Fluid-metal interactions within the shear zone appear to be dominant in governing the forces necessary to shear metal with the shear tester, an instrument which simulates ideal low-speed metal cutting. The conditions causing shear arise only just before shear. Fluid activity along the tool-chip interface is of secondary importance. The fluid's role in promoting shear is based on the stabilization of microcracks at the tool tip by the fluid, or lubricant, enabling shear to occur at reduced levels of stress and metal deformation. Thus, the lubricant embrittles the metal. Any action of the lubricant along the tool-chip interface is also related to the stabilization of microcracks, occurring in this case within junctions formed when asperities penetrate the lubricant layer and weld together. Thus, lubricants reduce friction between moving surfaces by embrittling asperity welds, i.e. by promoting shear at relatively low stress levels before extensive growth of the weld occurs. The role of the lubricant in stabilizing microcracks at reduced stress levels is explained by the tendency of high concentrations of impurity atoms to pile-up dislocations at the metal-lubricant interface during deformation of the metal. This action leads to shear at reduced stress. The increased shear angle observed in low-speed metal cutting is interpreted in terms of one of the real properties of metals, i.e. their ability to workharden.