Aircraft parts manufacturers are required to hold some of the tightest tolerances in the metalworking industry. For example, Boeing has a program called Accurate Fuselage Assembly (AFA), which requires that its suppliers meet ultratight tolerances that allow fuselage components to literally snap in place like Legos. Northrop Grumman Applied Digital Technologies Group (ADT, Hawthorne, CA) needed to meet these "snap-together" assembly demands.

The company bought a new machine tool and fitted it with the MP10 machine tool probe from Renishaw Inc. (Hoffman Estates, IL). The probe is used for in-cycle gaging of critical assembly hole locations. By accurately finding hole positioning, scrap was reduced and throughput speed of flight-critical components increased.

"Our working tolerances use to be 0.03 inch or better on hole location," said Jeff Howard, ADT manager for Northrop Grumman's Aerostructures Business Area. "To comply with Boeing's AFA program, we now must work with 0.01 inch true position."

These tight specifications tested the limits of conventional sheet metal frame fabrica- tion methods. Northrop Grumman needed to reduce the variability of formed sheet metal to a level comparable to computer numerical control (CNC) machined parts. These parts, called Z-frames, are arc sections of a frame that make up the barrel of the 747 fuselage. These parts are typically 10 to 18 feet in length and are made from roll-formed sheet metal that is stretch formed, trimmed and drilled. The parts have a Z-shaped cross section with a thickness of 0.063 inch.

Because the existing frame supplier could not produce parts to meet its specifications, Northrop Grumman decided to do the frame fabrication in-house. Because of thermal changes and setup difficulties on the shop floor, the large sheet-metal part could not be fixtured accurately, and holes could not be drilled repeatedly within specification.

With the MP10 probe, the fixtured part is measured before cutting, making machine axes compensations to put the part into the center of the tolerance band. "If the part is too wide or necked-down, we can compensate," said Matt Turner, engineer at Northrop Grumman. "There was no way we could do this through fixturing."

According to Turner, engineers determine if a part is good before it is machined. "We don't waste time on a raw part that can't meet specification," he said.

The MP10 touch-trigger probe acts as an omnidirectional switch, effective in the X, Y and Z axis directions. The probe is automatically spindle loaded and then placed against the workpiece or fixture. Each contact generates a signal, relating the contact point to a common datum. The probe has a repeatability of + or - 1 micron.

Northrop Grumman uses two probes to measure holes. The probes allow the company to locate the part and adjust the machine's axes to get in the middle of the part's tolerance band. The smallest holes have a diameter of 0.098 inch and to measure them, a 2-millmeter styli is used, Turner said. These holes are "determinant" holes used to bolt the parts together, while the rest of assembly is riveted together along mating surfaces.

The second probe measures the outer contour of the barrel and has a larger stylus. "There is strong angularity on the outer contours, and to make sure we're not hitting the shank of the probe, we have to use the larger stylus," said Howard.

According to Howard, Northrop Grumman is working on software to streamline the process. The software will take real time statistical process control data on the floor and give operators instant feedback.

However, inspection of parts with the probe is at 100%, and currently parts take about 10 to 15 minutes to machine, with probing taking up half of that time. The company has reduced probing time.

"The results have been impressive," said Turner. "Probing with the MP10 has eliminated assembly problems with the Z-frames and has helped us achieve Boeing's goal of a snap-together fuselage."

Renishaw Inc.
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