A shimming method based on outer mold line control of thin-walled parts

Abstract

Aerospace framework products typically have a thin-walled structure. To ensure the aerodynamic performance of the product, it is necessary to precisely control the shim clearance between the understructure and the skin. However, it is difficult to meet the requirements of product design by digital measurements and calculations of the shim clearance after installing parts on the final assembly or special test stand. To improve the efficiency and accuracy of the shimming process, a method to determine the shimming quantity by a flexible virtual assembly without a physical assembly process is proposed. The proposed method measures the initial shape of the assembly using digital technology that analyzes the assembled shape using a finite element model and calculates the shimming space using the assembled shape. First, the geometric information of the understructure, skin inner mold line (IML), and outer mold line (OML), or the actual pre-assembly shape, are obtained using a three-dimensional laser digitizer. The geometric features are reconstructed and the coordinate transformation matrix is then calculated. The coordinate system is unified based on the positioning features of each part, such as the positioning surface and positioning hole. Finally, the OML of the measured model and the theoretical OML are flexibly fitted to simulate the ideal state of the assembly. The shimming space between the IML and the understructure is calculated and the OML is precisely controlled by shimming. The physical verification results show that the deviation between the analysis results and the actual situation is within 0.1 mm. This method can predict the space of shimming when the OML reaches the ideal state without real assembly thereby reducing the time required for a repeated trial assembly of products and the cost of special measuring tools and conformal tools.