Nonlinear Modeling of Surface Roughness in Turning of Nitinol: Influence of Cutting Forces, Temperature, and Negative Rake Angle Tools

This study investigates the influence of cutting parameters on surface roughness and process conditions during turning of NiTi (Nitinol), a difficult-to-machine shape memory alloy. Special attention was given to the coupling between mechanical and thermal phenomena governing surface formation. Experiments were conducted using cubic boron nitride (CBN) and ceramic tools with negative rake angle geometry. A two-factor design of experiments (DOE) was applied, with cutting speed (vc) and feed rate (f) as input variables, while cutting depth remained constant. Cutting force (Fc), temperature (T), and surface roughness parameters (Ra) were measured. Based on experimental data, a nonlinear regression model was developed. The model parameters were determined using the least squares method following a prior logarithmic transformation of the equation, which enabled the nonlinear regression problem to be reduced to a linear form. Results indicate that feed rate is the dominant factor affecting surface roughness, while cutting speed primarily influences temperature. Ceramic tools generate higher forces and temperatures but promote more regular surface geometry. In contrast, CBN tools provide lower forces and improved surface roughness, though with increased smearing effects. The study confirms that surface roughness in NiTi machining is governed by coupled thermo-mechanical interactions. The proposed model enables improved prediction and optimization of machining parameters, particularly for high-precision applications.