Aerospace structures are typically joined to form larger assemblies using screw lock or swage lock fasteners or rivets. Countersunk fasteners are used widely in the aerospace industry on flying surfaces to reduce excrescence drag and increase aircraft performance. These fasteners are typically installed to a nominal countersink value which leaves them flush to the surface before being locked into position.
The Northern Ireland Technology Centre (NITC) at Queen’s University Belfast has developed and demonstrated two processes which enable high tolerance flush fastening of countersunk fasteners:
The ‘Flush Install’ process produces countersunk holes based on the specific geometry of each individual fastener;
The ‘Fettle Flush’ process accurately machines fasteners to match the surrounding surface.
Flushness values well within the allowable tolerances have been demonstrated for both Flush Install and Fettle Flush processes. The Flush Install process uses a physics based, experimentally verified constant ‘ζ’ based on the material types and thickness of the stack being assembled. The Fettle Flush process uses a proprietary toolpath to ensure low machining forces, optimum flushness and excellent surface finish. Both processes were developed by the NITC to TRL4 before transferal to the Manufacturing Technology Centre (MTC) in Coventry for use within a TRL5 application, a Ground Based Demonstrator (GBD) wing at the MTC which further demonstrated the two processes.