On design and simulation of passive damping solutions for milling of thin-walled parts

Machining of thin wall structures is a challenging area due to the problems such as chatter and forced vibrations that arise due to low stiffness of such structures. Up to now, most of the research was driven by need to improve productivity in manufacturing of aircraft components such as airframe panels; hence, straight thin walls such as cantilever structures were considered for research in machining dynamics. However, thin wall casings and particularly thin wall assemblies that are encountered in jet engine casings are not well studied. Not only their dynamics is quite different from that of straight thin walls, they present a different set of challenges for fixturing to minimise machining vibrations. Hence, there is a two-fold need of research: (i) to understand the effect of dynamics of thin wall casings on interaction of tool and workpiece and (ii) to design and validate damping solutions to minimise machining vibrations in such structures. In this work, both the above objectives are addressed. Initially a coupled dynamic interaction of tool and workpiece is studied through experimental analysis of machining vibration signal. This study was carried out both on a straight thin wall cantilever and on a thin wall casing to present the effect of variation of dynamics on dynamic coupling between tool and workpiece. The machining vibration signal was analysed in frequency and time-frequency domain to see which of the elements (tool or workpiece) are dominant during machining. For providing fixturing solutions to minimise machining vibrations, initially tuned mass dampers were studied. Tuned dampers with increased mass ratio were studied to see their effect in minimising vibrations. Subsequently, two novel surface damping solutions - viscoelastic based surface damper and torsion spring fixture - were proposed and validated experimentally. The design philosophy behind both these solutions is to improve mass and stiffness of the casing in addition to imparting damping. Finite element modelling was carried out to predict the dynamic response without and with proposed solutions