Advanced Technologies Supplement: Composites Challenge Cutting Tools
Better tools and machining techniques open up possibilities for aerospace composites
By Mike Tolinski
At first it might seem that machining fiber-filled composites with hard cutting tools would be a recipe for disaster. Developers of cutting methods and tools for composites face all kinds of problems. A composite's fiber layers can delaminate from the machined surface; the fibers or other hard reinforcements are abrasive and reduce tool life considerably; and the combination of hard and soft materials that make up a composite complicates the best choice of tool and machining parameters.
However, in aerospace manufacturing's drive to lighten aircraft, more composites applications are on the way, made from carbon, metal, and ceramic-based composites. Many complex composite components are made with near-net-shape processes, but some machining is unavoidable. Machining may become even more important for tight-tolerance mechanical components like turbine engine parts. So given the new applications and materials, manufacturers will benefit from having more options in machining to choose from, along with abrasive waterjet and lasers, for cutting perfect surfaces and shapes.
Along with civilian aircraft from Boeing and Airbus (see the March 2006 Advanced Technologies supplement to Manufacturing Engineering), new military aircraft are awash with composites. Carbon-fiber composites have been used for years in advanced aircraft, but stronger, lighter composites are pushing the capabilities of existing machine tool technology.
Tooling companies have had some success using existing tools for machining these composites, but aerospace companies keep upping the ante. "Their needs are constantly changing, and their materials are constantly changing, so it's very difficult to hammer out a standard offering of tools," says Michael Standridge, aerospace business development specialist for Sandvik Coromant (Fair Lawn, NJ). "New materials are being developed as quickly as we can make tools and tool processes for them."
Machining composites is made harder given all the possible material designs, notes Francois Gau, aerospace and defense industry global-segment manager for Kennametal Inc. (Latrobe, PA). Fiber layout and orientation complicate the choice of cutting-tool geometry for milling or drilling. Because of all the variations encountered in fiber size, weave, direction, and content, Gau characterizes composites as "almost like wood — every tree is different."
For current and future aircraft structures, milling and drilling is critical for finishing trimmed edges of panels, cutting windows or openings, or making accurate holes to rivet pieces together. Aircraft orders are expected to grow gradually over the next few years, but machining applications for composite parts may double by 2010, says Gau, with the average composites content expected to rise from 7.5 to 13% of aircraft weight.
These developments in advanced materials have motivated 54 machine and tooling companies (aided by government funding) to join an alliance to investigate advanced machining processes, says Glenn Sheffler, outreach manager for the National Center for Defense Manufacturing & Machining (Latrobe, PA). The NCDMM is a manufacturing productivity development center established in 2002 on Kennametal's headquarters campus.
Along with machining of high-tech alloys, the NCDMM has investigated a variety of composite materials, including carbon-fiber and ceramic-matrix composites, and composite-stack materials that combine metal and composite layers for aircraft structures. "There are various ways in which they can be machined," says Sheffler. "Depending on the customer or the application of the materials, there could be stipulations regarding heat-affected areas, water infiltration, or other issues that limit the method or methods that can be used."
Because of security concerns, reportable details about military applications can sometimes be limited, but there are some reasonably clear trends in the development of practical solutions for machining advanced materials.
The F-35 Joint Strike Fighter (JSF), for example, is filled with advanced materials. The aircraft's structural components must be machined to extremely tight tolerances to provide the low-observability (that is, stealth) characteristics required for its outer shape, explains Michael Christie, JSF program manager for BAE Systems in Samlesbury, England. BAE supplied the first F-35's aft fuselage, and is part of the multicountry effort to develop and build the JSF. "Techniques used in the machining of this first production component will fall within 0.007" [0.2 mm]."
Major F-35 carbon composite components include the wing skin supplied by Lockheed Martin Aeronautics Co. (Fort Worth, TX). The wing skin requires edge machining with hard cutting tools, but the material is abrasive, and initially it could only be machined with a process with very limited tool life, as well as unacceptable part edges and scrap rates.
Lockheed requested a new approach from the NCDMM. Glenn Sheffler says the edge-of-part trimming project for the JSF addressed some basic problems involved in using cutting tools with fiber composites. When using tools not specifically designed for these materials, machining can delaminate, rip, and tear the composite, as well as create excessive heat, because the composite matrix does not conduct heat well. Cutting tools designed for metals have been used for composites, but "these tools can create results that may cause failures in components that, in turn, contribute to increased cost, rejections, and time required to do the job," Sheffler remarks.
The NCDMM's focus for projects like the JSF wing skin is to evaluate or select a "real deal" solution for edge-of-composite trimming, according to Sheffler. This effort involves evaluating tools that deliver increased life and productivity, while reducing scrap of what can be extremely expensive materials.
Finding the optimal choice of tool and technique was based on down-selecting different tools, and comparing their cutting forces with a dynamometer. Project goals included low cutting forces, consistent speeds and chip loads, fast material removal, and better tool cooling.
Sheffler says specific tooling companies were selected to provide the optimized tooling technologies. "One supplied the carbide substrate for the tool, one supplied the geometry for the tool, and one provided the coating for the tool per our recommendations." The three partner tooling companies were determined after initial tests based on tooling and process background information from the project's client.
The results of the project "were staggering," says Sheffler. With changes to tool geometry, material, and coating, delamination was eliminated and scrap reduced. And cutting-tool life increased from nine linear feet (2.7 m) at one-third material thickness to 57 linear feet (17.4 m) at full material thickness. This reportedly reduced the number of tools per job from 24 to two, saving $80,000 per aircraft, or about $223 million over the projected build life of the F-35 JSF.
Other kinds of composites can replace metal and save weight for very different kinds of aircraft parts. For example, the NCDMM studied a carbon/silicon-carbide composite that's five times lighter than the high-temperature nickel alloy typically used for a turbine-engine component, which must resist 1400°F (760°C).
The turbine component is ring-shaped and flanged with holes on its outer diameter. Even when made from Inconel, it's not an easy part to cut; however, researchers were able to machine it from a block of C-SiC composite using a process optimized with "state of the market tooling" for low tool wear and high productivity, says Sheffler. Once the composite part is engine-tested and proved out in service, this is the kind of composite application that could become common in aircraft manufacturing.
Some aircraft structures use stacks of fiber composites and aluminum or titanium, and these present unique machining challenges. Abrasive waterjet machining can be a good solution for trimming the edges of composites and other materials, but throughholes in stacks typically require hard cutting tools and multistep drilling methods. (But even this is tricky, since chips from the metallayer drilling can damage the composite layer, for example.)
"Currently, waterjet machining is not accurate enough to produce critical features or create final dimensions," says Michael Standridge of Sandvik Coromant. "Holes for rivets must still be made with conventional cutting tools designed for composite material."
The company has been focusing on holemaking in material stacks for structural aircraft components for fuselages, doors, and beams. To maintain structural integrity around riveted holes, aerospace manufacturers demand holes without any delamination or pulled threads in the composite layers. "To try to approach all that with one tool is difficult, because each material layer requires a different approach and unique tool geometry," Standridge says.
In response, the company is developing a multistep process for opening up a hole: drilling through each metal layer with standard metal tools, coring out an undersize hole in each composite layer with a helical cutting process to minimize delamination and burrs, and then reaming the composite hole to finished size with a diamond-coated reaming tool. The need for counter-sunk or chamfered holes may add another tool to the process.
Kennametal's approach to holemaking in composite stacks is based on a partnership with Novator AB (Spanga, Sweden). Here, after creating a small hole in the stack layer, a "TwinSpin" spindle revolves the orbital milling tool to expand the hole cleanly, without delamination. (For more information on Novator's orbital drilling technique, see "Holemaking With Precision," Manufacturing Engineering, November 2002, p. 51.)
"The cutting action is in the radial direction, so it basically cuts through the material in an 'orbital' motion," says Kennametal's Francois Gau. "It's the same principle as milling interpolation, but in a very small diameter. You get a double motion: the motion of the drill itself around its axis, and the motion of the drill around a bigger axis." Gau adds that aircraft manufacturers are interested now in reducing cutting forces and achieving better surface finishes with orbital drilling, which could save them as much as 50% in assembly costs.
Tool life can be severely shortened by carbon or glass fibers, so machining processes benefit from the most wear-resistant tool materials. Accordingly, manufacturers are interested in better ways of applying PCD (polycrystalline diamond) on tool-edge surfaces.
MegaDiamond, a division of Smith Technologies (Provo, UT), is focusing on long-life PCD helical end mills, particularly for machining carbon-fiber-reinforced plastics (CFRP) and other composites. In the company's V-tec process, PCD is sintered directly into veins that form true helical geometries. The company's Matt Collier says the process creates tools for CFRP that are more effective than standard PCD/carbide tools.
"The ideal form would be to couple a complex geometry such as a helical end mill that can be produced in solid carbide with the properties, abrasion resistance, thermal conductivity, and performance of a material such as PCD," says Collier. Geometric complexity is limited in traditional PCD end mills that have straight brazed PCD segments; in contrast, MegaDiamond flutes a solid carbide rod with veins, and fills them with sintered PCD.
Such advancements are needed to support the aerospace market's increasingly more specialized needs. Because cutting-tool companies have mainly focused on metalcutting, more strategic partnerships may be needed for them to serve aerospace's alternative material applications.
Strategic changes in aerospace manufacturing are in store for aerospace manufacturers at all levels, notes Francois Gau. In February, Kennametal sponsored its Aerospace Machining Symposium, in which both large aircraft manufacturers and their smaller contract-manufacturing suppliers and tooling suppliers discussed how their ways of doing business must adjust to changes in aircraft materials and manufacturing.
Presenters from Lockheed Martin and other companies stressed the value of partnerships and joint-research agreements with OEMs as a good way to make real progress in aerospace manufacturing. For composites machining, at least, the reason is simple: process development might require a $100,000 investment just for a stock of composite test material—not an easy cost for one company to absorb. Moreover, determining the proper cutting tools requires a carefully structured design of experiments with several tools and trials. Gau says using partnerships for this kind of development creates demonstrable cost and cycle-time savings.
Presenters also stressed that aerospace companies both large and small will need to respond more rapidly to customer needs. Lockheed Martin announced a goal of making a plane every eight hours, which will require manufacturers to incorporate design features to allow faster assembly, says Gau. These will include faster techniques for inspecting composite parts, for instance. Suppliers will also need to speed up their response to customers, such as by supplying quotes in a day or two, which is currently not common in the industry.
Gau adds his own wakeup call for the aerospace industry, urging it to move away from being like a craft industry to more managed-production-type manufacturing, as in the automotive industry. He advises machining companies to "look backward" and optimize their current processes supporting older aircraft that are still being produced, rather than just focusing on new projects. Their goals should include 50–60% cycle-time reductions and 30–50% reduced time-to-market. Then they'll be healthier in the longterm, and ready to competition from low-cost emerging global manufacturers.
This article was first published in the April 2007 edition of Manufacturing Engineering magazine.