Abstract:
A percussive assisted rotary drill includes a top sub for connection with a drill pipe. The drill pipe imparts torque to the drill and also supplies motive fluid to the drill. The drill includes a shank adapter to facilitate affixing a rotary drill bit to the drill. The motive fluid is divided between a bit flow which flows through the bit to clear debris at the bottom of the drill, and an actuator flow. An actuator, which may be in the form of a reciprocating piston, moves within the drill under the influence of the actuator flow to impart cyclical blows to the shank adapter. The blows are transferred to the drill bit through the shank adapter to provide a relatively high frequency low amplitude percussive force on the rotating drill bit to assist in the drilling operation. At least a portion of the actuator flow portion of the motive fluid is exhausted through the top end of the drill. The relative flow rates and volumes of the bit and actuator flows can be adjusted with a check valve in the actuator flow exhaust path.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/369,579, filed Feb. 11, 2009, the content of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The two most common methods for drilling rock involve either quasi-static loading of rock as used in rotary drilling, or high intensity impact loading as used in down-the-hole (DTH) drilling. DTH applications include a hammer assembly having a piston or actuator that reciprocates within the drill casing and applies a cyclical impact on an anvil. The anvil is typically part of or directly connected to the drill bit so that impact forces of the piston striking the anvil are transferred through the drill bit into the rock being drilled. The piston typically reciprocates in response to motive fluid (e.g., compressed air) alternatingly raising and lowering the piston. All motive fluid is typically exhausted from the drill through the drill bit after actuating the hammer assembly. Exhausting motive fluid through the drill bit clears cuttings and other debris from around the drill bit and carries such debris up out of the hole or bore being drilled. Hybrid rock drills (called percussive assist rotary drills or PARD) that utilize a DTH hammer assembly to impact a rotary drill bit are also known, and also exhaust all motive fluid through the drill bit. 
     When motive fluid is exhausted through the drill bit, it flows over an exterior surface of the drill bit (“flows over” and variations thereof meaning in this specification that the motive fluid flows across and in contact with the drill bit exterior surface) and up the bore being drilled. In known DTH hammer assemblies having reverse circulation configurations, the motive fluid is actually exhausted above the drill bit, flows down over the drill bit exterior, and then flows up through the center of the drill bit, drill assembly, and drill pipe or drill string to the surface. In this specification, the term “through the bit” and “bit exhaust” are intended to include exhausted motive fluid that flows over the drill bit exterior surface, whether flowing out of the bit and up the bore or flowing in a reverse circulation direction. 
     In the present application, the terms “down hole hammer,” “hammer,” and “hammer assembly” refer to a drilling arrangement using the impact forces of a reciprocating piston or other moving actuator, whether such drilling arrangement is present in a DTH application, a PARD arrangement, or another arrangement, and regardless of whether the drilling arrangement includes a standard bit, drag bit, rotary bit, or another cutting surface. 
     The present invention relates to a down hole hammer that exhausts at least a portion of the motive fluid through a portion of the drill other than the drill bit. For drilling operations in which the drill bit is at or near the bottom of the drill assembly, the invention may be termed a down hole hammer having a portion of motive fluid exhausted above the drill bit or a down hole hammer having elevated exhaust. The invention also relates to a down hole hammer in which motive fluid is divided into a portion that is exhausted through the drill bit or elsewhere such that it flows over a portion of the drill bit&#39;s exterior, and a schematically parallel portion that operates the piston and is exhausted above the drill bit such that it does not flow over the drill bit&#39;s exterior surface. 
     SUMMARY 
     In one embodiment, the invention provides a down-hole drilling tool comprising: a housing; a bit connected to an end of the housing and adapted to drill rock; a piston comprising a central piston bore and at least one conduit communicating with the central piston bore; a control tube including at least one port, the control tube receiving a flow of motive fluid comprising an actuator supply portion and a bit flow portion; a drive chamber above the piston; and a return chamber between the piston and the bit; wherein the control tube extends through the central piston bore and the piston reciprocates along the control tube; wherein reciprocation of the piston along the control tube periodically places the at least one conduit in the piston in communication with the at least one port in the control tube; wherein periodic communication between the at least one conduit and at least one port causes the actuator supply portion of the motive fluid in the control tube to be supplied to the drive chamber and return chamber in alternating fashion, to cause the piston to respectively move into impact with the bit and lift away from the bit; wherein the actuator supply portion of the motive fluid becomes actuator exhaust upon flowing out of the drive chamber and return chamber, the actuator exhaust flowing along an actuator exhaust path and being vented above the bit; wherein the bit flow portion of the motive fluid in the control tube flows along a bit exhaust path and is vented through the bit; and wherein the bit exhaust path is separate from and schematically parallel to at least a portion of the actuator exhaust path. 
     In one embodiment of the invention, reciprocating movement of the piston at least temporarily cuts off communication between the drive chamber and the actuator exhaust path while placing return chamber in communication with the actuator exhaust path, and at least temporarily cuts off communication between the return chamber and the actuator exhaust path while placing the drive chamber in communication with the actuator exhaust path. In another embodiment, the invention further comprises a drive exhaust port communicating between the drive chamber and the actuator exhaust path; and a return exhaust port communicating between the return chamber and the actuator exhaust path; wherein reciprocating movement of the piston at least temporarily cuts off communication between the drive chamber and the actuator exhaust path by covering the drive exhaust port with a portion of the piston; and wherein reciprocating movement of the piston at least temporarily cuts off communication between the return chamber and the actuator exhaust path by covering the return exhaust port with a portion of the piston. In another embodiment, the at least one port in the control tube includes a drive supply port and a return supply port; wherein the piston includes a drive supply conduit and a return supply conduit; wherein reciprocating movement of the piston at least temporarily places the drive chamber in communication with the actuator flow path by aligning the drive supply port with the drive supply conduit; and wherein reciprocating movement of the piston at least temporarily places the return chamber in communication with the actuator flow path by aligning the return supply port with the return supply conduit. In another embodiment, the invention further comprises a flow plate at least partially defining a throttle chamber; and check valve within the throttle chamber; wherein adjustment of the check valve at least partially controls the ratio of the bit flow portion to the actuator supply portion of the motive fluid. In another embodiment, the invention further comprises a top sub defining a top end of the drilling tool and adapted for connection to a drill pipe; wherein the actuator exhaust path vents the actuator exhaust through the top sub; and wherein the flow plate is adapted to be clamped to the drilling tool by attachment of the drill pipe to the housing. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a percussive assisted rotary drill assembly embodying the present invention. 
         FIG. 2  is an exploded view of the drill assembly. 
         FIG. 3  is a cross-sectional view of the drill assembly in a bottomed-out standby condition. 
         FIG. 4  is a cross-sectional view of the drill assembly at the end of the drive stroke and beginning of the return stroke. 
         FIG. 5  is a cross-sectional view of the drill assembly in the middle of the drive stroke and return stroke. 
         FIG. 6  is a cross-sectional view of the drill assembly at the beginning of the drive stroke and end of the return stroke. 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
     For the sake of simplicity and consistency in this specification, the term “axial” means in a direction parallel to a central axis  10  of a percussive assisted rotary drill assembly  25  illustrated in the drawings. All of the main elements of the drill assembly  25  discussed below are generally ring-shaped or cylindrical and therefore all have inner and outer surfaces. The term “inner surface” means the surface facing toward the central axis  10  or generally toward the inside of the drill assembly  25  and the term “outer surface” means the surface facing away from the central axis  10  or generally away from the inside of the drill assembly  25 . All elements also have first and second ends which, using the convention of the illustrated embodiment, will be referred to as “top” and “bottom” ends with respect to the typical operating orientation of the rotary drill assembly  25 , which orientation is illustrated in  FIGS. 2-6 . Also, terms such as “above” and “elevated” describe a relative position while the drill assembly  25  is in the typical operating orientation. 
     While the invention is illustrated in the drawings and described below in the embodiment of a PARD (i.e., having both rotary and impact aspects to the drilling operation), such embodiment is not limiting to the scope of the invention. The invention may also be embodied in a pure DTH drill arrangement in which there is no rotary component. The invention may be embodied in drilling arrangements using substantially any type of drill bit, including a standard bit, drag bit, rotary bit, or another cutting surface suitable for or adaptable to impact loading. The invention may also be embodied in substantially any other down hole hammer application in which at least a portion of the motive fluid is exhausted somewhere other than through the drill bit. 
       FIGS. 1 and 2  illustrate a flow plate  15 , a check valve  20 , and a percussive assisted rotary drill assembly  25 . The drill assembly  25  includes the following basic components: a rotary tool joint or top sub  30 , a control tube  35 , a cylinder head  40 , a cylinder  45 , a piston or actuator  50 , an outer sleeve  55 , a snap ring  60 , a bit bearing  65 , a bit retainer or split ring  70 , a washer  75 , a chuck  80 , and a shank adapter  85 . A hammer assembly of the tool  25  includes the illustrated reciprocating piston  50  or other actuator and other components that control the flow of motive fluid to actuate the piston  50  or other actuator. 
     The top sub  30  includes an American Petroleum Institute (“API”) male threaded connector  90  that is adapted to be threadedly received within a drill pipe DP. The top sub  30  also includes a main body  95  that includes a large diameter cylindrical portion  100  and a small diameter cylindrical portion  105 . A step or shoulder  110  is defined between the large and small diameter cylindrical portions  100 ,  105 . The top of the large diameter cylindrical portion  100  defines an exhaust face  115  around the API connector  90 . The bottom end  120  of the small diameter cylindrical portion  105  has a reduced diameter. A top sub bore  125  extends axially through the center of the top sub  30 . The main body  95  includes multiple exhaust bores  130  arranged around and generally parallel to the top sub bore  125 . 
     The flow plate  15  and check valve  20  are ring-shaped and surround the API connector  90  of the top sub  30 . In the illustrated embodiment, the flow plate  15  is pressed or clamped against the exhaust face  115  by the drill pipe DP when the drill pipe DP is threaded onto the API connector  90 . In other embodiments, the flow plate may be part of or integral with the back head. The flow plate  15  includes exhaust holes  135  that communicate with the space around the drill assembly  25  and drill pipe DP. The check valve  20  is free to move axially within the space defined between the flow plate  15  and the top sub  30  (the throttle chamber, as will be discussed below). As will be discussed in more detail below, the flow plate  15 , check valve  20 , or the combination of the flow pate  15  and check valve  20  operates as a throttle for operation of the piston  50 . 
     The control tube  35  includes an enlarged mounting end  140  received within the top sub bore  125 . The control tube  35  defines an axially-extending control bore  145 . A plurality of o-ring seals  150  ( FIG. 3 ) provides a substantially air-tight seal between the top sub bore  125  and the outer surface of the enlarged mounting end  140  of the control tube  35 . Consequently, fluid flowing through the top sub bore  125  is substantially prevented from flowing around the outer surface of the enlarged mounting end  140 , and is instead forced to flow into the control bore  145 . The control tube  35  also includes drive supply ports  155  and return supply ports  160  communicating through the sides of the control tube  35 . 
     The cylinder head  40  includes a ring-shaped flange  165 , a ring-shaped support surface  170  that is surrounded by and recessed with respect to the flange  165 , and a depending skirt  175 . The support surface  170  defines a central hole  180  through which the control tube  35  extends. The enlarged mounting end  140  of the control tube  35  and one of the sealing o-rings  150  abut against the support surface  170  to create a substantially air-tight seal between the control tube  35  and the support surface  170 . Consequently, there is substantially no fluid flow through the central hole  180  of the cylinder head  40  except through the control bore  145  of the control tube  35 . The bottom end  120  of the small diameter cylindrical section  105  abuts the support surface  170  of the cylinder head  40 , which positions the bottom ends of the exhaust bores  130  adjacent the flange  165 . Exhaust fluids flowing around the cylinder head  40  can flow into the exhaust bores  130  of the top sub  30 . 
     The cylinder  45  includes drive exhaust ports  185  and return exhaust ports  190  that communicating through a side of the cylinder  45 . The bottom of the cylinder head  40  flange  165  abuts a top end of the cylinder  45 , and the depending skirt  175  of the cylinder head  40  extends into the cylinder  45 . A sealing member  195  ( FIG. 3 ) provides a substantially air-tight seal between the depending skirt  175  of the cylinder head  40  and the inner surface of the cylinder  45 . The top end of the cylinder  45  includes grooves  200  that permit exhaust fluid flowing around the outside of the cylinder  45  to flow past the top end of the cylinder  45 . 
     The piston  50  includes a central piston bore  210 , a drive end  215  having a beveled ring-shaped surface  220 , a return end  225  also having a beveled ring-shaped surface  230 , and an enlarged-diameter middle portion  235 . The piston bore  210  is closely dimensioned to receive the control tube  35  such that the piston  50  is free to slide along the control tube  35  while maintaining close tolerances and a substantially air-tight seal between the piston bore  210  and the outer surface of the control tube  35 . A plurality of drive conduits  240  communicate between the piston bore  210  and the beveled surface  220  on the drive end  215  of the piston  50 , and a plurality of return conduits  245  communicate between the piston bore  210  and the beveled surface  230  on the return end  225  of the piston  50 . As will be discussed in more detail below, as the piston  50  reciprocates along the control tube  35 , the drive conduits  240  are placed in communication with the drive supply ports  155  of the control tube  35 , or the return conduits  245  are placed in communication with the return supply ports  160  of the control tube  35 . The piston  50  is received within the cylinder  45 , and the enlarged-diameter middle portion  235  of the piston  50  is closely dimensioned to slide against the inner surface of the cylinder  45 . 
     An internal surface of the outer sleeve  55  includes threads at each of the top and bottom ends. The internal surface also includes internal shoulders and other surfaces (visible in  FIGS. 3-6 ) against which bear the top sub  30 , cylinder  45 , snap ring  60 , and chuck  80 . The external threads on the main body  95  of the top sub  30  thread into the threads in the top end of the outer sleeve  55 . The snap ring  60  is positioned against a portion of the inner surface of the outer sleeve  55 , and the bit bearing  65  and split ring  70  are stacked against the snap ring  60  within the outer sleeve  55 . 
     The chuck  80  includes an internally-splined portion  250  which has internal splines  255  and external threads, and an enlarged head portion  260  which defines a ring-shaped bearing surface  265  at the base of the internally-splined portion  250 . The washer  75  sits on the ring-shaped bearing surface  265  around the internally-splined portion  250 . The internally-splined portion  250  is threaded into the bottom end of the outer sleeve  55  until the bottom end of the outer sleeve  55  bears against the washer  75  and ring-shaped bearing surface  265 . The internally-splined portion  250  of the chuck  80  forces the split ring  70  and bit bearing  65  against the snap ring  60  as the chuck  80  is threaded into the outer sleeve  55 . 
     The shank adapter  85  includes an anvil  280  at its top end, an externally-splined portion  285  having external splines  290 , and a bit-mounting head  295  at its bottom end. An adapter bore  300  extends axially from the top end to the bottom end of the shank adapter  85 . The anvil  280  is received within the bit bearing  65 , with the control tube  35  extending into the adapter bore  300 . The anvil  280  includes external blow down grooves  305  that permit the blow down of exhaust fluid through the bit bearing  65 , split ring  70 , and chuck  80  to enable more quick stopping of the hammer assembly cycle. 
     The bit-retaining head  295  includes internal threads or other suitable connecting apparatus for receiving a rotary drill bit (e.g., a tricone) DB or other suitable work piece for rock drilling. In other embodiments, the entire shank adapter  85  may be integrally formed with the drill bit DB, instead of being provided as separate parts as illustrated. The drill bit DB includes an exterior surface or working surface that bears against rock or other material being drilled. 
     The external splines  290  of the splined portion  285  mesh with the internal splines  255  of the chuck  80  such that torque is transmitted from the chuck  80  to the shank adapter  85 , while the shank adapter  85  is permitted to move axially within the chuck  80 . Top edges of the external splines  290  and a bottom surface of the anvil  280  define stopping surfaces for axial movement of the shank adapter  85  with respect to the chuck  80 . The split ring  70  is assembled around the shank adapter  85  between the stopping surfaces. 
     The drill assembly  25  is assembled by extending the control tube  35  through the central hole  180  of the cylinder head  40 , placing the cylinder head  40  on the top end of the cylinder  45 , and positioning the piston  50  inside the cylinder  45  with the control tube  35  extending through the piston bore  210 . The top sub  30  is then positioned with the enlarged mounting end  140  of the control tube  35  inside the top sub bore  125  and is threaded into the top end of the outer sleeve  55  such that the bottom end  120  of the top sub  30  abuts against the support surface  170  of the cylinder head  40 . A gap exists between the shoulder  110  and the top of the outer sleeve  55 , which may be referred to as “stand off.” Then the snap ring  60  and bit bearing  65  are positioned within the outer sleeve and the subassembly of the split ring  70 , shank adapter  85 , chuck  80 , and washer  75  is inserted into the lower end of the outer sleeve  55 . The internally-splined section  250  of the chuck  80  is threaded into the bottom end of the outer sleeve  55 . Wrenches are then applied to flats  307  on the top sub  30  and shank adapter  85 , and torque is applied to both to cause the top sub  30  to further thread into the top end of the outer sleeve  55  such that the bottom end  120  pushes the cylinder head  40  into the top of the cylinder  45  and creates a clamping load to keep the cylinder head  40  and cylinder  45  locked together during heavy vibrations arising from use of the drill assembly  25 . 
     With reference to  FIG. 3 , when the drill assembly  25  is not being pushed against rock and is simply subject to forces arising from gravity, the shank adapter  85  bottoms out with the bottom surface of the anvil  280  resting on top of the split ring  70 . With reference to  FIGS. 4-6 , when the drill assembly  25  is engaged against rock, the shank adapter  85  is pushed up until it tops out when the tops of the external splines  290  abut the bottom of the split ring  70  and the bit-mounting head  295  bears against the enlarged head  260  of the chuck  80 . 
     As assembled, the drill assembly  25  defines a central bore consisting of the top sub bore  125 , the control bore  145 , and the adapter bore  300 . The drill assembly  25  also defines several passages and chambers. A drive chamber  325  is defined between the cylinder head  40 , the inner surface of the cylinder  45 , the outer surface of the control tube  35 , and the drive end  215  of the piston  50 . A return chamber  330  is defined between the return end  225  of the piston  50 , the inner surface of the cylinder  45 , the inner surface of the outer sleeve  55 , the top of the bit bearing  65 , the anvil  280 , and the outer surface of the control tube  35 . An annular exhaust chamber  335  is defined between the outer surface of the cylinder  45  and the inner surface of the outer sleeve  55 . A throttle chamber  340  is defined between the flow plate  15  and the exhaust face  115  of the top sub  30 . The check valve  20  is within the throttle chamber  340 . 
     The drill assembly  25  also defines a bit exhaust path, an actuator flow path, and an actuator exhaust path. The actuator flow path and actuator exhaust path are in series in the illustrated embodiment, and the bit exhaust path is schematically parallel to the actuator flow path and actuator exhaust path. As used with respect to flow and exhaust paths, the term “series” means that fluid flows from one path into the other, and the term “schematically parallel” means that the paths are not in series. The bit exhaust path includes the central bore downstream of the drive and return supply ports  155 ,  160 , and delivers motive fluid (e.g., compressed air) to the drill bit DB where it flows out of the drill bit DB, over the drill bit&#39;s exterior surface, and up through the bore between the drill assembly and bore wall as bit exhaust. In other embodiments, such as reverse circulation systems, the bit exhaust may flow out of the tool above the drill bit DB, flow over the exterior surface of the drill bit, and return to the surface through the bit bore and other conduits in the drill pipe DP. The terms “bit exhaust” and “through the drill bit” and similar terms are intended to cover exhaust that flows over the exterior surface of the drill bit, whether in a regular or reverse circulation direction. 
     The actuator flow path includes the drive supply ports  155 , drive conduits  240 , drive chamber  325 , drive exhaust ports  185  (these four components, collectively, the “drive side” of the actuator flow path), return supply ports  160 , return conduits  245 , return chamber  330 , and return exhaust ports  190  (these last four components, collectively, the “return side” of the actuator flow path). The actuator exhaust path includes the annular exhaust chamber  335 , the grooves  200  at the top of the cylinder  45 , and the exhaust bores  130 . Motive fluid flowing out of the actuator flow path through the drive side and return side becomes actuator exhaust which flows into the actuator exhaust path. The actuator exhaust path delivers the actuator exhaust to the throttle chamber  340 . 
     In the throttle chamber  340 , the actuator exhaust is restricted as it lifts and flows around the check valve  20 . Finally, the actuator exhaust flows out of the throttle chamber  340  through the exhaust holes  135  in the flow plate  15 . The flow of actuator exhaust out of the exhaust holes  135  in the flow plate  15  assists the upward flow of cuttings and debris being evacuated from the hole or bore being drilled. The check valve  20  blocks cuttings and other debris from falling into the exhaust path. 
     In other embodiments, the actuator exhaust path may include schematically parallel exhaust paths for the drive chamber  325  and return chamber  330  which may vent actuator exhaust at different elevated axial locations with respect to the drill bit DB. Alternatively, one of the schematically parallel exhaust paths could be in series with the bit exhaust path such that some of the actuator exhaust flows over the exterior surface of the drill bit DB. The illustrated actuator exhaust path may be advantageous over an exhaust path that exhausts one or both of the drive and return chambers  325 ,  330  over the exterior surface of the drill bit DB because it reduces the volume of fluid flow over the exterior surface of the drill bit DB. Reducing the volumetric flow over the drill bit DB and other external members may reduce wear rates of such components and increase component life. 
     It will be appreciated that, although the illustrated embodiment includes an actuator exhaust path that vents the actuator exhaust through the top of the drill assembly  25 , the invention is applicable to any embodiment that includes elevated exhaust, by which is meant exhaust holes above the drill bit DB or elsewhere to substantially avoid flowing any of the actuator exhaust over the exterior surface of the drill bit DB. For example, exhaust holes may be provided through the outer sleeve  55 . 
     In operation, a conventional rotational force drives rotation of the drill pipe DP. Torque from the drill pipe DP is transmitted to the drill bit DB through a torque path that includes the top sub  30 , outer sleeve  55 , chuck  80 , and shank adapter  85 . In the illustrated embodiment, all elements of the torque path are coupled by way of threaded interconnections, except between the chuck  80  and shank adapter  85  which is by way of the splines  255 ,  290 . In other embodiments, the elements in the torque path may be coupled in other ways than threaded and splined connections, so long as the essential purpose of torque transfer is met. 
     During standby ( FIG. 3 ) when the drill assembly  25  is not engaged against the bottom of a hole or bore being drilled, the shank adapter  85  is bottomed out under the influence of gravity and the piston  50  rests on the anvil  280 . In this condition, sometimes referred to as blow down, the drive supply ports  155  of the control tube  35  are not aligned with the drive conduits  240  of the piston  50  (they are, in fact, above the piston), and the return supply ports  160  of the control tube  35  are not aligned with the return conduits  245  of the piston  50  (they are blocked by the middle portion  235 ). Motive fluid is typically supplied through the drill pipe DP during standby. Such motive fluid flows through the bit exhaust path and the drive side of the actuator flow path (except that the motive fluid flows directly from the drive supply ports  155  into the drive chamber  325  without flowing through the drive conduits  240 ) and is exhausted as bit exhaust and actuator exhaust. The bit exhaust and actuator exhaust resist debris from entering the drill assembly  25  during standby, and provide sufficient flow paths to avoid significant pressure increase in the drill assembly  25 . 
     When the drill bit DB is lowered to the bottom of the hole and engages rock or other substance to be drilled, the shank adapter  85  is pushed up toward the position illustrated in  FIG. 4 . As the shank adapter  85  moves up, it pushes the piston  50  up as well. The return conduits  245  register with the return supply ports  160  as the shank adapter  85  approaches its topped out position. Once the return conduits  245  are placed in communication with the return supply ports  160 , the actuator flow is directed to the return side. The actuator flow alternates between the drive side and return side to cause the piston  50  to reciprocate and impact the anvil  280 . In other embodiments, the drive and supply sides may drive non-reciprocal piston operation. The bit exhaust continues to flush cuttings and other debris around the outside of the bit DB. The bit exhaust and actuator exhaust together push such debris up to the surface through the hole being drilled. 
     The cycle of piston  50  reciprocation is described below, with upward movement of the piston  50  referred to as the “return stroke” and downward movement referred to as the “drive stroke.” With reference to  FIGS. 4-6 , the motive fluid supply and fluid exhaust logic is controlled and timed by the relative positions of the drive supply ports  155  and return supply ports  160 , the drive conduits  240  and return conduits  245 , and the drive exhaust ports  185  and return exhaust ports  190 . 
     With reference to  FIG. 4 , during the terminal portion of the drive stroke and the initial portion of the return stroke, the middle portion  235  of the piston  50  covers the return exhaust port  190  and the return conduits  245  register with the return supply ports  160  while at the same time the drive exhaust ports  185  are uncovered by the middle portion  235  of the piston  50  (i.e., the drive exhaust ports  185  communicate with the drive chamber  325 ) and the drive conduits  240  are not registered with the drive supply ports  155 . Thus, during the terminal portion of the drive stroke, there is slight compression of fluid in the return chamber  330  but such compression is negligible and does not materially affect the momentum of the piston  50  and its impact on the anvil  280 , and such compression is dissipated by blow down through the grooves  305 . During the initial portion of the return stroke, there is a rapid build-up of pressure in the return chamber  330  due to motive fluid rushing in through the return conduits  245 . Additionally, initial upward movement of the piston  50  is not restricted by significant opposing pressure in the drive chamber  325  because fluid in the drive chamber  325  is exhausted through the drive exhaust ports  185  into the exhaust path described above. 
     With reference to  FIG. 5 , during the middle segment of the drive and return strokes, the middle portion  235  of the piston  50  covers the drive exhaust ports  185  and return exhaust ports  190 , and neither of the drive conduits  240  nor the return conduits  245  are registered with the respective drive supply ports  155  or return supply ports  160 . From this point until the end of the drive and return strokes, the piston  50  moves partially under the influence of pressure built up in the respective drive and return chambers  325 ,  330  during the initial portion of the stroke and partially under the influence of momentum. As volume in the drive and return chambers  325 ,  330  increases due to movement of the piston  50  in the respective drive and return strokes, the pressure-assist component of movement is reduced, and the piston  50  moves primarily under the influence of the momentum it gained during the initial portion of the stroke. 
     With reference to  FIG. 6 , during the terminal portion of the return stroke and the initial portion of the drive stroke, the middle portion  235  of the piston  50  covers the drive exhaust port  185  and the drive conduits  240  register with the drive supply ports  155  while at the same time the return exhaust ports  190  are uncovered by the middle portion  235  of the piston  50  (i.e., the return exhaust ports  190  communicate with the return chamber  330 ) and the return conduits  245  are not registered with the return supply ports  160 . Thus, during the terminal portion of the return stroke, there is slight compression of fluid in the drive chamber  325  to assist in arresting upward movement of the piston  50 . During the initial portion of the drive stroke, there is a rapid build-up of pressure in the drive chamber  325  due to motive fluid rushing in through the drive conduits  240 . Additionally, initial downward movement of the piston  50  is not restricted by significant opposing pressure in the return chamber  330  because fluid in the return chamber  330  is exhausted through the return exhaust ports  190  into the exhaust path described above. 
     The illustrated drill assembly  25  therefore has a rotary component (the drill bit DB rotates under the influence of the torque transmitted through the drill pipe DP and the drill assembly  25 ) and a percussive component arising from the piston  50  impacting the anvil  280 . The impact of the piston  50  on the anvil  280  is transmitted through the shank adapter  85  and bit DB to the rock or other substance being drilled by the drill assembly  25 , which assists in the drilling operation. The axially-directed impact on the anvil  280  is not borne by any other component of the drill assembly  25 ; the distance between the bottom of the anvil  280  and the top of the external splines  290  is selected to accommodate the largest expected deflection of the shank adapter  85  to prevent the shank adapter  85  from bottoming out. After impacting the anvil  280 , the piston  50  typically rebounds slightly, but the degree of rebound depends at least in part on the hardness of the substance being drilled. The return conduits  245  and return supply ports  160  are sized to register with each other in the instance of no rebound or a degree of rebound within an expected range. Once the return supply ports  160  and return conduits  245  register with each other, the cycle begins again. 
     Fundamentally, the volume and flow rates of the bit and actuator flows are defined by the relative resistance in the actuator and bit exhaust paths. The level of resistance to the actuator exhaust flow is affected by the size and shape of the exhaust holes  135  in the flow plate  15  or the size and shape of the check valve  20  or the interaction between the flow plate  15  and check valve  20 , or a combination of two or more of the these factors. A more restrictive actuator exhaust path (arising from, for example, a lower lift check valve  20  and/or more restrictive exhaust holes  135 ) will result in lower actuator power, while a less restrictive actuator exhaust path (arising from, for example, a higher lift check valve  20  and/or less restrictive exhaust holes) will result in higher actuator power. 
     As resistance to the actuator exhaust flow increases, so does the backpressure in the actuator exhaust path, which ultimately affects the rate at which actuator exhaust fluid is pushed out of or displaced from the drive chamber  325  and return chamber  330  through the drive exhaust ports  185  and return exhaust ports  190  during piston  50  reciprocation. Speed and frequency of piston  50  reciprocation is affected, at least in part, by the rate at which exhaust fluid is displaced out of the drive chamber  325  and return chamber  330  through the drive exhaust ports  185  and return exhaust ports  190 . The faster motive fluid can be exhausted from the drive and return chambers  325 ,  330 , the faster the piston  50  can reciprocate and the more impact power (“actuator power”) the piston  50  can deliver to the drill bit DB. 
     An operator of the drill assembly  25  may adjust the split between bit and actuator flow by changing the size or shape of the check valve  20 , the space within the throttle chamber  340  accommodating axial movement of the check valve  20 , the size or shape of the exhaust holes  135  in the flow plate  15 , or a combination of these factors. Because the flow plate  15  and check valve  20  are secured to the drill assembly  25  only by the drill pipe DP connection trapping and clamping the flow plate  15  against the top sub  30 , the flow plate  15  and/or check valve  20  can be removed and replaced by merely disconnecting the drill pipe DP, replacing the parts, and re-connecting the drill pipe DP. Other than disconnecting and reconnecting the drill pipe DP, there are no fasteners or other connections that must be removed or loosened in the process of changing the check valve  20  in the illustrated embodiment. 
     Additionally, replacement of the flow plate  15  and/or check valve  20  does not require disconnection of the outer sleeve  55  from the top sub  30  or chuck  80  or any other disassembly of the drill assembly  25 , because the flow plate  15  and check valve  20  are external parts. Also, changing the flow plate  15  and/or check valve  20  permits the actuator power output to be adjusted while maintaining supply pressure constant. Thus, the flow plate  15  and check valve  20  subassembly permits one to adjust actuator power independent of supply pressure by simply changing an external part and without requiring a change in bit nozzle, and the flow plate  15  and check valve  20  may be said to function as a throttle for the bit and actuator flows. 
     Operating the bit exhaust path schematically parallel with the actuator flow path and actuator exhaust path is advantageous compared to operating the paths in series. The piston  50  operates at full system pressure and thus develops more actuator power when driven by actuator flow that is schematically parallel with respect to bit flow, than when compared to actuator flow that is in series with the bit flow. The schematically parallel bit and actuator flows achieve the dual benefit of clearing cuttings and other debris with minimal bit wear via bit flow, and boosting the hole cleaning flow above the drill assembly  25  via elevated actuator exhaust to assist in removal of cuttings and other debris from the hole. The illustrated embodiment of the present invention therefore exhausts the entire actuator exhaust out of an elevated exhaust (out of the top of the drill assembly  25  in the illustrated embodiment) and the entire bit exhaust out of the bottom of the drill assembly  25  through the drill bit DB. In other embodiments, it is possible to exhaust only one of the drive side and return side (i.e., less than the entire actuator flow) through an elevated exhaust and the other side out the drill bit DB. 
     In a series arrangement in which actuator exhaust is recycled as bit flow, backpressure in the bit flow path can affect the flow rate of actuator exhaust which may unnecessarily reduce actuator power. A schematically parallel arrangement of the bit and actuator flows decouples backpressure in the bit exhaust path from the actuator flow path. 
     One advantage of the present invention is to provide higher frequency impact loads to the drill bit DB when compared to known DTH and PARD rigs at an equal pressure and similar outer dimension size of the tool. For example, and without limitation, while a standard eight inch DTH hammer may operate at a frequency of about 16 Hz at 100 psi, a similar sized down hole hammer according to the present invention operating at the same pressure may operate at about 25 Hz. The present invention will operate at a wide range of motive fluid pressures, with a typical range of operating pressures around 50-100 psi, but may also operate under higher pressure (e.g., about 150 psi) in rotary drilling environments or even much higher pressures if used in oil gas drilling environments. 
     Thus, the invention provides, among other things, a down hole hammer that exhausts at least a portion of the motive fluid through a portion of the drill other than the drill bit. The invention also provides a down hole hammer having schematically parallel bit and actuator flow paths. Various features and advantages of the invention are set forth in the following claims.