One of the first lessons any machinist learns is that sparks and flames in the machining area are bad—as in, push the big red button and run for the fire extinguisher is bad. And while this is generally sound advice, there are situations where a stream of red-hot chips flying from the workpiece is not only preferred, but is a prerequisite for productive machining. Welcome to the world of ceramic cutting tools.
One such “hotter is better” success story comes from Greenleaf Corp., Saegertown, Pa., where a customer reportedly reduced cycle time by an incredible 97 percent when milling a block of Inconel 625. “Previously, it required three solid-carbide end mills and more than 90 minutes to machine the part,” said Martin Dillaman, applications engineering manager for Greenleaf. “After switching to our XSYTIN-360 solid-ceramic end mill, it took just three minutes.”
Ceramic cutting tools are nothing new. As the University of Florida’s E. Dow Whitney explained in the book “Comprehensive Hard Materials,” the first ceramics began to appear in the early 1930s, when machinists used them to turn gray cast irons. Yet due to “the difficult manufacturing process of ceramics combined with unsuitable machine tools and lack of experience,” it would take another four decades of development work before these advanced cutting tools would begin seeing limited use in hard-turning applications.
Since then, ceramic cutting tool use has grown, and for many shops they’ve become the go-to solution for turning and milling alike. Cast irons, hardened steels, and—as Dillaman pointed out— Inconel, Hastelloy, and other heat-resistant superalloys (HRSAs) are all fair game for ceramic cutting tools. And while most don’t provide the 97 percent reduction in cycle time cited earlier, many come close, with metal removal rates far greater than that of their solid-carbide equivalents.
With productivity gains like this, it begs the question: why aren’t ceramic cutting tools more popular with machine shops? The answer, according to Dillaman and the other experts interviewed for this article, is that a) ceramic cutting tools are primarily limited to the tough, hard, or abrasive metals just listed, b) they require the use of very rigid machine tools and tooling, and c) they call for very high cutting speeds and feed rates, application parameters that some machine tools are incapable of achieving.
It’s this last point that Greenleaf has addressed recently with its XSYTIN-360 solid-ceramic end mill lineup, which, as Dillaman noted, can be applied at 20 to 25 percent lower cutting speeds than needed with traditional ceramics. “Thanks to its ability to better control heat, you don’t need to have as elevated of a temperature as you would with an aluminum oxide or whisker-reinforced ceramic,” he said. “For someone milling with a 3/8 or 5/16"- [9.53 or 7.94-mm] diameter cutter, it means running 20,000 vs 25,000 rpm or higher. The lower spindle speeds can make a big price difference for someone looking at a new piece of equipment.”
Fast spindle or no, Dillaman suggested that any CNC lathe or machining center equipped with ceramic cutting tools should be rigid and well-maintained, as should its workholding. Any vibration will have an adverse effect on ceramic inserts or solid tools, so success often depends on avoiding older equipment, where spindle bearings and way surfaces may have “just enough play” to create problems with tool life and unexpected failure.
“When you’re applying the higher speeds and feeds associated with ceramics, it definitely illustrates any flaws in the mechanical system,” he said. “In fact, we often see dramatic performance differences from one machine to the other, even though we’re running the exact same parameters and the machines themselves are otherwise identical.”
Luke Pollock, product manager at Walter USA LLC, Waukesha, Wis., also has some relatively new grades in his ceramic lineup. These include the MC275 and MC075 Prototyp milling cutters, both available in brazed head or modular “ConeFit” configurations, and both designed for roughing of nickel-based superalloys. Whether using Walter’s end mills or a competing brand, however, Pollock suggested that what’s needed most is a change in mindset. “Depending on the alloy, you might be running at cutting speeds of 3,000 to 4,000 feet per minute, and you’ll be doing so without coolant,” he said. “That means you’ll have this big fireball inside the machine; if it’s the first time you’ve seen it, you’ll probably be scrambling for the stop button. And instead of getting 45 minutes from an end mill, you might need to replace it after 10 minutes or even less, during which it will wear perhaps 0.02" [0.51 mm] or more on the diameter. It’s just a completely different experience than machining with carbide.”
Different or not, all of this is perfectly okay, he added, because once machinists get past the initial learning curve, they’ll quickly discover that they’re removing eight to 10 times more material per minute than they did with carbide. That said, it’s important to monitor cutting tools, whatever they’re made of. Pollock noted that, in the case of ceramic, an end mill might look “completely burnt up” but will still cut well.
Rather than changing tools based on appearance, operators should instead listen to the machine and watch the load meters for clues as to when a ceramic tool is ready for the recycling bin. “It’s surprising how much they can wear and continue cutting, but it’s important to pull them out before catastrophic failure occurs,” he said.
Justin Messerschmidt, technical manager of cutting tools at Ceratizit USA Inc., Warren, Mich., agreed on ceramic’s ability to withstand elevated cutting speeds and commensurately high feed rates. It is nowhere near as forgiving as carbide, however, and will generally not tolerate any vibration. Furthermore, ceramic must be applied within the manufacturer’s relatively strict operational parameters, or all bets are off.
“Ceramic’s ability to maintain edge strength at high temperatures is phenomenal, but since it is significantly more brittle than carbide, it might not be the best choice for interrupted cuts, or on less capable machine tools,” said Messerschmidt.
Despite these warnings, Product Manager for Cutting Tools Robert Bokram offered some tips for less than optimal machining situations. “Ceratizit’s very active in the automotive market, where castings and forgings are common,” he said. “Here, it’s important to get under the ‘skin’ on the first pass, and then vary the depth of cut on subsequent passes, so as to spread the wear evenly across the insert as much as possible. We also recommend using as large a nose radius as possible with ceramics, and try to maintain consistent tool pressure to minimize shock.”
Like Pollock, Bokram noted that catastrophic failure is more likely with ceramic due to its brittleness, and that operators should routinely inspect for wear until a stable process has been achieved. Some of this stability depends on good toolholder hygiene—thoroughly clean the pocket when changing inserts, replace worn clamps and shims sooner rather than later, and always use a torque wrench to tighten toolholder screws.
“Stability is critical with ceramic inserts, but they generally don’t have holes in them, and must be clamped differently than carbide,” Bokram said. “So whether it’s a top clamp or a wedge clamp, there’s some sort of claw design to hold it securely in place. But because the chips coming off the workpiece are so hot and abrasive, we’ve gone to an oversized carbide top clamp to prevent the premature wear that can occur with steel clamping hardware.”
High heat can cause other problems, especially since most ceramic machining is performed without cutting fluid. For instance, shops should always use a dust collection system to remove the smoke and small metal fines that result from dry machining—whatever the cutting tool or workpiece material—and it’s a good idea to have a fire extinguisher handy.
This last bit of advice became abundantly clear during a recent visit Bokram made to a shop that wanted to test a ceramic milling application in Inconel. The problem was they’d used cutting fluid on a different job the day earlier, and the dried coolant residue on the machine surfaces actually caught fire after coming in contact with red hot chips. “Be sure to wipe down the machine tool before cutting with ceramics, and make sure there’s nothing flammable around the work area,” he said.
Lothar Unglaub doesn’t necessarily agree on the “dry only” approach. The director of marketing portfolio management for inserts at Kennametal Europe GmbH, Rheinfall, Switzerland, he’s seen great success with high-temperature alloys and ceramics, but only when applied properly. “Cast iron machining is always done dry, but with aerospace materials, some customers have achieved up to ten times the tool life by completely flooding the cutting zone with high-pressure coolant,” he said. “The only caveat is that there must be no gaps in fluid coverage; otherwise thermal shock can occur, leading to rapid tool failure.”
Unglaub likens ceramic cutting tools to coffee cups, in that it’s possible to apply immense pressure from one direction with relative impunity, yet press from multiple sides simultaneously and ceramic will fail. This is why a rigid setup is so important, as it eliminates the vibration that destroys most cutting tools, but especially those composed of ceramic. To this end, always use the shortest tool possible, which is good advice no matter what you’re cutting or how you’re cutting it.
Toolpaths are similarly important, he said, recommending that certain programming methods be applied when machining with ceramic tools. For example, anyone attempting to mill a slot in a single pass or otherwise employ a “hogging” approach will generally not be successful. Instead, it’s far better to follow a trochoidal milling strategy, using smaller, shallower depths of cut but at greatly elevated feeds and speeds relative to carbide, therefore removing significantly more material in the same amount of time.
Other sources interviewed for this article seconded the idea, adding that shops should arc into and out of the workpiece to reduce shock, avoid burying tools in the corner, and do whatever is necessary to eliminate chip recutting—in most cases, this is best accomplished with a continuous compressed air blast. Unglaub also noted that negative rake tools are preferred in 90 percent of all ceramic machining applications, since these provide the greatest edge stability. Finally, train machine operators, not only in how to apply ceramic cutting tools, but also in what to expect when they push cycle-start.
“Every time I work with a new customer, they’re shocked when the sparks begin to fly,” said Unglaub. “They immediately try to reduce the speeds and feeds because they feel something is wrong. This, however, is the worst thing you can do, because ceramic has a fairly narrow operating range. So, as I and others have said, take care to eliminate any vibration and overhang, apply the recommended cutting parameters and toolpaths, and be especially careful to clean the insert pocket and apply the correct amount of torque when clamping the tool. These are the key points to assure success.”
Jason Wells is the president and chief executive officer of Kyocera SGS Tech Hub in Danville Va., the newly formed research and training arm of Kyocera SGS Precision Tools Inc., Cuyahoga Falls, Ohio. He is quick to point out that, while parent company Kyocera has extensive experience with ceramic turning tools, SGS has historically focused its efforts on carbide, and that ceramic round tools and milling applications have recently become fertile ground for this 70-year old company.
Said Wells, “We see tremendous potential for ceramic cutting tools in the aerospace and automotive markets, so we have partnered with several different organizations—NASA, Airbus, Siemens, and CCAM (the Commonwealth Center for Advanced Manufacturing in Richmond, Virginia) among them—to do research into advanced ceramics and determine the optimal geometries, grades, and coatings needed for maximum performance.”
Early results illustrate the importance of using the right equipment and tooling for ceramic machining—in one instance, a grade that “failed miserably” on a commodity machine worked perfectly well on one with higher performance characteristics, reinforcing what Greenleaf’s Dillaman said at the outset of this article. They’ve also found that cutter paths play a similarly crucial role in tool longevity, and that efficient heat evacuation is equally important.
To address this last point, Kyocera SGS has been working with Japanese tooling manufacturer MST on a special “vented” toolholder specifically for ceramic round tools. “By their very nature, high-temp alloys have low thermal conductivity, so heat build-up is a real concern,” Wells said. “The toolholder’s unique design is intended to eliminate the thermal growth in the Z axis that may occur due to the machine spindle absorbing excess heat during milling operations, while at the same time securely and accurately gripping the tool. It’s a great example of the efforts that we and other manufacturers are making in this area, as well as the opportunities that exist with ceramic cutting tools.”
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