Cutting with More Than One Edge

Blades narrower than the surface will form a slot or groove. A ball bearing rolling over the hard surface of its races forms a minute elastic groove that recovers after the ball passes by. The much larger deformations when one walks or rolls around on a mattress, water bed, or air-filled “bouncy castle” also recover. Above some limiting contact stress, but still within the elastic range, various types of crack appear to the side, and below, the track on the surface of brittle bodies. Unless the cracks intersect there is no material removal but, even so, the surface is deemed to be “damaged” by formation of regular patterns of cracks in the wake of the slider. Instead of multiple cracks, a permanent groove may be produced when scratching with a pointed tool, as relied on by the glazier when marking the line along which glass is snapped to size by bending. The formation of permanent grooves by pointed tools takes place by two distinct modes of deformation, depending on the geometry and attack angle of the tool. In the upper illustration, material displaced by the tool forms a prow above the original surface, through which material flows up from the groove to form ridges alongside the groove. In the other, the tool cuts away the material in the form of a chip that may be continuous or discontinuous.