A novel assembly-oriented measurement datum transformation and tolerance reallocation method

With the increasing requirements for aircraft assembly accuracy, pre-assembly analysis technology based on measured data has become one of the most important process precision compensation methods. How to accurately obtain and analyze the measured data of key features of parts according to assembly intention is the key step of pre-assembly analysis technology. Existing researches only explain the importance of the correct selection of measurement datum qualitatively, but none of them analyzed theoretically and provided specific methods when the measurement datum is inconsistent with the design datum, which is easy to misjudge the originally qualified parts. Therefore, this paper proposes a novel assembly-oriented measurement datum transformation and tolerance reallocation method for aircraft assembly. Firstly, based on the assembly type and geometric feature classification, the measurement datum of parts without Geometric Dimensioning and Tolerancing (GD&T) is identified, and the importance of correct selection of measurement datum is proved by a case theoretically. Secondly, when the measurement datum is not coincident with the assembly datum, a spatial homogeneous transformation method of measured data is proposed, which transforms the measured data from a temporary measurement coordinate system (TMCS) to an assembly datum coordinate system (ADCS). Finally, the tolerance redistribution of the measurement target features relative to the TMCS is carried out directly on the basis of the original tolerance. The experimental results show that the key features exceeding the upper tolerance limit of 0.042 mm are within the theoretical tolerance again after datum transformation, which solves the problem that the part has been misjudged as unqualified in the measurement process.