The Boeing 787 Dreamliner is a marvel of modern aviation. Boasting innovative features such as active gust suppression, hybrid laminar flow control (HLFC), and a “no-bleed” electrical architecture that reduces the need for traditional pneumatic systems, the Dreamliner family is among the most comfortable, fuel-efficient, and quietest commercial airliners ever built.
Composites receive much of the credit. That’s because the 787’s wings, tail, doors, and one-piece fuselage section contain approximately 32,000 kg of carbon-fiber-reinforced-plastic (CFRP) composite—80 percent by volume and 50 percent by weight. According to its manufacturer, the Dreamliner “makes greater use of composite materials in its airframe and primary structure than any of Boeing’s previous commercial airplanes.”
Impressive as that may be, composite use in aircraft is nothing new. In 1932, Amelia Earhart made aviation history by flying a Lockheed 5B Vega alone across the Atlantic Ocean, one of the first aircraft constructed from laminated layers of glue and plywood (which also is a type of composite). She called it her “Little Red Bus.” Fifteen years later, Howard Hughes’ Spruce Goose’s one and only flight was made possible through Duramold, a phenolic-based composite material developed during World War II.
Since then, CFRP and other composites are taking the pilot’s seat in aircraft design—for good reason. They are lighter than aluminum, stronger and more flexible—unlike metal, composites can absorb tremendous impacts without denting. And thanks to composites’ ability to assume practically any shape, aircraft designers have far greater design freedom than would otherwise be possible.
Because of this, industry leaders seek to increase composite use well beyond existing levels, and to manufacture future aircraft more sustainably and quickly. For instance, the Carbon Fiber Technology Facility at Oak Ridge National Laboratories is focused entirely on carbon-fiber innovation and the scale-up of emerging technologies. And NASA’s Hi-Rate Composite Aircraft Manufacturing project aims to construct composite-rich aircraft like the Dreamliner six times faster than possible with today’s layup and molding technologies.
They have their work cut out for them. Achieving these goals will require significant investments in manufacturing technology, well beyond what’s currently available (and, as you’ll see, is already quite impressive).
CFRPs are abrasive and prone to splintering and delamination. The end mills, drills, and routers used to cut them must not only be wear resistant but have geometries able to cleanly shear composite fibers without separating the individual layers.
Kyle Matsumoto, the “resident expert on aerospace fastener applications” at OSG USA Inc., St. Charles, Illinois, explained that holemaking in composite materials can be particularly problematic. Traditional drill geometries tend to lift the individual layers during tool entry, then push them away upon breakthrough. And since the fibers are apt to bend and pull out during drilling operations—especially as the tool dulls—holes often end up undersized.
OSG has developed a line of cutting tools that help alleviate such composite woes. Matsumoto said that ultra-fine crystalline diamond coatings, when applied to carbide drills and end mills using chemical vapor deposition (CVD), make the tools freer cutting and more wear resistant than their uncoated counterparts, often providing up to 10 times the life and far cleaner machined surfaces besides.
Dirk Dietsch of M.A. Ford Mfg. Co., Davenport, Iowa, offers similar solutions. Dietsch, the regional business manager for the Great Lakes region, said the company’s 239-series carbide router—also with a CVD diamond coating—is licensed by Boeing. In fact, one of the OEM’s suppliers uses the tool to trim flash and cut windows in Dreamliner fuselage components.
Dietsch is quick to note that not all composites are bound for aerospace use; the automobile industry has also begun using this game-changing material in passenger vehicles, and for the same reasons. “We have a customer in Israel that’s making composite bodies and substructures for Chevrolet Corvettes and Camaros, and the bed on the GMC Denali pickup is also carbon fiber.
These and many other manufacturers are doing whatever they can to take weight out of their products, and CFRP is a great way to do that.”
Iscar USA of Arlington, Texas, recognizes this, and has responded by establishing a CFRP machining with part and process development. Its capabilities include the design and grinding of special tools, a Kistler system to measure cutting forces, and high-speed cameras and high-resolution microscopy to analyze the machining process.
National product manager for holemaking Patrick Cline ticked off a number of composite-specific cutting tools, among them the SUMOCHAM replaceable head drill, SOLIDMILL burrs and end mills, and ADU-ready (automated drilling unit) tools for fastener hole drilling. As with the other cutting tool providers listed here, CVD diamond is one of Iscar’s go-to coatings for combating wear, as is DLC (diamond-like carbon) coating and the use of polycrystalline diamond (PCD) segments brazed to a carbide substrate.
A range of innovative geometries is available as well, each designed to reduce the fraying, chipping, and splintering nastiness described earlier. “I should also mention that we offer uncoated cutting tools, but these are primarily for prototype and low-volume work,” said Cline. “Composites are very abrasive and will destroy the edge on an uncoated drill or insert fairly quickly, which is why we generally recommend a diamond coating or, for production quantities, PCD tooling. It’s the most expensive of all cutting tool materials, but is well worth the price.”
Dave DenBoer, aerospace industry specialist at Sandvik Coromant US, Mebane, N.C., is in complete agreement. He began machining 787 Dreamliner components in 2005 for a well-known aerospace company and is currently part of the manufacturing application support (MAS) group at Sandvik Coromant’s Precorp facility in Spanish Forks, Utah, which pioneered the development of PCD-veined cutting tools. He and his colleagues spend their days helping customers make composite machining faster and more predictable, working to avoid expensive failures.
“For example, achieving clean breakout on the backside of a drilled hole can be challenging in any material, whether it’s titanium, aluminum, or composite,” said DenBoer. “The good news is that we make a wide variety of tools in this facility that do extremely well in this and other applications. We also spend a lot of time developing the proper speeds and feeds for different composite materials, which is just as important as tool geometry to achieving proper breakout without the risk of delamination.”
This last part is critical, he added, since a single delaminated hole can cost upwards of $5,000 to repair. Further complicating matters are the different types of composites, each with a unique combination of resin and fiber types, shapes, orientations, and concentration levels. There are also additives such as aluminum and silicon (which make composites even more abrasive) as well as complications like “lightning strike,” a copper mesh on the outside of the fuselage that prevents damage during lightning storms.
“The composite world is already huge and new materials are coming along all the time, which is why it’s critical to use the correct combination of cutting tool and machining parameters,” DenBoer said. The last thing any manufacturer wants to do is change an end mill in the middle of a 45-ft-long wing spar or fuselage component. That’s why we spend so much time on process development, and is why we strongly recommend PCD-veined tooling for many of these applications.”
Developing effective cutting tools and process parameters for composite materials is challenging work, but no less so than designing the CNC machinery, fixturing, and toolpaths needed to machine the components.
That’s the job of Jim Cunov, chief engineer for aerospace at PAR Systems LLC, a Shoreview, Minn.-based manufacturer that engineers, builds, and integrates custom equipment for a range of industries and applications, aerospace composite machining among them.
“On the aerospace side, we offer machines that perform conventional milling, drilling, trimming, and sawing operations using diamond cutting tools and high-speed spindles,” said Cunov. “But we also provide abrasive waterjet cutting for cured composites, often in the same machine. Most of these are gantry-style CNCs and many are quite large—for instance, we’ve built equipment that processes the upper and lower covers for Airbus A350 wings, which at the time of installation, were the largest composite panels in commercial aerospace.”
Cunov has spent nearly four decades at PAR. During the first half of his career, he struggled to understand why customers were seemingly unable to present him with parts that met the required geometric accuracy. “A stringer might be out of true or a panel have some slight warp to it, and I found myself asking how they expected us to cut good parts when they weren’t at the nominal shape to begin with,” he said. “After a lot of head pounding, we finally realized it’s the nature of composites. The parts are big, they have spring to them, and they move slightly during the curing process. We knew that we’d better find a way to fix it ourselves.”
He and his team did exactly that. Using vision systems and laser profilometers, PAR is able to map a composite component’s physical location and contours and then morph the CNC toolpath to match the geometry.
“This allows us to produce good parts even when the layup doesn’t meet the nominal condition,” Cunov explained. “We call it adaptive machining.”
A key piece of this adaptive machining strategy is workholding. For example, PAR uses a series of robots to grip a stringer along its entire 20-30’ [6-9 m] length. After mapping it as described, locational information is sent to the system controller that then morphs the tool paths to accommodate the actual location and contour of the part as it is held by the robots. This arrangement also allowed PAR to accommodate a complete family of stringers, machining close to 900 different part numbers with the same configurable robotic fixture.
Some of those composite parts may have come from a machine built by Electroimpact Inc. in Mukilteo, Wash. A manufacturer of aerospace tooling, fiber layup machinery, and advanced automation systems, Electroimpact is another composite machining solution provider that uses robots to great effect. Robotic systems lead Russ DeVlieg described one example of this, a patented technology that uses six-axis articulated arms to perform aerospace milling and drilling operations. They call it the Accurate Robot.
Aurora Flight Sciences in Bridgeport, W.V., a Boeing company, has two such robots. They’re named Parker and Ackley, both part of its flexible robotic composite manufacturing cell. In addition to “milling a couple hundred different composite parts,” the system also does in-process robotic inspection and automated part handling.
“Robot use in the aerospace market is definitely increasing,” said DeVlieg. “Aside from cutting and inspection, aircraft components are often assembled by robots. We have robots that climb along the outside of the fuselage while a human works inside, drilling holes and attaching threaded fasteners as a team—robots touch pretty much every surface on a modern airliner.”
These are impressive feats, yet Burton Bigoney, project manager at Electroimpact, said it’s too soon to discount traditional CNC machine tools. “Our robots are extremely capable, but once you cross a certain threshold in terms of part size or accuracy requirements, or move into metallic structures, you need large, dedicated machine tools. That’s why we recently built a huge gantry-style milling machine for one of our customers, and we’re also developing some composite trim and drill machines.”
When asked why an aerospace OEM wouldn’t use off-the-shelf machine tools and robots for this type of work, Bigoney offered several reasons. Chief among these is Electroimpact’s extensive experience with large metallic and composite aerostructures, followed by high-speed clamping systems, precision countersinking abilities, and in-machine vision systems to locate reference features for assembly. “We developed these types of technologies for our riveting machines long ago, and are now able to leverage them on our robots and machining equipment,” he said.
DeVlieg seconded this. “Looking at the whole package, it’s something that maybe a handful of integrators can take on and actually succeed with. And off-the-shelf robots, specifically, don’t have the positional accuracy needed for this type of work.”
Electroimpact is probably best known for its automated tape laying and fiber placement equipment. Andrew Purvis is a project manager for this area, and noted that even here, robots are on the rise. He pointed to the company’s new scalable composite robotic additive manufacturing (SCRAM) system as a perfect example: a continuous-fiber-reinforced 3D printer that enables the tool-less rapid fabrication of aerospace-grade integrated composite structures.
“SCRAM is yet another instance where we’ve combined multiple processes into a single hybrid manufacturing system,” Purvis noted. “In this case, we can print a tool using a water-soluble thermoplastic, then deposit fiber-reinforced tape or engineering-grade polymers like PEEK [polyetheretherketone] and glass or carbon-fiber materials on top of and around that tool. SCRAM can also stop and drill or mill features at any time during the build,” he continued. “When done, the tool is dissolved and the part is ready for assembly. And because there are six axes of motion, we can deposit material in whatever position provides optimal strength and stiffness. It’s one of the first true 3D printers, but one that also has subtractive capabilities. It’s unique to the industry.”
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