Thermal modelling of high speed machine tool spindles
Yalçın, Turgut Köksal (2016) Thermal modelling of high speed machine tool spindles. [Thesis]
Machining is the most widely used manufacturing method by far since its foundation and its development process still continues parallel to the technology. Demand for higher quality parts together with the lower machining time and cost is rapidly increasing. In order to meet this increasing demand, high speed machining processes and equipment are getting much more important compared to the traditional methods. High speed machine tools are the key elements of this new machining era; but making the machine tools faster while improving their overall performance requires high end technology together with advanced engineering applications. Positioning accuracy of a high speed machine tool is the most important metric among others because of its direct effect on the finished parts, which are measured as dimensional errors. The highly strong and nonlinear relationship between the positional accuracy and the thermal characteristics of the machine tools raises the importance of modeling the thermal behavior of the machine tools. The main aim of this thesis is to develop robust thermal models for high speed machine tool spindles, by considering the effects of built-in cooling systems, to be able to predict and then reduce the positioning errors related to the thermal behavior of the spindle unit. Fully analytical approaches are very complex for solving the nonlinear thermal behavior of the spindle units; but they are still powerful when they used together with the finite elements model of the complex spindle geometry. As the first step of the thermal model, the heat generated by the ball bearings, which is considered as the main heat source of the entire spindle unit, is calculated analytically. Calculated heat is used as an input to the Finite Elements Method (FEM) model for the heat transfer and thermal error calculations. Built-in cooling system of the spindle unit is also analyzed using the Computational Fluid Dynamics (CFD) approaches again using FEM models. Overall temperature distributions and thermal elongations leading to positioning errors are calculated by the FEM model. Simulation results are validated by temperature and thermal elongation experiments measured on a 5-axis CNC machine tool spindle. Cooling system parameters optimization is achieved by using the developed models as quick solutions to the positioning problems. On the other hand cooling system design improvements are also analyzed by the developed models and several different cooling channel designs are investigated for increasing the positioning accuracy of the high speed machine tool spindle used in the experiments. Overall good agreement is observed between experiments and simulations.
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