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TECHNICAL FIELD 
     The present invention relates to a well drilling tool for use on hollow continuous non-rotating drill strings and in particular to a percussion tool powered by pressurized fluids supplied through the hollow drill string in combination with a generic, fluid powered rotary drilling motor. 
     BACKGROUND OF THE INVENTION 
     Downhole rotary fluid powered drilling motors of several types are in use today and others are being developed for application on non-rotating drill strings in oil well servicing and open hole earth borings. Also, there are a number of different types of surface rotated percussion drills in use today mostly in water-well drilling, blast hole drilling and the like, which have proven to be effective, especially where weight forces on the bit are limited. When downhole motors alone are used on continuous coil tubing strings, weight forces on the drill bit are limited. Also in highly deviated or horizontal wells, bit weight is limited and drill bit penetration rates are consequently reduced. 
     Downhole percussion tools are presently used on surface rotated drill strings, where the extended length of tubing “winds-up” as a spring, under torsional loading. This quick reacting “wound-up” torque discourages axial slip in a splined connection, while at the same time, the elasticity and inertia of the long drill stem soften the sharp hammer blows and would otherwise break loose the static spline friction. Thus, a surface rotary drive is never adequately isolated from the axial hammering of a downhole percussion tool. 
     Presently several types of fluid powered downhole rotary motors are in use and others are being developed for drilling with non-rotating drill strings. Among these are the Moineau progressive cavity type, such as covered by patents U.S. Pat. Nos. 6,241,494 BI and 4,676,725. Experimental and limited usage motors such as the roller rod vane type of patents U.S. Pat. Nos. 5,785,509, 5,833,444, and 5,302,666 BI are other examples of such downhole motors. Yet other examples are the geared vane and geared turbine type of downhole motors described in Martini U.S. Pat. No. 6,520,271. For the purposes of this disclosure, all of these drilling motor types are considered generic as related to the present invention. 
     Therefore, a first object of the present invention is to provide a percussive tool for use in combination with existing gas powered, rotary drilling motors. A second object of the invention is to provide increased drill penetration rates, particularly with non-rotating drill strings and where bit weight is limited and yet a third object is to achieve effective motor isolation from percussive shock. 
     SUMMARY OF THE INVENTION 
     The present invention is an automatic fluid powered, reciprocating mass, percussion drilling tool adapted to drive a drill bit forward in sustained repetition when coupled with a generic fluid driven rotary motor so as to enhance the drilling operation in accordance to the afore stated objects. The industry is moving to increased usage of fluid powered downhole motors for bit rotation instead of surface drill string rotation and inasmuch as percussion drilling has proven to be effective, the present invention provides a means for adding the benefits of percussion drilling to such pre-existing drill motors. 
     The apparatus of the present invention provides for an improved method for drilling earthen boreholes with a fluid powered rotary motor mounted on the downhole end of a non-rotating, fluid conducting drill string. The central shaft of a fluid powered, axially reciprocating hammer is coupled to the motor by means of a rotary coupling having freedom for axial movement, so that the hammer will be rotated by the motor and the motor will be axially isolated from the reciprocating hammer. The percussion hammer comprises a reciprocating mass, in the form of an annular piston operating on the central shaft to impact the drilling bit on its down stroke. The motor is powered by fluid conducted from the surface through the drill string according to the well known practice of prior art. In the present invention however, drilling fluid discharged from the motor is utilized at lower pressure for and controlled by valving ports integral to the central shaft so as to power reciprocation of the piston. Upon discharge from the percussion hammer, the drilling fluid circulates through the bit to flush the borehole and carry the cuttings up the annulus around the the drill string to the well surface. 
     The present invention eliminates the extended drill string length between the surface motor and the percussion tool, so as to eliminate the quick reacting wound-up forces and axial forces which otherwise conspire to inhibit motor isolation. Thus, the percussive blows reach the splined coupling with a sharpness that overcomes static friction in the splines of a connecting coupling coupling and a heretofore unrealized degree of motor isolation is achieved. 
     Since the output torque and rotational speed of the generic motors varies with the particular design, operational pressure and other factors, the percussion hammer should be sized, tuned and made suitable for the characteristics of each motor as well as the field application. For example a higher speed lower torque output motor would require a relatively light, higher frequency hammer to bit impact while a higher torque lower speed motor could have a higher hammer to bit impact. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a center vertical section view of the lower end of a fluid powered drilling motor showing the bearing mounted rotary output shaft with drill bit, reciprocal piston and the ports and passages of the valving arrangement. 
     FIG. 2 is a cross section view taken at line  2 — 2  of FIG.  1 . 
     FIG. 3 is a cross section view taken at line  3 — 3  of FIG.  1 . 
     FIG. 4 is a cross section view taken at line  4 — 4  of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     This specification discloses an automatic bit hammering tool  100  for use in cooperation with a generic fluid powered well drilling motor. It has been demonstrated that impacting a drill bit during rotation will increase the penetration rate of the drilling operation. The motor is powered by drilling fluid supplied through the drill string, and may be any suitable drilling motor having an axial downward extending output shaft. The drilling fluid commonly used in the oil field is highly pressurized nitrogen. The drilling fluid first powers the rotation of the motor and, after bring discharged from the motor, powers the reciprocating percussion tool of the present inventions. Thus, the motor and percussion tool are each powered by a differential pressure drop, with each consuming a portion of the total pressure energy supplied. Both differential pressure drops are well within common oil field pressure capabilities even at very deep well depths. 
     FIG. 1 shows a preferred embodiment  100  of the percussion tool  5 , comprising housing  6 , bearing mounted rotary shaft  7 , with drill bit  8 , reciprocal piston  9  and automatic valving arrangement  10 . The housing is made up of barrel  11 , bottom sub  12 , middle sub  13  and sleeve  14  all in fixed relation. Sleeve  14  is close fitted to inside of barrel  11  and has extended lugs  15  on both ends to fit in recesses  16  of bottom sub  12  and middle sub  13 . Bottom sub  12  has sleeve bearing  33  and thrust bearing  34  for supporting shaft  7 , means for locking shaft  7  against rotation and is attached to barrel  11  by threaded connection  35 . 
     Motor  4 , which may be a low-speed, high torque motor or may be a high speed motor and include a geared reduction, is mounted in barrel  11 , with output shaft  39  connected to rotary shaft  7  by splined coupling  30 . Shaft  7  is also supported by flanged sleeve bearing  36  at its upper end in middle sub  13 . On its upper end  37 , shaft  7  fits coupling  30  for transmitting rotary motion and torque from motor  4  and isolates the motor from the axial percussive movements of shaft  7 . Coupling  30  may be internally splined for a slip fit with shaft end  37  or be of a coupling type that allows longitudinal shaft displacements. Also shaft  7  has a stepped impact receiving shoulder  17  intermediate its ends, fluid passageway  18  extending from the upper end to cross ports  20 , fluid passageway  19  extending upward from the lower end to intersect with cross ports  21  and  22 . Fluid passageway  19  also serves to conduct fluid to the face of the drilling bit for flushing cuttings away from the borehole bottom and up the annulus between the drill string and the borehole. Shouldered retaining nut  31  just below shaft upper end  37  contacts the flange of sleeve bearing  36  to restrain shaft  7  against falling from the assembly. 
     Piston  9  is made for a sliding fit with sleeve bearing  14  on its outside diameter and also with shaft  7  on its inside diameter, These sliding fits are close enough to provide sealing for movement of piston  9  under drilling fluid pressure. Chamber  28  above piston  9  and chamber  29  below piston  9  are alternately pressurized by valving arrangement  10  to provide fluid driven axial reciprocation. Surface  23  at the lower end of piston  9  strikes against shoulder  17  of shaft  7  on each piston cycle. Piston  9  has upper annulus  24 , with fluid passageway  25  connecting to the lower end of piston  9  and chamber  29  and lower annulus  26  with fluid passageway  27  communicating with the upper end of piston  9  and chamber  28 . 
     The valving arrangement  10  for piston  9  reciprocation consists of ports  20 ,  21  and  22  of shaft  7  cooperating with upper annulus  24  lower annulus  26  and the impact against piston end surface  23  to produce self sustaining piston reciprocation. When piston  9  is at the downward end of its reciprocating travel, ports  20  are aligned with upper annulus  24  so as to direct fluid to chamber  29  and drive piston  9  upwards. When piston  9  is at the upward end of its reciprocating travel, ports  21  are aligned with lower annulus  26  so as to direct fluid to chamber  28  and drive piston  9  back downwards. 
     The piston  9  cycle start and end position is shown in FIG. 1 where at the end of its down stroke strikes the shaft  7  shoulder  17  for inertial energy transfer and the valving arrangement  10  ports and passages are aligned for pressure charging chamber  29 ; for accelerating piston  9  upward; and for exhausting chamber  28  above the piston. As piston  9  travels upward it reaches a position where valving arrangement  10  exhausts fluid from chamber  29  through port  22  and simultaneously pressurizes chamber  28  above piston  9  with a compressible cushion of gas, which is trapped in chamber  28 . On the down stroke of piston  9 , valving arrangement  10  further charges chamber  28  for added piston acceleration. Near the end of piston  9  down stroke, valving arrangement  10  exhausts chamber  28  through annulus  26  and ports  21 . Simultaneously, through annulus  24  and ports  20 , chamber  29  is pressurized for immediate, piston acceleration upward after surface  23  strikes shaft shoulder  17  for a percussive blow that is transferred to bit  8 . Thus, ends one piston reciprocation cycle of piston  9 . Cycle frequency will vary from high to low as a function of drilling fluid supply pressure downstream from motor  4 . Screws  32  in bottom sub  12  are provided to be adjusted inwardly to engage recesses  33  so as to lock shaft  7  for changing bits. 
     The embodiments shown and described above are exemplary. It is not claimed that all of the details, parts, elements, or steps described and shown were invented herein. Even though many characteristics and advantages of the present inventions have been described in the drawings and accompanying text, the description is illustrative only. Changes may be made in the detail, especially in matters of shape, size, and arrangement of the parts within the scope and principles of the inventions. The restrictive description and drawings of the specific examples above do not point out what an infringement of this patent would be, but are to provide at least one explanation of how to use and make the inventions. The limits of the inventions and the bounds of the patent protection are measured by and defined in the following claims.

Summary:
Apparatus for an improved method for drilling earthen boreholes with a fluid powered rotary motor mounted on the downhole end of a non-rotating, fluid conducting drill string has a fluid powered, axially reciprocating hammer coupled to the motor by means of a rotary coupling having freedom for axial movement, so that the hammer will be rotated by the motor and the motor will be axially isolated from the reciprocating hammer as both are powered by fluid conducted from the surface through the drill string.