CFRP gets ‘tough’ love from microtools

A selection of CFRP-suitable microtools. Image courtesy Richards Micro-Tool.

Carbon-fiber-reinforced plastic (CFRP) is a corrosion-resistant and lightweight material used in everything from paintball guns to Formula One racecars. The composite material has an “ultimate tensile strength” approximately four times that of steel and unrivaled strength-to-weight ratio, meaning it’s not only stronger than alternative materials, like aluminum, but you also need far less of it by weight to do the same job.

Tiger Woods has CFRP in his golf clubs; the Boeing 787 Dreamliner has a CFRP skeleton; and, if you’re a superhero, you’ve probably got a CFRP crime-fighting suit hanging in your closet.

But CFRP is difficult to machine because it’s made of carbon fiber and epoxy resin. Not only is CFRP extremely abrasive, it’s prone to nasty things like delamination and fracturing. Think of it as machining fiberglass with a bad attitude. And the smaller the tool, the greater the challenge it is to keep that tool sharp and in one piece.

Given those challenges, machining costs for CFRP must be astronomical, right? “Not necessarily,” said Earl Benton, executive advisor for CFRP manufacturer Toray Composites (America) Inc., Tacoma, Wash. “There are always trade-offs. You have to look at it in the context of the total life-cycle cost.”

According to Benton, the machining challenges posed by CFRP are offset by its longer service life and the reduced number of manufacturing steps required to turn it into a useful product.

Compare a CFRP airframe to a traditional airframe, for example, where the different aluminum components must be joined with mechanical fasteners. “By using composites and utilizing cold-bonding or secondary bonding techniques, you can eliminate a lot of fasteners,” he said. This means less machining, lower assembly costs and a cheaper, stronger and longer-lasting airplane.

Anyone who flies loves the thought of a better plane, but that doesn’t help you cut this stuff. So how do you drill microholes in the material equivalent to Superman’s tights?

Jason Marsh, director of R&D for Kyocera Micro Tools, Costa Mesa, Calif., explained that the trick is to “sever the fibers and leave a clean edge, but without breaking down the surrounding resin structure. Otherwise, you’ll end up with rough edges around the hole or delamination of the substrate. And hanging fibers at the edge of the workpiece can promote wicking of water into the workpiece, which, due to possible freezing temperatures, could be a concern in aircraft use.”

Marsh recommends an up-sharp tool (no edge preparation), but because of the abrasive qualities of CFRP, the need for a sharp edge must be balanced against tool life. “For shorter runs, where tool life isn’t as critical, you could go with an amorphous-diamond or a DLC (diamond-like carbon) coating, which, due to their lubricious nature and high hardness, are well-suited to cut extremely abrasive materials like CFRPs. But it’s a fine line—too much coating and the edge won’t be sharp enough, and you won’t get a clean cut. It’s for this reason that DLC coatings are only a couple microns thick.”

When it comes to production machining of CFRP, diamonds also are a machinist’s best friend. Marsh explained there are several tool geometries available, anything from “diamond-cut” routers, which resemble miniature corncobs, to 1- and 2-flute routers. “We even have something that looks like an apple corer, called a half-moon cutter,” said Marsh.

But often the preferred choice for drilling and milling is a zero-helix PCD cutting tool. There are two reasons for this: Achieving a helical shape when using diamond tooling is quite expensive, and a typical tool helix generates a lifting force, leading to delamination.

Any old tool won’t do

Richards Micro-Tool Inc., Plymouth, Mass., makes custom cutting tools as small as 0.004" in diameter for medical, dental and industrial customers. When asked for a recommendation on what, in most materials, would be a simple drilling application—a series of ¹⁄₃₂"-dia., ¼"-deep holes—Richards Engineering Manager Steve Houbre quickly made it apparent that you can’t just plow in there with any old tool when drilling CFRP.

There are many variables to consider when designing a cutting tool, but they become more important when applying microtools to CFRP. “We’d usually go with a diamond-coated drill—one with a lower point angle at the tip and a lower helix angle to avoid chipping and delamination,” Houbre said. “But you also have to consider available rpm, coolant and the composite’s properties. You might need to try it on for size, get back to us with your results, and we’ll tweak the tool as necessary. It all depends on the specific application.”

A high-performance cutting tool for CFRP. Image courtesy Kyocera Micro Tools.

Whatever the machining difficulties, Jeff Davis, vice president of engineering for Harvey Tool Co. LLC, Rowley, Mass., said, “because of the superior physical characteristics of CFRP, the application potential for these materials is huge.” For product designers and CFRP manufacturers, this is great news, but for the people who have to cut this stuff for a living, watch out.

“Carbon fiber is a big buzzword in the industry,” said Davis. “But if you back off from that for a minute, you’ll see there are a whole slew of related materials—G-10, Aramid, PEEK and so on—which present similar machining challenges. They are all very abrasive, and they chew up tools left and right.

“It all comes down to cleanly cutting those fibers,” continued Davis. “Vertical lifting forces generated by the helix angle can cause a lot of problems, such as delamination and chipping. Also, as the tool dulls, the fibers aren’t cut cleanly and you end up with a fuzzy edge.” This is a problem on most workpieces, but especially so on microparts, where the length of the fuzz can exceed the size of the machined feature.

Here comes the fuzz: the result when your tool goes dull on CFRP. Image courtesy Kyocera Micro Tools.

To avoid delamination, Davis recommends compression spiral, or herringbone-style, milling cutters. These cutting tools feature an up-cut spiral on the bottom half of the tool and a down-cut spiral on the upper half. “By straddling the machined edge of the workpiece at the intersection of the two spirals, you’ll direct the cutting forces into the center of the material, rather than pulling away from it,” he said. “This is especially helpful for reducing delamination.”

CFRP is tough stuff. Not only is it a tool killer, but you also might be left with a cracked, fuzzy mess of a workpiece. This is especially the case on microparts, where the unwanted side effects of CFRP cutting can become almost as large as the microscale workpiece itself.

If you’re faced with cutting CFRP, all you can do is get good cutting advice, keep your tools sharp and make sure your workpiece edges are clean. And, most importantly, remember not to step on Superman’s cape. µ