UP-FRONT Quality

Manufacturing Engineering, Mar 2005 by Destefani, Jim

For high-value aerospace parts, the emphasis is on

"You can't inspect quality into a product" is an old axiom of manufacturing that's been taken to heart at GKN Aerospace (St. Louis), where the emphasis is on knowing machine capabilities up front to limit production of any bad parts.

Essentially a large aerospace job shop, the plant manufactures machined and sheetmetal parts for the F-22, F-15, and F-18 fighters; T-45 trainer; the C-17 cargo plane; and other US military aircraft. GKN's 750,000 ft^sup 2^ (70,000 m^sup 2^) machine shop houses, among other equipment, 54 large three- and five-axis gantry machines with a total of 162 spindles and bed lengths to 120' (36.6m).

The gantries formed the core of GKN's machining capabilities for aluminum and titanium alloy parts, but most of them were vintage 1970s machines and some had seen better days. Faced with the challenge of improving its machining capabilities to meet tolerances required for a new generation of aircraft such as the F-22, GKN engineers chose to rebuild/retrofit some of the gantries as well as bring in some new equipment.

To make the most of current machine capabilities and decide which machines to retrofit, managers instituted a machine capability testing program. Performed before any parts are machined, the capability studies have multiple benefits, according to quality specialist Ulysses Green. "The capability studies really determine the machine's potential accuracy without ever cutting a chip," he explains. "They allow us to determine what the capability of the machine is based on machine motion."

Armed with that knowledge, engineers can then match job tolerances to machines with more precision. "For years, any supervisor or seasoned machinist in any shop had an intuitive understanding of which machines could hold which tolerances," Green says, "but they didn't know why. These machine validation studies allow us to assign a number to that, so we can quantify what we're able to do and put jobs on the machines accordingly. And, if we know the machine's inherent accuracy, we can also make allowance for other variables, such as cutter runout." Depending on the part, tolerances can be as tight as ±0.004'' (0.1 mm), he adds.

Finally, the studies help in root-cause analysis in the event of a problem, Green says. "You can cut a part and have a failure, but then you'd have a hard time trying to determine what caused the problem," he says. "Was it spindle runout? Was it the program? Was it a worn cutting tool? There could be a variety of causes.. Using this method, we can eliminate the machine as a possible cause."

GKN's machine validation efforts revolve around use of a terahedral artifact placed at multiple locations on the machine bed. Quality personnel perform 84 measurements-one on each corner of the tetrahedron-in 21 locations on the machine bed in less than two hours, according to Green.

Called the Spatial Reference System, the artifact is supplied by a company called Metronom US (Ann Arbor, MI; see sidebar for more information). "The shape of the artifact and how we use it gives us squareness, straightness, scale, and all the measurements you'd get off a CMM, but a lot faster," Green says.

"So we're working with our maintenance team to footprint each machine, and we actually get a volumetric accuracy number for each machine. The number that we get out of our process is inaccuracy in X, Y, Z, A, B and everything else all rolled into one number," he says.

The actual checks combine procedures recommended in the ANSI B-89 standard for calibrating CMM machines and those contained in a corresponding VDI/VDE standard. Green says the configuration of the artifact dictates this approach.

"The B-89 standard calls for probing several points on spheres, while VDI/VDE specifies single measurements on step gages," he explains. "We're checking all the things outlined in the standards-squareness, straightness, scale, etc.-and we're taking both sphere and single-point measurements.

"Of course, CMM machines are three-axis machines," Green continues. "Most of our machines are five-axis. So we came up with our own method of checking the additional axes using laser measurement devices. We can thoroughly check a machine, all three spindles, in less than two hours, and we get a lot of meaningful information."

If Green's team-currently, four people are assigned to the validation project-finds a problem with a machine, it is turned over to GKN's machine maintenance department. "They determine the root cause and corrective action needed," Green says. "They're very good at doing that and documenting the fix. Then we can come back and check the machine again if necessary."

Data analysis and reporting also combine multiple approaches to give GKN engineers, managers, and other personnel a useful snapshot of machine capabilities. "We take all the data from our tests and bundle it together using multiple software packages," Green explains. "The most important are the Metronom software and a package called SpatialAnalyzer [3-D graphical metrology software from New River Kinematics, Williamsburg, VA]. The two packages work hand in hand, and we had to develop our own process to get the system to work."