Icing Hard Turning

Process developers report they have seen several improvements to surface quality with the IceFly process, while being able to maintain close tolerances.

Besides cooling the cutting tool, the IceFly process keeps workpieces chilled to specific temperatures.

Extremely low temperatures add strength and toughness to ceramic inserts.

A NEW HARD TURNING PROCESS IS about to debut, and its developers say it can knock rough grinding down to 15 minutes from as much as four hours. Moreover, the process is supposed to reduce tooling costs by 85 percent, while improving tolerances and the surface finish of the workpiece.

The process, developed by Hardinge Inc. (www.hardinge us.com) and Air Products and Chemicals Inc., introduces liquid nitrogen into the cut on turning operations. The liquid nitrogen cools the cutting tool to a very low temperature and checks temperature build-up in the workpiece, increasing the efficiency of the cutting operations to provide the huge benefits the companies say come from the process.

Called "IceFly," the process can be used to replace rough grinding operations to hold tolerances of 0.008 in. in roughing operations while increasing the depth of cut and spindle speed on materials that have hardness to 78 Rc, according to Ranajit Ghosh, technical leader for the IceFly process at Air Products.

Air Products (www.airproducts.com/Metals/default.htm) developed the process of applying liquid nitrogen to cutting operations about five years ago, Ghosh said in an interview. Using liquid nitrogen in cutting originally was developed to machine the heavy process rollers and back-up rollers used to make flat-rolled steel. Those rollers are made from hardened tool steels which are difficult to machine. Ghosh said his company's cooling process is being used on five machines in the U.S. to produce those rolls.

The partnership between Air Products and Hardinge is moving that process to wider markets and to smaller parts.

SMALL PARTS, BIG CHALLENGES
Providing cryogenic cooling to hard turning for a variety of parts is not simple. When extremely cold liquids are introduced to the machining process, the liquid tends to reach its boiling point quickly (in the case of nitrogen, a temperature of -320 degrees F or -196 degrees C). A gas layer forms between the liquid and the considerably warmer cutting tool and workpiece, where it acts as a heat shield.

Air Products found a solution to that problem, Ghosh said. "This process is not as simple as just pouring liquid nitrogen onto a workpiece. When you do that, you form the gas boundary layer, and that reduces the heat transfer efficiency, almost to the point of eliminating the ability to transfer heat (from the workpiece)."

Air Products developed a two-phase system with proprietary solid particles suspended in the liquid nitrogen. In combination with the velocity of the gas hitting the cutting tool and workpiece, the particles break up the gas boundary layer. The liquid nitrogen is applied to the cutting tool and workpiece through a nozzle that regulates its flow.

The nozzle is attached to the tool turret, and the liquid nitrogen is supplied to it from a reserve tank. It discharges the nitrogen directly onto the rake face of the cutting tool, Ghosh said.

BENEFITS
The cold liquid removes heat from the cutting process, keeping the workpiece at a constant temperature. Tom Sheehy, senior applications engineer for Hardinge, said keeping the workpiece at a constant temperature provides consistent cutting conditions, so that accurate tolerances can be maintained even at depths of cut into the workpiece that are deeper than usual.

Secondly, the nitrogen keeps the cutting tool extremely cold. Ghosh said cryogenic chilling changes the characteristics of ceramic cutting tools, making them harder and tougher. That allows ceramic cutting tools to be used to cut carbide materials with hardness ratings of more than 78 Rc with depth-of-cuts to 0.003 in. to 0.005 in., according to Sheehy. And, Sheehy added, the cryogenic chilling of the ceramic cutting inserts allows them to be used in interrupted cuts for those hard materials.

"Cryogenic cooling makes the ceramic tool stronger and tougher, so you can go from using a $100 cubic boron nitride tool insert to a $15 ceramic tool insert, and get better performance," Sheehy said.

A third processing benefit is that the cold nitrogen quenches the workpiece, improving surface hardness.

SURFACE IMPROVEMENTS
"We have seen improvements in the hardness of the surface and the subsurface, added compressive residual stress, and a reduced white layer," in machined parts, Ghosh said. He explained that the increase in compressive residual stress gives parts greater fatigue strength. "White" refers to a non-etching, brittle layer of heat-affected metal that is found at the surface of a machined part, which is believed to be produced in certain steel compositions, with certain machining conditions and cutting tools.

Increasing the compressive residual stress and reducing the white layer increases the wear-resistant and fatigue-resistant properties of the machined part, Ghosh said.

These benefits change the program for rough cutting hard materials, such as tool steels and irons or tungsten carbides and steels with high cobalt or chrome content. He says the process also can be used to machine porous materials such as powder metal products, and other hard-to-machine materials such as metal-matrix composites.

"There is no other process that can be applied to all of these applications," claims Ghosh.

Also, he adds that the process is environmentally friendly, and can be easily adapted as a retrofit to existing machines.

BETA TESTING
Air Products and Hardinge have put together a developmental lathe with the process at Air Products' facility in Allentown, Pa., where they are proving the concept as a beta-test, Jeff Thomason, manager for turnkey operations for Hardinge, said. "We see this as a process that can replace hard grinding," he notes.

Hardinge is looking at developing applications for the process in a variety of industries and for a variety of products but, because it deals so well with cutting extremely hard materials and exotic materials, the first applications may be in aerospace and medical markets or to produce tools from very hard materials. "This is not industry specific," Thomason says.

Thomason and Sheehy declined to talk about the specific costs to install and use the process, but they said costs would be "application specific." Also, the expected return-on-investment for the process would be application specific, Thomason said.

"This could have a huge payback immediately, if only because of the increased tool life and longer insert wear," he observed. However, Sheehy added that Hardinge has seen the process reduce the time needed to rough grind a part from four hours to 15 minutes, nearly a 95 percent reduction in roughing time. And, he said the turned part maintained the same tolerance as the part that was rough ground. Finally, he said in that job, the IceFly process used an inexpensive ceramic cutting insert, not a CBN tool.

"There is an added cost for using liquid nitrogen, but that is entirely offset by the reduction in cycle time and the lower cost of tooling," Sheehy said.

Ghosh said Air Products holds or has applied for nine patents related to the IceFly process, and that those patents cover both the process and the equipment the company has developed for the process. Thomason said the process is now available commercially for hard turning operations.

Ghosh said he believes the liquid nitrogen process can be developed for additional machining operations, such as milling, but development work needs to be done to accomplish that.