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Northrop Grumman Wins "Skins" Game

Pioneering twin-gantry, 5-axis router uses POGO® Flexible Tooling System Technology and 30,000 rpm HSK spindle to eliminate hard tooling, cut setup time by 2/3 and maximize part quality

HAWTHORNE, CA—"Configurable" tooling provided the right figures for Northrop Grumman in expanding its productivity and capabilities at aircraft skin processing. The Hawthorne plant ordered a new two-gantry, 5-axis Cincinnati Vertical CNC Router with adjustable CNC fixturing to bypass build and setup costs for hard tooling. Three adjustable beds of new POGO® Universal Holding Fixtures (UHFs) cut setup times by about two-thirds on trimming and hole drilling operations on more than 100 different fuselage skin parts.
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Fixturing such a large variety of parts with traditional vacuum chucks and fiberglass molds would have required not only lengthy setup burden, but also 75,000 sq ft (7000 m²) of floor space to hold the master tooling dock and 60 pick-up stations needed to store and retrieve so much tooling, said plant officials.

The pioneering approach takes advantage of the new POGO® UHFs built into the 10 x 50' (3 x 15 m) work tables under the company's new two-gantry, 5-axis vertical router. Consisting of hundreds of individually programmable, variable-position vertical supports, a POGO® UHF can be reconfigured for a new part in as little as two minutesas easily as calling up a new machine tool program.

Northrop Grumman's new router features three of the POGO® Flexible Tooling Systems, allowing the separate gantries to machine different parts simultaneously. The router is the first Cincinnati machine tool ordered with the new "soft" workholding technology. Developed by CNA Flexible Tooling Systems (Redmond, WA), POGO® Universal Holding Fixtures are sold exclusively by Cincinnati in the aviation, space and defense markets.

"Flexible tooling is ideal for this application," says Northrop Grumman industrial engineer Art McIntosh, who performed prove-out and "first article" verification for the new machine. "We run these parts in units of one, so flexible change-over between part numbers goes a long way toward streamlining our process."

First HSK Router
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The Hawthorne plant produces the skins for a commercial aircraft manufacturer. Covering the fuselage of a jumbo jet from behind the cabin to just before the empennage, the aircraft skins are aluminum panels averaging 84" (2.1 m) wide, and ranging from 220 to 260" (5.59 to 6.60 m) long and 0.049 to 0.500" (1.2 to 12.7 mm) thick.

The new twin gantry router complements existing fabrication operations performed by the Hawthorne plant on the part to extend value-added benefits. The additional operationsperiphery milling and drilling holes for locating stringers and shear tiesallow Northrop Grumman to deliver snap-together parts that minimize added labor during aircraft assembly.

The Northrop Grumman fabrication area has used similar Cincinnati 5-axis machines for profiling and pocket milling for over 20 years, McIntosh says. The new router addresses evolving requirements in two areas particularly, he notes:
*  Accuracy. "It's a high-precision machine," he says. Capable of ±0.002" (±0.050 mm) volumetric true position, the router easily holds Northrop Grumman's ±0.015" (±0.40 mm) tolerance on milled surfaces and hole locations.
*  HSK Spindles. "With HSK, we're able to take advantage of the full 30,000 rpm, and still hold our tightest tolerances," says McIntosh. The router is Northrop Grumman's first machine tool accepting tools with the HSK taper, designed to eliminate wobble up to the highest spindle rpms. The increased stability will let Northrop Grumman drill at full speed, without leaving its 0.003" (0.076 mm) total tolerance band on hole diameters.

Featuring 6000-30,000 rpm speed range on each of its two 17.5 hp (13 kW), No. 40 taper spindles, the new router feeds at 650 ipm (16.5 m/min) and rapids at 1200 ipm (30.5 m/min).

880 Actuators

The router's three POGO® UHFs look like beds of nails on the machining tables, where each "nail" is a pneumatic actuator ending in a swiveling vacuum end effector cup. Interfaced with the CNC, the POGO® actuators move to programmed heights with ±0.004" (±0.100 mm) accuracy and ±0.001" (±0.025 mm) repeatability, across a vertical stroke as long as 30" (0.76 m). When hundreds of the POGO® actuators move independently to form a pattern matching the contours of the part, the result is a support structure equivalent to a hard-tooled fixture.

Northrup Grumman's UHFs each consist of 880 POGO® actuators, situated among 1320 pockets. Though McIntosh believes his operation will allow them to remain stationary, he notes that any POGO® actuator can be relocated to any pocket for added flexibility. "It's just a four-bolt attachment," he says.

Saving on Setup

With configurations for dozens of parts stored in the CNC, one POGO® UHF can take the place of an entire inventory of hard-tooled fixtureseliminating not only the need to produce and store these fixtures, but also to move them around. This relocation is the main source of the setup savings, McIntosh says.

"All of the work associated with taking a fixture on and off the router's work table is gone," he says. "No more bolting the fixture in place, squaring it, and homing the machine to the tool. With the POGO® Universal Holding Fixture, most of the setup remaining consists of just craning the part to the machine."