A full-kinematic model of fixtures for precision locating applications

The conventional point-kinematic model of fixtures has only treated point geometry of the contacts between locators and workpiece. However, this model, which ignores the underlying surface properties of the locators-plus-workpiece system, is inherently incapable of capturing the effects of the geometric properties important to accurate positioning of the workpiece. In this paper, we present a fixture model based on the full kinematics of locator-workpiece contact. This model incorporates a "virtual" kinematic chain with meshing parameters of contact kinematics. It is shown that the conditions of deterministic localization are related to surface properties of both the workpiece and the locators, including the surface curvature, torsion and scale factors, as opposed to the conventional point-kinematic model. The full-kinematic model developed here has a strong implication for designing fixtures with high locating precision requirements.