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It is a well-known fact that, in a real engineering situation, fixtures are not ideally stiff, so numerical simulations using them are unlikely to present results that are consistent with the experimental ones. The present paper intends to describe a model updating methodology inserting translational and rotational springs in order to better represent the real clamping. For that purpose, the PSO stochastic optimization method will be used to determine the spring stiffness in an iterative way. In addition, uncertainties regarding the material properties, such as density and Young's Modulus, as well as workpiece dimensions, will also be taken into account in the optimization algorithm. Once the experimental natural frequencies and the geometry of the studied parts are known, the algorithm automatically updates the model, approximating the natural frequencies obtained from the numerical model to the experimentally obtained ones as closely as possible. In addition, the modal shapes of the updated simulation will be compared to the experimental data and to a rigid boundary simulation. Results will demonstrate that the proposed methodology efficiently represents the fixturing flexibility: both natural frequencies and mode shapes found were close to the real dynamic system.

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The 3-2-1 fixture design principle is the most used method to locate prismatic workpieces. To further increase location accuracy, the influence of friction and contact stiffness between workpiece and fixture parts should be minimized. This can be achieved by decreasing the structural stiffness of the locators in the tangential direction to the contact. Hence, based on flexure hinges, locators with two different stiffness values are investigated. To analyze the influence of locator stiffness, a special experimental setup is developed to study two locators positioned in the secondary plane of the 3-2-1 fixture. Experiments are carried out to compare the locators with flexure hinges to a rigid one. The displacement and rotation of the workpiece are investigated as a function of the clamping force. The experimental results show that the reduction in the tangential stiffness can improve the position and orientation accuracy of workpieces.