Cutting sand: tools for advanced ceramics

Ask any archaeologist excavating an ancient village and he will confirm that ceramics have been around a long time. Ceramic was first used for making pottery over 10,000 years ago. But today’s advanced, or “technical,” ceramics are being used in ways never imagined by the ancients. They have found their way into the military, medical, automotive, consumer and electronics industries.

In short, ceramics are everywhere.

But what is a technical ceramic, and what makes it different from that old robin-egg-blue bathroom tile hanging on the wall of your shower? With names like aluminum nitride, aluminum oxide, boron nitride, silicon carbide and zirconia, we must be talking about some pretty lofty stuff, right?

Ceramic cell-culturing chips after micromachining. Image courtesy Synkera Technologies.

Keith Costello, responsible for new business development at machine shop Ferro Ceramic Grinding Inc., Wakefield, Mass., explained that technical ceramics are engineered to provide a host of physical properties, including high density and dielectric strength, low water absorption and gas permeability, and excellent flexure, fracture, tensile and compressive strengths.

Costello noted that technical ceramics in powder form are readily cast or pressed into complex shapes, and, depending on the size of the finished product, can be injection-molded. They also can be extruded into rods or bars, grown into a crystalline form or deposited onto a substrate using chemical vapor deposition.

In their finished, or sintered, state, technical ceramics have several desirable properties. They’re chemically inert, making them corrosion resistant. They can withstand extreme temperatures and remain dimensionally stable. And they’re very, very hard.

Slow going

It is this last property that makes technical ceramics challenging to machine post-firing. Ken Sherwood, president of Kadco Ceramics, an Easton, Pa., job shop that specializes in making sintered ceramic components for the military and medical industries, pointed out that many of the products the shop processes are made from piezoceramics, which generate voltage under deformation.

Sherwood said machining technical ceramics is “a whole lot slower than metal” and recommends diamond tools and lots of coolant. “Use a light feed rate and a light depth of cut. You need to get the swarf out and reduce the heat on the workpiece. And you have to watch tolerances all the time.”

He cited one example where he applies a 0.100" diamond core drill to make a small square pocket in the face of a workpiece. “The feed rate here is a fraction of an inch per minute, and depth of cut is maybe a thousandth of an inch.” This means on a 1⁄8"-deep pocket, machining follows the same toolpath 100 times, with each pass just 0.001" deeper than the previous one. In short, machining fired ceramics is a painfully slow process.

Despite this, tool life can be quite good, and some cutters might last for years, Sherwood said. He added, laughing, “But then again, if you make a mistake, they might only last a minute.”

What sort of tool is needed to cut ceramics? It depends. According to Sherwood, only CVD-diamond-coated cutting tools will effectively cut post-fired ceramics. Depending on workpiece shape, this could mean CVD-diamond-coated core drills, endmills, grinding wheels or special form tools.

Explained Costello of Ferro Ceramics, the round tools typically have a flat cutting surface and act like a grinding wheel in that the exposed facets of the tool’s diamond plating are the actual cutting edges. These tool surfaces may be slotted to enhance the grinding operation by allowing coolant to be pulled into the cutting zone, which also helps to flush out swarf particles measuring 40µm and smaller.

Diamond grinding tool processing a sintered ceramic part. Image courtesy Kadco Ceramics.

One company offering diamond cutting tools is Harvey Tool Co. LLC, Rowley, Mass. The company offers three types: amorphous diamond coated, CVD diamond coated and PCD. Amorphous diamond is a smooth, lubricious layer applied microns thick to a carbide substrate. Because these tools have sharp cutting edges that resist wear, they are an excellent choice for cutting green ceramics. Harvey Tool’s amorphous-diamond-coated endmills are available down to 0.010" in diameter.

But, as previously mentioned, only CVD-diamond-coated tools are appropriate for machining fired ceramics. CVD diamond is “grown” on a low-cobalt-carbide substrate. This makes for a coating about 10µm thick and a less-sharp edge compared to one coated with amorphous diamond, but one that is suitable for the difficult task of machining fired ceramics. Harvey produces them in endmill sizes down to 0.015" in diameter.

Even then, warned Jeff Davis, vice president of engineering at Harvey Tool, “it’s a hairy proposition. I don’t know if anyone has a product line specifically for fired ceramics.” Despite this, Harvey Tool customers are having success using CVD tools.

Going green

It’s not all hard, however. Joe Brennan, president of CVD Diamond Corp., London, Ontario, said, “Green (unfired) ceramics are easy to machine, but they’re very abrasive. It’s a little like machining sand.” CVD Diamond manufactures CVD-diamond-coated cutters for machining green ceramics. “Our tools are made from tungsten carbide coated with a pure diamond film around 10µm thick, which can be applied on endmills down to 1⁄64" in diameter.” The diamond film extends wear rates when cutting green ceramics by a factor of 50, compared to uncoated tools, he said.

But it’s not all about the coating. “Our tools are designed for cutting ceramics,” Brennan added. “We specify the carbide as well as the geometry.”

When asked about feeds and speeds, Brennan said, “You have a broad range of cutting parameters with diamond-coated tools. They handle heat very well. The feed rate depends mainly on part geometry, as unfired ceramics have a tendency to crumble. You need a sharp tool, but you can run at spindle speeds maybe three to four times faster than you would with bare carbide, and can sometimes increase feed rates somewhat as well.”

Another company offering cutters for unfired ceramics is Kyocera Micro Tools, Costa Mesa, Calif. Kyocera produces endmills down to 0.005" in diameter and drills down to 0.0015" in diameter.

Scanning-electron-microscope image of micromachined structures in ceramic chips used for guided cell culturing. Image courtesy Synkera Technologies.

Jason Marsh, director of global operations for Kyocera’s KTC Division, cited a number of examples where the tools are being used. “One of our customers cuts a layered, silica-based ceramic for semiconductor manufacturing,” he said. “They use a multiflute router with a special point to drill down and mill pockets in the workpiece.”

Kyocera is also selling more ballnose endmills to cut implantable ceramic bone replacements and dental implants made from zirconia. When asked what speeds and feeds these shops are using, Marsh laughed. “This is an absurdly secretive business. There aren’t too many shops willing to share that information,” he said.

What’s not a secret is that technical ceramics are an integral part of our lives, and their use will continue to grow as engineers and part designers devise increasingly clever ways to exploit their unique properties. Considering the difficulties in machining them, toolmakers will have to move quickly to keep up. µ