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BACKGROUND OF THE INVENTION 
     This invention relates to a ground drilling or cutting tool, and to an attachment means for attaching cutting elements to ground drilling or cutting tools. 
     Some ground drilling or cutting tools comprise one or more cutting elements and it is desirable for those cutting elements to be replaceable. This enables damaged cutting elements to be replaced or worn cutting elements to be removed for re-work. 
     The working conditions for drilling equipment are exceedingly arduous and it is very difficult to construct drilling equipment so that the cutting elements are removable. The most reliable and easiest way of attaching cutting elements to a cutting tool is to fix them in place by welding. 
     For example, a cutting element may comprise a roller cone which is rotatably secured to an arm, where the arm is in turn welded to the body of the drilling or cutting tool. A number of such cutting elements may be spaced around the periphery of the drilling or cutting tool. 
     Welding of the cutting elements to the cutting or drilling tool presents some difficulty in maintaining the cutting element though. Massive rotary drilling tools can each have a large number of cutting elements that will each require, on a periodic basis, to be re-worked or replaced. Obviously, in the case of a welded cutting element, the re-working must occur with the cutting element in situ. Given that some drilling and cutting tools may be quite large, such an operation becomes quite a task. 
     In addition, when the cutting elements finally become unserviceable, then it is more likely for the drilling or cutting tool to be discarded in its entirety rather than attempting to rework the tool by removing cutting elements. 
     Cutting tools and elements of the above described type are well known to the applicant. The applicant&#39;s previous attempts to overcome at least some of the problems associated with the above described cutting tools and elements is disclosed in their earlier filed PCT application WO/2004/065752, also entitled ATTACHMENT MEANS FOR DRILLING EQUIPMENT, the content of which should be considered included herein by way of reference. This earlier application discloses an attachment means wherein there is a first surface on the cutting element, a second surface on the tool upon which the first surface locates, dowel holes in each of the first and second surfaces, dowels that locate in aligned dowel holes between said surfaces. Also provided are engagement surfaces on the cutting element and tool that extend substantially parallel to the longitudinal axis of the tool and engage to resist side loads applied to the cutting elements and fastening means that secures the cutting elements to the tool. This arrangement provides a means of easily removing the cutting elements from the tool for either replacement or maintenance work. 
     It is an object of the present invention therefore to provide a ground drilling or cutting tool having improved (at least when compared with the disclosure of the applicant&#39;s above mentioned earlier filed patent application) cutting tool retention, or at least provide an attachment means that is a useful alternative to known attachment means. 
     Other objects and advantages of the present invention will become apparent from the following description, taking in connection with the accompanying drawings, wherein, by way of illustration and example, an embodiment of the present invention is disclosed. 
     SUMMARY OF THE INVENTION 
     In one aspect of this invention, there is proposed a ground drilling or cutting tool comprising a cutting element removably attached thereto by attachment means, and releasable retention means adapted to maintain attachment of the cutting element to the tool in the event that the attachment means fails. 
     In one form, the ground drilling or cutting tool is a hole opener or reamer. 
     In one form, said retention means comprises: 
     a first surface on a side of the cutting element, 
     a mounting recess on the cutting tool in which at least a portion of said cutting element locates, 
     a second surface in a side of the mounting recess against which said first surface locates, 
     channels in each of said first and second surfaces that align so as to cooperatively define a dowel hole when said first and second surfaces locate together, 
     at least one dowel that locates in the dowel hole, and means for retaining the or each dowel within the dowel hole. 
     In one form, the means for retaining the or each dowel comprises a recess in the or each dowel, for each dowel a bolt hole extending substantially normal to and at least partially intersecting the dowel hole, for each bolt hole a bolt that locates in said bolt hole when and only when the recess in the dowel is aligned with the bolt hole, the bolt thereby securing the dowel in its dowel hole, and the dowel aiding to secure the cutting element to the cutting tool. 
     In one form, the attachment means comprises a surface on an underside of a base of the cutting element, and a further surface on a floor of the mounting recess upon which the base underside surface locates, a further set of dowel holes in each of the underside and floor surfaces, and a further set of dowels that locate in the aligned further set of dowel holes between said surfaces. 
     In one form, the cutting element includes an arm supported by a base, where said base locates in the recess and a side of said base is the first surface that locates against the second surface. 
     In one form, the recess in the dowel is a circumferential groove extending there around. 
     In one form, the recess in the dowel is a hole through the dowel. 
     In one form, the base comprises a pair of substantially oppositely facing side surfaces, and the recess comprises a pair of matching second surfaces in the sides thereof, where each pair of matching first and second faces cooperatively define dowel holes. 
     In one form, each of the two dowel holes accepts two dowels, and for each dowel there is a bolt hole receiving a bolt that registers in the circumferential groove of one of the four dowels. 
     In one form, there is also a provided a surface on the underside of the base of the cutting element, and a further surface on the floor of the recess upon which the base underside surface locates, a further set of dowel holes in each of the underside and floor surfaces, and a further set of dowels that locate in the aligned further set of dowel holes between said surfaces. 
     In one form, there are engagement surfaces on the underside of the cutting element and floor of the mounting recess that extend substantially parallel to the longitudinal axis of the tool and engage to resist side loads applied to the cutting elements, and fastening means that secure the cutting elements to the tool. 
     In one form, these engagement surfaces comprise a channel located on the floor of the mounting recess in the drilling or cutting tool and a projection to engage the channel on the underside of the base of the cutting element. 
     In one form, the engagement surfaces comprise channel located on the underside of the base of the cutting element, and projection to engage the channel on the floor of the mounting recess in the drilling or cutting tool. 
     In one form, the further set of dowels locate within each of the dowel holes and resist movement in a direction that is parallel to the longitudinal axis of the drilling or cutting tool. 
     In one form, the fastening means comprises a plurality of bolts which locate through corresponding holes in the underside surface and engage in threaded holes in the floor surface. 
     In one form, clearance is provided between the holes in the underside surface and the bolts so that minimal shear loading is applied to the bolts. 
     The second set of dowels locate within each of the dowel holes and resist movement in a direction that is parallel to the longitudinal axis of the drilling or cutting tool. In one form, the fastening means comprises a plurality of bolts which locate through corresponding holes in the underside surface and engage in threaded holes within the floor surface. Preferably, clearance is provided between the holes in the underside surface and the bolts so that minimal shear loading is applied to the bolts. Instead, shear forces along the longitudinal axis of the drilling or cutting tool are restrained by the second set of dowels. This prevents damage to the bolts which may in turn create difficulty with disassembly. 
     In a further aspect, the invention may be said to reside in an attachment means for removably securing a cutting element to a ground drilling or cutting tool, said attachment means comprising: 
     a first surface on a side of a cutting element, 
     a cutting tool including a mounting recess in which at least a portion of said cutting element locates, 
     a second surface in a side of the mounting recess against which said first surface locates, 
     channels in each of said first and second surfaces that align so as to cooperatively define a dowel hole when said first and second surfaces locate together, 
     at least one dowel that locates in the dowel hole, and means for retaining the or each dowel within the dowel hole. 
     In one form, the means for retaining the or each dowel comprises a recess in the or each dowel, for each dowel a bolt hole extending substantially normal to and at least partially intersecting the dowel hole, for each bolt hole a bolt that locates in said bolt hole when and only when the recess in the dowel is aligned with the bolt hole, the bolt thereby securing the dowel in its dowel hole, and the dowel aiding to secure the cutting element to the cutting tool. 
     In one form, the cutting element includes an arm supported by a base, where said base locates in the recess and a side of said base is the first surface that locates against the second surface. 
     In one form, the recess in the dowel is a circumferential groove extending there around. 
     In one form, the recess in the dowel is a hole through the dowel. 
     In one form, the base comprises a pair of substantially oppositely facing side surfaces, and the recess comprises a pair of matching second surfaces in the sides thereof, where each pair of matching first and second faces cooperatively define dowel holes. 
     In one form, each of the two dowel holes accepts two dowels, and for each dowel there is a bolt hole receiving a bolt that registers in the circumferential groove of one of the four dowels. 
     In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawing. The invention is capable of embodiments in addition to those described and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting. 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate certain embodiments of the invention, and together with the description, serve to explain the principles of the invention. 
     Those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for designing other structures, methods, and systems for carrying out the several purposes of the present invention. It is important, therefore, to recognise that the claims should be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of this invention it will now be described with respect to an exemplary embodiment which shall be described herein with the assistance of drawings wherein 
         FIG. 1  shows a perspective view of a cutting tool; 
         FIG. 2  is an exploded perspective view of the cutting tool of  FIG. 1 , where a cutting element and its attachment means are shown exploded from the cutting tool; and 
         FIG. 3  shows a detailed perspective view of the cutting element and attachment means illustrated in  FIG. 2 . 
     
    
    
     In the following description, like reference characters designate like or corresponding parts throughout the several views of the drawings. 
     DETAILED DESCRIPTION 
     The exemplary embodiments described and shown in the representations are generally used in relation to a drilling or cutting tool known as a hole opener or reamer. This cutting tool acts to open a small diameter hole to a larger diameter and uses multiple conical roller cutters to achieve cutting of the rock or earth. This exemplary embodiment relates to the attachment of those roller cutters to the main body of the hole opener or hole reamer. 
       FIG. 1  shows a drilling tool  10  incorporating the number of cutting elements  11  around the periphery of the tool  10 . In this embodiment, the cutting elements  11  comprise conical roller cutters. 
     As seen in  FIG. 2 , an attachment means is used to fix the cutting elements  11  to the drilling tool  10 . In this embodiment, the cutting elements  11  comprise an arm  21  extending from a roller cutter  22 , where the arm  21  is secured to a base, the base being in the form of a mounting box  12  (hereinafter therefore, the term mounting box will used in place of the word base). The arm  21  is secured to the mounting box  12  by welds. 
     The mounting box  12  comprises at least a first pair of generally planar surfaces on oppositely facing sides  30  thereof, and a generally planar underside surface  32 . 
     The drilling tool comprises a mounting recess  40  in which said mounting box  12  locates. This mounting recess  40  defines a pair of second or side surfaces  42  against which the first (side) surfaces  30  of the mounting box  12  locate. There are matching channels  50  in each the sides  30  of the mounting box  12  and second side surfaces  42  of the recess that align when said first  30  and second  42  surfaces locate together, so that these matching channels  50  cooperatively define a dowel hole. 
     Each dowel hole is sized to accept two dowels  60  with a light tap fit therein in an end to end fashion. In this exemplary embodiment, each of the dowels  60  is 1 inch diameter hardened steel having a circumferential groove  62  extending there around, where this circumferential groove  62  is closer to one end of the dowel  60  than the other, so that one end of the dowel is ‘a shorter end’. During assembly, each of the dowels  60  are inserted into the dowel holes formed by channels  50  so that this shorter end is directed inwardly. 
     There are two bolt holes  70  extending substantially through, normal to and at least partially intersecting each of the dowel holes formed by channels  50 . 
     A bolt  72  may be located in each bolt hole  70  when and only when the recess  62  in the dowel  60  is aligned with the bolt hole  70 , the bolt  72  thereby securing the dowel  60  in its dowel hole, and the dowel  60  aiding to secure the cutting element  11  to the cutting tool  10 . 
     The bolt holes  70  within the mounting box  12  are counter-sunk and are drilled to a diameter to provide clearance between the shaft of the bolt and the bolt hole  70 . In use, this assists in preventing or minimising shear loads being applied to the bolts  72 . 
     The bolts  72  are then retained in their respective bolts holes  72  by cir-clips  74 . 
     The underside surface  32  of the cutting element&#39;s  11  mounting box  12  comprises a generally planar face  13  that abuts against the floor surface  44  of the mounting recess  40  on the drilling tool  10 , which comprises an abutment surface  14 . A second set of dowel holes  15  are formed in both the planar face  13  and the abutment surface  14  on both the mounting box  12  and the drilling tool  10 . A second set of dowels  16  locate within the second set of dowel holes  15  and both the planar face  13  and abutment surface  14 , and are a light interference fit with the second set of dowel holes  15 . In this exemplary embodiment, this second set of dowels are 1.25 inch diameter hardened steel. 
     The abutment surface  14  has a channel  17  and the second set of dowel holes  15  are located centrally within the channel  17 . The planar face  13  of the mounting box  12  has an elongate projection  18  that fits tightly within the channel  17 . Clearly, this arrangement could be reversed with the abutment surface  14  incorporating a respective elongate projection which mates with a corresponding channel located on the planar underside face  13  of mounting box  12 . 
     The fastening means in this embodiment comprises a plurality of M24 grade steel bolts  19  and washers  24  that locate through the mounting box  12 . Washers may be of the standard slit lock or serrated variety. In this embodiment NORD-LOCK™ self locking washers are used. Bolts  19  threadably engage within apertures  23  in the abutment surface  14 . The apertures  20  within the mounting box  12  are counter-sunk and are drilled to a diameter to provide clearance between the shaft of the bolt  19  and the aperture  20 . In use, this assists in preventing or minimising shear loads being applied to the bolts  19 . Instead, the dowels  16  resist the majority of any shear load which results from drilling forces applied to the cutting elements  11  along the longitudinal axis of the drilling tool  10 . 
     In order to assemble the attachment means to the drilling tool  10 , the second set of dowels  16  are located within the second set of dowel holes  15  in the abutment surface  14 . The mounting box  12  is then positioned so that the second set of dowels locates within second set of dowel holes  15  in the planar surface  13  and so that the elongate projection  18  locates within the channel  17 . The bolts  19  are then engaged in the apertures  20  and tightened. Cir-clips  80  are then inserted in order to ensure that portions of bolts  19  cannot escape their respective bolt holes  20  in the event that the bolt  19  fails. 
     The first set of dowels  60  are then inserted into the holes for these so that half of the width of each dowel  60  is in the mounting box  12  and the other half is in the drilling tool  10 , and such that their short ends are directed innermost until their longer ends are flush with the ends of the dowel hole. The dowel bolts  72  are then inserted into their respective holes  70  and tightened. 
     In use, any drilling forces longitudinal to the drilling tool  10  are transmitted as shear forces primarily to the second set of dowels  16 . The clearance between the bolts  19  and the apertures  20  minimise any shear loading applied to the bolts  19 . As the drilling tool rotates, any side forces applied normal to the longitudinal axis of the drilling tool are resisted by the first set of dowels  60  and the engagement of the elongate projection  18  within the channels  17 . 
     Moreover, the first set of dowels  60  resist loads that would otherwise be imparted upon the bolts  19  as tension loads, thereby significantly reducing the likelihood that the bolts  19  will fail in this way. 
     The applicant refers to the first set of dowels  60  as “safety dowels”, as one function of these is to ensure that the cutting tool  11  is retained in its mounting recess  40  in the event that bolts  19  fail. Without dowels  60  registering in aligned channels  50 , the cutting element  12  would break free of the cutting tool  11  in the event that bolts  19  failed. 
     The cutting elements  11  can be easily removed from the drilling tool  10  by reversal of the above-described assembly process. To further facilitate removal of cutting element  11 , jacking bolts  25  are screwed into the middle pair of apertures after removal of bolts  19 . This pair of apertures is modified to include a screw thread which threadably engages with the shafts of jacking bolts  25 . As jacking bolts  25  are screwed in they function to force mounting box  12  from abutment surface  14 . This thereby simplifies the removal process for maintenance purposes particularly when compared with mounting of the cutting elements  11  to a drilling tool  10  by welding. 
     In a second embodiment (not illustrated), the arm of the cutting element  11 , instead of being welded to a mounting box  12 , may be arranged to secure directly to the drilling tool  10 . In this case, the planar face  13  and associated elongate projection  18  would be on the arm  21  of the cutting element  11  and therefore would mount directly to the abutment surface  14 . 
     Further, as an alternate to the use of a channel  17  and elongate projection  18 , both the mounting box  12  in accordance with the first embodiment or the arm of the cutting element  11  in accordance with the second embodiment may locate within a channel having side walls that abut against either the mounting box  12  or the arm of the cutting element  11  so as to resist sideways loads. 
     With both of the embodiments, it is possible to easily remove the cutting elements for either replacement or maintenance work. As minimal loads are directly applied to the bolts holding the cutting element to the tool, there is less possibility of damage being caused to the bolts resulting in them being easier to remove. 
     In this specification unless the contrary is expressly stated, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not to be construed as an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge; or known to be relevant to an attempt to solve any problem with which this specification is concerned.

Summary:
This invention relates to a ground drilling or cutting tool comprising a cutting element removably attached thereto by attachment means, the tool further including retention means adapted to maintain attachment of the cutting element to the tool in the event that the attachment means fails. The retention means comprises a first surface on a side of the cutting element, a mounting recess in the cutting tool in which at least a portion of said cutting element locates, a second surface in a side of the mounting recess against which said first surface locates, channels in each of said first and second surfaces that align so as to cooperatively define a dowel hole when said first and second surfaces locate together, at least one dowel that locates in the dowel hole, and means for retaining the or each dowel within the dowel hole.