|Title:||Modeling and experimental investigation of spindle dynamic errors and surface generation in ultra-precision diamond turning|
|Advisors:||Lee, W. B. (ISE)|
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Industrial and Systems Engineering|
|Pages:||xxvi, 171 pages : color illustrations|
|Abstract:||The aerostatic bearing spindle (ABS) is a vital component in ultra-precision machine tools. The performance of the ABS is an important factor in determining the machining quality in ultra-precision machining. Many studies have been conducted to measure the spindle error motions, and three typical measurement techniques, which are reversal, multi-probe and multi-step approaches, have been extensively utilized. However, few studies focus on the spindle dynamic errors. Motivated by this research gap, this study adopts theoretical and experimental approaches to study the spindle dynamic errors and their effects on the machining quality in ultra-precision machining. The study was divided into two parts. In the first part, the spindle dynamic errors were investigated with theoretical and experimental methods. Firstly, a spindle dynamics model of ABS was developed to characterize the spindle dynamic behavior. A series of experiments, including groove cutting with diamond turning and fabrication of microstructures with slow slide servo (S3) machining, were conducted. The theoretical and experimental results indicate that the unbalance induced eccentricity has significant effects on the machining accuracy in ultra-precision machining, even though the amplitude of spindle error motion is in the range of several tens of nanometers. The spindle has a double frequency vibration under excitations, including spindle unbalance and cutting force. The double frequency vibration can generate a low frequency enveloping phenomenon. In addition, the results indicate that the axis average line (AAL) of the spindle rotation axis will drift away from the bearing center with increase of spindle speed due to the hydrodynamic effect of the ABS. This drift of the AAL of the spindle rotation axis with increase of spindle speed is nonlinear. The drift makes significant contribution to the machining accuracy in the fabrication of microstructures with S3 machining, where two different spindle speeds are employed.|
In the second part, a comprehensive dynamic surface generation model with consideration of the spindle dynamics, cutting mechanism and machining error is proposed. Firstly, an algorithm for the cutting force calculation and surface generation is developed. This algorithm takes into account the effect of minimum chip thickness and elastic recovery. A groove cutting experiment was conducted to verify the effectiveness of the algorithm. The experimental results indicate that the algorithm is capable of addressing the minimum chip thickness and elastic recovery in the micro cutting process. This algorithm is integrated into the comprehensive dynamic surface generation model. The simulated and measured surface topographies indicate that the low frequency enveloping phenomenon due to double frequency vibration of the spindle has a significant effect on the surface topography. The surface topography in cylindrical turning changes with different spindle speeds, even though the feed rate per revolution remains unchanged. In ultra-precision machining, the requirement of ultra-high machining accuracy and ultra-smooth surface roughness makes the effects of spindle errors on form accuracy and surface finish of machined components highly significant, even though the spindle errors can be down to the nanometric range. Thus, it is very significant for this research to investigate the effect of spindle errors on machining accuracy and surface roughness with experimental and theoretical methods. Based on the investigation, the spindle unbalance induced eccentricity, double frequency vibration, as well as the position drift of the AAL of the ABS have been identified. The development of the comprehensive dynamic surface generation model with consideration of spindle dynamics, effect of tool edge radius and machining error can enable optimization of the cutting conditions in ultra-precision diamond turning.
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