The Importance of Considering Size Effect Along the Cutting Edge in Predicting the Effective Lead Angle for Turning

The concept of an effective orthogonal cutting edge in turning is considered. The orientation of this edge in the radial-longitudinal plane, as commonly modeled through an effective lead angle, is studied. The methods of effective lead angle prediction used in numerous previously developed force models are plagued with large errors over ranges of process inputs, in particular feed rate and depth of cut. Four previously developed methods of effective lead angle prediction are reviewed and compared to a new method presented here. This new method accounts for the size effect as introduced through the variation in chip thickness along the cutting edge, especially along the tool nose region. The difference in the new method is that the effect of continuous chip thickness variation along the cutting edge is included when evaluating the specific machining energies rather than using an average chip thickness, which has been used in the other methods. Therefore, the differential normal and friction force components acting on the rake face are functions of chip thickness through both the elemental chip load and the specific energies. Their directions are characterized by the orientations of the rake face and edge. By numerically integrating the differential force components modeled in this fashion, a significant improvement in effective lead angle prediction accuracy is realized. This improved accuracy is verified using experimental data obtained for 1018 steel and 304 stainless steel at varying levels of feed rate, depth of cut, cutting speed, nose radius and tool lead angle. Introduction The demand for improved quality and reduced cost of manufactured products has lead to a need for better means to predict the outputs of machining operations. Surface error, a strong determinant of quality, and machine dynamic response, an indicator of the most cost-effective machining conditions avaliable, are process outputs directly related to the magnitude and direction , of cutting forces. Accurate force prediction is therefore a key step towards achieving high quality, low cost designs. When modeling the turning process, as well as boring and face milling for which the fundamental tool edge and rake face geometry are the same, the forces acting on the tool are three dimensional. One of the force components is the tangential force which lies in the direction of the cutting velocity and hence is analogous to the cutting force in orthogonal cutting. The other two force components in the longitudinal and radial directions are usually modeled with …