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Painting by numbers

BUILDING L-64 AT Marietta is not a paint shop, although it is here that F-22s will be finished before being towed across the road for radar cross-section verification ready for first flight. Each F-22 is expected to spend about 20 days in this building, where the elaborate, low-observable, finish will be applied by robot. "It's not a paint job," says Wally Plumley, finishes team leader at Lockheed Martin Aeronautical Systems.

Building L-64 is the "coatings application facility", and represents an investment of about $16.5 million. Here aircraft will be finished using a $5 million paint robot while, in a second bay, a smaller robot will be used to paint parts. The large, nine-axis robot is a modification of a system developed by Pratt &Whitney Waterjet for robotic paint-stripping.

The robot is mounted on an automated guided-vehicle (AGV), developed by Mentor Technologies, that follows a horseshoe-shaped path around the aircraft. A rotating vertical column carries a jointed arm on which is mounted a small Fanuc six-axis spray-head robot.

The spray head carries two small, high-efficiency guns. Two incompatible resin systems are used on the F-22, epoxy primer and urethane coating, and this requires two guns, explains Plumley. Only one gun is run at a time.

 

ROBOT VISION

The position of the spray head relative to the aircraft has to be accurate to within 0.75mm. The aircraft cannot be positioned to this tolerance, so the robot is designed to orient itself to the aircraft. The wire-guided AGV moves to a pre-programmed position and lowers jacks to stabilise the robot, which then looks at key features of the aircraft using a Perceptronics laser system. "The system knows what it is looking at, knows where it is relative to the aircraft, and adjusts the [paint] program accordingly," says Plumley.

A finishes-application mock-up, built by ATI, has been used to check out the robot. This is a full-scale model of the F-22, accurate to within 6mm, with an aluminium frame and glassfibre-reinforced plastic skins over foam core machined to shape using the CATIA design system.

The typical spray-gun stand-off distance is 250mm, reducing to 150mm for fine details. Camouflage is applied using a "gun-break" technique, to avoid masking, Plumley says. F-22 coatings are designed using CATIA, and the three-dimensional "skin" data is used to program the robot to apply material at deposition rates and to thicknesses developed in advance. "There is little or no on-line programming," says Plumley. Rather than paint over one colour with another to achieve the camouflage effect, "holes" are left in the base coat which are filled in with a darker colour. This makes savings in and time and money, as well as weight, he says.

Air in the paint bay is continuously replaced and cleaned, although Plumley says that "very little" overspray will be created by the single small gun that will be operational at any one time. The temperature in the paint bay can be increased to around 50íC to accelerate curing of the "fairly thick" coatings, he says.

Repeatability and reliability, and not cost, are the reasons for using a robot, he says: "We are aiming for reliable location, thickness and smoothness every time." The intention is to develop a robust paint process which will eliminate the need for inspection. "We want to check it at the outset, and know that it will work," he says. Paint will be mixed "on the fly" to eliminate the potential for error in manual mixing.

About half the work involved in finishing the F-22 is in surface preparation, such as filling gaps and fairing edges, which will be performed on the assembly lines, when assembly work is not under way. Areas that would be hard to reach after assembly, such as the inlet ducts, will be painted by robot in Building L-64 before mating.

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