Abstract:
A sectional drive system for transmitting rotational power to an output, which system includes multiple splined drive segments which are nested and interlocked as a drive string that is rotatable in a selected straight or curved path. The top one of the drive segments cooperates with a drive mechanism to effect rotation of the nested drive segments in concert, and the bottom one of the drive segments connects to a suitable output such as a drill bit. Multiple, tapered and truncated exterior splines on each of the drive segments mesh with complementary interior splines on the adjacent drive segment to enable slight angular positioning of the drive segments on each other and facilitate dampening of drive vibration and bending of the drive string in or out of the chosen path in any desired direction as the drive string transmits rotational power in a curved path of desired magnitude from the drive mechanism to the output. The drive segments are typically connected internally by means of a cable or shaft, or may be connected by floating “collars”, to form the drive string. In a preferred embodiment, a drill bit having a removable center bit insert is mounted on the bottom one of the drive segments on the drive string, and is characterized by multiple interior splines which engage the companion exterior splines of the drive segment.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to drive systems for transmitting rotational power to an output and more particularly, to a sectional drive system which is characterized by multiple, splined, interlocking drive segments that each include truncated and tapered exterior splines extending from one surface of a round segment base and interior splines extending from the opposite surface of the segment base and having interior spline seats located between the interior splines, which interior spline seats are substantially complementary to the configuration of the exterior splines. The drive segments are nested and interlocked by inserting the exterior splines of one drive segment in the congruent interior spline seats between adjacent interior splines of the adjacent drive segment, and are rotated in concert as a drive string in a selected path. The path may be straight or curved and in the latter case, the interlocking drive segments are capable of slight angular shifting on each other while maintaining a drive configuration of high integrity to dampen the drive vibration, define the chosen curved drive path and facilitate transmission of rotation from a drive mechanism to an output device with considerable torque and thrust. The nested drive segments can be interconnected interiorly by means of a cable or rod or exteriorly by floating collars and therefore, can be used as a drive train in any application in which a transfer of rotation is required in a straight line or at substantially any angle or deviation from a straight line. The sectional drive system may, for example, be used to effect horizontal drilling or coring of producing hydrocarbon intervals in oil and gas wells, utilizing the multiple, stacked and tapered, interlocking drive segments driven by a downhole drilling motor at one end of the drive string to operate a drill bit connected to the opposite end of the drive string. Retrieval of the drive string from the interval may be typically facilitated by a cable extended through openings in the drive segments. Consequently, the sectional drive system of this invention can be used in a downhole drilling apparatus to more efficiently effect drilling deviation in a controlled manner from a vertical well bore and provide a primary horizontal deviation or a lateral deviation from an existing vertical well bore. In a preferred embodiment of the invention, a drill bit having a removable center bit insert is mounted on the bottom one of the drive segments on the drive string, and is characterized by multiple interior splines which engage the companion exterior splines of the drive segment. 
     While capable of being operated in an extremely efficient manner to permit horizontal or angular drilling of drain hole perforations in oil wells, the sectional drive system of this invention can also be implemented to transmit rotational power from substantially any drive system to an output apparatus, drive or other system under circumstances where the rotational power is to be transmitted in an offset or a curved line. Accordingly, the sectional drive system of this invention is preferably designed with truncated drive segments and is capable of being used to transmit rotation from an engine, motor or other power source to automobiles, mud motors and like apparatus and equipment, as well as to dental drills, robotic devices and material-handling equipment, in non-exclusive particular. 
     2. Description of the Prior Art 
     Conventional techniques for effecting the transmission of rotational power between a power source and an output under circumstances where the power is to be transmitted in an offset or curved manner, includes the use of coupling mechanisms such as a universal or “CV” joint which are well known to those skilled in the art. For example, many devices have been designed for lowering into an oil or gas well for the purpose of boring and drilling holes at right angles to the well bore at the production interval, but many problems have been encountered using these systems. Typically, the relatively low bit rotational speed generally necessitated by using curved shafts of various design sometimes requires excessive time to achieve significant penetration, and increasing the bit rotational speed and torque load frequently causes failure of the shafts. Accordingly, these conventional horizontal drilling devices have not proved capable of sustaining the high compressive loads necessary to penetrate the well casing, concrete sheath, rock and producing interval in a well within an economical time frame without failure. Other problems have been encountered, such as impediments to bit retrieval and reduced freedom of rotation of the drilling string in such application. 
     Among the directional drilling apparatus designed to achieve this function are those detailed in the following U.S. Patentes: U.S. Pat. No. 1,367,042, to Granville; U.S. Pat. No. 2,516,421, to Robertson; U.S. Pat. No. 2,539,047, to Arutunoff; U.S. Pat. No. 2,726,847, to McCune; U.S. Pat. No. 2,778,603, to McCune; U.S. Pat. No. 3,667,556, to Henderson; U.S. Pat. No. 3,903,974, to Cullen; U.S. Pat. No. 3,958,649, to Bull et al; U.S. Pat. No. 4,051,908, to Driver; U.S. Pat. No. 4,185,705, to Bullard; U.S. Pat. No. 4,368,986, to Cousins; U.S. Pat. No. 4,442,908, to Stenbock; U.S. Pat. No. 4,601,353 to Schuh et al; U.S. Pat. No. 4,625,815, to Spies; U.S. Pat. No. 4,658,916, to Bond; U.S. Pat. No. 4,699,224, to Burton; U.S. Pat. No. 4,880,067, to Felsma; U.S. Pat. No. 5,337,839, to Warren et al; U.S. Pat. No. 5,373,906, to Braddick; U.S. Pat. No. 5,392,858, to Peters et al; U.S. Pat. No. 5,413,184, to Landers; U.S. Pat. No. 5,699,866, to James E. Cousins et al; U.S. Pat. No. 5,911,283 to James E. Cousins. 
     It is an object of this invention to provide a sectional drive system for transmitting rotational power in a straight path or a deviated, curved or offset path to an output of selected character. 
     Another object of this invention is to provide a sectional drive system for transmitting rotational power in a straight path or in a curved path offset from a source of power to an output, which sectional drive system includes multiple, splined, interlocking drive segments that are stacked and nested to rotate as a drive string responsive to application of rotational power to one end of the drive string in order to rotate the output at the opposite end of the drive string. 
     A still further object of this invention is to provide a sectional drive system of selected length and size, the drive system including multiple, splined, interlocking drive segments which each includes a round segment base having multiple protruding, tapered and truncated exterior splines, as well as alternating interior splines and interior spline seats in the base for receiving the projecting, tapered and truncated exterior splines of an adjacent drive segment. Multiple exterior base splines provided at the bases of the exterior splines on one drive segment engage multiple interior base splines provided on the adjacent drive segment. The drive segments are stacked and nested as a drive string within or without a guide path such as a tube, with the segments typically interconnected by a cable, rod or floating “collar” for dampening drive mechanism vibration and transmitting rotational power between the drive system and an output. 
     Yet another object of this invention is to provide a sectional drive system having drive segments with asymmetrical splines and coupled to a drill bit for drilling one or more drain holes of selected depth and angle into a producing interval of an oil or gas well to increase the flow of hydrocarbons or gas from the interval into the well bore. 
     A still further object of this invention is to provide a self-contained sectional drive system characterized by multiple, splined and interlocking drive segments which can be stacked and nested, optionally on a cable, shaft or rod as a drive string, or fitted with locking grooves and cooperating external floating collars, in a straight or curved guide path. One end of the drive string is connected to a drive apparatus such as a mud motor and the opposite end to an output such as a drill bit. The drive string is typically rotated by the mud motor to drill a hole through the well casing, cement sheath and damaged formation and undamaged production formation and increase the flow of hydrocarbons into the well bore of an oil or gas well. 
     Still another object of the invention is to provide a transverse down-hole drilling system which is self-contained and includes multiple, cable-mounted, splined and interlocking drive segments. The drive segments each have a round base and multiple, asymmetrical, tapered and truncated exterior splines each having a drive face and spline support face of unequal area projecting from one surface of the base, and companion interior spline seats which alternate with interior splines that extend from the opposite surface of the base. The drive segments nest and rotate in concert as a drive string, and adjacent drive segments are capable of pivoting or positioning at an angle on each other in a curved guide path to define a corresponding configuration of the drive string while maintaining an interlocking configuration of high integrity. The curved guide path may be shaped in such a manner as to permit sufficient lateral movement to traverse a path bend at any predetermined angle with an output such as a drill bit attached to the lower end of the string and an input such as a downhole electric or hydraulic drilling motor coupled to the upper end of the drive string for effecting rotation of the drive string and drill bit. The drive segments may be interconnected by means of an internal cable, a rod or shaft or multiple internally-flange floating collars, to define the drive string. 
     SUMMARY OF THE INVENTION 
     These and other objects of the invention are provided in a new and improved sectional drive system for transmitting rotational power from a drive source or apparatus of selected character to an output device of selected design, such as a drill bit, in a straight guide path or in a curved guide path in any angle from 0 to 90° under circumstances where the drive apparatus and the output device are misaligned. The sectional drive system is characterized by multiple, splined and interlocking drive segments which typically include eight spaced-apart asymmetrical, tapered and truncated exterior splines extending from one surface of a round base and each having a drive face and an angular spline support face of unequal area. Typically eight asymmetrical interior splines extend from the opposite surface of the base and define interior spline seats between the interior splines for receiving the congruent or complementary exterior splines of an adjacent drive segment in driving relationship. Multiple exterior base splines are provided at the bases of the respective exterior splines, and the exterior base splines of one drive segment mesh with respective interior base splines provided at the extending ends of the respective interior splines of an adjacent drive segment. The drive segments may be optionally slidably mounted on a cable or rod or externally connected by internally flange floating collars mounted in corresponding locking grooves of the drive segments and stacked and nested as a rotatable drive string, one end of which drive string is attached to a drive mechanism and the opposite end to an output device. In a preferred embodiment of the invention, the output device is a drill bit having a removable center bit insert, and is characterized by multiple interior splines which engage the companion exterior splines of the terminal drive segment of the drive string. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood by reference to the accompanying drawings, wherein: 
     FIG. 1 is a side view of a drive string of a typical sectional drive system of this invention, connected at one end to a suitable drive apparatus for rotating the entire segment string and an output device on the opposite end of the drive string, with the drive segments of the drive string connected by a segment cable; 
     FIG. 2 is a side view of a typical sectional drive system, with the drive segments of the drive string connected by a rod or shaft; 
     FIG. 3 is a rear perspective view of a typical drive segment element of the sectional drive system illustrated in FIG. 1; 
     FIG. 4 is a front perspective view of the drive segment element illustrated in FIG. 3; 
     FIG. 5 is a side view of the drive segment illustrated in FIGS. 3 and 4; 
     FIG. 6 is a sectional view taken along line  6 — 6  of the drive segment illustrated in FIG. 3; 
     FIG. 7 is a rear view of the drive segment illustrated in FIGS. 3-5, more particularly illustrating the interior splines and intervening interior spline seats of the drive segment; 
     FIG. 8 is a front view of the drive segment illustrated in FIG. 7, more =particularly illustrating the exterior splines and intervening exterior spline seats of the drive segment; 
     FIG. 9 is a rear perspective view of a preferred embodiment of a drill bit element of the sectional drive system of this invention; 
     FIG. 10 is a perspective view, illustrated in phantom, of the sectional drive system, with the drill bit illustrated in FIG. 9 mounted on the output end of the drive string of the sectional drive system; 
     FIG. 11 is a front perspective view of the drill bit illustrated in FIG. 9; 
     FIG. 12 is a sectional view, taken along line  12 — 12  in FIG. 11, of the drill bit, more particularly illustrating a preferred technique for mounting the drill bit on a segment cable extending through the drive string of the sectional drive system; 
     FIG. 13 is an exploded, perspective view of the drill bit, more particularly illustrating a preferred, retainer bolt and retainer washer technique for removably mounting a center bit insert in the drill bit; 
     FIG. 14 is a sectional view of the drill bit, more particularly illustrating a preferred technique for mounting the drill bit on the segment cable of the sectional drive system; 
     FIG. 15 is an enlarged sectional view, taken along section line  15  in FIG. 3, of a drive section element of the sectional drive system; and 
     FIG. 16 is an enlarged sectional view, taken along section line  16  in FIG. 9, of the drill bit element of the sectional drive system of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIGS. 1-8 of the drawings, the sectional drive system of this invention is generally illustrated by reference numeral  1 . The sectional drive system  1  is characterized by a drive string  10 , formed by stacking multiple, splined drive segments  17  on a segment cable  33 , as illustrated in FIG. 1, or alternatively, on an elongated rod or shaft  45 , as illustrated in FIG.  2  and hereinafter described. Each of the drive segments  17  includes a flat, disc-shaped segment base  18  and multiple tapered, truncated, asymmetrical exterior splines  19 , extending from a flat front base surface  18   a  of the segment base  18 , as illustrated in FIGS. 4-6. In a preferred embodiment of the invention, each of the drive segments  17  is shaped to include eight exterior splines  19 , each having a drive face  19   a  and an angular spline support face  19   b . The exterior splines  19  define eight intervening exterior spline slots  20  in a repetitive, geometric pattern which resembles an eight-point star when viewed from the front as illustrated in FIG.  8 . As particularly illustrated in FIGS. 4 and 5, each of multiple sloped base surfaces  34  angles into the segment base  18  from the flat front base surface  18   a  to form a sloped boundary of each exterior spline seat  20 , and each sloped base surface  34  extends between the spline support face  19   b  of one of the exterior splines  19  and the facing drive face  19   a  of the adjacent exterior spline  19 . Each of the sloped base surfaces  34  further defines an exterior base spline  28  at the base of the drive face  19   a  of each exterior spline  19 . As illustrated in FIG. 5, the plane of each sloped base surface  34  is disposed at an angle “B” of from about 15 degrees to about 20 degrees, and preferably, about 17 degrees with respect to the plane of the corresponding adjacent front base surface  18   a . As illustrated in FIG. 8, the bottom edge of each spline support face  19   b  which meets the corresponding sloped base surface  34  is typically disposed at an angle “D” of from about 10 degrees to about 45 degrees with respect to the bottom edge of the drive face  19   a  for optimum strength. The exterior splines  19  taper from the front base surface  18   a  to a flat truncated tip  21 , which is coplanar with the converging sets of exterior splines  19 , and a tip aperture  22  is typically provided in the center of the tip  21 , as illustrated in FIGS. 3 and 6. As further illustrated in FIG. 8, the angle “C” defined by the drive faces  19   a  of adjacent external splines  19  is typically about 45 degrees for optimum driving characteristics. Moreover, the vertical drive face angle “LF” (FIG. 5) measured between the plane of the front base surface  18   a  and the plane of the drive face  19   a  of each exterior spline  19 , is typically about 90 degrees, whereas the plane of each spline support face  19   b  of each exterior spline  19  is disposed at an obtuse angle with respect to the corresponding sloped base surface  34 . As illustrated in FIG. 8, the angle “F” defined by each drive face  19   a  and the flat edge of the corresponding exterior spline  19  is typically about 90 degrees. 
     Referring again to FIGS. 3-7 and to FIG. 15 of the drawings, multiple interior splines  25  and intervening spline dividers  35  (FIG. 15) extend from the flat rear base surface  18   b  of the segment base  18  of the drive segment  17 , to define a central segment interior  23  which communicates with the tip aperture  22  of the drive segment  17  as illustrated in FIG.  6 . As illustrated in FIG. 15, each of the interior splines  25  is characterized by a flat drive face  25   a  and a flat spline wall  25   b . An interior spline seat  24  is defined between adjacent interior splines  25 , by the recessed interior divider face  35   b  of each intervening spline divider  35 , the drive face  25   a  of one interior spline  25  and the facing spline wall  25   b  of the adjacent interior spline  25   b . As illustrated in FIG. 5, an exterior spline face  25   c  of each interior spline  25  is disposed at an angle “A” of from about 15 degrees to about 20 degrees and preferably, about 17 degrees, with respect to an exterior divider face  35   a  of the corresponding adjacent spline divider  35 , which angle “A” is the same as the angle “B” between each sloped base surface  34  and the front base surface  18   a . The sloped exterior spline face  25   c  of each interior spline  25  defines an interior base spline  30  adjacent and substantially perpendicular to each exterior divider face  35   a . As illustrated in FIG. 7, the plane of the drive face  25   a  of each interior spline  25  is typically disposed at an angle “D” of from about 10 degrees to about 45 degrees with respect to the exterior edge  27  of the facing spline wall  25   b  of the adjacent interior spline  25   b . The interior splines  25  and intervening spline dividers  35  extend outwardly from the rear base surface  18   b  of the segment base  18 , to substantially conform to the taper angle of the exterior splines  19 . Moreover, the exterior splines  19  are complementary in shape to the interior spline seats  24 , and the interior splines  25  are complementary in shape to the exterior spline seats  20 , respectively. Accordingly, the drive segments  17  will nest, stack and interlock and yet are capable of being positioned at an angle on each other in driving relationship to shape the drive string  10  as illustrated in FIG. 1, with the exterior splines  19  and interior splines  25  of one drive segment  17  inserted in the interior spline seats  24  and exterior spline seats  20 , respectively, of respective adjacent drive segments  17 . Furthermore, the exterior base splines  28  of one drive segment  17  engage the respective companion interior base splines  30  of an adjacent drive segment  17 . This interlocking registration of the drive segments  17  is not rigid, but permits pivoting movement of the drive segments  17  in the interlocking and nested configuration, such that the drive string  10  can easily bend to conform to the bend illustrated in FIG. 1, and yet maintain an interlocking driving relationship of high integrity due to the drive faces  19   a  of the exterior splines  19  and matching drive faces  25   a  of the interior splines  25   b . The interlocking registration of the drive segments  19  also serves to dampen any excessive vibration of a mud motor (not illustrated) or other drive mechanism (not illustrated) at the input  12  of the drive string  10 . 
     Referring again to FIGS. 7 and 8 of the drawings, each drive segment  17  is designed such that the internal spline seats  24  are rotatably offset with respect to the respective exterior splines  19  thereof. This offset is preferably at a rotational angle, “G”, illustrated in FIG. 7, in the range of from about 0.5 degrees to about 12 degrees and most preferably, about six degrees. This rotational angle “G” facilitates proper meshing of the exterior splines  19  of one drive segment  17  and the interior spline seats  24  of an adjacent drive segment  17  in the drive string  10 . In application as hereinafter described, torque is applied to the top of the drive string  10  by means of an input  12  of selected design to rotate the drive string  10  and the output  42  attached to the opposite end of the drive string  10 , as further illustrated in FIG.  1 . 
     Referring again to FIG. 6 of the drawings, in a most preferred embodiment of the invention the angle “H” of taper of the spline dividers  35  and the exterior splines  19  with respect to the plane of the truncated tip  21  of the exterior splines  19 , is in the range of from about 20 degrees to about 40 degrees and most preferably, about 30 degrees, when one of the drive segments  17  is viewed as illustrated in FIG.  5 . This structuring of the drive segment  17  facilitates a drive string  10  which is capable of bending with a separation angle “E”, illustrated in FIG. 1, of from about zero to about 10 degrees for each one of the drive segments  17  utilized in the drive string  10 , to facilitate traversal of the bend illustrated in FIG.  1  and yet maintain optimum interlocking contact between the exterior splines  19  of each drive segment  17  and interior splines  25  of the adjacent drive segment  17 , to effect driving rotation of the selected output  42  responsive to power applied to the drive string  10  by the selected input  12 . 
     Referring now to FIGS. 1 and 2 of the drawings, in a preferred embodiment of the invention a set of drive segments  17  may be slidably strung on the flexible segment cable  33  (FIG. 1) or on the stiff segment shaft  45  (FIG. 2) and nested with each other, with the exterior splines  19  (FIGS. 4 and 5) of each drive segment  17  inserted in the respective interior spline seats  24  (FIG. 15) of the adjacent drive segment  17 , and the exterior base splines  28  engaging the respective interior base splines  30 . One end of the segment cable  33  or the segment shaft  45  may be fitted with a cable stay or anchor (not illustrated) or otherwise fixed inside the output  42  and the other end threaded through the registering tip apertures  22  of the nested drive segments  17  for similar attachment to the input  12  to maintain the drive segments  17  in nested configuration in the drive string  10 . The drive string  10  utilizing the straight rod or shaft  45  illustrated in FIG. 2 is typically rotated in a straight guide path or tube of selected design under circumstances in which the input  12  is in an aligned position with respect to the output  42 , with the drive faces  19   a  (FIG. 4) of the exterior splines  19  on one of the drive segments  17  engaging the drive faces  25   a  (FIG. 15) of the respective interior splines  25  of an adjacent drive segment  17 , and the exterior base splines  28  engaging the respective interior base splines  30 . Alternatively, the drive string  10  can be rotated in a curved guide path or tube of desired curvature under circumstances in which the input  12  is disposed in an offset position with respect to the output  42 , as illustrated in FIG.  1 . When the output  42  is configured as a drill bit, for example, one or more lateral or horizontal drain holes (not illustrated) can be drilled in a hydrocarbon formation (not illustrated) in a vertical oil or gas well, according to the procedure outlined in U.S. Pat. No. 5,699,866, and the drive string  10  can be retrieved from the drain hole by application of the segment cable  33 , illustrated in FIG.  1 . In a most preferred embodiment of the invention the drill bit may be typically about twenty percent larger than the drive segment  17  to better facilitate retrieval of the drive string  10  and to facilitate removal of debris from the drain hole as the drive string  10  and the drill bit are removed from the drain hole. In a preferred embodiment of the invention, the tip aperture  22  of each drive segment  17  is about ½″ in diameter, whereas the segment cable  33  or shaft  45  is about ½″ in diameter to facilitate sufficient clearance between the segment cable  33  or shaft  45  and the edge of the tip aperture  22  for the passage of drill fluid (not illustrated) through the drive segments  17  of the drill string  10  for purposes which will be hereinafter described. It will be further appreciated by those skilled in the art that the drive segments  17  illustrated in FIGS. 1-8 of the drawings may alternatively be used in connection with multiple “floating collars” of the design and in the manner outlined in our U.S. Pat. No. 5,911,283, for the purposes outlined in that patent. Accordingly, under circumstances in which the drive string  10  is to be left in the drain hole and not retrieved and the drive segments  17  are connected by means of the “floating collars”, the segment cable  33  may be omitted. 
     Referring again to FIGS. 7 and 8 of the drawings, it will be further appreciated by those skilled in the art that substantially any number of exterior splines  19 , exterior spline seats  20 , exterior base splines  28 , interior spline seats  24 , interior splines  25  and interior base splines  30  can be provided in the design of the drive segment  17 . However, in a most preferred embodiment of the invention, eight exterior splines  19 , exterior base splines  28  and exterior spline seats  20  and matching interior spline seats  24 , interior base splines  30  and interior splines  25  are provided for each one of the drive segments  17 , as illustrated. In a most preferred embodiment, the taper of the eight exterior splines  19  and the configuration of the interior splines seats  24  are complementary, as heretofore described, and the exterior splines  19  and interior splines  25  are typically about two percent to about five percent smaller than the interior spline seats  24  and the exterior spline seats  20 , respectively, for optimum smoothness and meshing during bending of the drive string  10  while operating the sectional drive system  1  typically as illustrated in FIG.  1 . 
     It will be further appreciated by those skilled in the art that other applications of the sectional drive system  1  may include the application of torque and thrust in a straight line or along a deviation from a straight line up to or even beyond ninety degrees, wherein the drive segments  17  shift or pivot on each other, utilizing either the segment cable  33 , the segment shaft  45  or the floating collars (not shown) as described in U.S. Pat. No. 5,911,283, in any desired direction. Torque may also be applied to the drive segments  17  as the latter lie in a curved guide tube or path (not illustrated), as desired. Accordingly, typical applications include “CV” joints and mechanical couplings in vehicles, mud motors and other applications involving misaligned drive and driven systems. Application to dental drills may also be effected under circumstances where the dental drill drive train must be curved over a selected adjustable or fixed radius from the drive motor to the application or drill end. The device may also be used in tools such as flexible-shaft screwdrivers and similar applications, in non-exclusive particular. 
     It will be appreciated by those skilled in the art that the drive segments  17  can be constructed of substantially any desired material, depending upon the application. Furthermore, the drive segments  17  are typically applied where the deviation, offset or curved between the input  12  and the output  42  of the drive string  10 , is significant. 
     Referring next to FIGS. 9-14 and  16  of the drawings, in a preferred embodiment the drill bit of this invention is generally illustrated by reference numeral  51 . The drill bit  51  is designed for attachment to the drive string  10  (illustrated in phantom in FIG. 10) of the sectional drive system  1  heretofore described with respect to FIGS. 1-8, as hereinafter described. The drill bit  51  is characterized by a substantially cylindrical drill bit head  52 , having a convex or dome-shaped cutting face  52   a  typically studded with multiple diamond bits  52   b  in a selected pattern, in conventional fashion. Multiple water course grooves  57  are typically provided in spaced-apart relationship in the circumference of the drill bit head  52  for facilitating passage of drilling fluid (not illustrated) between the drill bit head  52  and the well casing, cement sheath, producing interval (not illustrated) or other medium as the medium is drilled using the drill bit  51  as hereinafter described. Additional diamond bits  52   b  are typically provided on the drill bit head  52  between the water course grooves  57 . As illustrated in FIG. 13, a center bit bore  53  extends centrally through the drill bit head  52  and receives a center bit insert  75  having an insert head  76  which is tapered in cross-section as illustrated in FIG. 12, and an insert shaft  77  extends from the insert head  76 . Opposing drive lugs  58  of the drill bit head  52  protrude toward each other into the center bit bore  53  for engaging complementary lug grooves  78  provided in opposite sides of the insert shaft  77  and insert head  76  of the center bit insert  75 . As illustrated in FIG. 12, an annular bit shoulder  54  is defined by the drill bit head  52  between broad and narrow portions of the center bit bore  53 , and the insert head  76  of the center bit insert  75  seats on the bit shoulder  54  in the broad portion of the center bit bore  53  whereas the insert shaft  77  extends through the narrow portion of the center bit bore  53 , beyond the bit shoulder  54 . A cable ball cavity  56 , the purpose of which will be hereinafter described, continues rearward extension of the narrow portion of the center bit bore  53  in the drill bit head  52 , and a retaining washer shoulder  55  is defined between the cable ball cavity  56  and the smaller-diameter narrow portion of the center bit bore  53 . As illustrated in FIGS. 12 and 13, the center bit insert  75  is typically removably mounted in the center bit bore  53  by means of a retaining bolt  81 , extended through a retaining washer  80  and threaded into the insert shaft  77  of the center bit insert  75  with the retaining washer  80  engaging the retaining washer shoulder  55 . As illustrated in FIG. 11, the lug grooves  78  (FIG. 13) of the center bit insert  75  define a lug space  73  between the center bit insert  75  and the drive lugs  58 , which lug space  73  communicates with the cable ball cavity  56  of the drill bit head  52  to facilitate flow of drilling fluid (not illustrated) through the drill bit head  52  as hereinafter described. 
     As illustrated in FIGS. 12 and 14, an annular base flange  71  having a tapered interior  71   a  extends rearwardly from the drill bit head  52 , and the complementary tapered base nose  62  of a cylindrical drill bit base  59  is typically threaded (FIG.  14 ), welded (FIG. 12) or otherwise fixedly or removably attached to the base flange  71  of the drill bit head  52  at the tapered interior  71   a . As illustrated in FIG. 14, the drill bit base  59  includes a central bushing seat  63  which extends through the base nose  62  and is provided with multiple interior seat threads  64  and extends rearwardly from the cable ball cavity  56 . Multiple, spaced-apart interior splines  25 , adjacent ones of which are separated by a spline divider  35  defining an interior spline seat  24  (FIG. 16) between adjacent interior splines  25 , extend rearwardly from the base nose  62  of the drill bit base  59  in surrounding relationship to a base interior  60  which communicates with the bushing seat  63 . The interior spline seats  24 , interior splines  25  and interor base splines  30  of the drill bit base  59  are similar in number, size and configuration to those respective elements of the drive segments  17  of the drive string  10  heretofore described with respect to FIGS. 1-8 and  15 . Accordingly, the exterior splines  19  (FIG. 4) of the drive segments  17  are complementary in size and shape to the interior spline seats  24  of the drill bit base  59 . Thus, the exterior splines  19  of the terminal drive segment  17   a  (FIG. 10) on the drive string  10  are capable of insertion in the complementary interior spline seats  24  between the adjacent interior splines  25  of the drill bit base  59 , with the exterior base splines  28  of the terminal drive segment  17   a  engaging the respective interior base splines  30  of the drill bit base  59 , to engage the respective interior splines  25  and rotate the drill bit  51  with the rotating drive string  10 , as hereinafter described. As further illustrated in FIGS. 12 and 14, multiple water course passages  31  typically extend through the base nose  62  of the drill bit base  59 , and communicate with the cable ball cavity  56  (which communicates with the lug spaces  73 , FIG. 11) and base interior  60  to facilitate flow of drilling fluid (not illustrated) through the drill bit  1  during use as hereinafter described. 
     Referring again to FIGS. 10,  12  and  14  of the drawings, in typical application the drill bit  51  is typically removably mounted by means of the segment cable  33  (FIG. 1) on the drill string  10  of the sectional drive system  1 , which segment cable  33  extends through the registering tip apertures  22  (FIG. 4) of the nested drive segments  17  in the drive string  10 . That portion of the segment cable  33  protruding from the tip aperture  22  (FIG. 4) of the terminal drive segment  17   a  (FIG. 10) extends through a cable bushing  67  (illustrated in section in FIGS. 12 and 14) and terminates on a cable ball  33   a , typically welded or otherwise attached to the segment cable  33 . The cable ball  33   a  is extended through the threaded bushing seat  63  of the drill bit base  59  and positioned in the cable ball cavity  56  while the cable bushing  67  is threaded in the bushing seat  63  by operation of the seat threads  64  and companion bushing threads  68  on the cable bushing  67 . The exterior splines  19  of the terminal drive segment  17   a  (FIG. 10) on the drive string  10  are inserted in the respective interior spline seats  24  (FIG. 16) in the base interior  60  of the drill bit  51 , and the exterior base splines  28  (FIG. 5) of the terminal drive segment  17   a  engage the respective interior base splines  30  (FIG. 9) on the respective interior splines  25  of the drill bit  51 . In application of the sectional drive system  1  and drill bit  51  typically according to the procedure outlined in U.S. Pat. No. 5,699,866, the drive faces  19   a  (FIG. 4) of the exterior splines  19  of the terminal drive segment  17   a  in the drive string  10  engage the drive faces  25   a  (FIG. 16) of the respective interior splines  25  of the drill bit  51  as the drive string  10  is rotated by means of the input  12  (FIG.  1 ). The weight of the rotating drive string  10  and other drilling components (not illustrated) engaging the input drive segment  17   b  (FIG.  10 ), attached to the input  12  shown in FIG. 1, bears against the drill bit  51  at the drill bit base  59  as the rotating drill bit  51  drills a perforation or drain hole (not illustrated) in a hydrocarbon-producing interval (also not illustrated). It will be appreciated from a consideration of FIG. 12 that the drill bit  51  is securely mounted on the drive string  10  since the cable ball  33   a , having a larger diameter than that of the cable bushing  67 , is prevented from pulling out of the cable ball cavity  56  by seating against the cable bushing  67  as the terminal drive segment  17   a  remains nested in the base interior  60  of the drill bit base  59 . 
     It will be appreciated by those skilled in the art that drilling fluid (not illustrated) can be continuously circulated through the drive string  10  and attached drill bit  51  during operation of the sectional drive system  1 , for purposes of cooling and preventing accumulation of drilling fragments in the drive string  10  and drill bit  51 . Accordingly, the drilling fluid (not illustrated) is injected through the tip aperture  22  (FIG. 4) of the input drive section  17   b  of the drive string  10 , and flows through the registering tip apertures  22  of the remaining drive segments  17  and terminal drive segment  17   a . The drilling fluid then enters the multiple water course passages  31  of the drill bit base  59  and flows through the cable ball cavity  56  and finally, from the drill bit head  52  through the lug spaces  73  (FIG.  11 ). The drilling fluid is capable of removing particulate drilling fragments from the hydrocarbon-producing interval as the drilling fluid flows between the drill bit head  52  and the interval, through the water course grooves  57  in the outer circumference of the drill bit head  52 . 
     Referring again to FIGS. 12 and 14 of the drawings, it will be appreciated by those skilled in the art that a worn or damaged center bit insert  75  of the drill bit  51  can be removed from the drill bit head  52  and replaced, as desired, by initially unthreading the cable bushing  67  from the drill bit base  59  and removing the cable ball  33   a  from the cable ball cavity  56 ; unthreading the retaining bolt  81  from the center bit insert  75 ; removing the center bit insert  75  from the drill bit head  52  and securing a replacement center bit insert  75  in the drill bit head  52  using the retaining bolt  81  and retaining washer  80 ; and replacing the cable bushing  67  in the drill bit base  59  and the cable ball  33   a  in the cable ball cavity  56 . By increasing the “rake angle”, or cutting angle, of the drill bit  51  at the center relative to the peripheral areas of the cutting face  52   a , the center bit insert  75  enhances the cutting efficiency of the drill bit  51  relative to drill bits having a constant cutting angle across the entire diameter of the cutting face thereof. It is understood that any mechanism known to those skilled in the art other than the retaining bolt  81  and retaining washer  80  described above can be used for removably mounting the center bit insert  75  in the drill bit head  52 . It is further understood that the drill bit base  59  can be constructed in one piece with the drill bit head  52  or alternatively, either removably attached to the drill bit head  52  typically at the base flange  71  typically by threaded attachment, or welded or otherwise fixedly mounted on the drill bit head  52  typically at the base flange  71 . 
     While the preferred embodiments of the invention have been described above, it will be recognized and understood that various modifications may be made in the invention and the appended claims are intended to cover all such modifications which may fall within the spirit and scope of the invention.