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END USER: Bear Manufacturing LLC, (832) 272-1245. CHALLENGE: Eliminate insert corner failure when ID turning Inconel 725 parts. SOLUTION: An advanced carbide insert grade and geometry. SOLUTION PROVIDER: Sandvik Coromant Co., (800) SANDVIK, www.sandvik.coromant.com/us

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Machined metal parts for the offshore oil industry must be made of materials that withstand the high pressures they're subjected to in the subsea environment. "They may have to go down a mile of water before they even hit the ground and start drilling," said Mark Bear, owner of Bear Manufacturing LLC. The Houston job shop has specialized in the offshore, down-hole oilfield industry for the 7 years it has been in business.

Such a demanding environment often requires parts made of difficult-to-machine, heat-resistant, high-nickel superalloys, such as Inconel 625, 718 and 725. One such part was an Inconel 725 lower bonnet, and Bear Manufacturing was tasked with producing about 1,000 of them.

According to Bear, the biggest challenge was ID turning, in part because the workpiece tends to generate long, stringy chips that promote tool failure as they collect in the hole. "You're flirting with disaster when Inconel starts stringing up," he said.

ID turning involved roughing on a Mori Seiki SL-4 CNC lathe and finishing on the lower-horsepower SL-2 lathe. The 2¼ "-deep ID starts at 17⁄8 " and finishes at 11⁄8 " after roughing. Unfortunately, rough ID turning the Inconel 725 pushed the corner off of the CNMG carbide inserts Bear was applying. "It would crush them," he said.

Courtesy of Sandvik Coromant

When ID turning Inconel 725, Bear Manufacturing found Sandvik Coromant's grade-4225 CNMG 431QM carbide insert achieves the best results.

The amount of machine downtime varied according to whether Bear had to simply index the insert or take the tool apart to replace a broken carbide shim and, possibly, a bent pin. "Sometimes, we got pretty drastic failure," Bear said, noting that it might take 15 to 20 minutes to repair a tool before machining resumed.

Bear experimented with feed rates to get the inserts to wear rather than break, but the results were unacceptable.

"When I first started this job, I was, 'Oh my gosh, there's no way I'm going to be profitable making these parts unless I find something that works,'" he said. "I was going through inserts left and right. Basically, my goal was to finish roughing one whole ID with one corner at the maximum surface footage and feed rate."

Bear called Sandvik Coromant Co. because he had previously worked with the Fair Lawn, N.J.-based toolmaker. Jerry Guerra, Sandvik Coromant's productivity engineer in Houston, brought eight different newly developed insert grades for turning high-temperature alloys and spent a couple days running them for ID and OD operations.

The insert with the best results was the grade-4225 CNMG 431QM carbide insert. "The QM chipbreaker broke the chips into really nice sixes and nines," Bear said, adding that the CNMG 431SM in grade 4225 showed similar results. "In this material, that's like a dream come true."

The QM geometry provides sufficient edge strength when machining difficult-to-cut materials while also enabling effective chip control, according to Sandvik Coromant. Although Inconel is typically cut with fine- or extra fine-grain carbide tools having a hard PVD coating, the grade-4225 medium-grain carbide, CVD-coated insert with QM geometry and a gradient zone proved effective. In addition, compared to grade 4225's 12 percent cobalt content, typically applied PVD-coated grades have 10 percent cobalt. As a result, they do not perform well when cutting difficult-to-machine materials, partly because of their lower toughness, the toolmaker stated.

With the new insert, Bear is able to rough turn a part's bore without insert corner failure at up to a 140-sfm cutting speed, a 0.008-ipr feed rate and a 0.080 " DOC, with a run time of about 8 minutes. The predictable run time eliminated worrying about stopping the machine to fix a damaged insert and tool body. "It freed me to do something else with my time besides having to sit there and baby the machine because I wasn't sure if a corner was going to last or not," he said.