Sharpness and Bluntness: Absolute or Relative?

This chapter provides an introduction to the concept of sharpness. It is particularly important when the cutting edge is always in contact with the newly separated material ahead of the tool. When cutting ductile metals with offcut formation by shear, the tool remains in intimate contact with the workpiece at the point of separation between cut surface and chip. During the transient indentation start to a cut, tool sharpness will concentrate the stresses and strains in the tip region as the load is increased, leading to the build-up of localized material damage. Eventually, at the appropriate load, the hydrostatic stress/effective strain criterion for fracture will be satisfied and a bifurcation in deformation mode occurs, from the plasticity and friction of inclined indentation to the plasticity, friction and separation of cutting. Sharpness of tools is important in guillotining and punching of ductile metal plates, not only in determining load levels but also in controlling the direction of the paths along which separation occurs and hence the quality of the cut edge. Sharpness of tools is particularly important for cutting highly extensible and flexible solids that are very difficult to separate by simple tearing. Whereas a stiff sheet of glass will break at a surface scratch, a sheet of some highly extensible material will not, as its deformation characteristics limit the degree of stress and strain concentration that is achievable.