An approximate machining theory is described in which account is taken of the temperature and strain-rate dependent properties of the work material. A feature of the theory is that the strain rates in the zones of intense plastic deformation in which the chip is formed and along the tool/chip interface are determined as part of the solution. The theory is applied to make predictions for two plain carbon steels and a range of cutting conditions by using flow stress data obtained from high speed compression tests and excellent agreement is shown, for example, between predicted and experimental cutting forces. The values of tool/chip interface plastic zone thickness predicted by assuming a minimum work criterion are shown to agree well with experimental values, both experiment and theory indicating a marked decrease in thickness with increase in cutting speed. It is also shown how the temperatures and strain rates in this zone can be used to determine the conditions that cause a built-up edge to be formed on the cutting tool and good agreement is again shown with experimental results.