From forming operations, it is known that the fracture strain under different stress conditions can be predicted by using ductility curves. These curves can be determined easily by two simple material tests. It will be shown that these curves can also be used to understand chip breaking. The work clarifies a part of the role of the cooling liquid, and also the phenomenon that the tool life can sometimes be increased by using a harder workpiece material. A lower friction between chip and tool can be more effective than a harder tool material. Application of this idea teaches us the effect of Molybdenum sulphide (MoS2 ) as a tool coating and the fine cutting of steel with diamond tools. Another workpiece material property that influences the life-time of tools is its strain rate dependency on the flow stress. Its prediction is not easy and strongly depends on the material.