The research work in this thesis involves an experimental and theoretical investigation for difficult-to-machine materials. An experimental program was carried out to determine the cutting forces, chip formation, the secondary deformation zone thickness and surface roughness at different cutting conditions using various nose radii ceramic tools and CBN tools for work materials. A theoretical model is developed by taking into account the flow stress properties of the AISI 4140 to use with the Oxley Machining approach. To find the flow stress data for AISI D2, the Johnson and Cook empirical constitutive equation is used as the constitutive model. In addition, to account for the effect of nose radius edge in hard machining, a simplified geometrical method is used to model the parameters for application in the Extension to Oxley Model and works for the cutting conditions considered here.