Análises numérica e experimental de vibrações do conjunto eixo-árvore de um centro de usinagem

Abstract

Machine tool vibrations are one of the most important factors affecting surface quality of machined components. As a consequence of high level vibrations, the costs associated with subsequent finishing operations are usually high and can lead even to the components discard. For this reason, there is a need of a continuous development of computational tools which allows characterizing the machine tool dynamic behavior. In this work, a Timoshenko beam finite elements procedure for calculation of natural frequencies and the relative vibration modes is implemented ina free software, aiming at the development of an open source computational tool to assist machine tool design. Experiments are conducted to point out the most relevant dynamic aspects and to allowa better understanding of the vibrational response of stable milling operation, where no resonance neither self-excited chatter vibrations are observed. The machine tool spindle unit modeled allows the user to calculate with good accuracy its natural frequencies under free rotation. Nevertheless, milling operation represents a phenomenon of high complexity and the analysis of modal parameters based on the stiffness defined for null rotation is not capable of precisely describing thespindle-unit dynamic behavior. Therefore, the dynamic effects involved, which cause a scatter of the natural frequencies around a central value and also its displacement, must be considered. Although several authors point out the roller bearing stiffness variation and the gyroscopicmomentum as the main dynamic effects, those are only significant at speeds far above the speed range of the analyzed machine. The results presented in this work show that, for the analyzed machine, those effects can be considered negligible in comparison with the stiffness inductionassociated with the deformation of the drive system and with the milling operation itself. The adjustment of the input parameters related to this induced stiffness allows the calculation of the first four natural frequencies with maximum error inferior to 5%. Investigating the vibrational response in milling, experimental results show that a longer tool implicates in higher vibrational levels, as well as in a greater sensibility to other factors, such as rotation and cutting direction. Finally, the spectral response analysis shows that the predominance of certain natural frequencies may be related, among other factors, to the width of cut.