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The Increasingly Perfected Science of Machining Composites

Jim Lorincz
By Jim Lorincz Contributing Editor, SME Media
A 1965 Shelby Cobra 427 shown at the Detroit Auto Show was additively manufactured on a Cincinnati BAAMCI machine by DOE’s Oak Ridge National Laboratory (ORNL), one of seven founding members of the Institute for Advanced Composites Manufacturing Innovation. The Detroit IACMI branch will get $70 million to develop a robust supply chain to improve materials, handling, and machining properties for automotive composites.

As use of composites matures in aerospace, how to machine the advanced materials grows up too, with advanced tooling solutions and tested approaches

Thanks to the aerospace industries and large quality-intensive programs such as the Airbus 380 and Boeing 787, the business of machining composites has moved into the mainstream of the manufacturing industry. There’s a bevy of tools, specialty tools and know-how about how to machine composites today, which helps make the complex materials ripe for expansion into other areas of industry.

The auto industry is the next big growth area for the lightweighting, strength, and design flexibility that composites offer. BMW’s Vision Future Luxury concept car introduced last year showcased how “subtractive modeling” could be used to design interior structure components with carbon fiber reinforced plastic (CFRP) and structural load-bearing layers of aluminum and decorative and functional wood and fabric—milled down to reduce total weight.

More car and truck components made with composites, and a fully developed supply chain to support them, are on the way. When the Strati 3D-printable car rolled out of IMTS 2014 last September, it represented 44 hours of additive manufacturing on Cincinnati Inc.’s BAAMCI (Big Area Additive Manufacturing Machine) and a full day of subtractive milling by Local Motors Inc. (Phoenix, AZ) of the ABS plastic/carbon fiber composite on a CNC router. Local Motors says 24 hours of AM is a realizable goal.

At the Detroit Auto Show, ORNL, one of seven founding members of the Institute for Advanced Composites Manufacturing Innovation (IACMI), showcased a 3D-printed 1965 Shelby Cobra 427. $70 million of the Institute’s $259 million public-partnership of 122 member companies will fund IACMI’s Detroit branch to develop a robust, risk-free supply chain for advanced composite materials—to improve materials, handling, and machining properties for automotive applications.

Automotive Chases Lightweighting Promise of Composites

According to Randy Von Moll, technical sales director, Fives Cincinnati (Hebron, KY), automotive manufacturers are following the lead of the aerospace industry exploring the potential of composites to reduce weight and increase fuel efficiency. “We may see the main chassis of the car being built from composites one day in the future. The hood is a good example of a part that weighs a lot and can benefit from lightweighting,” said Von Moll.

“Automakers are striving to manufacture composite parts to net shape. They can get pretty close to net shape, but virtually every part that is built from composites needs some form of machining to finish it. There are only a couple of parts in composites that I can cite that can be made all the way to net shape. Usually there’s a certain amount of excess material, around the periphery of the part in aerospace because of the way that the composite part is built up that it always has to have a profile trim done on it and typically holes drilled,” said Von Moll.

“For the larger aircraft parts, our re-engineered PMT [precision, mill, trim] traveling gantry-type machine can extend the X axis for a large single part or be set up in multiple work zones with light-curtain dividers or physical barriers between zones. All of the new aircraft designs are getting designed more accurately to minimize or eliminate the hand fitting that has historically taken place during the aircraft manufacturing process. The aircraft producers want to build airplanes more like the car builders build cars and pull them together on a moving line and they’re making a lot of progress toward that. Today virtually all the newer Boeing models are built on moving lines,” said Von Moll.

“There is a wide variety of tooling for different types of material that is being cut. Compresson-type cutters featuring both left-hand and right-hand helixes have been developed to machine laminated stacks of material without delamination. A lot of machining that is done of composites also involves other materials. It isn’t unusual for us to supply a solution for a composite material that has carbon fiber, Kevlar, usually a nonmetallic honeycomb core or foam, a copper mesh for lightning strikes, a titanium sheet in there for additional design purposes. It would be more unusual for us to have just a uniform material type. The sandwich structures are getting more complex,” said Von Moll.

Machining CFRPs Not Like Cutting Metal

According to Don Graham, Seco Tools Inc. (Troy, MI), carbon fiber reinforced plastics (CFRP) are machined by a series of brittle fractures in which the cutting action shatters the abrasive hard carbon fibers producing powder or brush-type chips. “Machining optimization of composite materials in drilling has to resolve problems of fiber pullout, uncut fibers and delamination, especially in exiting the material by addressing issues resulting from dull edges, improper prep, feed rates that are too aggressive, and wrong drill angles. For milling applications, fiber pull out, uncut fibers, delamination, chatter/vibration, and torn fibers benefit from some combination of replacing dull cutters, remedying prep, reducing aggressive feed rates, changing too high helix angles, improving cutter angle of attack and better fixturing,” said Graham.

“Tools can fall into one of two categories,” said Graham. “End mills with a double helix—both a left and right-handed helix—for cutting action use a guillotine action and are particularly useful for honeycomb materials and fiber reinforced materials as well. The idea is that you shear the fibers, not fray, split, or delaminate them,” said Graham. Seco Tools’ Jabro JPD and JC end mills were developed specifically for composite materials. The JPD group of solid carbide end mills features brazed polycrystalline diamond plates with through-coolant channels for chip and dust evacuation and are available in square, ballnose, and compression-type end mills. The JC group includes the 875 solid carbide router with a special edge treatment and diamond coating.

For drilling CFRP materials, Seco Tools offers drills with two different geometries: C1 geometry has an optimized drill point for carbon fiber and in stacked titanium/carbon fiber or aluminum/carbon fiber when exit is in the carbon fiber. The diamond-coated drill has a double chamfer and two different angles for exiting on the carbon fiber without splintering and delamination. C2 geometry is used for stacks entered on carbon fiber and exited on titanium or aluminum, controlling chips so that the metal chips don’t roughen the carbon-fiber surface. C1 and C2 drills are coated with a CVD Dura coating.

Many Industries Are Affected by Composites

When it comes to composites, everybody thinks of aerospace first, according to Adrian von Rohr, KOMET of America Inc. (Schaumburg, IL). “That is completely wrong. If you look around composites are becoming more and more interesting for applications where a heavier material component made from steel or cast iron can be replaced with a composite that requires less energy to move the object. Also, composites allow extreme positive features and physical hardness and toughness to be designed into components. In the automotive industry, weight reduction of composites results in fuel efficiency. BMW, for example, has launched a car that is completely made out of composites, except for the engine and drive train. It’s not a prototype, and BMW has a manufacturing line for that car,” said von Rohr.

Fiber reinforced plastics (FRP) are durable, but also lightweight, reducing loads, drag, and energy consumption. However, composite materials present their own special machining challenges. They are brittle, and the glass fibers within them are particularly abrasive. The glass sections of FRP require particularly sturdy drills and tools, while the plastic pockets need to be machined with care. “KOMET’s portfolio of tools includes drills, mills, and routers for opening holes up, milling slots, and cleaning trimming and drilling to get to the final shape. These tools are used both on manual and CNC machine tools. “The real challenge is to match the tooling solution with the right coating, tool geometry and tool body for the composite application based on the information that the customer supplies to us,” said von Rohr.

Full Range of Composite Machining Products

There is major growth in the use of composites in virtually every industry, including aerospace, automotive, and medical, according to Jeffrey Stephens, OSG USA Inc. (Glendale Heights, IL). “We have a number of tools designed specifically for aircraft production. The EXOPRO AERO BNC router with our patented ultrafine diamond coating is designed for trimming carbon fiber and glass cloth. It features a fine nicked geometry for roughing as well as overlapping with finishing. The nick is actually a flat portion making the tool highly efficient in producing clean cutting edges without delamination or leaving fibers uncut when used on CNC machines. The CNC router is the workhorse, capable of plunge and helical interpolation,” said Stephens.

“If you look at our current composite products, we’ve developed roughing and finishing routers and drills for CNC applications that are solutions tailored to the customer’s requirements. There are still a lot of applications in aircraft where the parts are awkward, sometimes the fixturing isn’t very good. They might use a vacuum frame or even adhesives to hold the part down and we have to develop cutting tools that reduce the cutting forces,” said Stephens. “The EXOPRO AERO HBC router features a diamond-coated herringbone for high-feed CNC applications that produces excellent surface finish. The EXOPRO AERO drill features diamond coating for clean entry and exit from the composite material.”

Machining honeycombs is an entirely different matter. “In our original herringbone compression router design, we featured a 30° helix. Compression-style routers compress the laminate together with a left-hand and right-hand helix, leaving a clean edge on both top and bottom. What we found was that the 30° didn’t work in all cases, especially for fibers with higher strength. So we developed a  45° helix with a higher shearing capability for more difficult fibers. For the most difficult to cut Kevlar honeycombs, we developed a 60° helix that performs like a pair of scissors to cut the Kevlar very cleanly,” said Stephens.

PCD Veined Drills Feature Unique Solution

Many of the most difficult challenges still originate with aircraft production generally and with jet engine applications specifically. The latest surge in jet engine design and production for new aircraft and for retrofit applications will ramp up demand for machining the toughest of materials, ceramic composites, with the hardest of materials, diamond.

Sandvik Coromant (Fair Lawn, NJ) has developed its unique solution in a series of PCD-veined cutting tools for drills, mills, and end mills. “Series 85 geometry cutting tools provide a real alternative to brazed diamond-tipped tools, especially for ceramic composites that require the hardest material to drill,” according to Linn Win, composites product specialist for Sandvik Coromant. “Our PCD-veined series 85 geometry cutting tools are monolithic in structure. That means that they can be ground with various geometries and features that were once difficult to achieve in the past using conventional brazing methods,” said Win.

The monolithic PCD-veined tool is formed by slitting a carbide nib in precise location and configuration in accordance to the customer’s requirements, filling the carbide nib with diamond powder, and then sintering the tool under high pressure and high heat. Once the nib is extracted from the pressure and heat vessel, the diamond has bonded with the carbide as one monolithic structure leaving no brazing weak points and allowing more options when grinding the cutters face geometry. “The monolithic structure allows cutters to be reground multiple times, so that tools  have the durability of diamond as well as the versatility of carbide,” said Win.

This article was first published in the April 2015 edition of Manufacturing Engineering magazine.

 

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