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Bearing stiffness exhibits non-linear characteristic when the bearing is subjected to a certain external load, which will inevitably cause changes in the spindle stiffness, since bearing stiffness has a decisive impact on the spindle stiffness. In this paper, the axial stiffness softening and hardening characteristics of machine tool spindle were studied. A novel spindle axial load automatic applying device and the spindle stiffness test bench were proposed. Meantime, an axial stiffness model for the experimental spindle was established based on the bearing load-displacement model and mechanical analysis of spindle. Then, the axial stiffness of fixed position preload spindle under different preload was measured experimentally and the evolution mechanism was analyzed. The results show that when bearing preload reaches a certain relatively large threshold, a "sag" shape occurs in the axial stiffness curve, indicating the "stiffness hardening" characteristic of the spindle. On the other hand, for a small preload, no "sag" shape occurs in spindle stiffness curve, indicating the "stiffness softening" characteristic of the spindle. This phenomenon is of great significance for the acquisition of the excellent spindle stiffness properties. [ABSTRACT FROM AUTHOR]

High-speed machining spindles are high-precision mechanisms with a complex and very sensitive behavior. Frequency response functions are required to avoid unstable cutting conditions that lead to premature failure of spindle and tool. However, FRFs are affected by stiffness loss of the bearings at high speed. Indeed, the rotor's behavior is driven by its boundary conditions which are the preloaded bearings. In order to obtain an accurate model of the preloaded bearing system, this paper focuses on the axial spindle behavior. An analytical model that computes the equilibrium state of the shaft, rear sleeve, and bearing arrangement is presented. A model enrichment method is presented with several new physical phenomena: the macroscopic deformations of the shaft and bearing rings as well as the rear sleeve's complex behavior. The significance of these phenomena is evaluated with a sensitivity analysis and used for the model updating to obtain a just accurate enough model. The contributions of these enrichments are presented for a case study performed on an industrial spindle. A good agreement between the simulation and the experimental results are achieved that validates the model updating strategy and the phenomenologically enriched model. [ABSTRACT FROM AUTHOR]

Chatter vibration has been a troublesome problem for a machine tool toward the high precision and high-speed machining. Essentially, the machining performance is determined by the dynamic characteristics of the machine tool structure and dynamics of cutting process of spindle tool. Prediction of the dynamic behavior at spindle tool tip is therefore of importance for assessing the machining stability of a machine tool at design stage. This study was aimed to evaluate the machining stability of a vertical milling system under the interactive influence of the spindle unit and the machine frame structure. To this end, a realistic finite element model of a vertical milling tool was generated by incorporating the spindle-bearing model into the head stock mounted on machine frame. The influences of the dynamics of spindle-bearing system and the machine frame structure were investigated respectively. Current results show that the machine tool spindle system demonstrates different dynamic behaviors at different frequency ranges, which are also characterized as structural modes and spindle modes, respectively. In particular, the maximum compliance of spindle tool tip was found to occur at the bending vibrations of spindle shaft and vary with the preload amount of spindle bearing. The machining stabilities were predicted to different extent, depending on the exciting modes which could be related to the influences of machine frame and spindle unit. [ABSTRACT FROM AUTHOR]