Assembly tolerance analysis method based on the real machine model

Abstract

The position errors of an objective feature of a machine is affected comprehensively by the geometric errors and assembly errors of the parts on the assembly chain. In order to investigate the effect of assembly datum errors on the transformation and propagation of the geometric errors in an assembly chain, a modeling and analysis method of assembly error accumulation based on the real machine model is proposed. In this approach, the calculation process of the real position of geometric feature is consist of four steps. Firstly, the deviation geometric feature are represented by the control point variation model , and the deviations of geometric feature from its normal position are simulated by the Monte Carlo method. Secondly, the normal position of the deviation geometric feature in the datum reference frame (DRF) is determined by theoretical dimension. The third step, the DRF establishment method is proposed, and the measure DRF of the objective feature is established with the datum geometry example. The fourth step, the assembly contact model is proposed, the relationship between the measure datum reference frame and assembly datums is calculated according to the assembly sequence and the deviation geometry example of assembly datum surface. The real position of all related parts on the assembly chain in machine coordinate system are calculated by above procedures, and the transformation and propagation of the geometric errors of assembly datum in an assembly chain are obtained. The procedure to simulate the real machine assembly is decomposed into four modular processes, which is useful in realizing the automation of assembly error analysis. A assembly error accumulation example of pin-hole assembly with the three orthogonal datum planes is investigated, and the proposed approach is verified.