Asymmetric gage insert for an earth boring apparatus
This is a continuation, of application Ser. No. 671,535, filed Mar. 29, 1976 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to the art of earth boring and more particularly to a cutting element for an earth boring apparatus. 
     Earth boring apparatus having hard insert elements mounted in a cutter member body are utilized in the boring of holes in the earth because of the ability of the hard insert cutting elements to penetrate the earth formations. A problem has been encountered with this type of apparatus, and generally with all earth boring apparatus, in maintaining the desired diameter or gage of the hole being bored. This is important in the boring of raise holes and tunnels as well as being critically important in the drilling of oil and gas wells and the like. For example, in the drilling of a deep well wherein more than one bit will be used in the well, the gage cutting inserts must maintain the hole at the full diameter. Otherwise, it would be necessary for the next bit being lowered into the hole to ream the undersized hole out to the desired diameter before the new bit reached drilling depth and could begin drilling its length of assigned hole. Such reaming action would reduce the useful lifetime of the second bit because by the time the second bit reached its assigned drilling depth, a substantial part of the lifetime of the gage cutting elements would be exhausted. 
     The inserts in the gage row are exposed to the most rigorous drilling. They must drill a larger area of the hole. In addition, the formation outwardly of the gage row of inserts is not being drilled and hence provides some degree of lateral support for the formation being drilled by the gage row. It will be appreciated that an improvement in the ability of an earth boring apparatus to maintain gage will be an improvement of the entire earth boring apparatus and contribute significantly to the performance efficiency, economy, and life of the earth boring apparatus. 
     BRIEF DESCRIPTION OF PRIOR ART 
     In the prior art the accepted method of determining the exact bit diameter was to grind the outer or gage surface of the gage compact. This would produce a flat on the surface of the gage insert. The flat would contact with the wall of the hole. It is impractical to grind the outer or gage surface of the existing gage inserts to the extent necessary to contact the wall of the hole with the majority of the length of their extended surfaces. In addition, the grinding of the inserts reduces the overall strength of the insert. 
     In U.S. Pat. No. 3,442,342 to F. H. McElya and R. A. Cunningham patented May 6, 1969 a specially shaped insert for compact rock bits and rolling cutters and rock bits using such inserts is shown. The original inserts of cemented tungsten carbide had hemispherical cutting tips, and rock bits using such inserts were used to drill the hardest abrasive formations, such as taconite, bromide, and chert. This shape is not particularly effective for the drilling of abrasive formations of medium hardness, e.g., hard shales, dolomite, and some limestones, and the inventors herein have developed inserts with more of a chisel or wedge shape to cut such rock. At the same time, they avoid the pitfalls of the &#34;roof-top&#34; style of cutting tip, one in which there are two flanks with flat surfaces converging to a flat crest. 
     Two basic shapes of cutting tips are disclosed: (1) a modified chisel with convex flanks converging to a crest which is convex along both its elongated lengths and its uniform narrow width, the flanks being normal to a common plane passing through the axis of the insert so that their projected intersection is a curve normal to such axis: and (2) a wedge shape in which the flanks are twisted or canted away from each other so that there is no single plane through the insert axis which is normal to both flanks and the projected intersection is not normal to the axis, the result being that the crest formed normal to the axis increases in width from one end to the other. 
     In all forms rounded intersections are provided to avoid the sharp corners and sharp edges which cause high-stress concentration. The inventor&#39;s theory is that their rounding and their convex surfaces distribute the operating load over the cutting edge of the insert and direct such load to the center of the insert, thus avoiding the high-stress at the edges which they believe to be responsible for the shipping and breaking of roof-top inserts. 
     In U.S. Pat. No. 2,990,025 to M. L. Talbert and W. E. Scarborough patented June 27, 1961 an improved arrangement of wear-resistant inserts to maintain the hole being drilled at gage is shown. A first circumferential row of gage cutting wear-resistant inserts is situated at the heel of the cutter. A second circumferential row of wear-resistant inserts is spaced inwardly of the first row toward the longitudinal axis of the head with the spacing between the first and second rows being such that the track of the second row on the bottom of the hole being drilled overlaps the track of the first row. The first row is situated at a substantially zero oversized angle and the second row is situated at a larger oversized angle than is the first row so that the second row effects disintegration of the earthen formations closely adjacent the wall of the hole at a level below the first row, whereby the formation to be disintegrated by the first row is left without substantial inner lateral support thereby facilitating cutting the hole to gage by the first row. 
     In U.S. Pat. No. 3,800,891 to A. D. White and A. E. Wisler patented Apr. 2, 1974 a hardfacing composition and gage hardfacing on rolling cutter rock bits is shown. This patent relates to a tooth-type bit rather than an insert bit, however, the patent points out the importance of maintaining the proper gage. Beginning at column 1, line 49, the importance of maintaining gage is discussed as follows &#34;the importance of such gage maintaining function in an oil well can scarcely be exaggerated. Since all subsequent operations such as running in casing and cementing it in place depend on having a full gage hole, the customer demands and obtains it in one way or another. If a bit drills an undersized hole, the following bit must be used to ream the hole to full gage, even if in so doing the second bit becomes useless for further drilling. Needless to say, the bit which drilled the undersized hole will not be reordered if a better one is available. Thus, the gage surface of a rolling cutter used in oil field drilling is completely unlike many other bits used in drilling rock, and must even be better than the bottom--cutting structure of the same rolling cutter on which it is employed. Wear of a gage surface cannot be tolerated, whereas it makes little difference if the teeth which cut the inner part of the hole gradually wear away, so long as they continue to penetrate effectively.&#34; 
     In U.S. Pat. No. 2,774,570 to R. A. Cunningham patented Dec. 18, 1956 a roller cutter for earth drills is shown. The rolling cutter includes an annular series of cylindrical inserts of hard wear-resistant material having their axis extending outwardly and substantially normal to the surface of the body and presenting protrusions at the surface thereof to affect disintegrating action and to maintain gage of the well bore being drilled. 
     SUMMARY OF THE INVENTION 
     The present invention provides more surface on the gage row inserts for contacting the wall of the bore hole. This decreases wear on the gage inserts and therefore increases the ability of the earth boring apparatus to maintain a full gage hole. The insert of the present invention contacts the wall of the hole with the majority of the length of its extended surface and with the same angle as the gage angle of the earth boring apparatus and maintains maximum hole gage retaining ability. The earth boring apparatus includes at least one cutter member for forming a hole in the earth. The cutter member has an annular gage row of inserts mounted in sockets in the cutter member body for cutting the gage of the hole. The inserts have a shape prior to assembly in the sockets that includes an asymmetric head with an extended gage contacting face. The gage contacting face is planar and is substantially larger than any other planar face on the head. The above and other features and advantages of the present invention will become apparent from a consideration of the following detailed description of the invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cut-away perspective illustration of a three-cone rolling cutter rock bit embodying the present invention. 
     FIG. 2 is an enlarged side view illustration of a gage row insert of the bit shown in FIG. 1. 
     FIG. 3 is an end view of the insert shown in FIG. 2 showing the gage cutting surface. 
     FIG. 4 is a side view of another insert constructed in accordance with the present invention. 
     FIG. 5 is an end view of the insert shown in FIG. 4. 
     FIG. 6 is an illustration of yet another insert constructed in accordance with the present invention. 
     FIG. 7 is an end view of the insert shown in FIG. 6. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and to FIG. 1 in particular, a rotary rock bit generally designated by the reference character 10 embodying the present invention is illustrated. The bit 10 includes a bit body adapted to be connected at its pin end to the lower end of a rotary drill string (not shown). The bit body includes a passage providing communication for drilling muds or the like passing downwardly through the drill string to allow the drilling mud to be directed to the bottom of the well bore and pass upward in the annulus between the wall of the well bore and the drill pipe carrying cuttings and drilling debris therewith. 
     Depending from the body of the bit are three substantially identical arms. Arms 11 and 12 are shown in FIG. 1. The lower end portion of each of the arms is provided with a conventional bearing pin. Each arm rotatably supports a generally conical cutter member. The cutter members being designated 13, 14, and 15 in FIG. 1. The bearing pins carrying the cutting members 13, 14, and 15 define axis of a rotation respectively about which the cutter members rotate. The axis of rotation are tilted downwardly and inwardly at an angle. The direction of rotation of drill bits is in a clockwise direction so that the threads making up the various joints of the drill string are constantly tightened by the forces exerted as the drill string rotates the bit 10. 
     Each of the cutter members 13, 14, and 15 includes a nose portion that is oriented toward the bit axis of rotation and a base that is positioned at the intersection between the wall of the well bore and the bottom thereof. The cutting action of the base defines the diameter or gage of the well bore. 
     Each of the cutter members 13, 14, and 15 includes annular rows of inserts 16 for destroying the inner portion of the hole. Each of the cutter members 13, 14, and 15 also include annular rows of inserts 17 that are located adjacent the base of each cutting member. The inserts 17 cut the intersection between the well bore wall and the bottom thereof. The annular rows of inserts 17 are generally referred to as &#34;gage rows&#34; and the inserts 17 are designated &#34;gage inserts.&#34; The gage row inserts are subjected to the most rigorous drilling action. 
     The present invention affords more surface for the gage row inserts to contact the wall of the hole. This decreases wear on the gage inserts, therefore increasing the ability of the bit to maintain a full gage hole. Applicants have provided an insert which contacts a wall of the hole with the majority of the length of its extended surface and with the same angle as the gage angle of the bit. This insert is believed to have the maximum gage retaining ability. 
     In the prior art the accepted method of determining the exact bit diameter was to grind the outer or gage surface of the gage compact. This would produce a flat on the surface of the gage insert. The flat would contact the wall of the hole. It is impractical to grind the outer or gage surface of the existing gage inserts to the extent necessary to contact the wall of the hole with the majority of the length of their extended surfaces. In addition, the grinding of the inserts reduces the overall strength of the insert. 
     Referring now to FIG. 2, a side view of one of the gage inserts 17 is shown enlarged and in greater detail. The outer or gage angle α of this compact before gage grind is within 1° 30&#39; of the gage angle of the bit. It is not necessary to grind the outer or gage surface excessively to bring the insert gage angle to the bit gage angle. The inner angle β of this compact is considerably less than the outer or gage angle α. This difference between the inner and outer angles allows the length of the crest 19 to approximate that of conventional gage inserts. The sides or flank surfaces of the gage insert can be flat or convex surfaces, convex surfaces on the flanks result in a larger flat area on the outer angle than do the flat angled flanks. 
     Referring now to FIG. 3, an end view of the insert 17 shown in FIG. 2 is illustrated. The insert 17 contacts the wall of the hole with the majority of its extended surface 21 and with substantially the same angle as the gage angle of the bit. The surface 21 is the largest plane surface on the cutting head of the insert 17. The plane surface 21 contacts the wall of the hole and performs the gage cutting function. Since the surface 21 is relatively large compared to other surfaces on the insert 17, the lifetime of the insert 17 will be increased. 
     The insert 17 is formed by pressing granules of a wear-resistant material such as tungsten carbide together with granules of a binder such as cobalt. The wear-resistant material granules and binder granules are pressed together with wax and formed in the desired insert shape. The head of the insert may be formed in a die. For example, the head of the insert may be formed by a punch member which molds the end of the insert into the desired finished shape. The inserts are de-waxed in a furnace and sintered at a higher temperature in a furnace. The insert is then press fit into the body of a cutter member with the asymmetric head oriented so that the extended plane surface of the insert is at gage. Very little, if any, gage grinding is required. 
     The foregoing should be contrasted with prior art inserts having symmetrical heads. The prior art inserts are pressed into the cutter member and subsequently a gage surface is ground around the gage of a cutter producing ground flats on the gage inserts. The inserts of the present invention are pressed into the cutter with the pre-formed plane gage contacting surface located at substantially the gage angle of the bit. 
     Referring now to FIG.4, a side view of another embodiment of an insert 22 is shown in some detail. The insert 22 includes a cylindrical body portion 23 adapted to be mounted in a socket in the cutter body. The head of the insert 22 includes an inner surface 24 and an outer or gage surface 26. The outer or gage surface 26 is substantially larger than the inner surface 24. The roof top or crest 25 of the insert has substantially the same length as that of prior art gage inserts. 
     Referring now to FIG. 5, an end view of the insert 22 shown in FIG. 4 is illustrated. The insert 22 contacts the wall of the hole with the majority of its extended surface 26 and with substantially the same angle as the gage angle of the bit. The surface 26 is the largest plane surface on the cutting head of the insert 22. The plane surface 26 contacts the wall of the hole. Since the surface 26 is relatively large compared to the other surfaces on the insert 22, the lifetime of the insert 22 will be increased. 
     Referring now to FIG. 6, a side view of another embodiment of a gage insert 27 constructed in accordance with the present invention is illustrated. The insert 27 has a generally spherical formation contacting head 29 and a generally cylindrical body portion 28. The body portion 28 is adapted to fit within sockets in the cutter body. The outer or gage angle of the gage connecting surface 30 of this compact before gage grinding is within 1° 30&#39; of the gage angle of the bit. It is not necessary to grind the outer or gage surface extensively to bring the insert gage angle to the bit gage angle. 
     Referring now to FIG.7, an end view of the insert 27 shown in FIG. 6 is illustrated. The insert 27 contacts the wall of the hole with the majority of its extended surface 30 and with substantially the same angle as the gage angle of the bit. The surface 30 is the largest plane surface on the cutting head of the insert 27. The plane surface 30 contacts the wall of the hole. Since the surface 30 is relatively large, compared to other surfaces on the insert 27, the lifetime of the insert 27 will be increased.