High-Speed Fluid Bearing Micro-Spindles for Meso-Scale Machine Tools (mMTs)

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Three critical problem areas pertinent to the development of precision high-speed spindles for meso-scale machine tools (mMTs), driven by air turbines, constitute the focus of this work. These are the development of the spindle, of a new tool clamping device, and of a measurement system for spindle error motions. For developing the micro-spindle, a complete analysis of the turbine dynamics including the speed, torque, and power, was performed to ascertain the critical design factors that satisfy the operational requirements of mMTs. The influence of back pressure as well as supply pressure on the performance of the turbine was investigated. As the foundation for the analysis of the spindle's fluid bearings, the validity of basic equations for a solver has been evaluated considering the size effect. As a result of the analysis, hydrostatic air bearings were chosen for the bearing system to meet the required error motions and load carrying capacity. Based on the analysis of the turbine and of the bearings, a prototype mMT spindle was designed, built and tested. During experiments, the back pressure building-up in the exhaust branches of the spindle was identified as the main reason for a lower than predicted speed. To alleviate this problem, the original design was modified to provide enlarged exhaust passages. After the modifications, a rotational speed of 422,400 RPM was reached. The radial error motions at this speed reached 3.2 um. The maximal static torque was 19.7 Nmm. To alleviate tolerance stack-up problems prevalent with traditional collet-chuck mechanisms, a shape memory alloy (SMA) based tool clamping device was conceived. Two conceptual designs, one using a SMA ring and the other a SMA collet, were developed and evaluated analytically and experimentally. A methodology for calculating the clamping force was given along with guidelines for designing the clamping system. For more accurate measurements of the spindle's error motions, on-line measurement system for spindle error motions was proposed that takes into consideration error sources that were neglected in the conventional methods. A rigorous analytical model of the new seven-probe system has been formulated. The feasibility of the system has been experimentally verified

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  • 05/21/2018
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