Optimisation of variable helix end milling tools

High productivity, low cost and high profits are important issues in aerospace, automotive and tool/die metal manufacturing industries. Machining processes are widely used in manufacturing operations for metal manufacturing rather than casting and forming. However, the dynamic deflection of tool and workpiece systems generates unstable cutting forces when machining with high material removal rate. Here, sudden large vibration amplitudes occur when energy input exceeds the energy dissipated from the system, leading to self-excited vibration or chatter. This thesis focuses on the avoidance of milling chatter by using variable helix milling tools. Since milling chatter is strongly influenced by the frequency response function of the dynamic system, a preliminary study is first presented to assess the feasibility of non-contacting electromagnetic modal analysis for milling tools. It is shown that this approach shows some promise for use in real machining problems where traditional modal hammers have some drawbacks. In particular, the amplitude dependency of the frequency response function could be qualitatively illustrated. The main focus of this thesis is the optimisation of variable helix tool geometry for improved chatter performance. A semi-discretisation method was combined with Differential Evolution to optimise variable helix end milling tools. The target was to reduce chatter and maximise performance by modifying the variable helix and variable pitch tool geometry. The performance of the optimisation routine was benchmarked against a more traditional approach, namely Sequential Quadratic Programming. Numerical studies indicated that the Differential Evolution optimisation performed much better than Sequential Quadratic Programming due to the nonlinearity of the optimisation problem. The numerical study predicted total mitigation of chatter using the optimised variable helix milling tool at a low radial immersion. However, in practice, a five-fold increase in chatter stability was obtained, compared to traditional milling tools. In addition to this practical contribution, this study has provided new insight into the experimental nonlinear dynamics of variable helix milling tools, which exhibit period-one bifurcations under certain conditions. There have been very few previous studies that have investigated variable helix milling tools. However, one previous study proposed that the so-called ‘process damping’ phenomenon is particularly important for variable helix milling tools. Consequently, the final contribution of this thesis is a study of process damped milling and the influence of different tool geometries. Testing was performed for tools with different rake and relief angle, edge radius and variable helix/pitch. It was found that variable helix/pitch had the greatest influence on the process damping phenomenon.