Effect of non-proportional damping on the dynamics and stability of multi-cutter turning systems

Parallel turning operations offer considerable potential in increasing productivity, since they ensure high material removal rates and high feasible accuracy simultaneously. According to preliminary calculations, the stable parameter region for a 2-cutter turning system can significantly be extended by its dynamical detuning via modifying the natural frequency of one of the tools. Mechanical models are presented for a 2-cutter turning tool fixture to predict the dynamics and stability properties of the system. First, a 2-degree-of-freedom (DoF) model with dynamically uncoupled cutters is analysed. A test fixture incorporating 2 cutters was built, in order to verify theoretical predictions on cutting stability. Measurement results resulted in the extension of the mechanical model with another DoF, which captures the physical coupling between the tools. Experiments indicate that for close natural frequencies, the corresponding modal dampings are essentially different, consequently, the damping properties of the 3-DoF model can properly be modelled by non-proportional damping only. It is also shown that the presence of this non-proportional damping results in a significant increase in the robust chip width limit of stable cutting, which enhances the effect of detuning the tool frequencies in the 2-cutter turning system even further.