Razor-sharp operation

Manufacturing a heatshield for the Joint Strike Fighter required a rare feat of engineering
accuracy in machining a titanium panel tapering to a fraction of an inch. Christopher Sell reports

Engineers at BAE Systems have hailed the design and production of a key component for the Joint Strike Fighter (JSF) programme as one the most complex feats of machining ever undertaken.

The titanium heatshield — which is located near the exhaust of a power pack on the plane’s aft fuselage — is to be fitted to the F-35, the first JSF to roll off the production line.

Titanium has been used because the heat-resistant properties required could not be achieved using carbon fibre, from which the outer surface of the aircraft is predominantly made.

Titanium is one example of a heatresistant, high-strength material used in aircraft to counter the effects of the heat barrier — the high temperatures caused by friction of air against the outer surfaces of the aircraft. The heatshield is designed to protect the carbon-fibre panels from temperatures up to 260º.

What started life as a 1.8m x 1.2m rectangular metal bar, 14cm thick and weighing over a tonne, now resembles a giant razor blade. It has been designed and machined to measure around 1.2m x 0.9m but narrowing to a tiny fraction of a centimetre at its edges, a stipulation to ensure it did not affect the special characteristics of the JSF.

This, according to Tony Ladley, team leader, engineering, at BAE Systems in Samlesbury, Lancashire, was the most unusual aspect of the project — working to an accuracy rarely seen in manufacturing.

‘The unique part of this design was the 3D edge profile, where the cross-section of the part is machined down to 7,000th in thick. I have never done one like that before,’ he said.

Ladley explained that the piece, custom-designed for the JSF, was machined in two distinct stages, with an initial roughing of the titanium piece, which required the team to adapt conventional machining techniques, before the secondary phase of moulding. Here, special vacuum holding fixtures were developed specifically for the project to hold the piece stable before the five-axis machining tool machined the block using a continuous spiral milling technique.

To gauge whether BAE Systems’ own facility could achieve this, a feasibility study was carried out with the help of Sheffield University’s engineering department, which assessed whether the tool would be capable of producing such an acute edge.

‘As the 3D shape was being evolved from the solid block it was important to keep the very keenest edge,’ said Ladley. ‘The most important factor was to ensure that the tool remained in contact at all times rather than breaking off and starting again in a patchwork effect.’

Once the design of the component and the manufacturing preparations had been completed, the machining of the part was carried out on a flexible overhead gantry — a device usually used to produce mould tools rather than components of an exacting tolerance.

Ladley said that while it was unusual to attempt such a procedure, the team’s cautious approach and initial tests indications showed that the result was satisfactory. The heatshield will be joined to the other sections of the aircraft by Lockheed Martin, which is carrying out the final assembly next month.