This invention relates to isogrid casing structures, and in particular to a method of manufacturing thin-walled isogrid casings by chip machining processes.
Isogrids are used for reinforcing thin-wall components such as gas turbine engine casings or for forming lightweight lattice type structures, for example for use in space vehicle applications. An isogrid is a structure which comprises a triangular pattern of ribs arranged in rows of equal sided triangles. Isogrids are used to increase the stiffness of thin-wall structures while minimising weight. Isogrids have found particular application in gas turbine aero engine applications where thin-wall engine casing ducts are reinforced with isogrids to provide additional stiffness for supporting ancillary units and components.
One method of manufacturing an isogrid cylindrical casing type structure is described in U.S. Pat. No. 3,940,891 in which a frusto-conical structure having an isogrid reinforcement on its internal surface is formed from a plurality of substantially identical panels. Each panel is manufactured from a metal plate having the thickness of the panel to be formed. The metal plate is positioned in a numerical controlled milling machine and the material between the ribs is milled away to form triangular pockets. NC milling can be used to reduce the wall thickness of the pockets to 1 mm and where wall thicknesses of less than 1 mm are required the pockets are further machined by chemical milling to the final desired thickness. Webs, flanges and other features are protected by masking during the chemical milling process. When machining is complete, the panel is rolled or formed by other means to its desired shape. The panels are secured together to produce the conical casing structure.
A method of chemically machining isogrid casings is described in U.S. Pat. No. 5,122,242 where it is mentioned that chemical machining can be used for producing pocket wall thicknesses and rib widths to a minimum of 0.5 mm.
In chemical machining metal removal is achieved by a reverse electro plating process which produces a metal hydroxide of the metal being removed suspended as an emulsion in the electrolytic solution. Removal and disposal of the metal hydroxide emulsion is both hazardous and expensive and this combined with other factors results in significant additional cost to the machined casing.
U.S. Pat. No. 5,485,723 discloses a cylindrical isogrid fan containment case for a gas turbine aero engine in which a mill type cutting tool is used to remove material from the pockets of the isogrid being formed. In this document a first tool is used to remove material to the desired pocket wall thickness and a second tool is used to mill around the periphery of the pocket so formed. In this document the pockets have a wall thickness of between 2.03 mm and 5.72 mm.