An Investigation on Assembly Accuracy Adjustment of a Parallel-Kinematic Machine at Multiple Positions

Machines based on parallel kinematic mechanisms (PKMs) have been attracting industrial interests in recent years. As PKMs require higher assembly accuracy, this study is aimed to develop a systematic approach for accuracy analysis and adjustment at assembly, using a single arm of a PKM as an illustration. Parts and feature decompositions of the arm are firstly performed to construct its tolerance network. Tolerance stack-up is analyzed at 17 different locations in the workspace. Tolerances are adjusted according to sensitivity and contribution to effectively constrain the cumulative tolerances. Error stack-up in the arm assembly of components, with fabrication errors, are analyzed at the 17 locations. While the result was not satisfied, adjustment of toleranced items were further conducted based on sensitivity. A principle for evaluating items at various locations are proposed for accuracy improvement. In the PKM arm example, six dimensional tolerance items, among 33 tolerance items, were quickly selected for adjustment using this approach. The result shows that the three-axial errors are greatly improved, from 10.3% to 95.2%, at different positions in working space. The overall deviations are reduced to less than 0.02 mm with more than 25% reduction after adjustment by the proposed method.