A cutter tool that includes a shank by which the tool is releasably retainable within a tool holder, an enlarged shoulder or heel provided at one end of the shank and adapted, in use, to abut a seating surface of a tool holder and limit the penetration of the tool holder into a receiving aperture of the tool holder, and an integral blade provided beyond the enlarged shoulder or heel and terminating in a carbide or other tip. The tool is provided with a zone of weakness such that if subjected to fracture forces, a fracture will be encouraged to propagate from the zone of weakness in a particular desired manner.

PRIORITY 
     This Application claims the benefit of British Application Serial No. 1113591.0 filed 8 Aug. 2011. 
     FIELD OF THE INVENTION 
     This invention relates to a cutter tool, primarily for use in mineral winning, such as coal mining, but also useable for other underground purposes such as tunnel or roadway driving, or above ground for civil engineering works, road planing, trench cutting, whether with drums or endless chains, both on land and sub-sea. 
     BACKGROUND OF THE INVENTION 
     In coal and other kinds of mining by the longwall technique, it is conventional to provide steel roof supports either as components of a series of hydraulically powered roof supports extending along, and advanceable with, the mineral face as the latter is extracted, or permanent supports in the form of rings or arches at roadways that are conventionally provided at each end of a longwall face. 
     Mineral is removed by a single or double ended ranging drum shearer, which traverses the mineral face, with a rotary cutting head carried by the, or each, ranging arm to follow the seam, each drum being provided with 50-100 cutter tools, each having a shank by which each tool is replaceably located in a tool holder. 
     However, despite the best efforts of the operatives, the drums, and specifically the tools, frequently strike a steel roof bar, or hard inclusions in the roof or floor of the mineral seam, and the tool is fractured—typically its head or blade, is sheared from its shank. 
     Tools need regular replacement, and removal of a broken or damaged tool is frequently difficult or sometimes impossible, such that removal cannot be effected until the drum is eventually taken to the surface, or a safe underground area, for refurbishment. 
     In civil engineering works, the tool may strike an unknown buried obstruction such as concrete reinforcement bar, a steel railway line, etc. 
     OBJECT OF THE INVENTION 
     A basic object of the present invention is the provision of an improved cutter tool. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided a cutter tool comprising a shank by which the tool is releasably retainable within a tool holder, an enlarged shoulder(s) or heel(s) provided at one end of the shank and adapted, in use, to abut a seating surface of a tool holder and hence to limit the penetration of the tool into a receiving aperture of the tool holder, and an integral blade provided beyond the enlarged shoulder(s) or heel(s) and terminating in a carbide or other tip. The tool is provided with a zone of weakness such that if subjected to fracture forces, a fracture will be encouraged to propagate from the zone of weakness in a particular desired manner. 
     The zone of weakness needs to be such that the normal, e.g. mineral winning, operation of the tool is not impaired, but the tool will fracture in the manner desired when the tool is overloaded by striking, for example, a steel support. However, as the inadvertent striking of e.g. a steel member of some form or other is largely unavoidable, the provision of a zone of weakness and thus the management of the zone of fracture, ensures that the residual portion of a broken tool is easier to extract in underground conditions, than would otherwise be the case. 
     In one aspect, a cutter tool in accordance with the present invention includes a shank releasably retainable within an aperture in a tool holder, and a head to contact the material to be engaged during use. The head is provided at one end of the shank to abut a seating surface of a tool holder and limit the penetration of the cutter tool into the aperture of the tool holder. The cutter tool has a front surface that faces in the direction of travel during use, an opposite rear surface, and side surfaces that connect the front and rear surfaces. A slot opens only in the front surface and in the side surfaces to the depth of the slot to define a frangible portion. The frangible portion remains intact during normal operations. However, when the cutter tool is subjected to fracture forces, a fracture will be encouraged to propagate from the slot through the frangible portion. 
     In another aspect, a cutter tool in accordance with the invention includes a shank releasably retainable within an aperture in a tool holder, and a head to contact the material to be engaged during use. The head is provided at one end of the shank to abut a seating surface of a tool holder and limit the penetration of the cutter tool into the aperture of the tool holder. The tool includes a zone of weakness to define a frangible portion that, in relation to the expected striking force, has less strength than the interface between the head and the shank in relation to the expected striking force such that when the cutter tool is subjected to fracture forces, a fracture will be encouraged to propagate through the frangible portion. 
     In another aspect of the invention, a cutter tool includes a shank releasably retainable within an aperture in a tool holder, and a head to contact the material to be engaged during use. The head is larger than the shank and provided at one end of the shank so as to define an interface between the shank and the head. The head abuts a seating surface of the tool holder and limits the penetration of the cutter tool into the aperture of the tool holder. A zone of weakness is provided at the interface between the shank and the head such that when the cutter tool strikes a hard member a fracture will be encouraged to propagate through that interface. 
     The zone of weakness is preferably provided by a slot (e.g., a slit or groove cut or forged) formed into the tool to provide a frangible portion within the tool, but could be provided by other means. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevation of one of Applicant's prior art, standard shearer tools. 
         FIG. 2  corresponds to  FIG. 1 , but shows in accordance with one aspect of the invention, a slot provided at the junction between a forward face of the shank, and the forwardly extending, enlarged shoulder. 
         FIG. 2A  shows a second embodiment of a cutter tool with slot at the junction between a forward face of the shank and the forwardly extending enlarged shoulder. 
         FIG. 3  also corresponds to  FIG. 1 , but shows in accordance with another aspect of the invention, a slot provided at the junction between a base of the front face of the blade, and the forwardly extending, enlarged shoulder. 
     
    
    
     In all three figures, like components are accorded like reference numerals. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1  is illustrated a typical shearer pick  1  in common use, worldwide, comprising a non-circular shank  2  adapted to be releasably located within a corresponding aperture in a tool holder or pick box, and releasably retained against inadvertent loss by a latching means, such as a multi-ribbed, synthetic plastics insert in the double “O” aperture  3 . At a front or leading face  4  of the shank  2  is provided a blind aperture  5  to receive an additional, or alternative, resilient, shank retaining button (not shown). From the upper end of the front face  4  of the shank  2  a forwardly directed enlarged shoulder  6  extends having an underside  7  to seat on a support surface (not shown) of the associated tool holder or pick box in the well known manner. Further forward, the shoulder  6  is provided with a notch  8  for engagement by an extraction tool, such as a screwdriver or drift, when extraction of the pick  1  is required. At a trailing face side  9  of the shank  2  is provided a heel  10  also having a support surface  11 , and in addition a through hole  12  to accommodate a portion of a water spray nozzle (not shown). Beyond the shoulder  6  and heel  10  extends an integral blade  13  provided with a carbide tip  14 . Shoulder  6 , heel  10  and blade  13  comprise the head  22  of the cutter tool  1 . 
     In accordance with a first embodiment of the invention, a slot  15  is provided in cutter tool  1 A at a 90° junction between the leading face  4  of the shank  2  and a portion  16  of the enlarged shoulder  6 . In this embodiment, the slot  15  extends at a 45° angle to the front face  40  of shank  2  and to the underside surface  7  of shoulder  6 . Slot  15  is provided to influence that the cutting tool  1  will break the head  22  from the shank  2  so that the shank can be removed downward when used in a tool holder that provides a removal opening such as disclosed in British Application Serial No. 1113669.4 field Aug. 9, 2011. 
       FIG. 2A  discloses a preferred slot formation at the junction between the shank and the shoulder. In this embodiment, slot  15 A extends inward at the junction along the top of shank  4  to define a frangible portion  20  that is sufficiently robust to withstand normal operation, but will break when the cutter tool  1 B strikes a steel member or other hard object. The provision of slot  15 A dictates that the cutter tool when striking a steel member will, in most cases, break cleanly along the top of the shank without bending or distorting the shank  2  or leaving a portion of head  22  remaining connected to the shank. An unbent shank can be easily tapped downward and out of the tool holder for tool holders with a lower removal opening such as disclosed in British Application Serial No. 1113669.4 filed Aug. 9, 2011. 
     Slot  15 A extends from a front surface  24  of the cutter tool  1 B. In one preferred embodiment, slot  15 A extends perpendicular to front face  4  and opens only in the front surface (i.e., in the direction the tool is driven during operation) of the cutting tool, which in this embodiment is the front face  4  of shank  2 , and in the sides to its depth of extension into the tool from the front face  4  in order to more effectively provide a clean break without distortion of the part remaining in the tool holder; i.e., it is considered beneficial for directing a clean fracture for the slot to open only or primarily in the surface which receives the primary loading. Slot  15 A defines a frangible portion  20  in the remaining thickness of the material. Frangible portion  20  is formed to resist normal loading and remain intact during normal operation of the cutter tool, and to provide a clean break of the head  22  from the shank  2  without distortion of the shank when a steel member or other hard object (e.g., a steel member) is struck. The minimum and maximum thickness will be determined based on the intended application as well as the design and material of the cutter tool. 
     As stated previously, a not uncommon occurrence is for the blade  13  to strike a steel obstruction, such as a roof support bar, or buried tramway rail or pipeline. In this situation, the provision of the slot  15  or  15 A is aimed at propagating a break from the slot to completely separate the head  22  from the shank  2  without distorting the shank for easier removal of the shank from the tool holder. 
     In another embodiment illustrated in  FIG. 3 , a slot  17  is provided in a front face  18  of the blade  13  in a zone  19  where the blade  13  meets the enlarged shoulder  6 . As with the earlier embodiments, the slot  17  preferably opens only in the front face of the cutting tool, which in this case is the front face  18  of blade  13 , and in the sides to the depth of the slot into the head  22 . As with the earlier embodiment, the slot forms a frangible portion  20  having a certain thickness. The frangible portion  20  has a certain “dynamic strength,” which is defined as the force required to break the frangible portion when the force is an impact load applied to the tip of the cutter tool. The dynamic strength of the frangible portion is, then, a factor of (i) the cross sectional size of the frangible portion where the break is intended (typically the most narrow cross section), (ii) material of the cutter tool at this cross section frangible portion, and (iii) the vertical distance between the frangible portion and the tip where the impact load is applied during use. 
     In accordance with this one aspect of invention, the dynamic strength of the frangible portion  20  is less or weaker than the dynamic strength of the interface between the head  22  and the shank  2 . In a preferred embodiment, the dynamic strength of the frangible portion is at least about ten percent less than the dynamic strength of the interface between the shank and the head in order to reliably (i.e., in most cases) direct the breaking of the cutter tool along the frangible portion rather than the shank-head interface when the cutter tool strikes a steel member or other hard member. In this way the shoulder  6  and removal notch  8  are preserved even if a steel member is struck by the tip  14  of the cutter tool to enable removal of the shank  2  from the tool holder so that a replacement cutter tool can be inserted. The dynamic strength of the frangible portion could be much less than 10% weaker than the shank-head interface so long as the frangible portion remained intact during normal operation. 
     In alternative constructions (not shown), a zone of weakness or frangible portion can be defined by means other than a slot defined in the cutter tool. In addition, other kinds of slots can be used even though they are formed in ways other than disclosed above. For example, a slot can be formed around the periphery or by being partially formed in both the front and the rear surface. In these alternative slot constructions, the slot is preferably primarily formed to extend from the front surface, i.e., that the majority of the depth of the slot extends from the front surface. The slots  15 ,  15 A,  17  are shown as narrow and linear gaps in the cutter tool, which is the preferred construction. Nevertheless, the slots could have a non-linear configuration, have a wider (not narrow) width, and/or have an irregular shape. The term “slot” is intended to have a broad construction to define a gap in the cutter tool having a wide variety of possible shapes. The shape or size of the opening can vary considerably.