Optimization And Reliability Analysis Of The Machine Tool Spindle Assembly

In comparison with its classical counterpart, a modern machine tool highlights its performance advances in high speed, high accuracy and high reliability. Spindle Assembly is the most important components of machine tools. Its stiffness, one of the most important performance indexes of a spindle system, indicates the ability of the spindle to resist external loads, which affects the machining accuracy and the cutting ability of whole machine. In the traditional spindle system design, parameters or variables such as material characteristics, geometric dimensions and external loads are usually considered as constants, without taking into account the dispersion effect of these parameters on the spindle. In order to meet a qualified design, the design results should be modified by safety factors derived from empirical statistics. However, in practice this safety factor design method fails to quantitatively analysis the reliability of the design.In this thesis, the static stiffness of a spindle assembly is studied with advanced computer-aided design methods, as follows:Firstly, a review of the study on optimization and reliability of machine tools is given. According to the previous work, the stiffness of a spindle assembly is analyzed with Monte Carlo simulation method, taking the statistical properties of geometric dimensions, external loads and material characteristics into account.Secondly, the static performance of the spindle assembly is analyzed using finite element method in the Ansys Workbench platform. The bearings are modeled with combinl4element. The largest deflection and the stiffness are calculated and compared with theoretical results.Thirdly, the original design of the spindle assembly is optimized, with the stiffness as optimization object. The optimization design variables consist of tool tip overhang, the span between bearings, inner diameter and outer diameter of the shaft. The optimization constraint is the mass of the shaft. The optimization result shows that the stiffness could be improved considerably while the mass keeps constant.Finally, the reliability of the optimized spindle design is analyzed using the Six Sigma module of Ansys Workbench. The statistical properties of geometrical dimensions, material characteristics, external loads and bearing stiffness and their individual contribution to the reliability of the spindle are studied. The reliability analysis enables designers to understand how the spindle performs under uncertainty. The analysis result shows that the optimized design archives a high level reliability.By combining the optimization design and reliability analysis, a more reliable design has been implemented in this thesis, which increase the shaft stiffness as well as the structure reliability. The result is of much implication significance, since it provides an effective and practical method for spindle design.