Patent ID: 7866041

Claim:
A method of manufacturing a duct for a gas turbine engine or the like from sheet metal material, wherein the duct has an irregular final duct shape in which either end of the duct is substantially elliptical and the ends are joined by a continuous surface, the method comprising the steps of: developing a ruled geometry by dividing each end ellipse into an equal number of divisions and joining corresponding points on a periphery of the ellipse by straight lines; fabricating a template of sheet material; marking on the sheet metal template a plurality of ruled lines to form a ruled surface geometry corresponding to the ruled geometry, rolling or folding the sheet metal template along the ruled lines, and during a rolling process, ensuring that the templates are fed through rolls of a rolling apparatus with the ruled lines maintained parallel to an axis of the rolls; joining the template to form an approximate duct shape; positioning the joined template on a mandrel, the shape of which corresponds to the desired irregular duct geometry; the mandrel material having a greater thermal expansion coefficient than the sheet metal material; and heating the joined template and mandrel substantially to a stress relieving temperature of the sheet metal material such that differential thermal expansion of the sheet metal and mandrel at the stress relieving temperature causes the joined template to distort and adopt the shape of the mandrel surface on which the joined template is located.