by Jim Barnes
Abrasion, laminate stacks and proprietary products make composites a machining challenge
“When it comes to working with composites, you can pretty much take everything you’ve learned about cutting metals and throw it out the window,” says Andrew Gilpin, business development manager, Amamco Tool, Duncan, SC. It’s a strange new world for machinists used to working with metals. “There are no recipe books that tell you what parameters to use for a given piece of composite.”
Composites, including the widely used carbon-fibre reinforced polymer (CFRP), are spreading from their strong base in aerospace to specialty products in automotive, sports, marine and other applications. The reason is simple: for a given strength, a composite part might be 20 per cent the weight of steel.
“It’s a pretty mature sector. The rate of entry is fairly slow. People tend to start it with small jobs,” says David Vetrecin, product manager, holemaking, Iscar Tools, Oakville, ON. It’s an area where experience pays off and shops often develop processes by trial and error.
Tooling suppliers are eager to work on development programs. Often, “our customers and users actually do not use our tooling to its full potential,” says Paul Ratzki, marketing manager, Sumitomo Electric Carbide Inc., Mount Prospect, IL. In a course the firm hosts, users are asked to suggest feeds and speeds for a given application. “They were under-running the tool 90 per cent of the time,” he says.
The coating on the tool makes or breaks the machining operation. Several types are available, depending on the specific operation.
Uncoated carbide tools are used widely, especially for low volumes. A carbide tool is less expensive than a coated tool and can support a wide range of geometries. As well, it can readily be re-sharpened.
Diamond coatings rule the roost for composites.
CVD, or chemical vapour deposition technology, offers good wear resistance, conducts heat well, is chemically inert so it does not react with materials, has a low coefficient of friction, and offers more options in tool geometry. However, such coatings are thicker than PCDs, which may impact sharpness.
PCD, or polycrystalline diamond, is another popular option, offering a lot of wear resistance and sharpness at the tool edge.
The two most common approaches are brazed-in inserts (where the PCD is required only at the cutting edge) and PCD-coated tools.
As well, some manufacturers are at work on “full-nib” PCD, which has a larger, solid piece of PCD at the tip of the drill. Because the blank is much thicker, there are more options in geometry, notes Gilpin. Another approach is PCD “veins,” in which PCD is bonded into grooves in the carbide tool body to support more complex geometries.
Development is ongoing. Ratzki notes that his firm is at work on a new form of monocrystalline diamond that surpasses the performance level of PCD without jumping to the monocrystalline price level.
DLC, or diamond-like carbon, is another approach to making diamond-hard tool surfaces that is winning interest. Again, specifying a coating depends on the application. The best coating for your tool can likely be identified in a test lab.
Generally speaking, a tool geometry that minimizes cutting pressures works well with composites. However, there are a lot of variables in selecting a geometry. A geometry that worked well in an overhead gantry machine might not be a good choice for a hand-held application.
“Once we move into the CNC realm, everything is a little more rigid, and you can go to more conventional high performance tooling in that arena,” says Ratzki.
“We don’t know all the variables of the material, your machine, or your operator. Those are all factors that can influence the geometry,” says Gilpin. “[All the tool suppliers] know three or four geometries that are working pretty well right now. Which one is best? You have to test it.”
It gets more complicated. If you are drilling a laminate stack, the demands are stricter. For example, you might use a tapered drill with a long point angle to drill plastic. However, if the last layer in the stack is aluminum or titanium, a high-shear drill with a sharp point angle might be recommended. It will exit more cleanly and leave fewer burrs, according to information provided by Iscar.
Developing an effective process is a key to profitability. There is very little that is off-the-rack in the world of composites. In fact, every job you get might involve a different composite–a material whose composition might be a trade secret, notes Randy McEachern, product and application specialist, holemaking and tooling systems, Sandvik Coromant, Misssissauga, ON. Besides layers of metal, the laminate stack might include honeycomb composite, an additional machining challenge. Often, “standard” tools are just a starting point for these jobs.
Normally, you’ll get a test sample. “Sometimes, the material is so valuable or rare they won’t even give us the material,” says Gilpin. The composite supplier tests the tools themselves in their own facilities.
Size that attitude up against the “time is money” reality most shops work with. “Not everybody can afford to spend weeks or months in the test lab trying to figure out the best machining parameters. Everybody has to work to production schedules,” notes Gilpin. “A lot of times, they’ll drill a hole, they’ll look at it and say ‘that’s good enough.’ Then they go into full production.” Their machining parameters may or may not be the best parameters for that process. “If they went to or three more steps down the road, they’d be able to see a lot more value,” says Gilpin.
“If you’re not using a tool as it is intended, tool life goes way down. Productivity loss is a big problem,” notes Ratzki.SMT
Jim Barnes is a contributing editor. [email protected]