August 2007 Edition

MACHINING

The Coolest Machining Around

Cryogenic machining, using liquid nitrogen at -320° F is a cutting-edge technology in more ways than one

Cryogenic machining started as a laboratory curiosity and has found application on the shop floor. It offers a number of advantages and has moved from the exotic to merely the out-of-the-ordinary.

Using -320°F liquid nitrogen as a coolant – instead of water, oil, synthetic coolant, or dry machining – was seen in academic and research circles as an extreme way of lowering cutting zone heat, especially when working with hard-to-turn materials. The challenge was to bring the technology out of the lab and onto the shop floor.

Tom Sheehy, team leader and applications engineering at Hardinge Inc., Elmira, NY, is a member of the group working on Project Icefly. Icefly is cryogenic hard turning on Hardinge Quest CNC turning centers.

“We were presented with a carbide part that the company’s European subsidiary had been trying to cut,” Sheehy said. “The part was 11 percent cobalt, roughly 8.5" long and 2.75" in diameter. The part was rough ground and then finish ground. It took approximately four hours for the rough-grinding operation. Using hard turning would save time, but they didn’t have much success.”

American Ingenuity

“The Europeans tried everything to turn the part. The best they could do was hard turn approximately 2" on the 8.5" piece, before the tool would disintegrate. We put it on a Hardinge Quest 8/51 with Icefly. With a little testing and adjusting the speed and feeds, we were able to take six complete passes down the length of the part for about 48", with only one edge of the insert.”

The Icefly team achieved 80 sfm with each pass taking about four minutes.

“If we were trying to take 0.0006" off,” Sheehy said, “that would be three passes. About 12 to 15 minutes hard-turning time. A significant savings compared to four hours of rough grinding and then the additional finish-grinding operation.”

In selected applications, cryogenic machining decreases machining time, reducing cycle time and costs. Using liquid nitrogen coolant, a $15 off-the-shelf, four-tipped ceramic insert can replace a $50 carbide-tipped insert. Icefly technology makes the ceramic insert stronger through the introduction of liquid nitrogen at the cutting zone.

A test of liquid nitrogen cooling for another customer involved running a 0.5" round radius with a 0.25" diameter tool. The part had a hardness of 75 R_c. Fifteen minutes of Icefly machining replaced four hours of grinding.

Time = Money

As the epigram states: Time is money. In a shop, machine time is even more money. Shops are under pressure to increase productivity and reduce costs. Creating savings is difficult when working with hard-to-machine materials, such as hardened steel, wear-resistant alloys, and tungsten-carbide parts. Icefly can reduce cutting time, increase tool life, and increase productivity.

Cubic boron nitride – CBN – and polycrystalline cubic boron nitride – PCBN – inserts are the tools of choice for hard turning applications, but may look cost-prohibitive at first glance. The Icefly coolant system offers longer insert life, faster cutting rates, and more affordable hard-turning insert options.

Air Products, Lehigh Valley, PA, originally developed the Icefly cryogenic coolant system. The Project Icefly team at Hardinge refined the system for delivery to users. The system provides a jet of -320°F liquid nitrogen directly to the insert during turning operations. The liquid nitrogen raises insert hardness, which significantly reduces the thermal softening effect that an insert may experience as a result of hard turning’s inherent high cutting temperatures.

The steep temperature gradient between the chip/tool interface and insert body also helps remove heat from the cutting zone. The cooling maintains insert edge integrity to prevent “smearing” a part’s hot, compressed surface layer, providing a quality surface finish. Liquid nitrogen insert cooling extends insert life and allows use of low-cost ceramic inserts for hard turning operations.

200 Percent Increase

Unlike CBN and PCBN, ceramic inserts tend to wear unevenly and are prone to fracturing when hard turning dry or with water- or oil-based coolants. Increased fracture toughness resulting from low-temperature liquid nitrogen cooling provides gradual, more predictable flank wear for ceramic inserts. It also increases cutting speeds up to 200 percent. Predictable flank wear permits the use of alumina ceramic inserts in critical finishing operations, instead of PCBN inserts.

The advantages of the Icefly system include:

• Increasing cutting speed and material removal rates as much as 200 percent;
• Increasing tool life up to 250 percent;
• Reducing capital expenditures, labor, tool replacement, and inventory costs;
• Enhancing chip removal from the cutting area and eliminating oily residue on machines, parts, and chips;
• Improving cutting reliability with ceramic inserts while maintaining desired dimensional tolerances; and
• Improving the part quality by preventing mechanical and chemical degradation of machined surfaces.

Icefly is an option on Hardinge Quest turning centers. Liquid nitrogen can be stored in a small, dedicated cylinder near a machine, or a supply tank serving multiple machines. Programming is similar to a traditional coolant delivery system. A flexible line attaches to the machine’s turret via a rotational coupling. A delivery nozzle clamped to the tool directs the liquid nitrogen to the insert tip.

The system works with hardened steels, hard composites, and powder metal parts. Because the relatively inert nitrogen vaporizes after contact with the insert, it leaves no residue. This is an advantage for porous powder metal parts, which often require cleaning to remove coolant residue.

“Stay Cool, Boy”

The Icefly machining system aims a small amount of liquid nitrogen directly at the cutting insert engaged in material removal. The jet of low-temperature liquid removes heat without affecting the workpiece dimensions. Tool wear rates are lowered, even at high cutting speeds. The Icefly system reduces ceramic insert fractures.

Nitrogen is a noncombustible, noncorrosive gas. It makes up 79 percent of the Earth’s atmosphere. It is colorless, odorless, and tasteless, often used as an inert gas due to its relatively nonreactive nature with many materials.

Liquid nitrogen is a commonly used cryogenic fluid, used in food freezing and preservation, plastic and rubber grinding, inerting and blanketing, heat treating atmospheres, and tire inflation. The liquid nitrogen used in the Icefly machining system also evaporates, leaving no residue to contaminate the workpiece, chips, machine tool, or operator and there is no disposal cost.

“Once we understood the process and subtleties,” Sheehy said, “we learned a lot about this technology. I’m a believer after seeing carbide cut. Next is investigating cryogenic fluid in turning hardened ceramics on a lathe.” Hardinge, Inc.

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