Variation conscious assembly fixturing methodologies for the preliminary weld processes of segmented ring structures

This project was sponsored by Rolls-Royce, who had an interest in using scientific principles to improve the geometric accuracy (circularity) of large welded annular structures. This project therefore critically examines and models the mechanisms by which manufacturing variation in wedges components stack and propagate to influence the final assembly geometry. The Trent 900 (T900) Front Bearing Housing (FBH) Ring of Vanes (RoV), was used as an illustrative test-piece, so that the typical sources of variation in large annular assemblies could identified. Also examined was the interplay between assembly gaps created by component variation played and the thermal-mechanical distortions of the Preliminary Welding Process (PWP). Based on the fundamental sight gained form the illustrative test-piece a number of novel Variation Conscious Assembly Fixturing (VCAF) methodologies are proposed. The VCAF methodologies: the Circumscribed Geometric (CG), Inscribed Geometric (IG) and Translation Build (TB) are based on mathematical models of the wedge component and their assembly. The VCAF approaches generate in-fixture build conditions that are idealised for circularity by correcting the positions of the assembly locators in response to component variations. To experimentally validate the proposed VCAF fixturing methodologies, a ring assembly comprising of ten wedge-shaped components was designed and manufactured. In addition to the ten nominal assembly components (containing only implicit manufacturing variation), a number of explicitly oversized and undersized assembly components were also produced. Furthermore, a flexible test-bench fixture was also designed and built, its flexibility allowed it to utilise any of the proposed VCAF build methods, while also accommodating traditional fixed location diameter builds. This allowed each of the proposed and established fixturing methodologies to be tested in the same fixture, keeping fixturing errors consistent. Additionally, as the assembly input variation of the components could be controlled; so by purposely inducing an error into the assembly, and measuring the resultant circularity, the response of each build method to the input variation could be assessed. Working in this way the new CG method was shown to be the most effective at controlling circularity of annular assemblies, showing a 5x circularity improvement over the traditional fixed build diameter location approaches. The relationship between manufacturing variation and thermo-mechanical weld distortion is also presented in this research. Using a series of fixtured and free-state plate assemblies, the influence of inter-component gaps on PWP distortion has been experimentally studied and evaluated. The knowledge generated during these tests was then used to augment a Finite Element (FE) simulation of the welding process. Once the nature of the gap/weld interdependence was identified, a series of ring structures were welded. These ring assemblies demonstrated that with an understanding of the gap/weld interdependence PWP distortion could be substantially reduced