Bridging the gap between design and manufacturing specifications for non-rigid parts using the influence coefficient method

Abstract

The manufacturing process may lead non-rigid parts to endure large deformations which could be reduced during assembly. The manufacturing specifications of the single parts should refer to their free state or “as manufactured” state; the functional specifications should instead address the “as assembled” state. Therefore, a functional geometrical inspection requires dedicated fixtures to bring the parts in “as assembled” state. In this paper, through a linearized model that considers fixturing and elastic spring-back, we aim to correlate the functional specification to the manufacturing specifications. The model suggests a hybrid approach called “restricted skin model” that allows to reduce the degrees of freedom considering the form error when relevant. Firstly, the framework is verified in a mono-dimensional test case. Subsequently, it is verified including FEM simulation and actual measurement for two simple assemblies. The results show that the model can correctly interpret the theoretical assembly behaviour for actual applications. The use of the “restricted skin model” approach shows a negligible difference when compared to full FEM simulation with differences of 2.1 · 10−7 mm for traslations and 6.0 · 10−3 deg for rotations. The comparison with actual measurement values showed an error of ±0.2 mm at the assembly features. Furthermore, the linearized model allows a possible real-time application during production that enables to adjust manufacturing specification limits in case of process drifting.

Tolerance Specifications Management Integrated into the Product Development Cycle

“…Recently, a formal procedure to correlate the "manufacturing specification" to the "functional specification" for deformable components was proposed. In this framework, the "manufacturing specification" refers to the "as manufactured" state, therefore, the "free state" while the "functional specification" refers to the "as assembled" state, in which the part can be consider rigid [46,47].…”

“…The functional and manufacturing specifications should then be related by tolerance stack-ups, where the functional tolerances are the critical dimension limits, and the manufacturing tolerances are process-dependent variables. If parts are non-rigid, meaning that they will reach a stable configuration only once assembled, the correlation shall take into consideration deformability [46,47]. If more manufacturing steps are required, this transfer will be created between the functional specification and the manufacturing specification relevant to the last manufacturing step.…”

Nonlinear Tolerancing: Variation Simulation and Assembly Analysis with Regard to Contact Interaction of Parts

“…In this case, however, it is assumed that contact between the assembled structural elements occurs only at predetermined points (for example, at welding points), which makes MIC a purely linear method of analysis and significantly reduces the reliability of results in many application areas. However, the simplicity and speed of calculations have made MIC an extremely popular research method in many fields, including aircraft manufacturing [7][8][9][10][11][12].…”