The main aim of the work described in this dissertation has been to develop a description of the interface between the chip and the rake face of a cutting tool. This interface is of great importance in many manufacturing operations. The experiments are conducted in a vacuum planing machine, which provides precise control of the atmospheric conditions during cutting. The chip/tool interface is studied directly, using a transparent (sapphire) cutting tool to machine various ductile metals, such as lead and aluminium, in air and in vacuum. On the basis of these observations a new classification is made of the zones of contact at the interface in continuous cutting. This description is extended to steel tools and harder workpieces such as copper. The influence of oxygen on the rake face interaction is investigated. Previous workers have noted apparently anomalous behaviour; it is suggested that recent discoveries about the adhesion between metals and oxides help to explain these anomalies. The transparent tool is used to study the access of cutting lubricants to the chip/tool interface and the role of lubricants is examined in the light of the new description of the interface. Continuous cutting with built-up edge formation, experienced with materials such as Duralumin and steel, and discontinuous cutting, experienced with materials such as magnesium and free-machining brass, are studied by means of these new techniques. Some suggestions are made concerning the material properties responsible for these types of cutting behaviour. The conditions of contact at the rake face of the tool are also studied. A study is made of the influence of crystallografhic orientation of the workpiece on the cutting process. Pure single crystals of aluminium, copper and magnesium are machined in various orientations. Some speculations are also made about the source of the instability in continuous cutting and on the mechanism of chip curl.