Non-contact measurements and modelling of milling machine tool vibrations

Sammanfattning: This thesis concerns the development of non-contact measurement methods and analysis of rotors. The methods have been verified and applied to milling machine spindles in order to investigate the speed dependency in the milling machine spindle dynamic. The research was financed by the Swedish Agency for Innovation Systems (Vinnova). Turning operations like milling are common in the automotive and aerospace industries where large metal work pieces are reduced to a fraction of their original weight when creating complex thin structures. During these operations it is important that unwanted behaviours such as excessive tool vibrations be avoided (this is normally called "chatter"). Chatter causes poor surface finish and/or material damage and can expose machine operators to annoying and/or dangerous noise levels. In order to predict processes parameters for a chatter free milling operation, knowledge of the properties of the dynamic system are essential. Normally the system dynamics are measured during no rotation; in order to include the influence of the spin speed the system must be analysed for all spindle speeds intended for the milling operation. This can be done either by measurement or modelling. Non- contact measurement techniques are however, often based on displacement sensors which do not have the same sensitivity as velocity or acceleration based methods. To improve the sensitivity in non-contact measurements of rotors a laser Doppler Vibrometry (LDV) based method has been developed. The developed LDV method is based on the reduction of the rotor surface structure and makes it possible to use single beam LDV measurements of rotors. These types of measurements were previously considered inaccurate but now have become feasible through the use of the method described in this research. Furthermore the dynamic properties of a high-speed-milling machine spindle were studied by a contactless dynamic spindle tester (CDST) developed by SKF. The measurements were substantiated by simulations using a finite element model (FEM) which confirmed the measurement results. The CDST measurements could be performed without violating safety regulations regarding human interaction with high speed spindles through the use of a magnetic excitation method. In the measurements conducted by the CDST a speed dependency in the spindle dynamic could be detected. By performing FEM simulations the major source of this dependency could be identified. The centrifugal force of the balls in the angular contact ball bearings was shown to have the largest influence on the overall dynamics compared to the gyroscopic moment of the rotor. The study performed indicates that predictions of high-speed-milling stability must include consideration of the speed dependency in the dynamic.