An improved jaw for a rod gripping device is provided including a front, rod engaging face with a plurality of teeth or studs made of a material harder than the jaw body, such as tungsten carbide, with the studs positioned to engage an outer rounded rod surface. In one embodiment, the studs are oriented radially inward toward a longitudinal axis defined by the rod engaging surface. The rod engaging face of the jaw may have a concave shape in its lengthwise direction, thus being adapted to grasp a cylindrical member such as a rod or pipe section. A tool for gripping a rod including a pair of jaws is also provided.

TECHNICAL FIELD OF THE INVENTION 
     The invention relates to an apparatus for grasping cylindrical bodies such as pipes and rods, in particular to a rod gripping jaw for use in directional boring. 
     BACKGROUND OF THE INVENTION 
     In order to meet the need for boring under or through obstructions such as roadways, concrete lined waterways and large underground utilities, systems for underground directional boring were developed. A directional borer generally includes a series of drill rods or pipe sections joined end to end to form a drill string. The drill rods or pipe sections are typically connected with threaded couplings. The drill string is pushed or pulled though the soil by means of a powerful hydraulic device such as a hydraulic cylinder. See Malzahn, U.S. Pat. Nos. 4,945,999 and 5,070,848, and Cherrington, U.S. Pat. No. 4,697,775 (RE 33,793). The drill string may be pushed and rotated at the same time as described in Dunn, U.S. Pat. No. 4,953,633 and Deken, et al., U.S. Pat. No. 5,242,026. A spade, bit or head configured for boring and steering is disposed at the end of the drill string and may include an ejection nozzle for water to assist in boring. 
     The pipe sections used in drill strings are typically steel pipe having a diameter of from about one to eight inches, more often four to eight inches. During drilling, rod or pipe sections are added as the bore is advanced and removed when the tool is retracted for replacement or repair. Thus, as the pipe or rod is being advanced, it is necessary to couple and tighten additional sections of pipe to the drill string. Conversely, when the drill string is removed from the bore, it is necessary to loosen and decouple adjacent pipe sections. 
     Making and breaking joints between adjacent pipe sections in a drill string is generally accomplished with a hydraulic vise that having two adjacent pairs of jaws that grip adjacent sections of pipe and rotate the front section to engage or disengage a threaded pipe coupling. Such coupling and decoupling apparatus is known in the art. The rod gripping jaws should securely grasp of the pipe section without applying enough force to crush it or damage its surface. 
     Conventional pipe or rod gripping jaws are typically fabricated from steel or a steel alloy and have knurled surfaces to aid in grasping the pipe section. However, the effective life of conventional steel jaws for use in directional boring machines is limited. While the use of steel projections or knurled surfaces on rod gripping jaws may aid in gripping the surface of a steel pipe section, the steel-on-steel gripping action rapidly wears the gripping surfaces of the jaw. Thus, there exists a need for an improved rod gripping jaw, and in particular, an improved rod gripping jaw adaptable for use in connection with directional boring machines. 
     SUMMARY OF THE INVENTION 
     The invention provides an improved jaw for gripping a cylindrical object such as a drill rod used in a directional boring machine. The body of each jaw has a front face on which a plurality of teeth or studs are mounted. These teeth are made of a material harder than the jaw body and are positioned to engage an outer rounded rod surface. The improved jaw of the invention is particularly suitable for use in connection with directional earth boring machines which must grasp, couple and decouple section of drill string during the drilling operation. The teeth are preferably arranged in an array or formation that permits the teeth to grip a semi-cylindrical portion of a rod or pipe surface evenly, and the number of teeth is such that each tooth tip digs into the outer surface of the pipe or rod a sufficient depth to securely hold the rod, but not so deeply that the rod surface is scored excessively. In one embodiment, the teeth are oriented radially inwardly toward a longitudinal axis defined by a lengthwise concave recess in the front face of the jaw. 
     In another aspect, a tool for gripping a rod including a pair of jaws is provided. Each of the jaws comprises a steel jaw body with a concave surface in its lengthwise direction, thereby being adapted to engage a curved surface. Each of the jaws is provided with a plurality of studs having conical tips with points configured to penetrate and grip an outer surface of a curved cylindrical steel member. The studs are made of a material harder than the jaw body and configured to project from the jaw to provide a clearance between the concave surface of the jaw and the outer surface of the curved cylindrical member upon engagement of a rod or similar body. 
     A clamp assembly according to the invention configured for use in a directional boring machine includes a U-shaped clamp having a pair of first and second arms. A movable stem jaw is mountable against an inside wall of the first arm, and a vise jaw which may be movable or fixed is mountable against an inside wall of the second arm in a position opposed to the movable jaw. Suitable means may then be provided for securing the vise jaw in position against the inner wall of the second arm. Similarly, means such as a annular flange welded to the rear face of the stem jaw extends through an opening in the first arm for movably supporting the stem jaw for movement towards and away from the vise jaw. The vise and stem jaws are each provided with teeth or studs according to the invention as described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures in which corresponding numerals in the different figures refer to corresponding parts and in which: 
     FIG. 1 is a top perspective view of a rod-gripping jaw including a concave face with a plurality of studs for grasping and engaging a cylindrical member such as a rod or pipe section; 
     FIGS. 2A-2E are top, front end, rear end, right side and left side views of the jaw of FIG. 1, respectively; 
     FIG. 3 is an enlarged view of FIG. 2C, partly in section; 
     FIG. 4 is an end view similar to FIG. 3 of an alternative jaw according to the invention; 
     FIG. 5 is an end view similar to FIG. 3 of another alternative jaw according to the invention; 
     FIG. 6 is an end view, partly in section, of a rod gripping tool utilizing jaws of the invention; and 
     FIG. 7 is an exploded view of a clamp assembly according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention and are not to delimit the scope of the invention. 
     Referring now to FIGS.  1  and  2 A- 2 E, in one embodiment, a rod-gripping jaw  10  of the invention includes a generally rectangular block or jaw body  20  having a longitudinally extending, concave recess  12  in one rectangular side face  14 . Recess  12  is curved in its widthwise direction and may have a circular (arcuate) or non-circular profile in cross section. A number of studs or teeth  16  are embedded in or set onto recess  12 . Studs  16  which are set into holes in the jaw body are preferred over teeth which are merely welded or otherwise attached to the surface thereof. Such studs  16  are arranged in one or more longitudinal rows  17  generally aligned with a longitudinal axis  18  of jaw  10 . 
     Each stud  16  has a tip  22  produced from a material harder than the material from which body  20  and/or the rod to be held is formed. In a preferred embodiment, each stud  16  is a unitary pellet made of a hard, wear resistant material which is not excessively brittle and has a hardness greater than a conventional  4140  steel alloy, for example, high carbon tool steel, diamond, or a ceramic such as tungsten carbide. If stud  16  includes a tip  22  as a separate insert, the stud also includes a cylindrical holder made from a conventional steel such as  4140  alloy, and only the tip is made of the hardened material as described. However, since each stud  16  is relatively small, use of a unitary pellet is most preferred. Studs  16  may also comprise steel or ceramic inserts which have been surface coated on at least tip  22 , as by sintering and other methods known in the art, with a thin layer of hard material such as diamond. 
     As illustrated in FIG. 1, studs  16  are preferably of uniform size, and are distributed in a generally uniform manner on the surface of recess  12 . Depending upon the application, studs of different sizes or shapes may be used in a single jaw, or the studs may be positioned in a non-uniform manner on the jaw. As illustrated, approximately fifty-six studs  16  (seven rows of eight studs each) are shown distributed over concave recess  12 . Typically between three (e.g., 1 row of 3) to one hundred (e.g., 10 rows of 10) studs will be distributed in parallel rows  17 . Studs  16  in adjoining rows  17  are staggered as shown so that spacing between studs  16  is uniform, or approximately so. Rows  17  are located on the recess  22  to form an array or formation of studs  16  that can grip a circular rod evenly and leave a clearance between the surface of recess  12  and the rod surface. If the cross-sectional profile of recess  12  is circular or parabolic, it is preferred to have each row  17  of studs  16  substantially perpendicular to the adjoining surface in which it is mounted, so that lengthwise axes of studs  16  at the same cross section intersect at a common point or focus F, for example studs  16 A,  16 B and  16 C in FIGS. 1 and 3. 
     A greater or lesser number of studs can be used depending upon the diameter of the rods to be engaged, the torque required to unscrew them, and similar considerations. The usual minimum is at least one perpendicular bottom stud and at least two side studs inclined in opposite directions relative to the bottom stud, for example, from about 10-60 degrees, wherein the angle is most preferably the same for each pair of studs in the same longitudinal row  17  or in symmetrical positions on opposite sides of jaw axis  18 . For example, in a minimal configuration, three studs  16  could be used, including an upright bottom stud, a first side stud left of the bottom stud and inclined right by an angle in the range of 30°-60°, 45 degrees as shown, and a second side stud right of the bottom stud inclined left by an angle in the range of 30°-60°. In an expanded configuration with at least three rows  17 , all of the studs in the same row  17  (e.g., as studs  16 A,  16 B,  16 C respectively) are preferably angled in the same direction. 
     Referring to FIG. 3, stud  16  is bullet-shaped with a cylindrical bottom portion  30  and a conical upper portion  32  that tapers to point  22 . Each stud  16  may be press-fitted into a blind hole  33  in jaw body  12  and/or secured therein by known processes such as copper brazing. Conical portion  32  and tip  22  are exposed after the to stud is fully inserted into hole  30 . Preferably, conical upper portion  32  is formed at an included angle A of from about 30° to 120°, preferably 40° to 90°, centered on the stud axis. Tip  22  is preferably sufficiently small and pointed that it “bites” or penetrates a small distance into the outer surface of a rod or pipe section without excessive penetration that might deform or damage the rod or pipe section. Carbides with hemispherical or substantially hemispherical heads as used on rock drills to protect the bit from abrasion are not preferred for applications of the present invention wherein a high torque must be exerted, such as when uncoupling directional drill rods. 
     FIG. 4 illustrates a modified jaw  40  of the invention wherein the concave profile of FIGS. 1-3 is replaced by an outwardly flaring, trough-shaped recess  42  having a flat bottom  43  and a pair of straight, angled side walls  44 ,  46  that angle outwardly at angle generally from 30°-60° to approximate a concave curvature. FIG. 5 shows a stepped embodiment of a jaw  50  wherein the studs  16  are mounted in parallel on a series of parallel, offset flat walls  52 . Differences in stud lengths and/or the depth of stepped recess  54  could be used so that tips  22  approximate an arc comparable to the shape of the outer surface of the rod or pipe to be engaged. Similarly, it is even possible (though uneconomical) to eliminate the recess altogether and use teeth or studs of varying lengths to define an arc with tips  22 . Modifications of this sort are within the scope of the invention. 
     FIG. 6 illustrates a pair of rod-gripping jaws  10  of the invention clamped onto a rod section  60 . Jaws  10  clamp pipe section  60  by means of any suitable actuator, such as a hydraulic cylinder, with sufficient force to enable the studs  16  to bite into the surface of the rod  60 . As illustrated studs,  16  are oriented radially inwardly toward a centrally disposed longitudinal axis  64  of pipe section  60  that is parallel to the axes  18  of each jaw  10 . Although two jaws  10  are shown, more than two jaws could be used, for example, four jaws at 90 degree angles set in two pairs. 
     FIG. 7 illustrates a clamp assembly  69  according to the invention for use in a directional boring machine. Clamp mechanism has a stem jaw  70 A and a vise jaw  70 B having studs  16  according to the invention. Jaws  70 A,  70 B are configured for mounting in opposing positions on inner surfaces of arms  71 A,  71 B of a U-shaped lower or rear clamp  72 . Rear clamp  72  is in turn mounted by bar slides  73  into a clamp frame  74 . An upper or front clamp  75  can pivot by means of a pair of pivot slides  76  mounted in grooves  77  of frame  74 . A hole  79  provided on arm  71  A permits connection of an annular flange  81  extending from a rear face of jaw  70 A to a conventional clamp cylinder assembly, not shown, which extends and retracts stem jaw  70 A. Vise jaw  70 B is preferably removably held in a fixed position by any suitable means, such as a bolt assembly  80  which engages a central threaded hole  82  in jaw  70 B. A like mechanism is provided for front clamp  75 . 
     Front clamp  75 , which mounts another pair of jaws  70 A,  70 B (not shown), has an arm  78  which mounts an axle that is rotatably connected to a conventional hydraulic cylinder assembly which is engaged to pivot rear clamp  75  on slides  76  while front clamp  72  remains in place to unscrew one drill rod section from another. Such a clamp assembly of the invention is suitable for use in gripping drill string rods used by a directional boring machine, such as one of the Vermeer Navigator line. In addition, jaws of the invention can also be used in non-steering pipe pulling and pushing machines which operate using drill strings. 
     While certain embodiments of the invention have been illustrated for the purposes of this disclosure, numerous changes in the method and apparatus of the invention presented herein may be made by those skilled in the art, such changes being embodied within the scope and spirit of the present invention as defined in the appended claims.