In this study, a theoretical model was developed to predict cutting forces for machining hard materials that contain more than 0.58% carbon. The model is based on the Oxley machining theory, and it uses the Johnson-Cook empirical constitutive law for the flow stress data or the equation of a hard material and the tool geometric parameter, the nose radius. However, Oxley and his co-workers showed that the flow stress data obtained from high-speed compression tests for a range of only plain carbon steels (less than 0.58% carbon) is suitable for making analytical machining predictions assuming a perfectly sharp tool. In this study, an experiment was carried out to obtain the flow stress in the Johnson-Cook law and the geometry of the tool was simplified to obtain the nose radius. The cutting forces from the developed theoretical model were found to be in good agreement with those measured from an experiment of hard machining with an AISI 4140 steel heat-treated bar.