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 .