October 2008 Edition

Cutting tools

No Material too Tough

Machining a difficult material, a shop thought it had the right cutting insert, until a manufacturer offered one that increased tool life by 400 percent

Austenitic stainless steel is a grade of material that creates fear in the hearts of manufacturers. Users have often reported machining difficulties with this material. Problems such as poor surface finish and high tool wear are common. Optimum setting of machining parameters, such as cutting speeds and feed rates, is critical with this material, especially in today’s high-volume production environment.

Marine Machining and Manufacturing, Clinton Township, MI, is a company that knows the difficulties of machining this metal. The premium material the company uses in its drive shafts is a difficult austenitic stainless steel called Aquamet. The properties that make this material ideal for boat drive shafts make it a machining challenge. Chip control is a problem as is wear. Inserts used on this material must be both tough and wear-resistant.

Brian Jenich, owner of Marine Machining, has been making and repairing boat drive shafts for almost 20 years. He started out by accident after high school.

"I had a boat, but I couldn’t afford to pay for the repairs, so I had to learn how to do them myself. I built a straightening press in my dad’s shop and started fixing shafts on the side."

Now, Jenich’s company is an industry leader in marine machining, supplying specialty marine products to ship builders in North America, including drive shafts used in Coast Guard ships.

Nothing to Worry About

Marine Machining tried every major insert brand, chip breaker, speed, and feed to machine the Aquamet, and chose what seemed like the best insert grade and chip breaker, maximized depth of cut, feed rate, and SFM.

"We were satisfied with what we were using," Jenich said, "especially as sales reps continued to fail to improve on tool performance. Most reps would spend a day or two testing and then give up."

Jeff Braun, technical specialist at Seco Tools, Troy, MI, showed up with a brochure on the company’s grade TP2500 and made some bold statements about what it could do, according to Jenich.

"We worked with Marine Machining about a year ago on some threading issues," Braun said. "We were able to improve the threading process on the same material, but could not duplicate that success on the general turning process."

Jenich agreed to let Seco test the TP2500 expecting the same results the other brands produced.

Braun suggested depth of cut, speed, and feed parameters for the insert.

"When I asked Jeff if he would pay for the holder when it broke, he just smiled and said ‘I don’t think we have anything to worry about,’ " Jenich said.

At 50 percent greater depth of cut on the rough turn, the job ran at 50 percent greater SFM and 150 percent greater FPR. Chip control was perfect and tool life was four times that of the previous best insert. The results on the finish turn were improvements as well: the Seco wiper doubled the feed rate and improved the finish, while increasing SFM by 50 percent and tool life by 400 percent.

Re-writing the Rules

TP2500 is Seco’s first Duratomic turning grade. By modifying the coating process, Seco learned how to control the crystal growth to improve both toughness and wear resistance.

The individual crystals that make up the coating can be "tilted" to bring a more favorable crystallographic direction into the cut. This structural alteration of the aluminum-oxide layer creates a coating with improved life and increased cutting capability, according to Braun.

Conventional aluminum oxide coatings have a hardness of about 27.5 GPa. Duratomic hardness is closer to 30.5 GPa, almost an 11 percent increase. This translates to an increase in abrasion resistance and tool life. The coating also runs cooler – 35° C in a typical application, enough to reduce the tendency for the insert to crater.

"I know I’m a tough sell," Jenich said, "but this is a good insert. Last year I outsourced a lot of work because I didn’t have the in-house capacity. I’ve been shopping for new machinery. With the productivity improvements offered by this grade I may be able to postpone that machine purchase." Seco Tools

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