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For a busy aerospace machine shop, parting-off heat-resistant super alloy (HRSA) forgings was once a time-consuming job that risked expensive workpieces.
Multax Corp. is a subcontractor to major manufacturers of aircraft engines and industrial gas turbines. “We cut several different superalloys – Rene 41, Inconel, etc.,” explains Don Woerner, machining supervisor. Part-off operations on the company’s Mori-Seki horizontal lathe routinely cut tube-shaped forgings into shorter lengths for subsequent machining. “Depending on the job, we can get up to 14 pieces from a tube,” says Woerner. “It’s cheaper than forging individual parts.” Inconel Challenges Parting-off large flange rings from an Inconel 718 forging with conventional carbide inserts and knife-like grooving tools nevertheless posed recurring production challenges for Multax machinists. Two rings, 25" outside diameter and 23" inside, are separated from a single workpiece about 5" long in two sequential grooving steps. The forging is first grooved from the outside with one tool cutting about half-way through the 1" thick wall. An identical tool on a boring bar then completes the cut from the inside. Turning hard HRSA’s generates high heat that quickly deforms general-purpose cutting inserts. Super alloys are also often “gummy” when compared to common steels, contributing to built-up edge and faster insert wear. Inconel makes grooving especially difficult with an irregular microstructure. Overall, Inconel 718 has a moderate 320 Brinell hardness, but variations in the grain structure create hard spots. “Exotic materials vary from heat lot to heat lot,” observes Woerner. “One lot will run nicely while we will struggle with the next.” To part-off the difficult alloy with conventional grooving tools, Multax machinists turned the rings at just 95 sfpm surface speed and 0.003 ipr feed. Yet despite the slow speed and shallow feed, parting the Inconel forging took a toll on grooving inserts and toolholders. Cutting edges on the original “dogbone” insert lasted no longer than a single inside or outside groove. Worse, the thin, knife-like holders were prone to sudden failure. “The same grooving tool worked okay on stainless,” says Woerner. “On the Inconel, we were breaking a lot of blades.” Multax typically cuts 40 forgings at a time and, each batch was stopped once or twice by broken toolholders. Expensive Bottleneck Each toolholder cost about $70, and replacing a broken grooving knife wasted about 30 minutes of costly machine time. Potentially far more expensive, a broken parting tool could inject unwanted carbide and machining stresses into a critical aerospace component worth several thousand dollars. Together, slow speeds, shallow feeds, and frequent tool replacements made the parting operation an expensive bottleneck. “We’re always looking for cost savings, and tooling cost savings are a big objective. I had to look around for something better,” says Woerner. Hard, Hot, and Sharp Discussions with Sandvik Coromant Field Representative Robert Page quickly centered on rigid CoroCut grooving tools and Grade GC1005 cutting inserts. Both are concept tools optimized for specific operations and materials. “GC1005 was a brand-new grade made to cut HRSAs and stainless steels,” explains Page. “I knew it could give Multax more machine time and reduce part cost.”
While better-cutting inserts promised to increase feeds and speeds for more productive parting, they needed better holders to overcome the stresses of machining HRSAs. Rigid CoroCut toolholders are designed to withstand the high axial cutting forces encountered when grooving difficult materials. Rigid rails on the tool body prevent deflection. The strength and stability of the CoroCut toolholder enables machine operators to maximize grooving feeds and speeds without vibration. Faster Cut, More Parts Multax machinists established feeds and speeds for the new inserts and holders parting off the Inconel rings. The concept tools permitted a surface speed of 203 sfpm, more than twice that possible with the original grooving tools. Depth of feed was 0.005 ipr, two-thirds greater than the original. Cumulative cutting time fell from 25.3 minutes per forging to just 8.2 minutes. Using two cutting edges per insert, machine operators index the cutting edges between finished parts. With non-cutting steps unchanged, part cycle time fell 70%, from 41.3 minutes to 24.2 minutes. To validate the results, Woerner used the new grooving tools on two successive heat lots of Inconel rings. “Both times, they cut the forgings like butter,” he says. Significantly, the extra-strong concept tools with their free-flowing chips completed two batches of parts without once stopping production for a broken tool holder. “Don’t forget it takes a half hour to change a blade, so there’s a major time savings in itself.” While insert edge life was unchanged on the outside diameter cut, it tripled on the inside diameter. “Overall, I’ve cut my insert usage per part in half,” adds Woerner. Page is quick to point out the real measure of cutting tool productivity is throughput, not edge life. “Rule-of-thumb says just 3% of machining costs are attributed to tool cost,” he says. “The real payoff is getting more good parts faster.” Faster part-off on the Inconel rings, reduced tool cost, and more spindle time are projected to save Multax over $10,000 a year. Free Machining Faster cutting with free-flowing chips also made the groover’s messy job safer and easier. Woerner observes, “You still have to keep someone there to watch, but the amount of time to clear chips is a lot less. You can’t get away from it entirely, but it’s about half of what it used to be.” The success on the Inconel rings led to the same tools being tried on similar components made of another high-nickel superalloy even “gummier” than Inconel. The ring measures 35" in diameter, with Multax parts-off six pieces from each tube. While conventional tools could groove the forging at just 120 sfpm with 0.0035 ipr feed, Sandvik CoroCut tools with GC1005 inserts groove at 220 sfpm and 0.004 ipr. Thinner components are also more vulnerable to machining forces, and Woerner concludes the new Sandvik concept tooling should induce less stress. “I just feel this is a lot more free machining,” he says. “That means I’m getting better parts, faster.” Sandvik Coromant - September 2002 |
Multax Corp. (Morton, IL) found advanced “concept” grooving tools could trim cycle time 70% on one large Inconel component and eliminate unpredictable tool failures. With the help of a tool supplier, the company recovered machining time worth over $10,000 a year on one part. In addition, the combination of rigid CoroCut grooving tools and GC1005 coated, carbide cutting inserts, optimized for HRSAs, brought dramatic savings to other components.
The GC1005 cutting insert protects an extremely hard carbide substrate with a wear-resistant titanium aluminum nitride coating. The extra-thick, physical vapor deposited coating maximizes heat resistance to prevent plastic deformation of the ground grooving edge. In addition, the GC1005 insert incorporates a sharp Sandvik GF-style chip-breaking geometry to shear clean chips from the sticky superalloy and prevent stringers.