This application is a continuation-in-part of 09/122,610, filed Jul. 24, 1998 now in 6,209,665. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is related to earth drilling equipment, and particularly to down hole, pneumatic, percussive hammer drilling systems. As noted in my related co-pending applications, Ser. No. 08/674,123, filed Jul. 1, 1996, and Ser. No. 09/122,616, filed Jul. 24, 1998, to which the present application is a continuation-in-part, underreamers are used for the formation of radially enlarged areas extending about a pilot bit for insertion of a casing. 
     Eccentrically mounted underreamers are known which include an arm which travels in an orbit for underreaming operation, and which are retractable toward the hole axis for tool removal purposes. However, eccentrically mounted underreamers can be diverted off-axis if the underreamer encounters rock fragments, buried metal objects, etc. Any diversion of a large drill bit is unacceptable in most drilling operations, and particularly where a series of closely spaced holes are being formed. 
     The installation of casing in a drilled ground hole is also greatly hindered by any such diversion. 
     Other known underreaming equipment utilizes three bit mounted plates which are outwardly displaceable, but which incorporate a total working surface which is substantially less than the perimeter of the bore. Such undersized plates are subject to excessive wear and result in slow drilling operation. 
     Underreaming can also be achieved by use of a crown or ring bit, but components of those bits must be left in the underreamed area when drilling is complete, which is costly and otherwise unacceptable in some drilling operations. 
     Each of these problems is addressed by my co-pending U.S. application Ser. No. 08/674,123, and by the additional related underreamer embodiments disclosed and claimed below. 
     In addition to the foregoing problems associated with known underreamers, quick and efficient removal of drilling debris from the hole and drilling bits remains a problem. In my U.S. Pat. No. 5,511,628, which is hereby expressly incorporated by reference into this application, I disclosed a pneumatic down-hole drill with a central evacuation outlet. The apparatus of U.S. &#39;628 permits continuous evacuation of large debris fragments through a central axial bore formed in the bit and through a central evacuation tube attached thereto. Compressed air is directed downwardly through peripheral channels, under the drill bit, and into a central evacuation tube. The flow of compressed air through the central evacuation tube provides continuous and efficient removal of earthen fragments from the bore, including rapid removal of fragments that would be too large for removal through peripheral pathways along the casing. 
     However, a need remains for a reverse circulation pneumatic drill which provides for underreaming of the bore, continuous evacuation of drilling debris fragments from the drilling face in the bore, and for ready removal of the drill bit through the casing during or after completion of the drilling operation. 
     SUMMARY OF THE INVENTION 
     The present invention is embodied in a reverse circulation system that addresses the shortcomings of the prior art. 
     It is therefore an object of the invention to provide an underreamer that includes a pilot bit on which are mounted underreamer arms which can be extended and retracted by relative rotation between the pilot bit and the underreamer arms. Each underreamer arm includes a strengthening boss. The strengthening boss includes axial bearing surfaces that engage corresponding axial surfaces of the pilot bit. The bearing surfaces of the arm bosses and the bits include surfaces shaped to extend the arms as the pilot bit is rotated relative to the pilot bit. Surfaces are also provided for locking the arm in its extended underreaming position. As the bit is rotated in the opposite direction, the locking surfaces disengage and the arm can be retracted without vertical movement of the driver. 
     In another aspect of the invention, provision is made to continually flush the bit with compressed air which is exhausted from the down hole hammer. The flow of exhaust air is routed through porting in the bit assembly into the central evacuation tube. A second flow of compressed air may also be provided to continually flush the perimeter region of the bit. In one embodiment, the perimeter flushing air is received from compressed air introduced at the well-head to pressurize the casing. 
     These and other aspects of the invention will be described in further detail with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial cross-sectional view of a drilling assembly according to the present invention. 
     FIG. 2 is an expanded partial cross-sectional view of the assembly shown in FIG. 1, showing the power head assembly, compressed air inlet collar, and the upper terminus of the dual wall pipe assembly. 
     FIG. 3 is an expanded cross-sectional view of the assembly shown in FIG. 1, showing the casing driver in greater detail. 
     FIG. 4 is an expanded cross-sectional view of assembly shown in FIG. 1, showing the dual wall pipe assembly and the box and back head assembly connecting the lower terminus of the dual wall pipe assembly to the down-hole pneumatic hammer. 
     FIG. 5 is a cross-sectional view of the down-hole pneumatic hammer assembly, including the bit assembly. 
     FIG. 5A is a perspective view of an alternative design for the hammer barrel of the down-hole pneumatic hammer assembly. 
     FIG. 6A is an exploded perspective view of a first embodiment of a bit assembly according to the present invention. 
     FIG. 7A is a perspective view of the pilot bit on the embodiment of FIG.  6 A. 
     FIG. 7B is a bottom view of the pilot bit shown in FIG.  7 A. 
     FIG. 8 is a perspective view of an underreamer arm used in the embodiment shown in FIG.  6 A. 
     FIG. 9A is an end view of the underreamer arm shown in FIG.  8 . 
     FIG. 9B is an outer side view of the underreamer arm shown in FIG.  8 . 
     FIG. 10 is a bottom view of the bit driver of the embodiment shown in FIG. 6A, showing the axial surfaces which define the recesses which receive the underreamer arms, and the axial surfaces which bear against the underreamer arms for extension and retraction. 
     FIG. 11 is a bottom view of the bit driver in a second embodiment of the invention. 
     FIG. 12 is a bottom view of an underreamer arm of the embodiment referred to in FIG.  11 . 
     FIG. 13 is a bottom view of the arms depicted in FIG. 12 mounted in their retracted position on the bit driver shown in FIG.  11 . 
     FIG. 14 is a top view of a pilot bit for use with the bit driver and underreamer arms depicted in FIG.  13 . 
     FIG. 15 is a bottom view of the pilot bit depicted in FIG.  14 . 
     FIG. 16 is the bit driver and underreamer arms shown in FIG. 13 with the underreamer arms in their extended positions. 
     FIG. 17 is an enlarged partial view of the bit driver and underreamer arm shown in FIG.  13 . 
     FIG. 18 is a partial cutaway bottom view of the bit assembly depicted in FIGS. 11-17 showing the compressed air flow path. 
    
    
     DETAILED DESCRIPTION 
     Referring now to FIG. 1, a reverse circulation drilling system, shown generally at  10 , includes a head assembly  11 , a dual wall pipe assembly  12 , and a down hole pneumatic hammer  13  within a bore casing  14 . Turning to FIGS. 2 and 3, head assembly  11  includes a casing driver  15  for driving the bore casing  14  downwardly as the bit advances, and a power head assembly  16  of standard design for rotating the bore casing  14  it is driven downwardly. Casing driver  15  includes an annular hammer  17  which reciprocates vertically as compressed air is alternatively admitted to chambers above and below hammer  17 . Hammer  17  impacts on anvil  18 , which in turn impacts on casing cap  19 . Casing cap  19  is sealed against the inner surface of bore casing  14  to permit pressurization, through port  20 , of bore casing  14  between casing cap  19  and down hole hammer assembly  13 . Pressurization of the casing provides a downward flow of air between the casing and the down hole hammer, preventing upward migration of debris between the down hole hammer and casing, which can hinder the removal of the hammer. 
     In locations where there is a concern about the stability of the formation being drilled, use of a pressurizing fluid other than air is preferred. The alternative pressurizing fluid in such instances can be water, drilling mud, a polymeric liquid, or another substantially non-compressible fluid. When a non-compressible fluid is used to pressurize the casing, a portion of the fluid is discharged into the lower portion of the bore, and supports the surrounding formation, reducing the likelihood of the bore collapsing. 
     Power head assembly  16  is connected to anvil  18  through linkage assembly  21  to impart rotation to the dual pipe assembly and the down hole hammer. Power head assembly  16  is of a design generally known in the field, other than its central member  22 , that is threaded onto the upper end of dual wall pipe assembly  14 , includes a central bore in communication with the dual wall pipe assembly to extend the debris discharge path through the power head to the elbow  29 . The joint of central member  22  and the dual wall pipe  14  includes a port  23  for admitting air to the annulus  24  between the inner wall  25  and the outer wall  26  of the dual wall pipe assembly. Collar  27  is mounted around the joint, and includes air inlet  28 , through which compressed air is admitted into the dual wall pipe assembly for driving the down hole hammer as further described below. An elbow  29  is rotatably mounted and sealed to the upper end of central member  22 . Elbow  29 , central member  22  and the inner wall  25  of dual wall pipe assembly  14  together form a central drilling debris discharge tube for continuously discharging drilling debris from the down hole hammer as will also be described more fully below. 
     Turning also to FIG. 4, dual wall pipe assembly  12  is assembled from individual segments, each of which includes an inner pipe  31  and an outer pipe  33 . Each segment includes a threaded male connector  33  and a threaded female connector  35  at opposite ends. Male connector  14  and female connector  15  each includes air ports  36  and  37  respectively which are in communication with outer annulus  24  of dual wall pipe assembly  11 . At its upper end, dual wall pipe assembly is threaded in to central member  22  of power head  16 . At its lower end, dual wall pipe assembly  11  is connected to the box  38 , which in turn is threaded into back head  40  of down-hole hammer  13 . Ports  42  an  44  communicate with annulus  24  of the dual wall pipe assembly to route compressed air therefrom into the down hole hammer. 
     Turning now to FIG. 5, down-hole hammer  13  includes box  38  threaded onto back head  40 . A sleeve  41  and a hammer barrel  42  are threaded into back head  40 . A centrally located discharge tube  43  is pressed into sleeve  41 . A wear sleeve  44  is fitted around hammer barrel  40 , and press fitted over ring  45  and onto shoulder  46  of back head  40 . Sleeve  41  and barrel  42  define an annular upper air chamber  48 . Central evacuation tube  43  and barrel  42  define an annular lower air chamber  50 . The lower end of barrel  42  abuts bit driver  52 , and also includes a perimetrical lip  54  which engages wear sleeve  44  to center barrel  42  in the wear sleeve. Hammer  53  is slidingly fitted into barrel  42  for reciprocation. Bit driver  52  is slidably fitted into barrel  42  below hammer  53 , and over the lower end of central evacuation tube  43 . Bit driver  52  is retained in barrel  42  by a plurality of keys  56 , each of which is fitted into a keyway  58  and annular recess  60  of bit driver  52 . (See also applicant&#39;s U.S. Pat. No. 5,511,628, incorporated by reference above, for detail of an alternate barrel assembly incorporating a like key and keyway assembly for mounting the bit driver in the hammer barrel.) The key-keyway assembly permits the bit assembly to advance ahead of the dual wall pipe assembly during drilling. 
     A bit assembly according to the present invention is shown in FIG.  6 . Turning to FIG. 6, a bit assembly includes bit driver  52 , pilot bit  82 , and arms  88   a-c . Pilot bit  82  includes an upper shank  83  having a recessed chamfer  84 , camming surfaces  85   a  and  85   b , and a lower portion  86 . Lower portion  86  includes three peripheral recesses  87   a-c . Hardened drilling buttons, preferably made of a carbide material, are mounted on the peripheral and bottom surfaces of the pilot bit (FIG.  7 ). Arms  88   a-c  are nested atop pilot bit  82 , and slide thereon in an prescribed arcuate path as will be described. Each of the arms includes a raised boss  89  which is received into corresponding recess  90  of bit driver  52  (FIG.  10 ). Raised boss  89  serves several functions. First, impact forces from the hammer are transmitted downwardly to the pilot bit  82  through bit driver  52 , boss  89 , and arm  88 . Second, boss  89  is received and retained in recess  90 , where it rotates through a limited arc to extend and retract arm  88 . With arm  88  in its retracted position, surface  91  is adjacent camming surface  85   a . in this configuration, the overall diameter of the bit assembly is less than the inner diameter of the bore casing, permitting the bit assembly to be withdrawn from the bore. As arm  88  is rotated clockwise about pilot bit  82  by clockwise rotation of bit driver  52 , angled surfaces  85   a  engage surface  92  and urge arm  88  outwardly. The rotation and extension of arm  88  continues until surface  92   a  abuts surface  85   b  and surface  92   b  abuts surface  85   a , locking arm  88  in its extended position. To unlock and retract arm  88 , bit driver  52  is rotated in the opposite direction. In its fully retracted position, the overall diameter of the underreamer assembly is less than the inside diameter of the casing, permitting withdrawal of the entire underreamer bit assembly through the casing if necessary. This feature represents a significant advance over known underreamers, which cannot be retracted and withdrawn through the casing if necessary. 
     In operation, compressed air is delivered into annular chamber  59  through port  37 , radial ports  60 , annulus  62  and axial ports  64 . In FIG. 5, hammer  53  is shown during its downward stroke. Lip  66  is engaged with lip  68 , sealing off chamber  48 . Lip  72  is engaged with lip  74 , sealing off chamber  50 . Port  78  is closed. As piston  53  continues downwardly, port  76  is uncovered, exhausting chamber  48 . At about the same time, lip  74  disengages from lip  72 , admitting a fresh charge of compressed air into chamber  50  to raise piston  53  to its upper position after it has struck bit driver  52 . As piston  53  rises, port  78  is uncovered, exhausting chamber  50 . Lip  74  engages lip  72 , sealing chamber  50 . Port  76  is sealed by piston  53 , and lip  66  disengages from lip  68 , admitting a fresh charge of compressed air into chamber  48 . The fresh charge of compressed air in chamber  48  drives piston  53  downwardly to begin another stroke. The compressed air exhausted into ports  76  and  78  is collected in port  80  (FIG.  5 A), and discharged through the bit assembly into central evacuation tube  43 , carrying with it drilling debris and earthen fragments dislodged by the bit. As an added precaution against drilling debris becoming lodged between arms  88   a-c  and the pilot bit, in the bit assembly embodiment shown in FIG. 6B, port  91  is provided through which compressed air can be discharged to clear debris. The flow of compressed air through the bit assembly is essentially continuous, and provides a continuous evacuation of drilling debris from the drilling face of the bore. Moreover, the essentially constant diameter of the evacuation tube and inner wall of the dual wall pipe assembly provides a constant air velocity, which further aids debris removal. The continuous removal of debris through the central evacuation tube promotes continuous drilling. It is seldom, if ever necessary to stop drilling and raise the bit to clear debris from the bore. Significant improvements in drilling rates directly result. In addition, since debris is quickly removed as it is dislodged, it is possible to obtain a relatively accurate “core” sample from the bore. This aspect of the invention is useful in both exploratory and environmental applications. 
     In another aspect of the invention, pilot bit  104  advances into the ground with the underreamer arms locked in a deployed position below and radially beyond the advancing end of the casing at C. Casing movement is facilitated by the relatively large underreamed area, and if required, by the casing driver  15 . In one embodiment shown in FIGS. 1 and 3, if the drill bit assembly advances more than a predetermined distance ahead of the casing, linkage  21  operates a valve to provide compressed air to the pneumatic hammer  17  and associated porting casing driver  15 . 
     An alternative embodiment of the invention will now be described with reference to FIGS. 11-19. In this embodiment, the bit assembly also includes a bit driver  100 , arms  102   a-c , and pilot bit  104 , which are fitted together as described in the previous embodiment shown in FIG.  6 . In this embodiment, however, compressed exhaust air from port  80  is routed through internal ports in the bit driver, arms and pilot bit. Referring to FIG. 1, hammer exhaust air from port  80  flows into and through bit driver  100  via ports  106   a-c . The hammer exhaust air then flows through ports  108   a-c  formed in arms  102   a-c  respectively (FIG.  12 ). In FIG. 13, the arms are shown mounted on the bit driver in their closed and retracted positions. Exhaust air from ports  108   a-c  flows into ports  110   a-c  in pilot bit  104  (FIGS. 14,  15 ), through channels  112   a-c , ports  114   a-c , and into central evacuation tube  43  (FIG.  5 ). Ports  106   a-c ,  108   a-c  and  110   a-c  respectively are located so that they are all aligned when arms  102   a-c  are extended; i.e., holes  106   a ,  108   a  and  110   a  are aligned, holes  106   b ,  108   b  and  110   b  are aligned, and holes  106   c ,  108   c  and  110   c  are aligned. Referring to FIGS. 16 and 17, when driver  100  is rotated relative to pilot bit  104  to position arms  102   a-c  in their closed, retracted positions, ports  108   a-c  (through arms  102   a-c  respectively) are partially offset from ports  106   a-c  respectively; ports  110   a-c  (through the pilot bit) are offset from ports  108   a-c  (through the pilot bit) are entirely offset. To provide for a continuous flow of air through ports  106   a-c ,  108   a-c  and  110   a-c  when the arms are retracted, channels  112   a-c  are provided in the underside of arms  102   a-c . Turning again to FIG. 15, pilot bit  104  also includes axial recesses  114   a-c , and transverse channels  116   a-c . Recesses  114   a-c  and channels  116   a-c  provide a path for the discharge of compressed air from outside the bore casing to also be discharged through central evacuation tube  43 . 
     The foregoing description of the invention is intended to be illustrative rather than exhaustive. Those skilled in the art will appreciate that numerous changes in detail are possible without departing from the scope of the following claims.
A underreamer drill bit assembly including a pilot bit and extendable underreaming arms operatively connected to the pilot bit. The underreaming arms have an extended position for underreaming, and a retracted position in which the overall diameter of the underreamer drill bit assembly is less than the inside diameter of the well casing, permitting the entire bit assembly to be withdrawn through the well casing. In another aspect of the invention, the bit assembly is operatively connected to a dual wall pipe assembly. A supply of compressed air is conducted through the annulus of the dual wall pipe assembly to a down hole pneumatic hammer. Exhaust air from the down hole hammer is directed to the bit assembly for continuous removal of drilling debris through a central evacuation tube of the dual wall pipe assembly. In another aspect of the invention, a pressurized, incompressible fluid is injected into the annulus between the well casing and the downhole pneumatic hammer.