primary function of penetrating formation material to crush and disintegrate the material as cutters  21 ,  23 ,  25  roll and slide over the bottom of the borehole. 
     As described above, bits  11  having widely spaced teeth are designed to achieve high rates of penetration in soft, low compressive strength formation materials such as shale. Such a bit  11 , however, is expected to encounter hard, tough, and abrasive streaks of formation material such as limestones, dolomites, or sandstones. Addition of primary cutting structure, like heel elements  43  or the inner row inserts, assists in penetration of these hard, abrasive materials and helps prevent cutter shell erosion. But, this additional primary cutting structure reduces the unit load on each tooth or insert, drastically reducing the rate of penetration of bit  11  through the soft material it is designed to drill. 
     To insure that scraper element  51  functions only as secondary cutting structure, engaging formation material only when heel elements  43  are worn, or when large rock ribs form while drilling a hard, abrasive interval, the amount of projection of cutting edge  59  from heel surface  41  must be kept within certain limits. Clearly, to avoid becoming primary structure, cutting edge  59  must not project beyond heel surface  41  more than one-half the projection of heel element  43 . Further, to insure that scraper element  51  engages formation material only when large rock ribs form, the projection of cutting edge  59  must be less than 30% of the pitch between the pair of heel teeth that scraper element  51  is secured between. Pitch describes the distance or spacing between two teeth in the same row of an earth-boring bit. Pitch, in this case, is measured as the center-to-center linear distance between the crests of any two adjacent teeth in the same row. 
     The importance of this limitation becomes apparent with reference to FIG. 6, which depicts a fragmentary view of a portion of an earth-boring bit  11  according to the present invention operating in a borehole. FIG. 6 illustrates the manner in which heel elements  43  penetrate and disintegrate formation material  301 . Heel teeth  43  make a series of impressions  303 ,  305 ,  307  in formation material  301 . By necessity, there are buildups  309 ,  311  between each impression. Buildups  309 ,  311  are expected in most drilling, but in drilling hard, abrasive formations with bits having large-pitch, or widely spaced, heel elements  43 , these buildups can become large enough to detract from bit performance by engaging the cutter shell surface and reducing the unit load on each heel element  43 . 
     Projection P of heel elements  43  from heel surface provides a datum plane for reference purposes because it naturally governs the maximum penetration distance of heel elements  43 . Buildup height BH is measured relative to each impression  303 ,  305 ,  307  as the distance from the upper surface of the buildup to the bottom of each impression  303 ,  305 ,  307 . Cutter shell clearance C is the distance between the heel surface  41  and the upper surface of the buildup of interest. As stated above, it is most advantageous that clearance C be greater than zero in hard, tough, and abrasive formations. It has been determined that buildup height BH is a function of pitch and generally does not exceed approximately 30% of the pitch of heel elements  43 , at which point clearance C is zero and as a reduction in unit load on heel elements  43  and cutter erosion occur. 
     Thus, to avoid functioning as a primary cutting structure, scraper element  51  should not engage formation material until buildup  309  begins to enlarge into a rock rib or the depth of cut approaches projection P of heel elements  43 , wherein clearance C approaches zero. This is accomplished by limiting the projection of cutting edge  59  from heel surface  41  to an amount less than 30% of the pitch of the pair of heel elements  43  between which scraper element  51  is secured. 
     For example, for a 12¼ inch bit having a pitch between two heel elements  43  of 2 inches, and heel elements  43  having a projection P of 0.609 inch, scraper elements  51  have a projection of 0.188 inch, which is less than one-half (0.305 inch) projection P of heel elements  43  and 30% of pitch, which is 0.60 inch. In the case of extremely large heel pitches, i.e. greater than 2 inches, it may be advantageous to place more than one scraper element  51  between heel elements  43 . 
     FIG. 7 is a perspective view of an earth-boring bit  11  according to the preferred embodiment of the present invention. Bit  11  is generally similar to that described in connection with FIG. 1, but with the addition of a row of chisel-shaped cutting elements  61  secured to gage surface  31  of each cutter  21 ,  23 ,  25 . As is seen, each chisel-shaped cutting element  61  is formed similarly to scraper element  51  described above, but is positioned on gage surface  31 , rather than at the intersection or generally circular juncture of gage  31  and heel  41  surfaces. Preferably, chisel-shaped cutting elements  61  alternate with scraper cutting elements  51  to provide staggered rows of secondary and tertiary cutting structure. 
     As described in greater detail with reference to FIG. 8, each chisel-shaped cutting element  61  is surrounded by a generally circular counterbore  63 , which serves to provide an area around cutting element  61  that facilitates movement of cuttings and abrasive fines around cutting element  61  and up the borehole. Preferably, chisel-shaped cutting elements  61  are tilted toward heel surface  41  such that they are oriented in the direction of cut or advance of each cutter  21 ,  23 ,  25  as it rolls and slides on the bottom of the borehole. 
     FIG. 8 is a fragmentary section view of earth-boring bit  11  of FIG. 7 illustrating the superimposition of the various cutting elements on cutters relative to one another and to the bottom of the borehole. Inner row cutting elements are illustrated in hidden lines to emphasize the secondary cutting structure including scraper  51  and chisel-shaped cutting elements  61 . Scraper cutting element  51  is formed and positioned as described above. 
     Preferably, chisel-shaped cutting elements  61  have a cylindrical base interference fit in apertures in gage surface  31 . Chisel-shaped cutting elements  61  are formed similarly to scraper elements  51  and include a pair of surfaces  65 ,  67  converging to define a cutting edge or crest  69 . Surfaces  65 ,  67  are formed to be self-sharpening as described above with respect to scraper element  51 . Crest  69  is oriented circumferentially or transversely to the axis of rotation of cutters  21 ,  23 ,  25 . Cutting elements  61  and their axes are tilted toward heel surface  41  and away from backface  27  of cutters  21 ,  23 ,  25  to orient cutting elements  61  and crests  69  in the direction of advance of cutters  21 ,  23 ,  25  as they scrape the wall of the borehole. Cutting elements  61  and crests  69  are tilted such that a line drawn through the centers of cutting elements  61  and their crests  69  define an acute angle of between about 15 and 75 degrees with gage surface  31 , preferably 45 degrees, as illustrated. 
     The cutting mechanics of chisel-shaped cutting elements  61  are similar to those of scraper cutting elements  51 , but the cutting action is concentrated on the sidewall of the borehole, rather than at the corner. Chisel-shaped cutting elements  61  thus provide an aggressive tertiary cutting structure on gage surface  31 . According to one embodiment of the present invention, an outermost  67  of the surfaces of chisel-shaped element  61  is generally aligned with or parallel to gage surface  31  and projects beyond it. This configuration, in combination with counterbore  63 , provides effective scraping of the borehole wall by cutters  21 ,  23 ,  25 . 
     FIG. 9 is fragmentary section view, similar to FIG. 8, illustrating a variation of the cutting structure described in connection with FIGS. 7 and 8. In this variation, two rows of chisel-shaped cutting elements  61  are provided on gage surface  31 . Each row of chisel-shaped cutting elements is substantially similar to the single row described with reference to FIGS. 7 and 8. However, the second row of chisel-shaped cutting elements is closer to backface  27  of cutters  21 ,  23 ,  25 , and again provides an aggressive secondary and tertiary cutting structure on gage surface  31 . Additionally, outermost surfaces  67  of chisel-shaped cutting elements  61  are relieved between three and 15 degrees from the sidewall of the borehole to minimize frictional engagement therebetween and enhance the aggressiveness of the scraping action. 
     FIG. 10 is a fragmentary section view, similar to FIGS. 8 and 9, depicting an arrangement of chisel-shaped cutting elements  61  on a gage surface  31 ′ of a milled- or steel-tooth bit, in which the cutting elements, such as heel teeth  43 ′, are formed of the material of cutters  21 ,  23 ,  25  and hard faced to increase their wear resistance. In such a bit, gage surface  31 ′ can be considered to extend from backface  27 ′ of each cutter  21 ,  23 ,  25  to nearly the tips of heel teeth  43 ′. 
     Chisel-shaped cutting elements  61  again are secured to gage surface  31 ′ and tilted toward heel surface  41 ′ and are surrounded by counterbores  63 ′ to provide clearance for passage of cuttings and abrasive fines around chisel-shaped cutting elements  61 . Chisel-shaped cutting elements  61  are arranged in two rows, one being nearer and generally coinciding with the circular juncture between gage  31 ′ and heel  41 ′ surfaces, the other being nearer the cutter backface. In the row nearer the intersection between gage  31 ′ and heel  41 ′ surfaces, counterbore  63  extends into a heel tooth  43 ′. Like the arrangement illustrated in FIG. 8, the outermost  65  surfaces of chisel-shaped cutting elements  61  are aligned with and project beyond gage surface  31 . 
     FIGS. 11 and 12 are plan and elevation views, respectively, of a scraper cutting element  551  according to a preferred embodiment of the present invention. Scraper element  551  comprises a cylindrical body  553  formed of a hard metal such as cemented tungsten carbide. A cutting end extends from cylindrical body  553  and comprises a pair of flanks  555 , which converge to define a crest. According to the preferred embodiment of the present invention, an outermost surface  557  is formed by grinding or otherwise forming a generally flat surface at the outermost portion of element  551 . Outermost surface  557  preferably is formed at approximately 45° from vertical. Because the basic element is chisel-shaped, the intersection of outermost surface  557  is triangular or wedge-shaped. The intersection of outermost surface  557  with the crest defined by flanks  555  defines a plow point or edge  559 , which takes the form of a circular radius. In other configurations, plow point  559  could comprise a sharp corner or a chamfered point, as described in commonly assigned U.S. Pat. No. 5,346,026, Sep. 13, 1994 to Pessier et al. The edges of outermost surface  557  diverge at 45° from plow point  559  to permit flow of cuttings and material away from plow point  559  and cutting element  551 , as described more fully below. 
     According to the present invention, scraper cutting element  551  is secured to the cutter at the generally circular juncture between gage and heel surfaces  31 ,  41  such that outermost surface  557  is generally aligned with gage surface  31 . Outermost surface  557  may also be relieved between about three and about 15 degrees, such that it is not in parallel alignment with gage surface  31 . Alternatively, scraper insert  551  can also be secured to heel surface  41  to act as a more conventional heel element, but outermost surface  557  should still be generally aligned with gage surface  31 . 
     FIG. 13 is a fragmentary view, partially in section, of the cutting element of FIGS. 11 and 12 during drilling operation. As can be seen, upon shearing engagement with the sidewall of the borehole, cuttings are generated by the shearing action of plow point or edge  559  and outermost surface  557 . Because of the divergence of the edges of outermost surface  557  from plow point or edge  559 , cuttings and formation material move away from and around plow point or edge  559  and cutting element  551 , moving up the borehole freely. This action prevents packing of the cuttings in front of a broad or wide cutting edge, which can lead to balling of the cutting element and bit. 
     FIGS. 14 and 15 are plan and elevation views, respectively, of an alternative embodiment of a scraper cutting element  651  according to the present invention. In this embodiment, cutting element body  653  is a cylinder of hard metal, which is truncated at an angle to define an elliptical outermost surface  657  and a plow point or edge  659  at its uppermost extent. As with the embodiment of FIGS. 11 and 12, the edges or sides of outermost surface  657  diverge from plow point or edge  659  to provide for removal of cuttings or formation material. According to the preferred embodiment of the present invention, at least plow point  659  and a portion of outermost surface  657  are formed of super-hard material, such as polycrystalline diamond to enhance the wear-resistance of cutting elements  651 . 
     FIGS. 16A and 16B are fragmentary, longitudinal section views of earth-boring bits  800  and  900 , respectively. FIG. 16A illustrates earth-boring bit  800  wherein the teeth or elements are positioned in a typical arrangement. In particular, heel elements or teeth  843  are positioned at a 25 degree angle of deviation from true rolling (DTR). The DTR line is defined as a line connecting the apexes of the first two inner rows or the third and fourth row of teeth from the gage or corner of the borehole. By positioning teeth  843  at a 25 degree DTR, the outside of heel element base  845  is a sufficient distance away from the gage surface  857  to permit enough steel to be present between the outside of heel element base  845  and the gage surface  857  of the cutter to provide adequate retention of the tooth. 
     Heel elements or teeth  843  engage corner  849  of the borehole. Gage surface  857  extends generally parallel to sidewall  859  of the borehole. A first step  861  is formed by scraper cutter element  863 . Scraper cutter element  863  has a outermost surface  865  that defines a plow point  867  for shearing engagement with the sidewall  859  of the borehole. Plow point  867  forms a first step  861  in the sidewall  859  of the borehole. 
     FIG. 16B illustrates earth-boring bit  900  wherein the teeth or elements are positioned in an improved arrangement. In particular, heel elements or teeth  943  are positioned at an angle of less than 15 degrees, but preferably at a 12.5 degree angle of deviation from true rolling (DTR). By positioning teeth  943  at a 12.5 DTR, the outside of heel element base  945  is a sufficient distance away from the gage surface  957  to permit enough steel to be present between the outside of heel element base  945  and the gage surface  957  of the cutter to provide retention of the tooth. 
     Heel elements or teeth  943  engage corner  949  of the borehole. The first scraper cutting element  963  defines a first element surface  965  that extends generally parallel to sidewall  997  of the borehole. A first step  969  is formed by scraper cutter element  963 . Scraper cutter element  963  has an outermost surface  971  that defines a plow point  973  for shearing engagement with the sidewall  967  of the borehole. 
     A second scraper cutting element  975  defines a second element surface  977  that extends generally parallel to sidewall  957  of the borehole. A second step  979  is formed by second scraper cutter element  975 . Second scraper cutter element  975  has an outermost surface  983  that defines second plow point  985  for shearing engagement with the sidewall  967  of the borehole. Scraper cutter elements  975  and  985  may be wedge shaped, wherein the plow points  973 ,  985 , are a radius. Additionally, scraper cutter elements  963 ,  975  may be elliptical wherein plow points  973 ,  985  are a radius. Further, scraper cutter elements  963 ,  975  may be a three sided pyramid. 
     Gage element  949  forms a third step  991  on sidewall  967 . Gage element  949  has an outermost surface  993 . 
     By providing a stepped profile formed by first scraper cutter element  985 , second scraper cutter element  975 , and by gage element  949 , heel element  943 , may be positioned at a smaller DTR angle as discussed above. 
     With reference now to FIGS.  1  and  3 A- 16 B, the operation of improved earth-boring bit  11  according to the present invention will be described. Earth-boring bit  11  is connected into a drillstring (not shown). Bit  11  and drillstring are rotated in a borehole causing cutters  21 ,  23 ,  25  to roll and slide over bottom  201  of the borehole. The elements or teeth of cutters  21 ,  23 ,  25  penetrate and crush formation material, which is lifted up the borehole to the surface by drilling fluid exiting nozzle  19  in bit  11 . 
     Heel elements or teeth  43  and gage elements  33  or chisel-shaped cutting elements  61  cooperate to scrape and crush formation material in corner  203  and on sidewall  205  of the borehole, thereby maintaining a full gage or diameter borehole and increasing the rate of penetration of bit  11  through formation material. Scraper elements  51 , being secondary cutting structure, contribute to the disintegration of hard, tough, and abrasive intervals when the formation material forms enlarged rock ribs extending from corner  203  up sidewall  205  of the borehole. During drilling of the softer formation materials, scraper elements make only incidental contact with formation material, thus avoiding reduction in unit load on primary cutting structure such as heel elements  43 . 
     As heel elements or teeth  43  wear, scraper elements  51  become engaged, protect the cutter shell surface from abrasive erosion and conformity with sidewall  205  of the borehole, and cooperate in the efficient cutting of sidewall  205  of the borehole by gage elements  33  or chisel-shaped cutting elements  61 . Thus, earth-boring bit  11  according to the present invention is less susceptible to the rounded gage condition and the attendant increased lateral loading of cutters  21 ,  23 ,  25 , inefficient gage cutting, and resulting reduced rates of penetration. 
     Additionally, chisel-shaped cutting elements  61  on gage surface  31 , oriented in the direction of cut, aggressively cut formation material at the sidewall of the borehole, giving full coverage or redundance in the difficult task of generating the borehole wall. 
     The principal advantage of the improved earth-boring bit according to the present invention is that it possesses the ability to maintain an efficient and effective cutting geometry over the drilling life of the bit, resulting in a bit having a higher rate of penetration through both soft and hard formation materials, which results in more efficient and less costly drilling. 
     The invention is described with reference to a preferred embodiment thereof. The invention is thus not limited, but is susceptible to variation and modification without departing from the scope and spirit thereof.