Patent Publication Number: US-10323457-B2

Title: Down the hole hammer and systems and components thereof

Description:
This application is a national application of international application no. PCT/AU2015/000456, filed Jul. 31, 2015, which claims the benefit under 35 U.S.C. § 119 of Australian patent application Nos. 2014902955, filed Jul. 31, 2014; 2014902952, filed Jul. 31, 2014; 2014902951, filed Jul. 31, 2014;and 2014903285, filed Aug. 21, 2014. 
     TECHNICAL FIELD 
     A down the hole (DTH) hammer is disclosed. Also disclosed are systems and components of the DHT hammer. Such systems and components may also be used for or in fluid operated equipment other than a DTH hammer. 
     BACKGROUND ART 
     Many down the hole (DTH) devices such as but not limited to motors, pumps and DTH hammers operate by channelling a working fluid such as air, nitrogen, water, oil or mud through a drill pipe to operate the device. 
     The working fluid is pressurised and delivered at a pressure and rate dependent on many factors including the capacity of the driving compressor/pumping system; pressure losses through the drill pipe and DTH device itself; the mechanical limitations of the DTH device and environmental fluid pressure. 
     Consider for example a common DTH reverse circulation (RC) hammer (herein after “RC hammer”). The RC hammer comprises: an outer case which is coupled to a down hole end of the drill string; an inner tube; a porting sleeve; a hammer or piston which is able to slide along the inner tube and within the outer casing; and a drill bit. Fluid pumped down the drill string enters a porting arrangement between the outer case and the inner tube and reciprocates the piston to cyclically impact the drill bit. This transfers kinetic energy into the material at the toe (bottom) of the hole to fracture and displace the material. This material is delivered by residual fluid energy through the inner tube to the surface. This material can be analysed to provide information on the mineralogy and geology of the substrata. 
     Current RC hammers are deliberately designed to be inefficient with reduced mechanical energy output. This is to enhance mechanically reliable of the RC hammer when driven by compressors which provide excessive fluid pressure and flow rate to the work chamber when drilling short holes. It is known that inline pressure loses increase with hole depth (i.e. drill string length). So for short holes there is minimal inline pressure lose through the drill string. But the fluid pressure and flow rate provided by the compressors/boosters is designed for a deeper target depth. Consequently to protect the hammer as it progresses to the target depth current design practise is to reduce internal port efficiency and thus mechanical output of the hammer to protect bearing and striking surfaces of a piston and the bit from material and lubrication failures when used with excessive airpower. As a result of these in-built inefficiencies, when it is desired to drill deeper, then larger compressors are required especially to produce sufficient pressure differential at the piston (work chamber), to enable driving of the hammer drill bit and to compensate for additional in line losses as the length of the drill pipe increases. This leads to increased operational costs due to the power requirements of the compressors to do useful work at the bottom of deep holes, needing to overcome increasing additional line losses, and the designed and built-in flow restrictions. 
     The above described background art is not an admission that the art forms part of the common general knowledge of a person of ordinary skill in the art. Further, the above references are not intended to limit the application of the disclosed down the hole hammer and the systems and components thereof. Specifically, the above references are not intended to limit application of the systems and components to use only with or in a DTH hammer or DTH RC hammer. 
     SUMMARY OF THE DISCLOSURE 
     The disclosure is of a down the hole hammer and: an inner tube assembly; a fluid flow control system; a bit retaining system; a porting sleeve; and piston. Each of the an inner tube assembly; a fluid flow control system; a bit retaining system; a porting sleeve; and piston, can be incorporated individually or in any combination in the disclosed hammer. For example one embodiment of the disclosed hammer may have an embodiment of the disclosed fluid flow control system but otherwise a prior art bit retaining system; porting sleeve; inner tube and piston. In another example one embodiment of the disclosed hammer may have an embodiment of the disclosed piston but otherwise a prior art bit retaining system; porting sleeve; inner tube and fluid flow control system. Each of the inner tube assembly; fluid flow control system; bit retaining system; porting sleeve; and piston may provide operational or reliability or both benefits to an benefits to provide benefits to a hammer or other fluid operated device in which they are installed by themselves or in various combinations. 
     In one aspect there is disclosed a fluid flow control system capable of controlling the fluid flow form an upstream fluid supply to a work chamber or a fluid driven piston of a DTH device. The control system is connectable between a compressor or other fluid supply and the work chamber of the DHT device. The control system operates on the mass flow of fluid through a flow annulus which it forms immediately above or otherwise in close proximity to, and at a substantially fixed spacing from, the work-chamber and/or piston of the DHT device. Controlling the size of the flow annulus changes the pressure drop across the control system and the remaining pressure drop available across the work chamber. This has an effect on pressure available to other downstream restrictors, devices and flow paths. 
     As the disclosed control system is able to facilitate a variation in fluid pressure applied to the work chamber of the DTH device; it enables the DTH device to be originally designed for increased fluid flow efficiency while allowing fluid pressure across the work chamber to be regulated to enable matching of the supply of the fluid with the mechanical limits of operation of the DTH device and the environment in which the DTH device is deployed. In the case of the DTH device being an RC hammer the pressure control device can enable for example; tuning of the impact force and/or cycle rate of the hammer to best match: the flow rate and pressure of the driving compressor within the operational and reliability limits of the RC hammer itself; and, the nature of the ground in which it is deployed. 
     In a first aspect there is disclosed a fluid flow control system for a DTH device which can be coupled to a downstream end of a conduit and driven by a fluid supplied through the conduit from an upstream end of the conduit wherein the control system is capable of controlling fluid pressure to the DHT device; the control system being connectable between the conduit and the DHT device wherein the control system is maintained in a constant juxtaposition with the DHT device irrespective of length of the conduit. 
     In one embodiment the fluid flow control system comprises a flow path annulus through which a fluid for driving the DHT device flows, the flow path annulus having an annulus radial width Aw which is capable of being is changed. 
     In one embodiment the flow path annulus has an outer radius Ro and an inner radius Ri wherein Aw=Ro−Ri; and wherein at least one of the radii Ro and Ri is capable of being is changed to affect the change in the annulus radial width. 
     In one embodiment the at least one radii is automatically changeable in response to pressure differential of the fluid across the control system. 
     In one embodiment the fluid flow control system comprises an outer article having an inner surface that forms the outer radius Ro of the annulus and wherein the outer article is interchangeable with one of a plurality of user selectable other outer articles configured to produce a different annulus radius Ro. 
     In one embodiment the fluid flow control system comprises an inner article having an outer surface that forms the inner radius Ri of the annulus and wherein the inner article is interchangeable with one of a plurality of other user selectable inner articles configured to produce a different annulus radius Ri. 
     In one embodiment the fluid flow control system comprises an inner article having an outer surface that forms the inner radius Ri of the annulus and wherein the inner article is moveable in response to the pressure differential of the fluid across the control system between at least a first choke position and a second choke position to vary the inner radius Ri of the flow path annulus. 
     In one embodiment the fluid flow control system comprises an outer article having an inner surface that forms the outer radius Ro of the annulus and wherein the outer article is moveable in response to the pressure differential of the fluid across the control system between at least a first choke position and a second choke position. 
     In one embodiment the fluid flow control system comprises an inner article having an outer surface that forms the inner radius Ri of the annulus and wherein the inner article is interchangeable with one of a plurality of other user selectable inner articles configured to produce a different annulus radius Ri. 
     In one embodiment the fluid flow control system comprises an inner article having an outer surface that forms the inner radius Ri of the annulus and wherein the inner article is moveable in response to the pressure differential of the fluid across the control system between at least a first choke position and a second choke position to vary the inner radius Ri of the flow path annulus. 
     In one embodiment the outer article is a ring. 
     In one embodiment the article is a sub connectable between the DHT device and the conduit. 
     In one embodiment the fluid flow control system comprises a sub connectable between the DHT device and the conduit and wherein the sub arranged to retain the ring at a substantially fixed location relative to the DHT device. 
     In one embodiment the outer surface of the inner article has a different diameter at each of the choke positions. 
     In one embodiment the inner article is biased to move in an upstream direction with reference to a direction of flow of the fluid supplied through the conduit. 
     In one embodiment the fluid flow control system comprises a spring which biases the flow control body to move in the upstream direction and wherein the spring is retained within the flow control body. 
     In one embodiment the inner surface of the outer article has a different diameter at each of the choke positions. 
     In one embodiment the fluid flow control system h comprises a bypass path configured to enable a portion of the fluid downstream of the flow path annulus to be diverted from being able to drive the DHT device. 
     In one embodiment the inner article enables the bypass path to be open when the inner article is in the first choke position to facilitate bypassing of a portion of the fluid from driving the DTH device. 
     In one embodiment the the inner article closes the bypass path when the inner article is in the second choke position wherein all fluid passing through the flow path annulus can drive the DTH device. 
     In one embodiment the fluid flow control system comprises a bypass spacer which can be selectively coupled with the inner article to hold the inner article in the first choke position. 
     In one embodiment the bypass spacer is arranged to couple with the inner article in at least a first orientation in which the bypass spacer closes the bypass path and a second orientation in which the bypass spacer holds to the bypass path open. 
     In one embodiment the DHT device is a DTH hammer comprising an outer case, a piston and a hammer bit retained in the outer case, wherein the fluid flow control system is operable to control fluid pressure to the piston to enable tuning of the DTH hammer in terms of one or both of impact force and impact frequency of the piston. 
     In a second aspect there is disclosed a DTH hammer comprising:
         a fluid driven piston arranged to be capable of cyclically impacting a hammer bit; and   a fluid flow control system arranged to facilitate control of flow of fluid for driving the piston;   the fluid flow control system being in accordance with the first aspect.       

     In a third aspect there is disclosed a DTH hammer comprising:
         a hammer bit and a fluid drivable piston capable of cyclically impacting the hammer bit; and   a fluid flow control system arranged to facilitate control of fluid available drive the piston; the fluid flow control system comprising:
           a ring having an inner diameter which forms an outer radius Ro of a flow path annulus through which a fluid from an upstream fluid supply flows to drive the piston; and an inner article locatable with respect to the ring to form an inner radius Ri of the flow path annulus wherein the ring and the inner article together define the flow path annulus through which fluid from the upstream supply flows to drive the DHT hammer.   
               

     In one embodiment the ring is one of a plurality of user selectable rings of different inner diameter. 
     In one embodiment the inner body is movable between at least a first choke position and a second choke position to vary the inner radius Ri of the flow path annulus. 
     In one embodiment an outer surface of the inner article has a different diameter at each of the choke positions and the inner article is movable in an axial direction. 
     In one embodiment the inner article is biased to move in an upstream direction with reference to a direction of flow of the fluid to the piston form the supply. 
     In one embodiment the DTH comprises an inner tube which passes through the inner article and wherein the piston has an axial passage into the inner tube extends and along which the piston can reciprocate when impacting the hammer bit. 
     In one embodiment the inner tube comprises a first tube having an outer circumferential surface; and a second tube locatable coaxial with and around a portion of the outer circumferential surface of the first tube, wherein the portion of the outer circumferential surface of the first tube and an inner circumferential surface of the second tube are relatively configured to create a bypass path there between enabling a portion of the fluid to flow in an axial direction between the first and second tubes and through the axial passage. 
     In one embodiment the inner article is configured so that: when in the first choke position the inner article allows the bypass path to be open which enables diversion of a portion of the fluid downstream of the flow path annulus from being able to drive the piston; and when in the second choke position the inner article closes the bypass path so that substantially all of the fluid downstream of the flow path annulus is available to drive the piston. 
     In one embodiment the DTH hammer comprises a spacer which can be selectively coupled with the inner article to hold the inner article in the first choke position. 
     In one embodiment the DTH comprises a spacer which can be selectively coupled in either (a) a first orientation with the inner article to hold the inner article in the first choke position and close the bypass path and (b) a second orientation with the inner article to hold the inner article in the first choke position and open the bypass path. 
     In a fourth aspect there is disclosed a fluid flow control system for a DTH device which can be coupled to downstream end of a conduit and driven by a fluid supplied through the conduit from an upstream end of the conduit and the DTH device having a non-return valve to prevent a flow of fluid in an upstream direction past the non-return valve, the control system comprising:
         an orifice connectable between the DTH device and an upstream end of the conduit; and   a flow control body locatable downstream of the non-return valve and within a diameter of the orifice wherein the orifice and the flow control body together define a flow path annulus through which fluid supplied from an upstream end of the conduit passes to drive the DHT device.       

     In a fifth aspect there is disclosed a control system for a DTH device having an outer case and an inner tube and capable of being coupled to a drill pipe, the control system comprising:
         a sub configured to couple to one end of the outer case and at an opposite end to the drill string; and   a ring locatable within the outer case and retained by the sub, the ring having an inner diameter forming an outer diameter of flow path annulus through which a fluid supplied from an upstream end of the drill pipe flows to drive the DHT device, wherein the ring is one of a plurality of user selectable rings of different inner diameter.       

     In a sixth aspect there is disclosed a fluid flow control system for a DTH device which can be coupled to downstream end of a conduit and driven by a fluid supplied through the conduit from an upstream end of the conduit, the control system comprising:
         a ring locatable between the DTH device and an upstream end of the conduit, wherein the ring is one of a plurality of user selectable rings of different inner diameter.       

     n a second aspect there is disclosed a DTH drill comprising:
         a DTH hammer having an outer case, a piston and a hammer bit retained in the outer case; and   a control system arranged to facilitate control of fluid pressure to the DTH hammer;       

     the control system comprising:
         a ring having an inner diameter which forms an outer radius Ro of a flow path annulus through which a fluid supplied from an upstream end of the conduit flows to drive the DHT hammer; and an inner article locatable with respect to the ring to form an inner radius Ri of the flow path annulus wherein the ring and the inner article together define the flow path annulus through which fluid supplied from an upstream end of the conduit passes to drive the DHT hammer.       

     Embodiments of the disclosed a bit retaining system for a DTH hammer facilitates easy bit replacement. Embodiments of the disclosed bit retaining system may also facilitate uniform fluid flow distribution in a down hole direction on an outside of the drill bit. This fluid is subsequently used in the DTH hammer to convey drill cuttings through a central passage in a drill bit and up an associated drill string. The embodiments of the disclosed drill bit retaining system also enable retention of the drill bit in a region of increased diameter in comparison to the shank. This region is inherently stronger than the shank. Further, embodiments of the disclosed drill bit retaining system reverse the nature of the deceleration forces on the shank in comparison with the prior at. In the prior art at the end of a piston stroke any impact with a bit retention ring generates tensile forces to the shank. This is a leading cause of fracture of DTH hammer bits. In embodiments of the disclosed retention system, any comparable impact with the bit retention system occurs at a location on the bit adjacent a down hole end of the shank, and more significantly, in a larger cross-sectional area of the drill bit. Resultant deceleration forces now act as compressive forces on the shank from its up-hole end toward the end of the splines. 
     In a seventh aspect there is disclosed a drill bit retaining system for a hammer drill having an outer tube and a drill bit, the drill bit having a shank and a cutting face that extends from a first end of the outer tube and a plurality of splines that extend axially along the shank; the retaining system comprising: a shroud capable of coupling to the first end of the outer tube, the shroud being locatable over an intermediate portion of the drill bit, the shroud having an internal circumferential surface configured to provide an abutment surface for the drill bit to prevent the drill bit from falling from the outer tube, and facilitate substantially uniform fluid flow distribution in a down hole direction between the internal circumferential surface and an outer surface of the drill bit. 
     In one embodiment the internal circumferential surface of the shroud comprises a plurality of circumferentially spaced apart and radial inwardly extending protrusions, the protrusions forming the abutment surface. 
     In one embodiment the drill bit retaining system comprises a detent system capable of holding the shroud in a first fixed rotational position relative to the bit in which the abutment surface is capable of abutting a stop on the bit to prevent the bit form passing out of the shroud. 
     In one embodiment the detent system comprises a plurality of circumferentially spaced apart recesses formed in the internal circumferential surface of the shroud, the recesses axially spaced from the protrusions. 
     In one embodiment the recesses are evenly spaced about the internal circumferential surface. 
     In one embodiment each recess is axially aligned with a respective protrusion. 
     In one embodiment the protrusions are configured to enable axial alignment with respective splines on the bit when the shroud is in the first rotational position. 
     In one embodiment the drill bit retaining system comprises at least four protrusions and wherein the protrusions are evenly spaced about the internal circumferential surface. 
     In one embodiment the drill bit retaining system comprises one protrusion for each spline. 
     In one embodiment wherein there are more protrusions than recesses. 
     In one embodiment the internal circumferential surface comprises a circumferential band of reduced radius in comparison to an adjacent portion of the internal circumferential surface, and wherein the recesses are formed in the band. 
     In one embodiment the drill bit retaining system comprises a drive sub arranged to couple to the first end of the outer tube, the shroud being locatable over the drive sub and wherein the drive sub and the shroud are configured to enable clamping of the shroud between the first end of the outer tube and the drive sub. 
     In one embodiment the shroud comprises an outer circumferential surface provided with plurality of gaps arranged to enable the protrusions to pass there through in an axial direction. 
     In one embodiment the detent system comprises one or more members coupled to or provided on the drive sub and wherein the members are receivable in respective recesses. 
     In one embodiment the outer circumferential surface of the drive sub includes an outer band in which the gaps are formed and wherein the circumferential band of the sleeve has radius smaller the outer band thereby preventing the sleeve from falling from the drive sub. 
     In an eighth aspect there is disclosed a hammer drill comprising:
         an outer tube having a first end and a second end;   a drill bit provided with a central passage open at opposite ends of the bit, a shank, a cutting face that extends from the first end of the outer tube, and plurality of axially extending splines on the shank; and   a drill bit retaining system according to the seventh aspect, wherein the shroud is capable of coupling to the first end of the outer tube and arrange to prevent the drill bit form falling out of the outer tube.       

     In one embodiment the bit comprises an exterior surface portion between a down hole end of the splines and the cutting face the exterior surface portion of progressively increasing radius from a location adjacent a down hole end of the splines toward the cutting face. 
     In one embodiment an outer surface of the bit comprises a substantially smooth continuous fluid flow surface extending from an up stream end of the shank to the cutting face, the substantially smooth continuous fluid flow surface comprising a plurality of channels that lie between respective adjacent splines and a contiguous exterior surface portion between a down hole end of the splines and the cutting face the exterior surface portion of progressively increasing radius from a location adjacent a down hole end of the splines toward the cutting face. 
     One aspect of an embodiment of the disclosed inner tube assembly is the provision of a seat on the outer circumferential surface that acts to self-centre a component, such as a check valve, of the associated DTH hammer. The self-centring effect is achieved by forming the seat to have a radial face that is inclined to form at an obtuse exterior angle with the outer circumferential surface. 
     A further idea behind the disclosed inner tube assembly is to provide a mechanism by which some of the fluid used for driving the DTH hammer can bypass the piston of the DTH hammer via the inner tube assembly. This assists in reducing the impact force on the drill bit. This is particularly useful when drilling in soft ground such as clay. The reason for this is that high impact forces have a tendency to compact soft ground and force it into inlet holes in the bit. This results in clogging of the bit. When this occurs drilling must temporarily cease and the DTH hammer flushed to remove blockages. 
     In a ninth aspect there is disclosed an inner tube assembly for a DTH hammer comprising:
         a first tube having an outer circumferential surface; and   a seat extending in a radial direction from the outer circumferential surface, the seat having a radial face at one end that is inclined to form an obtuse exterior angle with a longitudinal axis of the first tube.       

     In one embodiment the seat is capable of bending to increase the obtuse exterior angle in response to the application of a force in an axial direction on the face. 
     In one embodiment the seat comprises an abutment that disposed circumferentially about the outer circumferential surface, wherein the face is a radial face of the abutment. 
     In one embodiment the abutment extends continuously about the outer circumferential surface. 
     In one embodiment the seat comprises a band axially spaced from the abutment. 
     In one embodiment the band comprises a circumferential groove. 
     In one embodiment a face of the band at an end distant the radial face extends perpendicular the longitudinal axis of the first tube. 
     In one embodiment the first tube has a first portion on one side of the seat and a second portion on an opposite side of the seat wherein an outer diameter of the first portion is different to outer diameter of the second portion. 
     In one embodiment the inner tube assembly comprises a second tube and wherein a portion of the outer circumferential surface of the first tube and an inner circumferential surface of the second tube are relatively configured to create one or more fluid flow paths enabling a fluid to flow in an axial direction between the first and second tubes when second tube is disposed on the first tube and covers the portion the outer circumferential surface. 
     In one embodiment the fluid flow paths are at least in part formed by profiling or configuring the portion of the outer circumferential surface of the first tube so that the radius of the portion of the outer circumferential surface is not constant. 
     In one embodiment the fluid flow paths are at least in part formed by recesses or flats on the portion of the outer circumferential surface. 
     In one embodiment the fluid flow paths are at least in part formed by profiling or configuring the inner circumferential surface of the second tube so that the radius of the inner circumferential surface is not constant. 
     In one embodiment the fluid flow paths are at least in part formed by recesses or flats on the portion of the outer circumferential surface. 
     In one embodiment the fluid flow paths are created by providing the portion of the outer circumferential surface of the first tube and an inner circumferential surface of the second tube with different profiles. 
     In one embodiment the inner tube assembly comprises one or more access paths formed in the second tube enabling fluid from outside of the second tube to flow into the fluid flow paths. 
     In one embodiment the one or more access paths comprise holes formed near and inboard of one end of the second tube. 
     In one embodiment the one or more access paths comprise comprises a circumferential recess or groove formed on the inner circumferential surface of the second tube onto which an inner radial end of each of the holes open. 
     In one embodiment the one end of the second tube is locatable against the seat on a side distant the radial face. 
     In a tenth aspect there is disclosed an inner tube assembly for a DTH hammer comprising:
         a first tube having an outer circumferential surface; and   second tube locatable coaxial with and around a portion of the outer circumferential surface of the first tube, wherein the portion of the outer circumferential surface of the first tube and an inner circumferential surface of the second tube are relatively configured to create one or more fluid flow paths enabling a fluid to flow in an axial direction between the first and second tubes when second tube is disposed on the first tube and around the portion the outer circumferential surface.       

     In one embodiment the fluid flow paths are at least in part formed by profiling or configuring the portion of the outer circumferential surface of the first tube so that the radius of the portion of the outer circumferential surface is not constant. 
     In one embodiment the fluid flow paths are at least in part formed by recesses or flats on the portion of the outer circumferential surface. 
     In one embodiment the fluid flow paths are at least in part formed by profiling or configuring the inner circumferential surface of the second tube so that the radius of the inner circumferential surface is not constant. 
     In one embodiment the fluid flow paths are at least in part formed by recesses or flats on the portion of the outer circumferential surface. 
     In one embodiment the fluid flow paths are created by providing the portion of the outer circumferential surface of the first tube and an inner circumferential surface of the second tube with different profiles. 
     In one embodiment the inner tube assembly comprises one or more access paths formed in the second tube enabling fluid from outside of the second tube to flow into the fluid flow paths. 
     In one embodiment the one or more access paths comprise holes formed near and inboard of one end of the second tube. 
     In one embodiment the one or more access paths comprises a circumferential recess or groove formed on the inner circumferential surface of the second tube onto which an inner radial end of each of the holes open. 
     In an eleventh aspect there is disclosed a piston for a DTH hammer drill, the piston arranged to impact a drill bit of the hammer drill, the piston comprising:
         a body having an axial passage and an outer circumferential surface provided with a maximum of three axially spaced apart circumferential porting bands.       

     In one embodiment the piston comprises a stop, wherein the porting bands comprise an upstream porting band, an intermediate porting band and a downstream porting band, and stop is located between the intermediate porting band and the downstream porting band, and wherein the downstream porting band has a constant outer circumferential surface for an entire axial length from the stop to a downstream end of the piston. 
     In an eleventh aspect there is disclosed A piston for a DTH hammer drill having an outer case and a drill bit supported by the outer case wherein the piston is capable of impacting the drill bit, the piston comprising: an outer circumferential surface configured to form a substantial fluid seal at maximum of three spaced axially spaced regions within an outer case of the hammer drill. 
     In one embodiment the piston comprises: a body having an axial passage and an outer circumferential surface provided with a maximum of three axially spaced apart circumferential porting bands respective bands at axially spaced locations, wherein respective bands are capable of forming a substantial seal at respective different sealing regions. 
     In one embodiment the porting bands comprise an upstream porting band, an intermediate porting band and a downstream porting band wherein the upstream band has an upstream edge adjacent an upstream end of the body and the downstream band has a downstream edge adjacent a downstream end of the body. 
     In one embodiment the piston comprises a stop located between the intermediate band and the downstream band and wherein the downstream band has a plain outer circumferential surface with a substantially constant outer diameter for an entire axial length from the stop to the downstream end of the piston. 
     In a twelfth aspect there is disclosed a porting system for a hammer drill having an outer case, a drill bit supported by the outer case and a fluid driven piston capable of reciprocating axially within the outer case and impacting the drill bit, the porting system comprising:
         an outer surface of the piston; and   an arrangement of surfaces configured to interact with the outer surface to provide a substantially uniform fluid pressure distribution on the outer surface such that the fluid pressure is able to hold the piston in a fixed axial position relative to the outer case when the hammer drill is in a blow down mode.       

     In one embodiment the fixed axial position coincides with a downhole most position of the piston in the hammer drill. 
     In one embodiment the outer circumferential surface comprises a maximum of three axially spaced apart circumferential porting bands, the bands being at axially spaced locations along the piston, wherein respective bands are capable of forming a substantial seal with the arrangement of surfaces at respective different sealing regions. 
     In one embodiment the porting bands comprise an upstream porting band, an intermediate porting band and a downstream porting band wherein the upstream band has an upstream edge adjacent an upstream end of the piston and the downstream band has a downstream edge adjacent a downstream end of the piston. 
     In one embodiment the porting system comprises a stop on the outer circumferential surface and located between the intermediate band and the downstream band, wherein the downstream band has a plain outer circumferential surface with a substantially constant outer diameter for an entire axial length from the stop to the downstream end of the piston. 
     In one embodiment the arrangement of surfaces comprises an inner circumferential surface of a porting sleeve disposed in the hammer drill and located such that an upstream end of the piston is maintained within the porting sleeve during operation of the hammer drill. 
     In one embodiment the arrangement of surfaces comprises an inner circumferential surface of a porting sleeve disposed in the hammer drill and located such that an upstream end of the piston is maintained within the porting sleeve during operation of the hammer drill, the porting sleeve having a plurality of openings inboard of a downstream end of the porting sleeve and wherein the inner circumferential surface of a porting sleeve has a first portion at the downstream end thereof with a first inner diameter and second portion upstream of the downstream portion with a second diameter being smaller than the first diameter and wherein the openings span the first and second portions; the upstream band of the piston and the second portion relatively configured to create between them an upstream sealing region when the second portion at least partially overlies the upstream band, the upstream sealing region substantially preventing fluid from passing through the openings and into an upstream end of the piston. 
     In one embodiment the first portion is configured relative to the outer circumferential surface to maintain a flow path that always remains open for all possible operational locations of the piston within the outer case wherein fluid is able to flow through the openings into an intermediate chamber located between the upstream band and the intermediate band. 
     In one embodiment the arrangement of surfaces comprises an inner circumferential surface of the outer case configured to form a with the intermediate band a bottom chamber seal when inner circumferential surface of the outer case at least partially overlies the intermediate band. 
     In a thirteenth aspect there is disclosed a hammer drill comprising:
         an outer case;   a fluid driven piston capable of reciprocating axially within the outer case and impacting a bit drill bit retained at an end of the outer case, the piston having an upstream end, a downstream end and an intermediate porting band between the upstream end and the downstream end;   a top chamber located between the outer case and the upstream end of the piston, the top chamber arranged to receive fluid for driving the piston in the downstream direction; and   a bottom chamber located downstream of the intermediate porting band and between the piston and the outer case;   wherein the top and bottom chambers are arranged to be in direct fluid communication with each other when the hammer drill is operated in a blow down mode.       

     In one embodiment the piston is configured to have:
         a downstream surface area being a total of the surface area of the piston looking in a downstream direction that is not parallel to a central axis of the piston and is within and between the top and bottom chambers; and   an upstream surface area being a total of the surface area of the piston looking in an a downstream direction that is not parallel to the central axis and is within and between the top and bottom chambers;   wherein the downstream surface area is greater than the upstream surface area.       

     In a fourteenth aspect there is disclosed a piston for a hammer drill, the piston arranged to impact a drill bit of the hammer drill, the piston comprising:
         a body having an axial passage and an outer surface, the outer surface of the body configured so that when the body is subjected to a substantially uniform fluid pressure field so that a net force acting on the piston by action of the fluid pressure is directed toward the drill bit.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Notwithstanding any other forms which may fall within the scope of the apparatuses, systems and devices as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic representation of one embodiment of the disclosed down the hole device in the form of a DTH hammer drill incorporating respective embodiments of the disclosed inner tube assembly, fluid flow control system, porting sleeve, bit retaining system lo and piston; 
         FIG. 2  is an enlarged view of the fluid flow control system shown in  FIG. 1  and illustrating an associated flow control body in a first choke position; 
         FIG. 3  is a view of the control system depicted in  FIG. 2  but with the flow control body in a second choke position; 
         FIG. 4  is a representation of the fluid flow control system shown in  FIGS. 1-3  but with the addition of an associated spacer and with the spacer in a first orientation; 
         FIG. 5  is a representation of the fluid flow control system shown in  FIG. 4  but with the associated spacer illustrated in a second orientation; 
         FIG. 6  is an enlarged side view of the flow control body and the spacer as well as a ring and housing incorporated in the fluid flow control system; 
         FIG. 7  is a longitudinal section view of the flow control body, ring, spacer and housing shown in  FIG. 6 ; 
         FIG. 8  is a perspective view of the flow control body ring, spacer and housing shown in  FIGS. 6 and 7 ; 
         FIG. 9  is a perspective view of a sub incorporated in an embodiment of the control system and down the hole device; 
         FIG. 10  is a perspective view of an adapter nozzle and filter screen associated with an embodiment of the down the hole device; 
         FIG. 11  is a section view of the porting sleeve shown in  FIG. 1 ; 
         FIG. 12  is a perspective view of the porting sleeve shown in  FIG. 1 ; 
         FIG. 13  is a cut away and enlarged view of a portion of the fluid flow control system, inner tube assembly, porting sleeve and piston in the DHT device illustrating air flow paths in one mode of operation; 
         FIG. 14  is a perspective and partly exploded view of the porting sleeve, an associated retaining ring and sealing ring incorporated in the disclosed down the hole device; 
         FIG. 15  is a side view of the inner tube assembly incorporated in an embodiment of the down the hole device shown in  FIG. 1 ; 
         FIG. 16  is a perspective view of the inner tube assembly of  FIG. 15  but illustrated in a disassembled state; 
         FIG. 17  is an enlarged section view of a portion of the inner tube assembly depicted in  FIG. 15 ; 
         FIG. 18  is a transverse section view of the inner tube assembly shown in  FIG. 15 ; 
         FIG. 19  is a cross section view of an embodiment of the disclosed bit retaining system shown in association with a downstream end of the DTH device; 
         FIG. 20  is an exploded side view of an embodiment of the bit retaining system together with an associated DTH hammer drill bit; 
         FIG. 21  is a perspective view of a shroud incorporated in the bit retaining system; 
         FIG. 22  is a perspective view of a drive sub incorporated in the bit retaining system; 
         FIG. 23  is a perspective view of a hammer drill bit that may be used in conjunction with the bit retaining system; 
         FIG. 24  is a schematic representation of one possible arrangement of components of the bit retaining system in an initial stage of coupling to the DTH device; 
         FIG. 25  illustrates an arrangement of the components of the bit retaining system in a subsequent stage of coupling to the DTH device where the shroud is in a bit release position and an associated detent system disengaged; 
         FIG. 26  illustrates an arrangement of the components of the bit retaining system in a subsequent stage of coupling to the DTH device where the shroud is in a bit retention position and detent system disengaged; 
         FIG. 27  illustrates an arrangement of the components of the bit retaining system in a subsequent stage of coupling to the DTH device where the shroud is in a bit retention position and detent system engaged but unlocked; 
         FIG. 28  illustrates an arrangement of the components of the bit retaining system in a subsequent stage of coupling to the DTH device where the shroud is in a bit retention position, the detent system is engaged but unlocked and the drive sub partially screwed into an outer tube of the DTH device; 
         FIG. 29  is a schematic representation of the bit retaining system fully installed in the DTH device where the shroud is in a bit retention position, the detent system is engaged and locked and the drive sub fully screwed into outer tube; 
         FIG. 30  is a schematic representation of a stage of decoupling of the bit retaining system to enable replacement of a drill bit where the shroud is in a bit retention position, the detent system is engaged but unlocked and the drive sub partially screwed into an outer tube of the DTH device; 
         FIG. 31  is a schematic representation of the juxtaposition of the shroud and the drive sub in a more advanced stage of the bit changing process where the shroud is in a bit release position and the detent system is disengaged; 
         FIG. 32  is a schematic representation of a drill bit being decoupled from the bit retaining system to enable replacement; 
         FIG. 33  is a side view providing a comparison between a hammer bit that can be used with embodiments of the present drill bit retaining system and a prior art hammer bit; 
         FIG. 34  is a cross section view of the bit retaining system installed on an end of the DTH device in the form of a reverse circulation hammer drill but in a different axial plane to that shown in  FIG. 19 ; 
         FIG. 35 a    is a schematic representation of embodiment of the disclosed piston shown in  FIG. 1 ; 
         FIG. 35 b    is a schematic representation of a prior art piston for a reverse circulation hammer drill; 
         FIGS. 36, 37, 37, 38, 39, 40, 41 and 42  depict an operational cycle of an embodiment of the disclosed the DTH device utilising the disclosed piston; 
         FIG. 43  illustrates an embodiment of the disclosed DTH hammer drill in blow down mode; 
         FIGS. 44 a  and 44 b    depict details of a porting arrangement incorporated in the disclosed DTH hammer drill. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
       FIG. 1  shows in longitudinal section an embodiment of the disclosed DTH hammer  10 . The specific embodiment of the DTH hammer is a reverse circulation (RC) hammer. However embodiments of the DTH device and the components and systems therefore are not limited to application in DTH RC hammers. 
     The hammer  10  incorporates embodiments of the disclosed: inner tube assembly  100 ; fluid flow control system  200 ; bit retaining system  400 ; porting sleeve  600 ; and piston  700 . Each of the: inner tube assembly  100 ; fluid flow control system  200 ; bit retaining system  400 ; porting sleeve  600 ; and piston  700  in their own right provide benefit to the overall operation and/or reliability of the hammer  10 . Also each of the inner tube assembly  100 ; fluid flow control system  200 ; bit retaining system  400 ; porting sleeve  600 ; and piston  700  may be incorporated individually (i.e. by themselves) in: a conventional DTH device/machine; or, fluid operated equipment to assist in the operation thereof. Greater operational and/or reliability benefits can be obtained by using two of more of these systems/devices, with the ultimate being having all of them as in the hammer  10  described hereinafter. 
     The hammer  10  has an outer case  12  with an up hole or upstream end  16  and a down hole or downstream end  18 . The inner tube assembly  100  extends coaxially inside the outer case  12 . The inner tube assembly  100  also has an up hole end  20  and an opposite down hole or downstream end  22 . A hammer bit  24  is retained within the outer case  12  by the bit retaining system  400 . The down hole end  22  of the inner tube assembly  100  extends into a central return passage  26  of the bit  24 . The piston  700  is also housed within the outer case  12  and slides along the inner tube assembly  100 . 
     Operating fluid such as air is delivered via a drill string (not shown) or other conduit which is coupled to the up hole end  16  of the outer case  12 . This fluid passes through the fluid flow control system  200  to a porting arrangement which has the effect of reciprocating the piston  700  cyclically to the strike the drill bit  24 . The impact force from the piston  700  is transferred via the drill bit  24  onto a toe of a hole being drilled. This fractures the toe of the hole. Hole cuttings/chips arising from this fracturing are carried up the return passage  26  and subsequently the inner tube assembly  100  via a return flow of the fluid (in this case air) which is used to drive the piston  700 . 
     All of the fluid for driving the piston  700  and providing a vehicle for return of the chips and hole-cuttings through the inner tube assembly  100  must initially pass through the fluid flow control system  200 . When the system  200  is used with the DTH hammer  10  this fluid is generally air and is supplied via compressors at the surface. Air is delivered at a pressure, (usually measured in pressure per square inch gauge “psig”) and flow rate, (usually measured in cubic feet per minute “cfm”) in accordance with the capacity of air compressors and/or boosters at the surface. (Commonly “flow rate” is simply referred to as “CFM”. This convention will be used in the remainder of this specification.) Generally the compressors and/or boosters are operated at full capacity to provide maximum pressure and CFM to drive the DTH hammer  10 . Pressure and energy losses will be experienced through the drill string prior to reaching the DTH hammer  10 . These losses increase as the length of the drill string increases with increased depth of drilling. The pressure and CFM available to drive the DTH hammer  10  is controlled down the hole by the control system  200  which is immediately upstream of the piston  700 . 
     Each of the main devices and systems incorporated in the hammer  10  will now be described. 
     Fluid Flow Control System  200   
     The fluid flow control system  200  ( FIGS. 1-8 ) is arranged to facilitate control of fluid pressure available to the DTH hammer  10 . As the control system  200  is part of the DTH hammer  10  it maintains a constant juxtaposition with the bit  24  irrespective of the length of the drill pipe. Thus the control system  200  enables the control of fluid pressure immediately adjacent to the upstream end of the piston  700  and associated work-chamber of the hammer  10 . 
     The control system  200  is arranged to define or form a flow path annulus  210  through which air  202  supplied from an upstream end of an associated drill pipe flows in order to subsequently drive the DTH hammer  10 . The flow path annulus  210  has an outer radius Ro and an inner radius Ri, (see  FIG. 2 ). The width of the flow path annulus is hereafter referred to annulus width Aw=Ro−Ri. 
     In broad terms the control system  200  enables variation of the annulus width Aw by enabling a change of either one or both of the radius Ro and the radius Ri. In the particular embodiment to be described below the radius Ro is defined by an outer article in the form a ring  212 . The ring  212  has an inner radius which forms the outer radius Ro of the flow path annulus  210 . The ring  212  may be one of a plurality of interchangeable rings of different inner diameter which can be selected for a particular use and coupled to the DTH hammer  10  while at the surface. For example the ring  212  may be one of a set of rings each having the same outer radius but with their respective inner radius incrementing or changing by a predetermined amount for example, but not limited to: 0.25 mm: or 0.5 mm or 1.0 mm, from ring to ring. 
     The inner radius Ri of the flow path annulus is created by an outer circumferential surface  214  of an inner article in the form of a flow control body  216 . Thus the flow control annulus  210  is defined by the ring  212  and the flow control body  216 . 
     The flow control body  216  is movable between at least a first choke position shown in  FIG. 2  and a second choke position shown in  FIG. 3  to vary the inner radius Ri of the flow path annulus  210 . As explained later this provides an automatic choke or airflow control feature of the control system  200 . 
     Referring in particular to  FIG. 2 , it can be seen that the flow path annulus  210  is formed between an inner surface  213  of the outer article or ring  212 ; and the outer surface  214  of the inner article or flow control body  216 . The flow path annulus  210  is formed in a diametric plane containing the ring  212 . 
     In addition to the ring  212  and the flow control body  216 , the fluid flow control system  200  also incorporates a bias mechanism in the form of a spring  218 . The flow control body  216  is slidably mounted on the inner tube  10 . The spring  218  encircles a portion of the inner tube assembly  100  and is retained within the flow control body  216 . The spring  218  acts to bias the flow control body  216  in an upstream direction counter to the direction of flow of the air  202 . However, the spring  218  also enables the flow control body  216  to move in a downstream direction in the event that the force supplied by the air  202  on the outer surface  214  of the flow control body  216  exceeds the force supplied by the spring  218  in the opposite direction. 
     When the force provided by the pressure of the air  202  acting on the outer surface  214  exceeds the force applied by the spring  218  on the flow control body  216  plus the force applied by air pressure downstream of the body  216 , the flow control body  216  moves from the first choke position show in  FIG. 2  to the second choke position shown in  FIG. 3 . Thus the flow control body  216  and the spring  218  form a pressure switch. Depending on the pressure differential across the flow control body  216 , the flow control body will reside primarily in either the first or second choke positions. It is to be understood however that there will be transition period when the flow control body moves between the first and second choke positions in the event that a threshold pressure differential is crossed. 
     This position of the flow control body  216  is limited in the first choke position by the provision of a radial flange  59  on the inner tube assembly  100 . When the differential pressure is exceeded then the flow control body  216  is forced to move or slide axially in a downstream direction compressing the spring  218  to the second choke position shown in  FIG. 3 . As is readily apparent from a comparison between  FIGS. 2 and 3  in the second choke position the flow path annulus  210  has a reduced width Aw. This has the effect of choking or limiting the pressure of the fluid downstream of the flow path annulus  210 . 
     The annulus width Aw and the threshold pressure differential can be varied by interchanging any one or more of: the ring  212 ; the flow control body  216 ; and, the spring  218  with like items of different physical characteristics. For example the flow path annulus  210  and more particularly the annulus radial width Aw can be varied by interchanging the ring  212  with another ring having a different inner radius Ro. Assuming that the flow control body  216  remains unchanged then changing the ring  212  with another ring having the same outer diameter but different inner diameter, will vary the annulus radial width Aw. Similarly changing the flow control body  216  with flow control bodies of different outer configuration will have a similar effect. Further, interchanging the spring  218  with springs of different spring constant will vary the differential pressure at which the flow control body  216  switches from the first choke position to the second choke position. This provides for great flexibility and tuning of the hammer  10  to operate at a desired efficiency and reliability level for a given set of ground conditions (e.g. hard ground soft ground, mixed ground) and compressor/booster availability and/or output. 
       FIGS. 2-5  also depict the provision of a bypass path  222 . The existence of the bypass path  222  is dependent on what type of inner tube assembly  100  is used.  FIGS. 2-5  depict a hammer  10  with a two piece inner tube assembly  100  which does facilitate the provision of a bypass path. (The two piece inner tube is described in greater detail later with reference to  FIGS. 15-18 .) However in an alternate embodiment the hammer  10  may be provided with a simple prior art one piece inner tube (not shown) which does not facilitate the provision of the bypass path  222 . 
     A portion the air  202  can flow through the bypass path  222  downstream of the flow path annulus  210  in certain conditions. The portion that can flow through the bypass path  222  is denoted in  FIG. 2  by arrow  202   b  and subsequent dots, with the remainder of the air for driving the piston  700  depicted by arrow  202   p.  In particular when the flow control body  216  is in the first choke position shown in  FIG. 2  the bypass path  222  is open. Therefore the portion of air  202   b  can flow through the bypass path  222 . 
     The bypass path  222  directs a portion of the driving fluid to exit the inner tube assembly  100  downstream to bypass a chamber in a head  711  of a piston  700 . As a consequence only the remaining portion of the air  202   p  drives the piston  700 . The air  202   b  exits the DTH hammer  10  between the outer surface of the bit  24  and the outer casing  8  and into the hole being drilled. The air  202   b  then re-joins the air  202   p  in the hole and flows back up the passage  26  and through the inner tube assembly  100  to carry cuttings and chips to the surface. 
     As shown in  FIG. 3  the bypass entry ports  226  are closed by the flow control body  216  when in the second choke position. Consequently the bypass path  222  is also closed by the flow control body  216 . 
     The entry ports  226  are formed as radial paths or holes in a housing  230  which is mounted on the inner tube assembly  100 . The housing  230  has an upstream end  232  which forms a bearing surface for an inside surface of the body  216  to slide when moving between the first and second choke positions. It will be also noted that the spring  218  abuts the upstream end  232  and is retained between the end  232 , the inner tube assembly  100  and the flow control body  216 . Thus the spring  218  is by and large isolated from the flow of air  202 . This provides benefits in terms of minimising or avoiding vibrations which can fatigue and/or otherwise damage the spring. The housing  230  is formed with a reduced outer diameter portion  233  ( FIGS. 6 and 7 ) near the end  232 . The reduced diameter portion  233  has an outer surface  235 . 
     The flow control system  200  optionally includes a spacer  234 . The spacer  234  is arranged to couple to the flow control body  216  to hold the flow control body  216  in the first choke position shown in  FIG. 2  irrespective of the pressure of the fluid  202  or pressure differential across the body  216 . As explained shortly the spacer  234  can be mounted in a first orientation or a second orientation. In the first orientation the spacer  234  is arranged to hold open the entry ports  226  and thus the bypass path  222  to divert a portion of the fluid  202 . In a second orientation the spacer  234  covers and closes the entry ports  226  thereby closing the bypass path  222 . 
       FIGS. 6-8  depict in greater detail various features of the control system  200  and the DTH hammer  10 . From these Figures it can be seen that the ring  212  is a simple annular ring having inner surface  213  which forms the outer radius Ro of the flow path annulus  210 . The ring  212  also has an outer circumferential surface  236  having a radius marginally less than the inner radius of the outer case  12 . This enables the ring  212  to sit inside the outer case  12 . 
     The flow control body  216  has a generally tubular form. The outer circumferential surface  218  of the body  216  forms the inner radius Ri of the flow path annulus  210 . With particular reference to  FIG. 8  the surface  218  is profiled so that the radius Ri varies at different axial locations along the body  216 . For a downstream portion  238  the outer surface  218  has a constant radius shown as Ria. However intermediate of the axial length, the body  216  has a portion  240  of increased outer diameter. This results in an increase in the inner radius Rib of the flow path annulus  210  and consequently a reduced annulus width Aw. Moving further in the upstream direction the outer surface  218  has a portion  242  of constant outer diameter. The outer diameter of portion  242  may be the same as or different to that of the portion  238 . There is a uniform and progressive transition in the outer diameter of the surface  218  from the portions  238  and  242  to the intermediate portion  240 . 
     The body  216  is also provided with a tapered upstream end  244 . On an inside surface of the end  244  the body  216  there is a circumferential groove  246  for seating an optional seal  248  ( FIG. 2 ). The seal  248  when used provides a substantial fluid seal against the outer surface of the inner tube assembly  100 . 
     The spacer  234  is in the form of a thin walled ring. The ring  234  is provided with a plurality of circumferentially spaced apart holes  250  which are closer to one end  251  and an opposite end  253 . The spacer  234  has an inner radius arranged so as to seat on the portion  233  of the housing  230  with a minimal clearance or tolerance from the corresponding surface  235 . Further, the spacer  234  has a material thickness T ( FIG. 7 ) substantially equal to the depth D of a shoulder  252  which demarks the portion  233  from the remainder of the housing  230 . 
     The housing  230  is formed with a plurality of outer circumferential grooves  254 ,  256 ,  258  and  260 . The grooves  254 - 260  are arranged to retain respective O-ring seals  262 ,  264 ,  266  and  268  respectively. An inner circumferential surface  270  of the housing  230  is formed with an intermediate circumferential groove  272  for seating an O-ring seal  273 . Downstream of the groove  272  the inner circumferential surface  270  is formed with a radial inward circumferential shoulder  274  leading to a band  276  of constant radius. On an upstream side of the groove  272  the housing  230  is formed with outer and inner circumferential grooves  278  and  280  respectively. The entry ports  226  extend in the radial direction and are located between the grooves  278  and  280 . 
     When the spacer  234  is used in the control system  200  in the orientation with end  251  closest to the shoulder  252 , the holes  250  are offset from the groove  278 . More particularly the holes  250  are located between the shoulder  252  and the groove  256  which contains the O-ring seal  264 . Thus in this configuration the holes  250  are unable to provide fluid communication with the entry ports  226 . This represents the second orientation described above of the spacer  234  in which the spacer  234  closes the bypass path  222 . This configuration of the control system  200  is shown in  FIG. 5 . 
     However if the spacer  234  orientated so that end  253  is closest the shoulder  252  as shown in  FIG. 4  the holes  250  will align with the groove  278 . Accordingly air can pass through the holes  250  into the groove  278  and subsequently through the entry ports  226  to enter the bypass path  222 . This represents the first orientation described above in which the spacer  234  opens the bypass path  222 . 
     The ring  212  is retained and held in place in the outer case  12  by a sub  282  (see in particular  FIGS. 2 and 9 ). The sub  282  is of a generally tubular construction and has an external thread  283  at a downstream end  285 . The thread  283  engages with a thread formed on an inner circumferential surface at the upstream end  16  of the outer case  12 . As depicted in  FIG. 2  an upstream end of the thread  283  is formed with a circumferential groove  284  which seats an O-ring seal  286 . The seal  286  forms a substantial fluid seal between the sub  282  and the outer case  12 . 
     Moving in an upstream direction the sub  282  is formed with a square shoulder  290  which forms an abutment surface for tightening of the sub  282  onto a radial end of the outer case  12 . A pair of opposed flats  294  are machined or otherwise formed in the outer surface of the sub  282  upstream of the end shoulder  290 . The flats  294  are provided to aid in gripping the sub  282  during tightening or breaking of the thread connection with the outer case  12 . An upstream end  296  of the sub  282  is formed with an internal screw thread  298  for engaging with an end of an adjacent drill pipe (not shown). A circumferential shoulder  300  ( FIG. 2 ) is formed on an inner circumferential surface of the sub  282  adjacent a downstream end of the thread  298  and extends in a radial inward direction. 
     The sub  282  seats an adapter nozzle  302  (see in particular  FIGS. 2 and 10 ). The adapter nozzle  302  abuts against the shoulder  300  and receives and itself seats the upstream end  16  of the inner tube assembly  100 . A downstream portion of the adapter nozzle  302  is formed with a plurality of circumferentially spaced apart fins or walls  304 . Mutually adjacent fins  304  define flow channels  306  for air delivered from compressors/boosters at a surface through the drill pipe connected to the sub  282 . Each channel  306  leads to a hole  308 . The holes  308  extend in the axial direction. Air  202  flows through the channels  306  between an outside of the adapter nozzle  302  and an inner circumferential surface of the sub  282  to the holes  308 . The holes  308  are defined by or in a circumferential base  310  of the adapter nozzle  302 . The base  310  is formed with a plurality of outer circumferential grooves  312  each of which seats a corresponding O-ring seal  314 . The seals  314  form a substantial fluid seal between the base  310  and the inner circumferential surface of the sub  282 . A portion  316  ( FIG. 2 ) of the inner surface of the wall  310  is tapered to increase in radius in a downstream direction. The surface  316  forms an abutment surface for a complementary surface portion  318  of a check valve  320 . 
     The check valve  320  operates to automatically close against the surface  316  to prevent a backflow of fluid in an upstream direction. The check valve  320  is slidably mounted on the inner tube assembly  100  upstream of the flow control body  216 . The check valve  320  includes a relatively light spring  322  which is arranged to compress and allow the check valve  320  to slide in a downstream direction to abut the flange  59  with the smallest expected fluid pressure and CFM. In this regard the check valve  320  is designed to remain open whenever the DTH hammer  10  is in use. 
     Returning to  FIG. 10 , the free end  324  of each fin  304  is formed with an intermediate recess  326 . Additionally, the surface of each free end  324  slopes toward the base  310  with increased radial distance away from a central axis of the adapter nozzle  302 . An upstream end  328  of the nozzle  302  is of a generally tubular form and, in one possible configuration as per this embodiment, is provided with a pair of spaced apart circumferential grooves  330  for seating O-ring seals (not shown). However not all embodiments require the provision of such grooves and associated O-rings. 
     An air filter screen  332  fits over the end  328  and sits on the free ends  324  of the fins  304 . The screen  332  is in the general shape of a frusto-conical shell and is formed with a plurality of holes  333 . When the screen  332  sits on the free ends  324  the recesses  326  provide fluid flow paths for air flowing through the holes  333  that align with the recesses  326 . This assists in minimising flow restrictions for the air used for driving the hammer  10 . 
     Returning to  FIGS. 2-4  the ability to control air pressure and in particular pressure drop and thus tune the DTH hammer  10  will be further explained. The ability to control the air pressure drop at the work chamber/piston  700  using the control system  200  resides in the ability to (a) interchange the ring  212  with other rings having a different inner radius therefore changing the outer radius Ro of the flow path annulus  210 ; (b) interchange the spring  218  with springs of different spring constant; (c) use or omit the spacer  234 , and if used, install it in either the first or second orientations; and (d) interchange the flow control body with other flow bodies of different outer surface configuration, (although it is envisaged that in practise it will be simpler to interchange the ring  212  than the body  218  to vary the annulus width Aw. 
     The purpose here is to introduce a variable/tunable parasitic pressure drop at the point of the control system  200  to remove energy from the airflow and reduce the available pressure drop across the work chamber in the hammer drill. Embodiments of the control system enable a hammer drill to be designed with maximum efficiency and then detune by introducing controlled pressure drops/loss as required to suit the power input and ground conditions. The controlled pressure drop is effected at a down hole location immediately adjacent the work chamber/piston. This is in contrast with the prior art of building in fixed air flow inefficiencies and pressure losses in the hammer drill to provide protection for mechanical components then increase power input as required to meet an application at hand. Therefore in the prior art the solution is to increase drilling rate is to add more air mass flow by adding a second compressor. This doubles the mass flow and will provide a substantial increase in pressure across the work chamber/piston. This in turn, in some circumstances, can cause the hammer to fail. 
     With embodiments of the present control system  200  pressure drop across the work chamber can be determined prior to drilling and arranged for example by appropriate selection of the size (inner diameter) of the ring  212 . But also an automatic down the hole pressure drop can be introduced by the control body  216  moving from the first choke position to the second choke positions if a predetermined excess pressure is introduced (for example by the adding of say a second compressor). The control system  200  also allows for the automatic pressure drop control to be disabled by the use of the spacer  234 . Further when the control system is used with the inner tube  100  the spacer  234  orientation can be varied between one position with allows additional pressure drop by opening the bypass path  222 . 
     Embodiments of the DTH hammer  10  also have superior air flow characteristics over prior art devices. In particular the hammer  10  has more streamlined air flow with changes or variations in air flow direction and larger air flow areas. For example in the hammer  10  there is no adapter sub between the outer case  12  and the sub  282 . Therefore there is greater inner diameter in the vicinity of the check valve  320  and the control body  216  for air flow. Thus not only is the available air flow annulus larger but there are also less changes and smoother changes in air flow in the upstream end of the outer case  12 . 
     Whilst the specific embodiment of the control system  200  have been in application within a reverse circulation hammer drill it can be applied to different types of down the hole devices such as a conventional hammer drill, or a fluid drive. The ring  212  which constitutes, at least in part, the flow path annulus  210  is illustrated and described as a separate component to the sub  282 . However the sub  282  and the ring  212  may be formed as a single integral unit. This can be achieved by simply extending the length of the sub  282  and machining its down hole end to have a configuration similar to that of the ring  212  with the inner circumferential band thereof defining the radius Ro of the annulus width Aw. 
     Porting Sleeve  600   
       FIGS. 1 and 11-14  depict a porting sleeve  600 . The porting sleeve  600  together with an inner circumferential surface of the outer casing  12  forms a porting arrangement directing the air  202  to the piston  700 . The porting sleeve  600  is of a generally tubular form and has an upstream end  602  and a downstream end  604 . Near but inboard of the upstream end  602  the porting sleeve  600  is provided with a shoulder  606  on its outer circumferential surface. Moving further in the downstream direction the sleeve  600  is provided with a plurality of openings  608 . The openings  608  are equally spaced around the circumference of the sleeve  600 . In this embodiment the openings  608  have a slot like shape and extend parallel to the axial direction. The slots  608  provide a passage for air  202  to flow from inside of the sleeve  600  to outside of the sleeve  600 . A downstream end  610  of each opening  608  is configured to form a corresponding ramp  612  such that the thickness of the sleeve  600  increases in a downstream direction from an inner circumferential surface  614  to the outer circumferential surface  616 . The ramps  612  assist in smoothing airflow and reducing turbulence as the air  202  flows from the inside to the outside of the sleeve  600  through the openings  608 . A circumferential shoulder  618  is formed near the ramps  612  on the inner circumferential surface  614 . The shoulder  618  projects in a radial inward direction and forms an abutment surface for the housing  230 . 
     Moving further in the downstream direction the sleeve  600  is provided with a plurality of porting holes  620  which direct the air  202  to flow inside of the sleeve  600  from which it is able to drive the piston  700 . A portion of the length of the sleeve  600  from a downstream end of the openings  608  to a downstream end of the ports  620  is in general radial alignment with a circumferential recess  622  (see  FIG. 13 ) on the inner surface of the outer casing  12 . This creates an annular gap  624  through which the air  202  can flow. Depending on the axial position of the piston  700  the air  202  flowing through the ports  620  can flow into longitudinal recesses  738  formed on an outer circumferential surface of the piston  700  to initially drive the piston  700  in a downstream direction. Alternately the air can flow into a well  709  formed in the head  711  of the piston  700 . This is depicted by the phantom arrow  628 . Thus, when the piston  700  is located so that the top of the head  711  is below an upstream end of the ports  620  the air  202  flows along the path  628  driving the piston  700  in the downhole direction to impact the bit  24 . 
     The inner circumferential surface  614  has a first portion  662  ( FIGS. 11, 44   a  and  44   b ) at a downstream end with a first inner diameter and a contiguous upstream second portion  664  with a second smaller inner diameter. Upstream of the portion  664  the inner circumferential surface  614  has a third portion  666  with a third inner diameter. The third inner diameter is larger than the second inner diameter of the portion  664 . The openings  620  extend from the second portion  664  into the first portion  662 . An internal shoulder  670  is formed at the transition of diameters between the first portion  662  and the second portion  664 . 
     With reference with  FIG. 13  the sleeve  600  is retained within the outer case  12  by: engagement of the shoulder  606  with a circumferential step  621  formed in the inner surface of the outer case  12 ; and the sub  282 . A space between the sub  282  and the sleeve  600  is packed with the ring  212  and a spring steel ring  635 . 
     The sleeve  600  provides enhanced air flow efficiency in comparison to conventional sleeves used in RC hammer drills. In this regard prior art sleeves are provided with a circumferential ring at their upstream end provided with a plurality of axially extending ports to direct air to flow from an internal surface at the upstream end of the sleeve to the outer circumferential surface of the sleeve. This row of ports in the prior art sleeves has been removed with the sleeve  600  and replaced the annular gap  624  between the sleeve  600  and outer case  12  to provide a larger and straighter flow path for the air. The sleeve  600  is relatively short in length (in comparison with a port sleeve with an integral top sub) and has only a relatively small annular area for its diameter. This results in the sleeve  600  being easy to make from hollow bar which can accommodate the piston  700 . This avoids the need to machine the sleeve  600  from a solid bar. Further, the sleeve  600  is relatively cheap to manufacture and easy to replace. As a consequence it can be replaced with the piston if required or desired to restore hammer performance. 
     Inner Tube Assembly  100   
     The control system  200  and indeed the DTH hammer  10  may be used in conjunction with either a single piece inner tube or a multi piece inner tube assembly  100 . When a single er  606  with a piece inner tube is used the bypass path  222  does not exist. In such an instance all of the air  202  which passes through the flow path annulus  210  passes through the gap circumferential step  624  and is available for driving the piston  700 . Notwithstanding that the single piece inner tube has no bypass path  222 ; the spacer  234  can still be used with a one piece inner tube in the event that it is desirable to always hold the flow control body  216  in the first choke position. Thus in this instance the spacer  234  holds the flow control body  216  in the first choke position irrespective of pressure differential thereby maintaining a constant annulus width Aw. In such circumstances, it is not necessary for the spacer  234  to be provided with the holes  250 . 
     The multi piece inner tube assembly  100  depicted in  FIGS. 2-5 and 15-18  can also be used without any modification to the control system  200  and the DTH hammer  10 . The inner tube assembly  100  has a first tube  30  having an outer circumferential surface  32 . A seat  34  (see in particular  FIG. 17 ) extends in a radial direction from the outer circumferential surface  32 . The seat  34  is located intermediate of the opposite ends of the first tube  30 . The seat  34  has a radial face  36  facing the upstream end  20 . The radial face  36  has an inclined or bevelled surface  38  which slopes toward the downstream when looking in a radial outward direction. 
     The inclination of the surface  38  provides a self-centring function for the adapter nozzle  302 . A circumferential groove or recess  44  is formed in the seat  34  near the radial face  36 . The recess  44  enables the seat  34  to bend toward the downstream end  22  of the inner tube assembly  100  in response to the application of a force in the down-hole transferred by the nozzle  302  when the sub  282  is connected to the case  12  and fully tightened with all of the operating parts present a drill string and inner conduit. 
     A series of axially spaced apart circumferential grooves  46 ,  48  and  50  is formed in the tube  30  on a downstream side of the face  36 . The grooves  46 ,  48  and  50  receive respective O-ring seals (not shown) which form a fluid seal against an inside surface of the adapter nozzle  302 . 
     Inner tube assembly  100  also includes a second tube  60 . The second tube  60  is shorter in length than the first tube  30 . The second tube  60  is configured so that it is locatable co-axially with the first tube  30  and overlies a portion  62  of the outer circumferential surface  32 . When the inner tube assembly  100  is installed within a DTH hammer  10  the second tube  60  abuts a circumferential shoulder  63  at an upstream end of the portion  62 . An opposite end of the second tube  60  is tapered to form a circumferential shoulder  65  which transitions from the outer diameter of the second tube  60  to the outer diameter of the first tube  30 . The flange  59  is located between the seat  34  and the shoulder  63 . 
     An inner circumferential surface  64  of the second tube  60  and the portion  62  of the inner circumferential surface of the first tube  30  are relatively configured or arranged so as to create the bypass path  222  enabling operating fluid to flow in an axial direction between the first and second tubes  30  and  60 . In particular the bypass path  222  is formed as a plurality of axial channels or gaps  66  between the tubes  30  and  60 . 
     As previously described and shown in  FIG. 2  incoming fluid  202  can be split to a stream  202   p  which flows to the top of the piston  700  providing impact force for the drill bit  24 ; and, stream  202   b  that flows through the bypass path  222  created by the channels  66 . The air that flows through the bypass path  222 /channels  66  does not contribute to the impact force of the piston  700 . Therefor when air flows through the bypass path  222  the impact force provided by the piston  700  is reduced. This can assist when drilling in soft ground for example ground comprising a significant proportion of clay. 
     In this embodiment the channels  66  are formed by profiling or otherwise configuring the portion  62  of the outer surface  32  of the first tube  30 . The profiling results in variation of the outer radius of the portion  62 . A relatively simple way of achieving this is to machine or otherwise form a plurality of flats  68  ( FIG. 16 ) in the axial direction of the portion  62 . Thus the portion  62  comprises a plurality of arcuate portions  70  of constant radius and intervening flats  68 . Depending on machining tolerances, the inner circumferential surface  64  of the second tube  60  may have a slight or marginal clearance from the arcuate portions  70  or alternately may be provided with a light interference fit. When the light interference fit is provided the flow paths  66  exist between the flats  68  and the inner circumferential surface  64 . If there is a clearance then the bypass path also includes this clearance. 
     In order for fluid to flow through to the bypass path  222  an access path  72  is formed in the second tube  60 . The access path  72  enables air to flow from outside of the inner tube  10  into the channels  66 . 
     In the present embodiment the access path  72  is in the form of radially extending holes  74  formed inboard of an upstream end  76  of the second tube  60 . The end  76  abuts the shoulder  63 . The access path  72  also includes in this embodiment a circumferential recess or groove  80  formed on the inner circumferential surface  64  of the second tube  60  onto which an inner radial end of each of holes  74  open. 
     The disclosed multi piece inner tube assembly  100  of  FIGS. 2-5 and 15-18  may be constructed in alternate forms. For example the channels  66  are described as being formed by the provision of flats  68  on the portion  62  of the outer circumferential surface of the first tube  30 . However as an alternative to or in addition to the provision of the flats  68 , one or more grooves may be formed axially along the portion  62 . Alternately, flats or grooves may be formed in the inner circumferential surface  64  of the second tube  60 . Further, the access paths  72  may be formed by the provision of openings such as slots that open onto the end  76  of the second tube  60  and feed to recess  80 . 
     Bit Retaining System  400   
       FIG. 19  depicts an embodiment of the disclosed drill bit retaining system  400  for the retaining the bit  24  present embodiment of the DTH hammer  10 . The bit retaining system  400  acts to retain a drill bit  24  within the outer case  12  and in particular acts to prevent the bit  24  from falling from the outer case  12 . The drill bit  24  has a shank  524  and a thickened head  525  having a cutting face  526 . The head  525  and cutting face  526  protrudes from the downstream end  18  of the outer case  12 . The drill bit  24  is also formed with a plurality of splines  528 . The splines  528  extend in an axial direction along an outside surface of the bit  24 . The drill bit  24  has a stop mechanism  530  at the down hole (or downstream) end of the splines  528 . The central passage  26  of the bit  24  is in fluid communication with the face  526  via a plurality of inclined feed passages  534 . Further features of the drill bit  24  will be described later. 
     The bit retaining system  400  includes a shroud  440 . The shroud  440  is capable of coupling to the downhole end  18  of the outer case  12 . The shroud  440  when coupled to the outer case  12  locates over an intermediate portion of the drill bit  24 . It will also be noted that the shroud  440  is on an outside, and at the downstream end, of the outer case  12 . The shroud  440  has an internal circumferential surface  442  which is configured to form an abutment  444  to prevent the drill bit  24  from falling from the outer case  12 . Further, the internal circumferential surface  442  is configured to facilitate substantially uniform fluid flow distribution in a down hole direction D between the shroud  440  and an outer surface of the drill bit  24 . 
     The shroud  440  is of a generally cylindrical configuration. The abutment  444  is formed near a down hole or downstream end  446  of the sleeve  440 . In this embodiment the abutment  444  is formed as a plurality of spaced apart protrusions  448  on the internal circumferential surface  442 . The protrusions  448  extend in a radial inward direction from the internal circumferential surface. The protrusions  448  can be equally spaced circumferentially. 
     A plurality of recesses  450  (see for example  FIGS. 19, 21 and 25 ) are also formed in the internal circumferential surface  442 . The recesses  450  are circumferentially spaced apart from each other and axially spaced from the protrusions  448 . The recesses  450  are evenly spaced in a circumferential direction from each other. Also the recesses  450  are in axial alignment with a respective protrusion  448 . However in this embodiment there are more protrusions  448  than recesses  450 . 
     The internal circumferential surface  442  comprises a circumferential band  452 . The circumferential band  452  is near an up hole or upstream end  454  of the shroud  400 . The recesses  450  are formed in the band  452 . The band  452  creates an internal shoulder  456  about the surface  442 . Each recess  450  extends in an axial direction and opens onto both the upstream end  454  of shroud  440  and the shoulder  456 . 
     The drill bit retaining system  400  also includes a drive sub  460 . The drive sub  460  couples the sleeve  440  to the outer case  12 . More particularly the drive sub  460  is able to clamp or otherwise retain the shroud  440  on or to the outer case  12 . The drive sub  460  is also of a generally tubular configuration. A screw thread  462  is formed on an outer circumferential surface  464  of the drive sub  460 . The thread  462  screws onto a thread  25  formed on the inner surface of the outer case  12 . This couples the bit retaining system (and the bit  24 ) to the outer case  12 . As shown in  FIGS. 20 and 22  the outer circumferential surface  464  is formed with a plurality of gaps  466 . The gaps  466  are evenly spaced apart and at locations that enable the protrusions  448  to pass there through. The gaps  466  are formed in an outer band  468  located at a downstream end  470  of the drive sub  460 . The outer band  468  extends in a radial outward direction and as such forms a thickened portion of the drive sub  460 . 
     A plurality of drive splines  472  are formed on an internal circumferential surface  474  of the drive sub  460 . The drive splines  472  extend in an axial direction and are evenly spaced apart. 
     The bit retaining system  400  also includes a detent system  476  that is capable of holding the protrusions  448  in a fixed first rotational position relative to the bit  24 . In the first fixed rotational position the protrusions  448  are capable of abutting the stop mechanism  530  on the bit  24  to prevent the bit  24  from passing out of the shroud  440 . The detent system  476  is a distributed system having components on both the shroud  440  and the drive sub  460 . The components of the detent system  476  on the drive sub  440  are the recesses  450 . The components of the detent system  476  on the drive sub  460  are in the form of members  478  on its outer circumferential surface  464 . The members  478  are evenly spaced apart and are able to register with the recesses  450 . 
     As shown in  FIGS. 24 and 25  in the present embodiment the members  478  are in the form of balls  480  which seat in hemispherical recesses formed on the outer circumferential surface  464 . The hemispherical recesses are disposed adjacent the outer band  468 . The hemispherical recesses are of a depth approximately equal to the radius of the balls  480 . Thus when the balls  480  are in the hemispherical recesses a substantially hemispherical portion of the balls  480  protrude from the outer circumferential surface  464 . 
     With particular reference to  FIGS. 19, 20 and 23  the stop mechanism  530  is in the form of a plurality of lugs  584  which are provided at a downstream end of each of the splines  528 . Indeed the lugs  584  can be considered as and are structurally formed as part of the splines  528 . The splines  528  are separated by axially extending grooves  586 . These grooves  586  form channels or passages for fluid to flow along the outside of the bit  24 . This fluid subsequently returns through the feed passages  534  and central passage  26  back up the DTH hammer  10 . 
     The lugs  584  are formed about a portion of the bit  24  that has an increased radius and material thickness in comparison to the shank  524 . A region  588  of the drill bit  24  between the lugs  584  and the bit face  526  is formed with a portion  590  in which the grooves  586  progressively reduce in depth to zero reaching a contiguous constant diameter portion  592 . 
       FIGS. 24-29  illustrate a manner of coupling the bit retaining system  400  to the DTH hammer  10 .  FIG. 24  illustrates the general juxtaposition of the components of the bit retaining system  400  and parts of the DHT hammer  10 . It should be understood however that when physically coupling the bit retaining system  400  to the hammer  10  it is not necessary to physically align each of the outer case  12 , shroud  440 , drive sub  460  and bit  24  at any one time in the juxtaposition shown in  FIG. 24 . 
     The first stage in assembly of the hammer  10  with the bit retaining system  400  is to slide the shroud  440  over the drive sub  460 . It is necessary for the end  446  of the shroud  440  to extend beyond the end  470  of the drive sub  460 . This requires that shroud  440  and drive sub  460  are rotated relative to each other so that the protrusions  448  align with and subsequently can pass through the gaps  466 . This arrangement is shown in  FIG. 25  passed there through.  FIG. 25  also depicts the bit  24  being partially inserted into the bit retaining system  400 . The bit  24  is pushed into the shroud  440  with the grooves  586  aligned with the protrusions  448 . The pins  478  and recesses  450  are spaced from each other and thus the detent system  476  is disengaged. 
       FIG. 26  illustrates a progression in the coupling process in which, starting from the arrangement in  FIG. 25 , firstly the bit  24  is pushed further in an up hole direction into the shroud  440  and drive sub  460 , and secondly the shroud  440  is rotated relative to the drive sub  460  and the bit  24  so that (a) the recesses  450  align with the members  478 /balls  480  and simultaneously (b) the protrusions  448  axially align with the stop mechanism  530 /lugs  584 . Although the members  478  and recesses  450  are aligned they are still spaced from each other and thus the detent system  476  remains disengaged. 
     Next, as shown in  FIG. 27 , the shroud  440  and drive sub  460  are moved in an axial direction relative to each other so that the members  478 /balls  480  are received within the recesses  450 . When this occurs the detent system  476  is engaged and the shroud  440  is in the first fixed rotational position relative to the drive sub  460 . The shroud  440  cannot fall from the drive sub  460  because the inner circumferential band  452  of the shroud  440  abuts against the thickened band  468  of the drive sub  460 . When in this first fixed rotational position the protrusions  448  are in axial alignment with the stop mechanism  530 /lugs  584 . While the detent system  476  is engaged, it is unlocked as it is possible to push the shroud  440  toward the outer case  12  and thus disengage the detent system  476 . 
     During the assembly steps shown in  FIGS. 25, 26 and 27  the drive sub  460  may have been partially screwed into the outer case  12 . Whether or not that had been the case, in order to assemble the hammer drill  10  the drive sub  460  is now fully screwed into the outer case  12  by way of engaging the respective screw threads  25  and  462 .  FIG. 28  depicts the drive sub  460  partially screwed into the outer case  12 .  FIG. 29  depicts a final use configuration where the drive sub  460  is fully screwed into the outer case  12 . In this configuration the shroud  440  is clamped between the case  12  and the drive sub  460  and the detent system  476  becomes locked because the shroud  440  cannot now be lifted away from the pins  478 . The shroud  440  retains the bit  24  in the outer case  12  and indeed in the drive sub  460 . This is also shown in  FIG. 19 . 
     When the DTH hammer drill  10  is in use the bit  24  is able to reciprocate in an axial direction in response to impacts from a fluid (most usually air) driven piston  700 . Further, in the event that a drill string to which the DTH hammer drill  10  is coupled is rotated, torque can be transferred to the bit  24  via the drive splines  472  in the drive sub  460  which reside within the grooves  586  and can subsequently push against the splines  528 . 
       FIGS. 30-33  depict the sequence of events for changing a bit  24  using the bit retaining system  400 . Initially as shown in  FIG. 30  the drive sub  460  is partially unscrewed from the case  12 . The drive sub  460  needs to be unscrewed only to the extent required to enable the shroud  440  to be slid toward a case  12  by distance sufficient to disengage the balls  480  from the recesses  450 . Subsequently, the shroud  440  can be rotated relative to the drive sub  460  to a second rotational position where the lugs  448  are aligned with the grooves  586  on the bit  24 . This configuration is shown in  FIG. 31 . 
     The bit  24  can now slide out of the outer case  12 , drive sub  460  and the shroud  440  as shown in  FIG. 32 . 
     A new bit can be coupled to the DTH hammer  10  by exactly the same sequence as explained above with the reference to  FIGS. 25-29 . 
     It will be appreciated by those skilled in the art that the above sequence enables very easy and fast replacement of bits  24  as it is not necessary to completely remove the entire drive-sub  460  from DTH hammer  10 . All that is required is the undoing of one thread and rotation of the sleeve  440  relative to the drive sub  460 . 
     Embodiments of the bit retaining system  400  confer numerous and substantial benefits and advantages over traditional methods of maintaining hammer bits. These are summarised as follows:
     (a) The bit retaining system  400  enables the hammer  10  to be made with or use bits  24  of a shorter length and with greater shank diameter than is ordinarily the case. By way of comparison reference is made between  FIG. 33  which depicts a prior art hammer bit  24 ′ and side by side with an embodiment of the bit  24  for use with the same DTH hammer drill  10 . The bit  24  utilised with the hammer  10  may have an overall length L 1  of 305 mm compared with 359 mm for the prior art hammer bit  24 ′. The difference in length results in the bit  24  having the weight of about 15 kg compared to 18 kg for the prior art hammer bit  24 ′.   

     This is significant in terms of transfer of impact forces from the piston  700 . In many prior art DTH hammers the piston is lighter than the bit. This difference is also often in the order of about 3 kg. Therefore use of embodiments of the bit retaining system  400  enables the use of a bit  24  of approximately the same mass as the piston resulting in better energy transfer.
     (b) As described previously the bit retention system  400  retains the bit  24  at the stop mechanism  530  at relatively large diameter location on the bit  24 . In comparison to the prior art bit  24 ′ the stop mechanism shown at item  530 ′ is at or near a free end of the shank  524 ′. As a result the forces generated in the shank  524  during deceleration are compression forces for bit  24  rather than tensile forces as in the prior art bit  24 ′. This reduces the likelihood of fracturing of the shank for bit  24 .   (c) Use of embodiments of the bit retaining system  400  further enable greater contact area between the drive splines  472  in the drive sub  460  and the splines  528  of the bit  24 . This arises because the bit  24  is able to be formed with splines  528  that have a greater active length (being the length along which they are contacted by the splines of the drive sub  460 ) in comparison to prior art comparable drill bit  24 ′. This is shown in  FIG. 33  by comparison of the active lengths L 2  of the splines  528  of bit  24  and splines  528 ′ of prior art bit  24 ′.   (d) The grooves  586  and surface portion  588  of the bit  24  provide a substantially continuous smooth surface for airflow from the upstream end of the shank  524  to the cutting face  526 . The smooth continuous nature of the airflow path provided by the grooves  586  and surface portion  588  are shown most clearly in  FIG. 34 . The arrows A in  FIG. 34  depict air flow while the shaded area S depicts the volume available for air flow between the shroud  440  and bit  24 . The air which previously powered the piston  700  exits from between the drill bit  24  and the shroud  440 . This air flows initially through each of the grooves  586  and subsequently across the smooth continuous surface portion  588 . When the bit  24  is at the top of its stroke substantially the entire surface portion  588  is encircled by the shroud  440  with only the head  525  and cutting face  526  outside of the shroud  440 . This is shown in  FIG. 34 . When the bit  24  is at the bottom of its stroke the flared surface portion  590  is covered by the shroud  440  but the contiguous surface portion  592  is exposed. This is shown in  FIG. 19 . In either case airflow is substantially smoother than can be achieved with the prior art bit  24 ′ (see  FIG. 33 ) which has square shoulders  501  between the shank  528 ′ and head  525 ′. Further the smooth nature of the surface portion  588  and the tapering of the surface portion  590  provide greater volume annulus for air flow between the bit  24  and the shroud  440 . This in turn reduces pressure loss and assists in reducing input energy required to lift cuttings to the surface.   

     It will be understood by those skilled in the art that the disclosed bit retention system  400  may be embodied in other forms. For example the bit retaining system  400  is described as having a detent system  476  comprising balls  480  that seat within hemispherical recesses. However the balls  480  can be replaced by cylindrical pins; or integrated ridges or keys formed on the outer circumferential surface  464  of the drive sub  460 . Further while the detent system is shown as comprising four recesses  450  and four members  478  alternate embodiments may have a different number of recesses and members. For example: only one of each; or only two of each. Alternately there may be more such as six or eight of each. 
     Piston  700   
       FIG. 35 a    illustrates an embodiment of the piston  700 . The piston  700  has a body  712  formed with a central axial passage  714  and an outer surface comprising an circumferential surface  716 ; an inner circumferential surface  717 , and opposite axial surfaces  719   a  and  719   b.  The end  719   a  is in the head  711  of the piston  700  and incorporates the well  709 . The outer circumferential surface  716  is provided with a maximum of three axially spaced apart porting bands namely: an upstream porting band  718 ; an intermediate porting band  722  and a downstream porting band  724 . The upstream porting band  718  is defined between respective upstream and downstream porting edges  726   u,  and  726   d  respectively. The intermediate porting band  722  is defined between respective upstream and downstream porting edges  730   u  and  730   d  respectively. The downstream porting band  724  is defined between an upstream porting edge and a downstream porting edge  732   u  and  732   d  respectively. A stop  734  is formed between the intermediate porting band  722  and the downstream porting band  724 . 
     The piston  700  has a nose  736  which in this instance comprises in combination the stop  734  and the downstream porting band  724 . 
     A plurality of axially extending and circumferentially spaced apart recesses or scallops  738  are formed in an upstream portion of the piston  700  extending from the upstream porting band  718  toward a central reduced diameter portion  744  of the piston  700 . A further set of recesses or scallops  740  is circumferentially spaced about the body  712 . The recesses  740  are located between the reduced diameter portion  744  and the intermediate porting band  722 . The upstream ends of the recesses  740  open onto the reduced diameter portion  744 . 
     The stop  734  has an outer diameter that is larger than the outer diameter of the downstream porting band  724 . A tapered shoulder  748  provides a smooth transition between the stop  734  and the downstream porting band  724 . A steeper shoulder  750  provides a sharper or quicker transition in outer diameter between the stop  734  and the intermediate band  722 . 
     By way of comparison  FIG. 35 b    illustrates a prior art piston  700   p.  The prior art piston  700   p  comprises a body  712   p  with an axial passage  714   p  and an outer circumferential surface  716   p.  The outer circumferential surface  716   p  is provided with four axially spaced apart circumferential porting bands  718   p,    720   p,    722   p  and  724   p.  The porting band  718   p  is the upstream porting band and is defined between a corresponding upstream porting edge  726   up  and a downstream porting edge  726   dp . The porting band  720   p  is a first intermediate porting band defined between an upstream porting edge  728   up  and a downstream porting edge  728   dp . The porting band  722   p  is a second intermediate porting band which is downstream of the first intermediate porting band  720   p.  The porting band  722   p  is defined between an upstream porting edge  730   up  and a downstream porting edge  730   dp . Porting band  724   p  is a downstream porting band and is defined between an upstream porting edge  732   up  and a downstream porting edge  732   dp . A stop  734   p  is also formed between the second intermediate porting band  722   p  and the downstream porting band  724   p.  The section of the piston from and including the stop  734   p  to the downstream porting band  724   p  forms a nose  736   p  of the piston  700   p.  A plurality of axially extending and circumferentially spaced apart recesses or scallops  738   p,    740   p  and  742   p  are formed in the outer circumferential surface  716   p.    
     As shown in  FIGS. 1 and 36-43  in the section of the outer case  12  in which the piston  700  reciprocates the hammer  10  is fitted with a spacer sleeve  701 . The spacer sleeve  701  is held between a locking ring  703  which sits in a groove formed in the inner surface of the outer case  12  and the drive sub  460 . The spacer sleeve  701  is formed with an inner circumferential surface  705 . The surface  705  is of constant diameter for a majority of its length except for its extreme axial ends both of which are tapered so as to gradually increase in inner diameter away from a midpoint of the axis of the sleeve  701 . 
     Notwithstanding the reciprocation of the piston  700  a portion of the nose  736  of the piston  700  is always within the spacer sleeve  701 . An up hole end, i.e. the head  711  of the piston  700  is also always within a downhole section of the porting sleeve  600 . 
     The outer case  12  has an inner circumferential surface  826  which in the section housing the piston  700  is profiled so that various regions have different inner diameter. Starting from an upstream end of the thread  25  the surface  826  has a constant diameter portion  830 . The constant diameter portion  830  is formed with a circumferential groove  831  for seating the locking ring  703 . Upstream of the portion  830 , the inner surface  826  has an increased inner diameter portion  832 . The portion  830  transitions via a shoulder  834  to the portion  832 . The portion  832  subsequently transitions with via a shoulder  836  to a reduced inner diameter portion  838 . Moving in the upstream direction the portion  838  transitions via a shoulder  840  to an increased inner diameter portion  842 . Upstream of the portion  842  is yet a further portion  844  of reduced inner diameter. A shoulder  846  transitions between the portions  842  and  844 . 
     The region of the outer case  12  coincident with the increased inner diameter portion  832  to may be notionally described as a bottom chamber  848 . The upstream contiguous portion of the outer case  12  coincident with the reduced inner diameter portion  838  may be notionally termed as an intermediate chamber  850 . A region within the outer case  12  on an inside of the porting sleeve  600  in which the piston  700  slides may be notionally described as a top chamber  852 . 
     The piston  700 , irrespective of its axial position within the outer case  12 , always has its upstream porting band  718  at least partially within the porting sleeve  600 . 
     A combination of surface portions of outer circumferential surface  716  of the piston  700 , the inner surface  614  of the porting sleeve  600  and surface portions of the inner circumferential surface of the outer case  12  form a porting system. The porting system acts to distribute the driving fluid in a manner so as to cause reciprocation of the piston  700 . The porting system is also arranged to hold the piston  700  in a fixed axial position when the hammer  10  is in the blow down mode. 
       FIG. 36  depicts the hammer  10  in an operational position and with the piston  700  having struck the bit  24 . The flow path for fluid used for driving the piston  700  is depicted by a series of dots in  FIG. 36 . It will be seen that at the time of the piston  700  striking the bit  24  fluid flows between the porting sleeve  600  and the outer case  12 ; through the openings  620 ; through the intermediate chamber  850  over the intermediate porting band  722  and into the bottom chamber  848 . The air is in substance prevented from escaping from the bottom chamber  848  because the inner surface  705  of the sleeve  701  overlies the downstream porting band  724  forming a substantial seal between these two surfaces. Further, an exhaust path that passes through the passage  714  of the piston  700  on an outer circumferential surface of the inner tube  100  is open. 
     Fluid pressure within the intermediate chamber  850  and bottom chamber  848  is substantially the same. However the surface area in the axial direction of the piston  700  upon which the fluid pressure in the bottom chamber  848  acts is greater than the surface area in the axial direction by which the fluid pressure in the intermediate chamber  850  acts. Accordingly the fluid produces a net force in the upstream direction on the piston  700 . Further, fluid is unable to enter the top chamber  852  because the upstream porting band  718  and the inner surface of the second portion  664  of the porting sleeve  600  are dimensioned to form a substantial seal there between. Therefore fluid passing through the opening  620  cannot pass through this seal into the top chamber  852  and can only flow into the intermediate chamber  850  and bottom chamber  848 . The net effect of this is that the piston  700  now commences to move in an upstream direction. 
       FIG. 37  depicts the hammer  10  at a point in the cycle subsequent to the point shown in  FIG. 36 . Here the piston  700  is moving in the upstream direction. At this point in time the upstream porting edge  730   u  of the intermediate porting band  722  has just passed the shoulder  836  and thus closes the fluid flow path entrance into the bottom chamber  848 . In effect the shoulder  836  can be considered as the inlet of the bottom chamber  848 . Fluid however is still able to flow from between the porting sleeve  600  and the case  12  through the openings  620  into the intermediate chamber  850 . Fluid is unable to pass into the top chamber  852  due to the substantial seal formed by the overlapping upstream porting band  718  and the inner surface of the second portion  664  of the porting sleeve  600 . An exhaust path  872  between the inner tube  100  and the surface  717  of the passage  714  remains open and therefore prevents build-up of fluid pressure within the top chamber  852 . Accordingly the piston  700  continues to move on this upward stroke. 
     With continued upstream movement of the piston  700  the piston reaches a position in its cycle depicted in  FIG. 38 . In this position it will be noted that the bottom chamber  848  is now open at its downstream end due to the downstream porting edge  732   d  being axially displaced from and no longer in radial alignment with the surface  705 . Thus fluid previously within the bottom chamber  848  is now free to exhaust to the hole between the drive sub  460  and the bit  24 . The overlapping of the surface  838  and intermediate porting band  722  maintain an upstream end of the bottom chamber  848  sealed or closed. 
     The driving fluid flows from between the outside of the porting sleeve  600  and the portion  842  of the case  12 . This fluid passes through the openings  620  and is now able to flow into both the intermediate chamber  850  and the top chamber  852 . Flow into the top chamber  852  is possible because the downstream porting edge  726   d  of the porting band  718  has passed the shoulder  670 . Accordingly the porting band  718  now resides within the third portion  666  of the porting sleeve  600 . A gap now exists between the surface of the portion  666  and the porting band  718  enabling fluid to flow into the top chamber  852 . Also, the exhaust path  872  is closed due to the shoulder  65  of the inner tube  100  now forming a seal with an inside surface of the passage  714 . Thus now fluid pressure commences to build in the top chamber  852 . 
       FIG. 39  depicts the piston  700  at the top of its stroke in the hammer drill  10 . A downstream end of the bottom chamber  848  is open while its upstream end is closed. A downstream end of the intermediate chamber  850  is closed but the intermediate chamber  850  and the top chamber  852  are subjected to the same fluid pressure as both are open and operating fluid is able to flow into both these chamber. As the exhaust path  872  is closed the fluid pressure within the top chamber  852  builds to the extent that it arrests the upstream motion of the piston  700  and commences the downstream motion or impact stroke of the piston  700 . The commencement of this is depicted in  FIG. 40 . 
       FIG. 40  depicts the piston  700  in the hammer drill  10  moving in its downstream or impact stroke. The top chamber  852  remains open and therefore continues to receive pressurised fluid. The intermediate chamber  850  is also open. An upstream end of the bottom chamber  848  is closed and the downstream end of the top chamber  848  has just closed due to the downstream porting edge  732   d  being in radial alignment or otherwise lapped by the surface  705 . The exhaust path  872  remains closed. While fluid within the closed bottom chamber  848  will become pressurised as the piston  700  moves further in its impact stroke the force supplied by this pressure is not sufficient to overcome the force applied by the fluid pressure in a top chamber  852  driving the piston  700  in the downstream direction. 
       FIG. 41  depicts the piston  700  in the hammer  10  in a subsequent stage of operation to that of  FIG. 40 . The piston  700  has now moved in a downstream direction to a position where the upstream porting edge  730   u  has just passed the shoulder  836  so as to open the upstream end of the bottom chamber  848 . A downstream end of the bottom chamber  848  remains closed due to the downstream porting band  724  forming a substantial seal with the surface  705 . Also, the exhaust path  872  is open allowing fluid pressure within the top chamber  852  to be exhausted through the central passage  714 . Thus prior to the piston  700  striking the bit  24  the pressure applied at the top chamber  852  is relieved. Also, the flow path of fluid into the top chamber  852  is shut due to the substantial seal formed between the upstream porting band  718  and the surface of the second portion  664  of the porting sleeve  600 . Accordingly there is an equalisation of pressure within the intermediate chamber  850  and the bottom chamber  848 . 
       FIG. 42  depicts a subsequent point in the operating cycle of the piston  700  in which the piston strikes the bit  24 . In effect this now returns to the operational state shown in Figure  36 . Thus the complete operating cycle of the piston  700  and hammer  10  has been described. 
     The operation of the present hammer drill  10  described above is for when the hammer  10  is in drilling mode with the drill bit  24  in contact with or cyclically impacting a toe of a hole. However at times the hammer drill  10  is operated in a blow down mode depicted in  FIG. 43 . In the blow down mode the hammer drill is lifted from the toe of the hole by a relatively short distance. The distance is sufficient to allow the drill bit  24  to slide in an axial direction until stopped by a retaining mechanism which comprises the combination of the lugs  584  of the bit  24  contacting the abutment  444  of the shroud  440 . In the blow down mode the fluid which would otherwise be used to drive the piston is passed directly to the hole being drilled to clear the hole. Cuttings and other debris at the bottom of the hole pass through the central passage  26  and the inner tube  100  to the surface. 
     Ideally in the blow down mode the piston  700  does not reciprocate. Indeed reciprocation when in the blow down mode is destructive of the hammer drill. From  FIG. 43  it can be seen that when in the blow down mode the piston  700  is seated against the sleeve  701  and the upper end of the bit  24  is spaced from the nose  736  of the piston  700 . Thus, if the piston  700  were to commence reciprocating when in the blow down mode the impact force of the piston  700  will be imparted to the sleeve  701 . Ordinarily this will cause damage to the sleeve  701  and the retaining ring  703  and render the hammer drill  10  inoperable. However embodiments of the piston  700  and piston porting system act to substantially prevent reciprocation of the piston  700  during the blow down mode and indeed hold the piston  700  in a fixed axial position relative to the outer case  12 . This fixed position is as shown in  FIG. 43  where the shoulder  748  of the stop  734  bears against an upstream end of the sleeve  701 . 
     When the hammer drill  10  is in the blow down mode with the piston  700  bearing against the spacer sleeve  701 , the downstream end of the bottom chamber  848  is open and provides fluid communication around the intermediate porting band  712  to a region  880  between the porting edge  730   d  and the sleeve  701 . Also at this time the intermediate chamber  850  is open at its upstream end and the top chamber  852  is open. The exhaust path  872  is also open. Thus fluid is able to flow into the top chamber  852  through the openings  620  into the exhaust path  872 , and subsequently between the drive sub  460  and the outside of the bit  24  and into the hole. The fluid is then returned through the central passage  22  and inner tube  100 . 
     Also, the fluid pressure acting on the outer circumferential surface  716  and more particularly the entirety of the outer surface of the piston  700  is substantially the same from the upstream porting edge  726   u  to the shoulder  748 . The region  880  is open at its upstream end and in fluid communication with the top chamber  852 . Thus the pressure in region  880  and the top chamber  852  is about the same. In other words the piston  700  is surrounded by fluid at substantially uniform pressure. There is no significant pressure differential between opposite ends of the piston  700 . Given the equalisation of fluid pressure the piston  700  is now held by the fluid pressure against the sleeve  701  in a fixed axial position. This arises because the surface of the piston is arranged so that when acted upon by a substantially uniform fluid pressure the net force applied by the fluid (being pressure×area) is directed in a downstream direction. Thus the piston is held against the sleeve  701 . The uniform pressure field exists due to the above described relationship between the piston and the porting arrangement. 
     More particularly in relation to surface areas the piston  700  has: (a) a downstream surface area being a total of the surface area of the piston  700  looking in a downstream direction that is not parallel to a central axis of the piston and is within and between the top and bottom chambers  852  and  848 ; and (b) an upstream surface area being a total of the surface area of the piston looking in an a downstream direction that is not parallel to the central axis and is within and between the top and bottom chambers. The downstream surface area is greater than the upstream surface area. Thus given pressure equalization within and between the top chamber  852  and the bottom chamber  848 , the force applied by fluid pressure in the downstream direction on the piston  700  is greater than that in the upstream direction, thus holding the piston  700  down and preventing back hammer. 
     It will be noted that with the current piston  700  and porting system in the hammer drill  200  there is no need for the present piston  700  to have recesses equivalent to recesses  742  in the prior art piston  700   p.  This is because in the current embodiment the region  880  always remains open. Since the nose  736  of the piston  700  can be made without recesses or indeed any functionally equivalent converging shapes, and planes, all of which provide stress raisers, it can be mechanically stronger and have a greater mass than the prior art piston  700   p.    
     In summary some of the substantive differences between embodiments of the present piston  700  and porting system and prior art are as follows:
         the bottom chamber  848  is turned ON (i.e. open at its upstream end) along with the top chamber  852  when hammer  10  is in the blowdown mode.   when in blow down mode, because greater force (pressure×area) exist in the top camber  852  than the bottom chamber  848 ; then a greater force holds the piston  700  down than is trying to push it up, thus holding the piston  700  down,   the drill hole pressure can change without changing the ratio of the top cylinder force to comparatively less bottom chamber force for comparable chamber pressures; as the force ratio is then only caused by the difference in chamber areas of the top and bottom chambers, any hole pressure changes will only change the force magnitudes not the force ratios that are only determined by the top and bottom chamber pressure areas,   the present embodiment operates by surrounding the piston  700  in an equal pressure field and relying on the difference in the size of the areas the pressure can act on to push the piston  700  to the downstream end of its stroke and hold it there (i.e. by the unequal application of force arising from the designed geometry of the piston  700 ),   in prior art hammer drills instead of turning both the top and bottom ON, the hammer is designed to turn the bottom cylinder OFF and bleed the pressure out, to stop the hammer from continuing to operate. This causes a problem in the event that the bottom cylinder doesn&#39;t bleed down fast enough or the hole pressure remains high. The problem being that the piston  700   p  can bounce and the hammer continues operating without the piston  700   p  being able to strike the bit, thus resulting in back-hammering.       

     In the claims which follow, and the preceding description, except where the context requires otherwise due to express language or necessary implication, the word “comprise” and variations such as “comprises” or “comprising” are used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the apparatus and method as disclosed herein.