Impact dynamics in milling of thin-walled structures

The development of reliable high-speed spindles and motion control systems has led to an increase in the industrial use of high-speed milling. One of the primary applications of this new technology is the manufacture of thin-walled aluminum components for aircraft. The flexibility of the tools and workpieces, the high spindle frequencies, and the inherent impact nonlinearities in the milling process can lead to complicated dynamic tool-workpiece interactions. An experiment was constructed to study the vibrations of a thin-walled part during milling. Time series, power spectra, autocorrelations, auto-bispectra, and phase portraits were examined. From this data, it is inferred that stiffness and damping nonlinearities due to the intermittent cutting action have a pronounced effect on the dynamics of the workpiece. Delay space reconstructions and pointwise dimension calculations show that the associated motions are characterized by a fractal geometry. The auto-bispectra suggest quadratic phase coupling among the spectral peaks associated with the cutter frequency. A mechanics-based model with impact-nonlinearities was developed to explain the observed results. The predicted results agree well with the experimental observations. The model predictions indicate that aperiodic motions are possible over a large range of control-parameter values. These analytical and experimental results have implications for the prediction and control of vibrations in milling.