Manufacturing the F/A-22

Manufacturing Engineering, Mar 2005 by Aronson, Robert B

Building the next-generation superfighter

For the Boeing sections of the F/A-22, most of the parts are being made by GKN Aerospace's St. Louis, MO manufacturing facility. Programs currently in the St. Louis portfolio include: the F-15, the C-17 Globemaster (a cargo carrier), F/A-18 (the Navy's replacement for the F-14), the F/A-22 Raptor, and the T-45 Goshawk trainer.

The site was initially founded by the United States Navy in 1939 and post-WWII became a part of the McDonnell Aircraft Corporation. It was home to the Phantom series of jet fighter aircraft and the AV-8B Harrier combat aircraft. McDonnell-Douglas Corp. merged with Boeing in 1997. On January 8, 2001 GKN Aerospace acquired the operation. It is now a Tier One supplier for Boeing IDS and other companies.

When contracts were awarded for the F/A-22, GKN Aerospace was faced with the challenge of bringing the metal structures operation up to the standards required for the new generation of aircraft. They had to develop the ability to rapidly machine titanium, work with large complex parts never before made, improve accuracy, and minimize scrap.

The improvements came in two forms:

First was working with existing machines. The shop had 54 Cincinnati Milacron gantry machines, some over 30 years old. Each is being evaluated for the company's gantry rebuild program. So far, six have been reworked and plans are in place to complete an additional 18 gantries.

Second, because of the complexity and precision required for other parts, the purchase of new equipment was necessary. This major investment included three Henri Line' high-speed profiling machines and three five-axis Mazak e-1060V Integrex machining centers.

Company engineers worked to improve a number of specific areas:

Titanium machining. One of the major features of the F/A-22's design is extensive use of titanium. The weight, stress, and survivability requirements made it a necessary element of the structure. Therefore, learning to machine titanium quickly and accurately became a prime requirement for the manufacturers. Essentially they achieved the speed and accuracy required by applying the same processes that gained them the ability to machine aluminum at high speeds. This was done by blending several elements: new cutters and cutter coatings, minimizing runout, and advanced programming techniques.

One particular challenge is cutting thin sections. In these cuts the designers try to avoid placing backup elements since that takes much more setup time, particularly with the large parts they work with. Instead they have derived a program that uses a combination of DOC, tool speeds, and feeds that allows the forming of sections less than 0.030" (0.8-mm) thick. The trick is to keep solid material around the pocket as much as possible.

Working with titanium not only means more rapid cutting but at the same time requiring greater accuracy. This is necessary because specifications demand it and because the cost of scrapped parts is very high.

With titanium roughing, it was found that cutter-tool selection and operating specs were concerns. These were addressed by adapting new programming techniques that influence the DOC, cutting feeds and speeds, and cutter size.

For finishing passes the main issue was the need to understand the impact of runout. It became understood that a bad pass with too much runout could easily ruin a part-or at least require extensive rework.

Spindles were reworked and toolholding methods were developed, including the use of shrink-fit on high-precision parts. In addition, they used smaller axial and radial DOC. This produced an overall runout of 0.001'' (0.03 mm) TIR measured from the cutting tool. Once runout is minimized it's possible to do more aggressive cutting, yet still hold tool life.

Training programs were necessary. The new and refurbished equipment requires a competent workforce. To ensure the availability of skilled operators, GKN Aerospace has linked with a local junior college which has a machinist training program.

They have a close relation with the school which is a potential source of not only machinists but maintenance personnel as well. There is a cooperative interchange between GKN Aerospace and the school faculty to ensure that course work matches the company's needs.

Deburring is essential for both ease of assembly and ensuring proper air flow over external mold-line surfaces. At GKN it's done in a vibratory media system. A recent addition is a unit that can take larger parts such as a wing spar measuring 8'' × 12'' × 20' (203 mm × 305 mm × 6 m). Part areas with very critical dimensions are masked to prevent dimensional changes.

Machine tools. The key machine is the Cincinnati Milacron (Cincinnati, OH) five-axis gantry. A gantry is the optimum machine to use for many of the F/A-22 parts.

These machines are undergoing a major refurbishing to bring them up to the standards needed for F/A-22 part production. All parts related to accuracy were reworked or rebuilt. Changes include the replacement of the original control with GE Fanuc 15Oi units, new drives and servos, and a full rebuild of the gearboxes, transmissions, and the 3600-rpm spindle packs. In the reworked machines quality, has improved 30% and there has been a 70% reduction in maintenance problems. The production C^sub pk^ is now averaging 1.3. The current goal is a runout of 0.0001'' (0.003mm).