Boundary conditions at the chip-tool interface in machining: Comparisons between seizure and sliding friction

Abstract A general model for the boundary conditions in metal cutting is presented. This incorporates some apparently conflicting viewpoints described in the literature by re-evaluating the terms seizure (or full sticking friction) and interfacial sliding. Seizure is defined in this work as a solid phase weld between the primary atomic bonds of absolutely clean metallic surfaces; the last layer of atoms is stationary and relative motion takes place in adjacent layers with the shear velocity gradually increasing until the bulk chip speed is obtained. Alternatively if the interface is composed of weaker secondary bonds, such as occurs when machining with an oxide tool, there will be relative motion (or sliding) between the chip material and the tool. The degree of sliding and hence the friction force can therefore vary considerably according to the cleanliness of and the binding forces between the surface structures. It is proposed that the interface should be viewed as a network of micro-regions each of which exhibits dynamic variations. At any instant some of these regions experience full seizure whereas others experience interfacial sliding. The proportion of seized areas As to real area of contact Ar is given by As = kAr. The constant k is controlled by such factors as the nature of the oxide layer on the tool, the cutting time, the cutting speed, the rake angle and the amount of secondary phases and free-machining additions in the work material. A wide variety of cutting conditions can be explained on this basis. Experiments in a vacuum planing apparatus show that when commercially pure copper is cut with a steel tool that has been premachined so as to be totally free of oxide full seizure arises with k ≈ 1. In contrast, the presence of a thin layer of oxide significantly reduces the proportion of seizure. The metallographic sections through these tools and adhering swarf and the evidence from quick-stop sections from turning tests show that a flow zone occurs at the interface when machining under conditions of seizure. The principles of boundary layer theory in fluid mechanics are invoked to describe these velocity profiles.