A rock bit for a down-the-hole drill provides auxiliary flow paths for flushing the working face of the drill bit to facilitate the removal of cuttings and debris through peripheral troughs in the drill thereby significantly reducing the abrasive effect of the cuttings and debris on the bit body. The exhaust fluid is supplied to the working face of the drill bit through several delivery channels spaced equidistantly about the periphery of the drill head where a first set of delivery channels are disposed at acute angles to the central axis of the drill bit and a second set of delivery channels are interposed directly between the central bore of the drill bit and the working face.

BACKGROUND OF THE INVENTION 
     This invention relates generally to a rock bit for down-the-hole drilling and more particularly the rock drill bit having passages for the discharge of cuttings using a fluid operated percussive drill motor. 
     A continuing problem exists in providing an adequately controlled flow of a flushing medium to the multicutting bit inserts provided in the face of a rock drill bit. Removal of cuttings and debris from the face of a rock bit is vital, if such bit is to continue to drill efficiently and to have an economical service life. Cuttings removal is customarily done by the introduction of an exhaust fluid into or near the working bottom of the drilled hole, usually through the drill bit body itself. In established down-the-hole drilling practices, fluid turbulence and constricted flow circuits for the flushing fluid serve to erode the bit body rather variably, and thus reduces the overall effective working time. 
     Existing bits that are employed with downhole, pneumatically driven rock drills (DHD's), typically are provided with one or more machined passages leading into the bit face or are located on the periphery of the face. Such directly engage the rock bottom of the drilling bore hole. The fluid outlets act as dual means to exhaust the operating air from the DHD and for flushing rock cuttings and debris away from the working face, permitting their continuous removal from the bore hole. 
     Since all of the spent air that operates the DHD is exhausted across the bit face, usually from about the bit axis to the outer periphery of the bit, the high volumetric air flows used and the consequent fluid flow velocity across the bit face and about the adjacent skirt become substantial, causing strongly abrasive wear of the bit body. This is especially so near the periphery of the bit body. 
     The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, this is accomplished by providing a rock bit for a down-the-hole drill which includes an elongated shank adapted to be operatively connected at its distal end to a drill string and to a source of fluid pressure which actuates the drilling action; a head having a generally planar working face, the head being preferably, but not exclusively, formed integrally with the shank, and being provided with an array of cutting inserts located about the face; an axially aligned, first bore-like passage connected to the fluid source and terminating within the shank at a point spaced apart from the working face, the first axial passage adapted for supplying exhaust fluid received from the drill motor to the drill bit head; two or more radially aligned, secondary passages connecting at their inner end from the first axial passage proximate its inner end to the periphery of the shank, and being sized for supplying a major percentage of exhaust fluid flowing to the bit head periphery; at least one axially aligned and substantially reduced diameter tertiary passage communicating between the inner end of the axial bore and the working face of the head for supplying a minor portion of exhaust fluid flow to the working face; a peripheral exhaust flow channel connecting between the outer end of each of the secondary radial passages and the working face for supplying a major portion of the exhaust fluid flow to the working face; two or more peripheral cuttings flow exit troughs adapted for passing cuttings-loaded exhaust fluid from the working face, with each of the exit troughs originating in the working face at a point intermediate of the axial center of the head and of its perimeter with such exit troughs terminating on the periphery of the shank between the peripheral exhaust flow channel; the flow area of the secondary radial passages coupled with the peripheral exhaust flow channel relative to the flow area of at least one tertiary axial passage is such that their countercurrent flow patterns maintain a positive fluid pressure across the working face relative to the flowing fluid pressure existing in the exit troughs; and a surface on the shank defining an inwardly oriented shoulder spaced back from the working face of the head and providing an annular channel about the shank adapted for receiving the cutting fluids passing from the working face through the exit troughs and for passing the same to the surface via the drill pipe annulus. 
     The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 is a perspective view illustrating an embodiment of a percussion drill bit body from the working end, depicting the machined surfaces of the invention; 
     FIG. 2 is an elevational end view (proximal) of the drill bit body of FIG. 1; 
     FIG. 3 is a vertical cross-sectional view of a first embodiment of the drill bit of FIG. 1, extending the length of the shank, taken, on line 3--3 of FIG. 2; 
     FIG. 4 is a second cross-sectional view of the drill bit, partially fragmentary, taken on line 4--4 of FIG. 2; 
     FIG. 5 is a third cross-sectional view of the drill bit, partially fragmentary, taken on line 5--5 of FIG. 2; and 
     FIG. 6 is a vertical cross-sectional view illustrating a second embodiment of the drill bit of FIG. 1 showing divergent central exhaust fluid inlet passages. 
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings in detail, wherein like numerals designate like elements, or parts, there is depicted in FIG. 1, a percussive drill bit 12 of the first embodiment, which comprises of a solid metal elongated shaft, or shank, 14 adapted for connection at its distal end to a drill string and a source of fluid pressure (not shown), conveniently high pressure air, sufficient to activate the drill bit. The proximal or working end has an enlarged head portion 16 presenting a generally planar working face 18. Symmetrically arrayed about the periphery of sidewall 19 of head 16 are curvilinear-shaped undercuts, 20, which define a peripheral passageway 22, between the recessed shoulder portion 24 of head 16 and the peripheral beveled edge 26 of the bit working face 18. Arrayed alternately with undercuts 20 in the head sidewall, and aligned in cater-corner fashion, are larger semi-cutting flow exit troughs 28, which bridge between the working face 18 and the sidewall 19 of head 16. 
     Mounted in a somewhat organized pattern across the drill working face 18 are a plurality of "buttons" 30, usually carbide tipped inserts, most of which are axially oriented so as to provide direct drilling contact with an opposing bore hole surface. The inner buttons are encompassed by a circular array of peripheral, or gage buttons, like 32, which engage the bore hole surface, but at an inclined posture relative to the bore hole sidewall. 
     Disposed on either side of the geometric axis of the drill bit face are a pair of exhaust ports, 34 and 36, representing the outer terminals of two linear bores (not shown) that are included within the shank with the main exhaust fluid supply bores and connecting with the main exhaust fluid supply bore to the bit head 16. These linear bores communicate with a larger axial bore, the configuration and function of which (exhaust fluid supply) will be described in relation to the sectional views of FIGS. 3 and 4. 
     In FIG. 2, the generally cylindrical drill bit head 18 is shown as encased in the bore hole 38. It will be seen that (of the four) peripheral exhaust flow channels 22a to 22d are preferably aligned in diametrically opposing fashion, and so to present an aggregate cross-sectional area (four main ports) for exhaust fluid supply into the working face 18. This flow area is several times greater than the cross-sectional flow area of the two central ports 34 and 36. The four somewhat larger exit troughs 28, also have cross-sections at their peripheral ends 29, that are somewhat larger than the combined exhaust flow channels 22 and central exhaust ports 34, 36. This variance in the flow areas facilitates the escape of cuttings-loaded exhaust fluid from the working face 18. 
     A plurality of inwardly pointing directional arrows 39-43 (FIG. 2), indicate how, under typical drilling conditions, entering exhaust fluid sweeps across the working face, generally inwardly, and thus largely avoids abrasive contact with the peripheral beveled edge 26, sidewall 19, and game button 32. The directional flow across the working face 18 permits only minimal overflow of the inner array of aligned buttons 30. All of the exhaust fluid introduced across the working face 18 then exits via the inclined troughs 28 to the head periphery through the trough ends 29. 
     To recapitulate the pathways of the exhaust fluid in flushing the bit face, reference should be made to FIG. 3. Exhaust fluid flows down via central bore hole 44 which itself dead ends well short of working face 18 on the head 16. Two linear passages 48 and 50 extend axially between the central bore end 46 and the exit, or exhaust ports 34, 36, permitting fluid flow to the working face. Also two generally radially aligned secondary passages 52, 54, both of which are preferably forwardly inclined, are coupled between the proximal closed end 46 of central bore 44 and to the machined channels 22 in the peripheral sidewall 19 of bit head 16. Channels 22 communicate between the proximal working face headroom 60 and the rearwardly-located annular channel 56, that is cut back in the distal sidewall of the bit head. The axially aligned cutting bits 30 span the receding gap 60 between working face 18 and bore hole facade 62 during drilling. 
     In the alternate sectional view of FIG. 4, the configuration of cuttings flow exhaust troughs 28 is better seen. They communicate directly with the rearward, annular channels 56 in the shank for facilitating transport of exhaust fluid and debris uphole to the surface. In this view, central bore 44 is fully isolated accessing the bit face only via the passageways 48, 50, 52, 54, exhaust flow channels 22 and exhaust ports 34, 36 of FIG. 3. 
     In the third sectional view of the drill bit (FIG. 5), the erosion-vulnerable inclined gage buttons 32 are noted as making an essentially abrasive contact between the drill bit peripheral edge 26 and the bore hole 62 perimeter. Such contact requires inlet exhaust fluid to enter the working gap 60 of bit face 18 mostly via the intervening peripheral channels 22 and the central exhaust ports 34, 36. The cuttings-loaded exhaust fluid then exits through the four cater-cornered troughs 28. 
     Referring now to FIG. 6, an alternate method of delivering the exhaust fluid to the working face 18 through the central exhaust ports 34, 36 is to arrange the central passages 48, 50 in a manner such that there exists a divergent angle between the passages 48, 50 from the end 46 of the central bore 44 and the working face 18 of the drill head 16. This divergence adds to the flow of the exhaust fluid across the working face 18 which will lengthen the service time of the drill inserts by reducing the abrasive force. 
     As to materials of construction, the bit inserts are customarily of tungsten carbide, as is well-known in the art. The drill bit and shank are of steel. 
     As to the configuration of the internally located flow inlet passages, radial bores 52, 54 are necessarily inclined toward the working face to an appreciable degree. The most acute angle, measured relative to the shank axis, is preferable to insure optimum inlet fluid flow and sweep efficiency across the working face periphery. The acute angle of radial passages 52, 54 is the maximal consistent with intercepting the peripheral channels behind the working face and between the carbide tipped bit inserts. Concurrently, at its distal (inner) end, the radial passages must be fashioned so as to avoid a weakened wall between the passage itself and the greatly reduced diameter of the drill shank somewhat rearward of the working face. 
     Comparative tests have shown that a drill bit fabricated in accordance with the present invention can offer appreciable practical advantages over a conventional DHD hammer drill fitted with a bit exhausting to the periphery. In preliminary trials of drills under simulated operating conditions, it was noted that erosion and wear of bits have been substantially reduced, in some instances increasing the service life of the gage buttons more than 50% compared to using state of the art bits.