Milton Luiz Polli

Analysis of high-speed milling dynamic stability through sound pressure, machining force and tool displacement measurements

High-speed milling is a manufacturing process that has great economical potential owing to high material removal rate, accuracy and quality of machined workpieces. However, chatter might arise especially in situations where there is lack of dynamic stiffness of the system composed by the machine tool, the cutting tool and the workpiece. This work analyses the dynamic stability of the high-speed end milling process. The evaluation of stability is based on sound pressure, machining force and tool displacements measured during the process. In order to investigate the transition from stable cutting to cutting with chatter, the experimentally measured tool displacements are used as input data into a computer simulation to estimate the chip thickness variation during the process. It was found that for unstable cuts, the tool shaft motions are elliptical, process dynamics are dominated by power at the chatter frequency and other vibration modes of the system can resonate with the harmonics of the chatter frequency. Through the computational simulation of the estimated chip thickness variation, it was possible to observe the transition from stable cutting to cutting with chatter, the phase differences between undulations left by subsequent teeth and their consequences in machining forces.

A methodology to mitigate chatter through optimal viscoelastic absorber

Chatter is an undesirable dynamic instability phenomenon, mainly due to the low dynamic stiffness, that, in turning processes, results in poor surface quality and in a reduction in the cutting tool life. To overcome this problem, viscoelastic dynamic absorbers can be employed. Those absorbers are able to introduce reaction forces and dissipate vibration energy. In previous works, the main difficulty in the use of viscoelastic dynamic absorbers was in tool modelling uncertainties and in inaccurate viscoelastic material models. To overcome those difficulties, this article presents a new methodology to identify the machine tool structure dynamic properties by using 1-degree-of-freedom equivalent model combined with a fractional derivative model to describe the behaviour of the viscoelastic material. As a result, it was possible to design an optimal viscoelastic dynamic absorber for chatter mitigation in internal turning using non-linear optimization techniques. The use of generalized equivalent parameters for the absorber allows obtaining a simple equation of motion for the compound system (primary system plus absorber). The numerical and experimental tests were performed, showing the efficacy of the proposed controlling method design.