You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to rock drilling equipment and more particularly drilling equipment utilised for drilling oil well bores and the like. 
     2. Description of the Prior Art 
     A problem which arises in operation of rock drilling equipment of the kind referred to is that occasionally the head of the operative end of the drill bit will break off from the remainder of the drill bit, due to the stresses arising in drilling and, in the absence of measures taken to prevent this, the broken off head of the drill will remain at the bottom of the bore being drilled and thus possibly many hundreds of feet below the surface. If the remainder of the drill string, with the remainder of the bit, is then withdrawn from the bore and the broken drill bit replaced, drilling of the bore cannot be resumed until the broken-off drill bit head is recovered, as otherwise the new drill bit would be bearing upon the broken drill bit head within the bore and not upon the rock. would present an inpenetratable barrier to the new drill bit and would merely result in the new drill bit in turn becoming hopelessly damaged. Various schemes have been proposed in the past for retrieving broken drill bit parts from bores or for ensuring that, should the operative part or head of a drill bit break off from the remainder, that drill bit head will nevertheless be retained on the end of the drill string allowing it to be retrieved from the bore with the drill string when the string is lifted from the bore. It is an object of the present invention to provide an improved arrangement of the latter sort. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a rock drilling apparatus including a drill string terminating in a drill chuck and a drill bit detachably fitted into the drill chuck, the drill bit having a shank received in the drill chuck, the drill chuck and the drill bit shank having complementary non circular cross-section portions allowing rotational movement about the drill axis to be imparted to the drill bit via the drill chuck whilst allowing limited longitudinal movement of the drill chuck relative to the drill bit, the drill bit shank and the chuck having complementary formations above a first location along the drill bit, for limiting downward movement of the drill bit in the drill chuck, the drill bit having a drill head at its lower end which is of greater diameter than said shank, said head having a retaining formation above said lower end and the below said first location which is of greater diameter than the drill bit is at said first location, which retaining formulation co-operates with complementary retaining means on said chuck so as to prevent the complete detachment of said drill bit head from the chuck in the event of a fracture of the drill bit at said first location, and wherein said retaining formation on the drill bit head and the complementary retaining formation on the chuck take the form of respective parts of a bayonet connection arrangement which is substantially unstressed in normal operation of the drill, the arrangement being such that, in normal operation, the weight of the drill string, and the weight of the drill bit during lifting of the drill string, are supported by said complementary formations, above said first location which likewise receive the stresses imparted to the drill bit to cause a rotational movement of the drill bit. 
     Preferably the bit head retaining arrangement includes means associated with the arrangement for preventing rotation of the drill bit head relative to the chuck assembly without disassembly of the chuck assembly. 
     An embodiment of the invention is described below by way of example with reference to the accompanying drawings in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG.  1 . is a view in axial section of the lower most part of a rock drilling string including a drill bit and a drill chuck assembly in accordance with the present invention, 
     FIGS. 2,  3  and  4  are a fragmentary axial section views to a larger scale as compared to FIG. 1, showing the drill bit and adjoining parts of the assembly in normal operation and (FIG. 4) during withdrawal of the assembly after fracture of the drill bit, 
     FIG. 5 is an exploded view of part of the assembly, 
     FIG. 6 is a perspective view of the drill bit in accordance with the invention, 
     FIG. 7 shows in perspective, parts of the chuck assembly, 
     FIG. 8 shows a perspective side view showing a first assembly stage, 
     FIG. 9 shows a perspective view showing a further assembly stage, and 
     FIG. 10 is again a perspective view showing a yet further assembly stage. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings, FIG. 1 shows the final, lower-most section  10  of a drill string of a drilling apparatus incorporating the invention. The final section shown is conventional apart from the drill bit, the bit-retaining part of the chuck, the bit catcher and the bit catcher gates to be described below. Accordingly, the conventional parts are not described in detail below. However it will be noted that the drill-string section  10  shown comprises at the rear end (i.e. the uppermost end in use) a tapered screw-threaded stub by which the section  10  can be attached to the remainder of the drill string (not shown). The drill string, of course, normally comprises a series of tubular sections connected together end to end by complementary screw threaded stubs and sockets, the string at its end remote from the drill bit being connected, in manner known per se, to means for rotating the drill string, and hence the drill bit, and for supplying compressed air to the section  10 , the compressed air, in use, operating a hammer  14  in the form of longitudinally reciprocable piston which, in operation, repeatedly strikes the upper end of the shank  18  of a drill bit  16 , again in a manner known per se. 
     The drill bit, also in known manner, has a central axial passageway leading to ports on the operative end face of the drill bit so that exhaust air from the hammer mechanism can assisting in flushing debris from the region of the drill face. The drill bit  16  comprises a head portion  22  which may be regarded as the region below/in front of a location A, indicated in FIG.  2  and the shank  18  which may be regarded as the portion above/behind the location A, in FIG.  2 . The bit  16  has its widest diameter at its free operative end face (its diameter at this point being somewhat greater than the maximum outer diameter of the remainder of the drill string) and the drill bit tapers somewhat from its operative end face and connects with a reduced diameter head part  26  via a radiused portion. The portion  26  is cylindrical apart from recesses which, as described in detail below, define part of a bayonet connection arrangement. The portion  26  in turn connects with the shank portion  18  of the bit via a further radiused portion adjacent said location A, the drill shank  18 , at the location A and for some distance rearwardly of location A being of reduced diameter compared to portion  26 . 
     As shown in FIG.  5  and FIG. 6, part  24  of the surface of the shank adjacent location A (i.e. nearer to the head portion) is a smooth continuous cylindrical surface for a short distance rearwardly of the head portion  26 , and the shank portion rearwardly of this smooth part  24  is longitudinally grooved to form a set of alternating splines and grooves. The splined portion terminates in an annular shoulder at the junction between the splined portion and a smooth cylindrical portion  31  of still further reduced diameter extends rearwardly from the splined portion. The portion  31 , in turn, is adjoined at its rear end by an end portion  34  which is of a greater diameter than the portion  31  but of somewhat lesser diameter than twice the radius of the splined part of the shank measured to the bottom of the gooves in the splined portion. A tubular air duct member  40  is secured in the rear end of the drill bit shank in the longitudinal central passage therethrough. 
     The final section  10  of the drill string comprises, in known manner, an outer tube or casing  30  which is internally screw threaded at its lower end to receive a complementary externally screw-threaded upper part of a main chuck part  32 . The main chuck part  32  is itself internally cylindrical having at its lower end a smooth internal cylindrical surface to receive, as a sliding fit, the smooth continuous cylindrical portion  24  of the drill bit shank immediately above the location A, the remainder of the internal surface of the main chuck part  32  being longitudinally splined to receive, as a sliding fit, the splined portions of the drill bit shank. Located within the tubular casing  30  above the main chuck part  32  is a bit retaining ring  35 , which may be a split resilient ring and which receives the portion  31  of the drill bit shank as a close sliding fit but which, in its installed state, is too small in diameter, internally, to pass over the end portion  34  of the drill bit shank. Also, mounted within the tubular casing  30  above the bit retaining ring  35 , is a bush  36  which can receive the end portion of  34  of the drill bit shank as a close sliding fit. At its lower end, the main chuck part  32  has a portion  33  of greater diameter externally than the remainder of the main chuck part to provide an upwardly facing annular shoulder. 
     A bit catcher  38 , in the form of a tubular sleeve, fits over the lower end of the main chuck part  32  and rests on the last-noted annular shoulder. More particularly, the bit catcher  38  has adjacent its upper end a portion which is internally smooth and cylindrical (apart from longitudinal grooves therethrough as discussed below) and of smaller internal diameter than the remainder of the bit catcher and which snugly fits over the externally cylindrical surface of the main chuck part  32 , (or at least over the region just above the end part  33 ). The bit catcher  38  has, adjoining its upper end portion, a cylindrical internal surface of slightly greater internal diameter which is a snug fit over the lower end portion of the main chuck part  32 . The remainder of the bit catcher  38  is internally cylindrical and of an even greater diameter such as to be a sliding fit over the cylindrical portion  26  of the drill bit head, apart from lugs  40  which project inwardly from the internal surface of the bit catcher and are located adjacent to the lower end of the bit catcher  38 . These lugs  40  are accommodated in the recesses, referred to above, in the surface of portion  26  of the drill bit head. As best shown in FIG. 7, in the embodiment illustrated there are three such lugs  40  around the interior of the bit catcher, spaced apart at 120° intervals around the drill axis, and corresponding to these, three similarly spaced longitudinal grooves in the upper portion of the bit catcher. The number of lugs  40  and corresponding grooves may, of course, be greater than or less than three. 
     As shown in FIG. 6, the recesses in the otherwise cylindrical surface of portion  26  of the drill bit head comprise a plurality (in the preferred embodiment 3) of formations each comprising a longitudinal groove part  44  which extends through the shoulder formed where the portion  26  connects with the shank  18 . Each groove  44  has a closed (blind) lower end. Each of these formations further comprises a generally rectangular recess  46  adjoining the respective groove  44  and extending parallel therewith, the recess  46  being blind at its upper and lower ends. Finally, each of these formations further comprises a connecting slot  48  which extends circumferentially from the groove  44  to the recess  46  and thus provides communication between the two. Each connecting slot is of lesser axial extent than the recesses  46  and is disposed about half-way along its respective recess  46 . 
     Further features of the recesses and grooves formed in portion  26  and of the lugs  40 , etc., in the bit catcher will be evident to those skilled in the art from the following brief description of the assembly sequence with reference to FIG. 5, and FIGS. 8-10. 
     In a first assembly stage, (FIG.  8 ), the main chuck part  32  is slipped over the bit shank  18  from the rear end thereof is such a way that the splines on the bit shank engage in the complementary grooves in the interior of the main chuck part, until the lower end of the main chuck part rests upon the shoulder formed at the junction of portion  26  of the bit head with the shank  18 , (adjoining location A). Thereafter, as shown as FIG. 9, the bit catcher  38  is slid over the rear end of the drill bit shank and over the main chuck part  32 . The bit catcher  38  is positioned angularly about the drill bit axis so that the lugs  40  correspond in position with respective ones of the slots of  44 . The bit catcher is then advanced until the lugs  40  come into register with the slots  48  and the bit catcher  38  is then rotated to move the lugs  40  into the recesses  46 . Thus, the connection between the bit catcher and the drill bit head can be regarded as a species of bayonet connection. 
     Finally, as shown in FIG. 10, retaining elements or gates  50  are inserted through slots in the rear (upper) end of the bit catcher  38  into respective ones of the grooves  44 , to prevent reverse rotation of the bit catcher such as would allow the lugs  42  to pass from recesses  46  into grooves  44 . As shown in FIG.  10  and FIGS. 2-4, these gates  50  are quasi-rectangular elongated strips which are of uniform width measured circumferentially of the bit assembly and of uniform thickness measured radially of the assembly, apart from their rear upper ends which are of somewhat enlarged radial dimension for retention in correspondingly enlarged upper end parts of the slots in the upper end of the bit catcher  38 . The end portion  33  of the main chuck part  32  also has longitudinal grooves across its periphery to receive the gates  50 . In the embodiment shown, in which the bit catcher has three lugs  40  and three slots in its upper end, there are three gates  50  and correspondingly three longitudinal grooves across the periphery of the end portion  33 . Again, the number of these items could, of course, be greater than or less than three. In the assembled condition when the gates  50  have been fully inserted into their respective grooves  44  and slots in the bit catcher  38 , the upper ends of the gates  50  are flush with the upper end of the bit catcher. A washer  60  is then fitted over the main chuck part to rest on the upper ends of the bit catcher and the gates  50  and the resulting bit/chuck assembly is screwed into the lower end of the casing  30  with the final position being as shown FIGS. 2 and 3 and the last-noted washer being clamped between the lower end of casing  3  and the upper end of the bit catcher  38 . 
     For convenience, the bit catcher  38  has been described herein and is referred to in the claims as being part of the chuck. 
     It will be appreciated that the bayonet-like connection between the bit catcher and the main chuck part might be reversed, i.e. the grooves, recesses and interconnecting slots could be formed in the internal cylindrical surface of the bit catcher sleeve and the lugs  40  could be formed on the exterior of bit head. 
     It should be appreciated that the dimensions of the various parts are selected so that, in normal use, i.e. unless the bit is broken, downward movement of the bit relative to the chuck is limited by engagement of the shoulder at the lower edge of portion  34  of the bit shank with the bit retaining ring  35  before any lug  40  engages the upper end of its recess  46  and that likewise, upward movement of the drill bit relative to the chuck main part  32  is limited by engagement of the lower end of the main chuck part  32  with the shoulder at the upper end of the bit head portion  26  before any lug  40  engages the lower end of its respective recess  46 . Consequently, the lugs  40  are unstressed as long as the bit remains unbroken. 
     In the event of the drill bit fracturing, which will typically occur in the drill shank rather than the greater-diameter drill bit head, and typically, as shown in FIG. 4, in the region of location, A (i.e. in the region where the drill shank connects with the drill bit head), the head portion of the drill bit will drop until it is retained by engagement of the lugs  40  with the upper ends of the recesses  46 . The gates  50  occupy the grooves  44  and thus prevent rotation of the drill bit head in relation to the bit catcher. As best shown in FIGS. 7 and 8, and as noted above, the outer peripherally of the lower end  33  of the main chuck part  32  is provided externally with longitudinally extending slots or grooves which receive the gates  50 . 
     In the event of the head of the drill bit breaking away from the bit shank, when the drill string is lifted, the head will slide down until it is retained by the lugs  40  within the bit catcher. The drill bit head will thus remain attached to the lower end of the drill string. The catcher arrangement disclosed makes it possible to rotate the drill string together with the broken drill bit head whilst withdrawing the drill string from the drill hole. This helps to prevent the drill bit head snagging in the hole on retrieval of the broken bit. 
     In the present specification “comprises” means “includes or consists of” and “comprising” means “including or consisting of”. 
     The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Summary:
A rock drilling apparatus includes a drill string terminating in a drill chuck ( 32 ) and a drill bit ( 16 ) detachably fitted into the drill chuck, the drill bit having a shank ( 18 ) received in the drill chuck, the drill chuck ( 32 ) and the drill bit shank ( 18 ) having complementary splines and grooves allowing rotation or movement of the drill chuck to be transmitted to the drill bit whilst allowing limited longitudinal movement of the drill chuck relative to the drill bit. The drill bit has a drill head ( 22 ) at its lower end which is of greater diameter than said shank, said head having a part ( 26 ) above that lower end and below the splined region. The part ( 26 ) and the surrounding part ( 38 ) of the chuck provides respective parts of a bayonet connection arrangement which is substantially unstressed in normal operation of the drill, the arrangement being such that, in normal operation, the weight of the drill string and the weight of the drill bit during lifting of the drill string are supported by complementary formations on the drill shank and the chuck, whereas the bayonet connection arrangement prevents complete detachment of the drill bit head from the chuck in the event of a fracture of the bit above the drill head.