An orthogonal cutting model is developed using the finite deformation theory of continuum mechanics. A family of flowlines is proposed to describe the chip flow during orthogonal cutting, and the shape of the flowlines is described in terms of three parameters, one of which is the shear angle. The velocity, Eulerian strain, and Eulerian strain rate distribution along the assumed flowlines are obtained analytically for the orthogonal cutting operation based on this model. The temperature distribution along the flowline is predicted via a finite difference method. Values for the three flowline parameters are selected that minimize the total power associated with primary shear zone deformation and chip-tool interaction using the Davidon-Fletcher-Powell optimization scheme. The model utilizes a general constitutive equation for material behavior, which is a function of strain, strain rate, and temperature. In Part I of this two-part paper, the continuum mechanics-based model for the orthogonal cutting process is established. Experimental assessment and adequacy checking of the model, including determination of the material constitutive equation using a split Hopkinson pressure bar technique, is presented in Part II of the paper.