The fact that aircraft design and manufacture represents a major technical challenge should come as no surprise to anyone. Less obvious, though, are the challenges confronting production engineers.
Traditionally, building an airplane was a highly labor-intensive process. However, those days are passing. Competition among aircraft builders is tougher than ever. Manufacturers are struggling to cut costs, all the while maintaining quality.
Indeed, the need for precision has become especially critical, thanks to the increased level of outsourcing. Back when the Boeing Co. (Chicago) built its first 747 aircraft in the late 1960s, workers spent weeks fitting parts built on-site with the help of shims. Today, assemblers like Boeing and the Airbus consortium (Toulouse, France) build even their largest aircraft in a matter of days. This means outsourcing major structures and systems that, in turn, must be manufactured to incredibly high tolerances if everything is going to fit together during final assembly.
The result has been the increased use of automation, especially in the area of installing mechanical fasteners. Even smaller aircraft require thousands of bolts and rivets, each of which needs to be positioned and installed with extreme precision. When it comes to building large commercial jets like Airbus' new A380 aircraft-set to go into service by the end of 2006-the numbers can truly be staggering.
The largest commercial aircraft in the world, the A380 has two complete passenger decks and is capable of hauling 150 tons of freight. (By comparison, the U.S. military's tank-carrying C-5 Galaxy aircraft can only carry a 135-ton payload.) Not surprisingly, given these loads, the A380 has extremely large wings. Measuring 119 feet in length and providing a total wingspan of 261 feet, they hold 41,000 gallons of fuel and have a total surface area of 9,100 square feet. Each wing is comprised of a framework of spars and ribs that support 20 separate aluminum panels. These panels, in turn, are reinforced by a series of aluminum stringers. In all, attaching the panels to both the stringers and underlying spar-and-rib framework requires positioning, drilling and then riveting or bolting about 180,000 holes.
Again, all this must be accomplished to a very high degree of precision. The A380's wings are manufactured at an Airbus plant in Broughton, England. They are then shipped to a plant in Toulouse, where they must be able to mate with the aircraft's other structural components during final assembly.
To help solve the challenges of precision, cost and productivity, Airbus hired aerospace tooling and automated assembly machine builder Electroimpact (Mukilteo, WA). Historically, aircraft companies have built their wings using large numbers of highly skilled workers performing a whole host tasks, including setting up workpieces; locating and drilling holes; pulling components apart for deburring and cleaning; applying sealant; and installing fasteners. However, Electroimpact-which prides itself on the fact that well over half of its more than 300 employees have engineering degrees-sped up the process and removed product variability by building a battery of highly sophisticated drilling, material handling and assembly machines.
Among these is what Electroimpact calls its E4380 system, a fully automated, four-story-high piece of equipment that Airbus uses for performing wing panel and stringer assembly-what the Broughton plant refers to as Stage 00 of its wing-building process. Initially, Electroimpact built four of the machines-two for assembling upper wing panels, two for assembling lower wing panels-each of which has its own 550-foot-long set of three separate in-line fixtures. Since the initial installation, Electroimpact has built four more E4380s, so that the Broughton plant now has eight machines.
Each machine deploys a number of different tools, including a servo feed drill; a servo spindle that shaves or reams; an electromagnetic riveter for rivet upset and collar swage; a sealant inserter; bolt inserter; coldwork tool; hole probes; and a resynch camera. All process tools are built in-house. A network of servomotors, linear guides, ballscrew systems and other linear-motion components from companies like Bosch Rexroth (Hoffman Estates, IL), THK Americas Inc. (Schaumburg, IL), NSK Corp. (Ann Arbor, MI) and IKO International Inc. (Parsippany, NJ) serves to position these various pieces of equipment. The machines can install rivets and bolts measuring from 0.25 to 0.5 inch in diameter, with a stack range of up to 2.5 inches.
According to Electroimpact lead mechanical engineer Ben Hempstead, P.E., his company was able to draw on earlier experiences with other wing panel assembly machines to improve the accuracy and performance envelope of the E4380.
"The result is a new generation of wing panel machines. Our design goal was to enable one operator to set up, load NC tapes, verify accuracy and configure fixtures," Hempstead says. The rest, what was once done by a gang of technicians, is now fully automatic.
A Challenge of PositioningNot surprisingly, given the size of the A380, creating the E4380 represented a unique challenge, especially in terms of positioning the various tools against the surface of each panel.
Specifically, Electroimpact had to accommodate an unprecedented amount of panel curvature, with some lower wing panels moving nearly 5 feet out of plane. In previous applications, Electroimpact's automated riveting machines were equipped with clamping heads that extend or retract to whatever distance is required to make contact and execute "clampup" with each panel. But, given the requirements of the A380 application, Electroimpact decided to try something different-providing the machine with an independent Z-axis.
All of Electroimpact's E4000 series machines include a large, U-shaped yoke that straddles the panel being riveted and is supported by what appears to be a single rigid structure moving down a pair of rails. In fact, this structure is comprised of two independent towers, each supporting one side of the 15-ton yoke by means of a trunnion that is attached to, and adjusted by, each tower via a vertical slide.
In operation, the vertical slides not only define the system's Y-axis, or vertical position, but also, by varying the heights of the trunnions, impart a rotational A-axis position to the yoke. Similarly, as the two towers move down the railway, they not only position the yoke in the X direction, but by positioning themselves in a staggered position, provide a B-axis rotational component.
By incorporating a horizontal slide feature into each tower to accommodate panel curvature, Electroimpact engineers created a situation in which clampup is always achieved at the same position relative to the yoke. This simplifies toolpoint alignment, because it minimizes the length of travel required of the clamping mechanism. In addition, because there is less movement involved, the new arrangement improves equipment longevity and cuts down on maintenance.
In all, the Z-axis ballscrew can shift the yoke through a horizontal range of 5.5 feet. Overall, the machine can access a very large work zone, measuring more than 13 feet vertically and 578 feet long, with ±15 degrees of rotation in both the A- and B-axes. The A-, B- and Z-axes on the machine are monitored using LC491F absolute encoders from Heidenhain Corp. (Schaumburg, IL). The Y-axis uses LB382 scales, also from Heidenhain. X-axis positioning is monitored using a pair of RG2 tape scales from Renishaw Inc. (Hoffman Estates, IL). In operation, the clampup pressure, which is load-cell controlled, can be programmed up to 3,500 pounds.
Currently, the eight automated machines produce enough wing panels for Broughton to build 20 pairs of wings in a year. At maximum capacity, the plant will be able to produce four pairs of wings each month. This may not sound like much to a ballpoint pen manufacturer. But, in the aerospace world, that kind of productivity is a major accomplishment.
"We ended up expanding the system's performance envelope and accuracy [over] earlier production panel machinery," says Hempstead. "The goal was not just to design machinery that automates manual tasks, but also to improve quality and reduce process time.... At this stage, speed, accuracy and operator safety are critical to success."