Finite Element Analysis for Fixture Stiffness

by Zheng, Yi.

With growing demands on improved product quality and shorter time to market, there is need for rigorous but practical tools to support the fixture design and analysis process. Computer-aided fixture design (CAFD), with predictable fixture stiffness, becomes a means to provide an appropriate solution in fixture design. The effectiveness of previous CAFD systems is not fully satisfactory partially because analysis of fixture stiffness has not kept pace with the development of CAFD. The dissertation research provides a model of fixture unit stiffness analysis and an experimental method of identifying contact stiffness parameters. The model and the method offer the potential for a more realistic analysis of fixture stiffness properties of a fixture–workpiece system, based on a fixture unit description. An FEA model of fixture unit stiffness is developed with contact elements for solving contact problems encountered in the study of fixture unit stiffness. The penalty function method is used to model the contact conditions in the energy equation of the general FEA and to describe the nonlinearity of connection shown in previous experiments. The contact and friction conditions are represented mathematically in the FEA model. The FEA model and the analysis procedure are validated by numerical simulation. An experimental study on contact parameters is carried out to identify contact stiffness, including normal contact stiffness and tangential contact stiffness, by both static and dynamic approaches. For normal contact stiffness, a static identification procedure is developed to estimate the contact parameters, using experimental data. Four factors - testing environment, contact area, surface finish of the specimen, and normal loads, - are I examined to see how they affect the behavior of the contact interface. A dynamic method is also used to identify normal contact stiffness. A scheme of eigenvalue analysis is developed to test the contact structure to estimate contact stiffness. The dynamic test results are compared with the results of static test under the same experimental condition and a reliable correspondence is presented. Similar to methods devised to identify normal contact stiffness, a frequency–domain identification system is developed to estimate tangential contact stiffness, using FEA and experimental data. A simulation study on vibration data from tangential contact model is presented in this study. The experimental study is carried out and tangential contact stiffness is estimated based on numerical simulation and experimental data. This research establishes the finite element model of fixture unit stiffness and develops the experimental approaches to identify contact stiffness. Based on this study, the database of fixture stiffness can be built up, and further used in CAFD. II