Cryogenic machining systems can extend tool life and reduce cycle times

Efficient removal of heat from the tool/workpiece interface is the main route to high-productivity machining. Several conventional methods are available to accomplish that—dry machining, minimum-quantity lubrication, flood coolant and high-pressure coolant—but cryogenic machining can be added to the list.

However, not all systems using cryogens take the same path to longer tool life and higher speeds and feeds. Presented here are three approaches, two of which use liquid nitrogen (LN2), while the other applies liquid CO2 mixed with air or an aerosol dry lubricant.

Watch for Flying Ice

At -321° F (-196° C), nitrogen is an effective, residue-free cryogen for combatting a cutting tool substrate’s worst enemy—heat. “If you don’t efficiently remove the heat, the result is thermal softening of the cutting tool and tool failure,” said Rana Ghosh, applications R&D manager for industrial gases at Air Products and Chemicals Inc. The Allentown, Pa., supplier of industrial gases developed the ICEFLY cryogen-delivery system and associated machining know-how to apply a cryogen at the cutting zone. The technology, which the company licenses, maintains a temperature from -250° F to 32° F (-157° to 0° C).

The system uses a proprietary coaxial, or tube-in-tube, geometry to flow high-pressure LN2 through the inner line and a low-pressure stream of LN2 through the outer line to minimize heat loss in the inner tube, Ghosh noted. The saturated, high-pressure LN2 stream is jetted through an external nozzle and immediately evaporates after impinging on the target surface, without leaving any residue.

According to 5ME, Lockheed Martin machined a medium-sized titanium (Ti6Al4V) aerospace structural component 52 percent faster with 5ME’s cryogenic machining technology than with coolant.

The ICEFLY cryogen-delivery system delivers nitrogen gas to cool the cutting edge.

One such nozzle, which can be retrofitted to virtually any vertical or horizontal machining center, is the Spidercool servodriven, programmable nozzle from Dimensional Control Inc., said Rick Knopf, owner of ICEFLY-licensee Industrial Cryogenic Technologies LLC, Macungie, Pa. The standard Spidercool, however, was redesigned with some new internal components to operate at such low temperatures without binding.

Ghosh noted ICEFLY is primarily targeted at five application areas: steel and iron harder than 45 HRC; sinter-hardened or heat-treated powdered metals; hard metal-matrix composites; cobalt chrome and other difficult materials for medical implants to avoid the use of traditional metalcutting fluids (long suspected as a source of contamination); and implants made of polymers, such as silicone, polyurethane and PEEK (polyether ether ketone).

When machining polymers, temperature control of the workpiece is key to prevent surface defects, such as cracking from overcooling and surface smearing and burr formation from undercooling, Knopf explained. Through optimal cooling of the uncut surface just prior to machining, the cryogen eliminates or greatly minimizes burr generation and enhances machinability. One example is a soft contact lens. “If you increase the modulus or stiffness of the workpiece surface, you can machine it,” he said. “Otherwise, you’re trying to machine a gummy bear.”

Unlike cutting metals, when machining polymers, the nitrogen stream is directed to the polymeric workpiece. To avoid overcooling and potentially introducing cracking when machined, Air Products developed a patented process that uses a mixture of liquid and gaseous nitrogen.

“We can dial in the optimal temperature of the fluid that needs to be jetted onto the material,” Ghosh noted, adding that the glass transition temperature of various polymers is a good starting point to establish cooling conditions with further optimization achieved through testing.

The cryogenic machining system from 5ME uses tube-in-tube, vacuum-jacketed feed lines to deliver LN2 from an external bulk storage tank to the cutting zone while protecting the integral machine components from being exposed to the low temperatures.

Applications expertise is a critical component of the cryomachining equation. For the majority of metalcutting, the focus is on cooling the tool and controlling the temperature and dimension of the workpiece, Ghosh explained. In situations involving workhardening materials or interrupted cutting, however, the focus is only on cooling the tool. For those cases, liquid nitrogen is injected from underneath the cutting tool. This enhances conductive cooling through the tool, while minimizing hardening of the unmachined work surfaces by overcooling.

According to Ghosh, cryogenic cooling can allow an end user to increase productivity 50 to 200 percent through a combination of higher cutting speeds and enabling the use of brittle cutting tool materials, such as alumina ceramics, in many applications where such tools cannot be used with flood coolant or when dry machining. However, greater gains are possible. In one wrought cobalt-chrome application, after switching from flood coolant and carbide tools to cryogenic machining and ceramic tools, the cutting speed went from 100 to 125 sfm (30.5 to 38.1 m/min.) to 550 to 700 sfm (167.6 to 213.4 m/min.).

Cryogenic machining also extends tool life, but manufacturing economics dictates that extended tool life shouldn’t be a primary goal. “Customers want to translate any tool life improvement into productivity gains because just having tool life improvement isn’t economical unless the tool material is PCBN or PCD,” Ghosh said.

Standard cutting tools are used, but Knopf pointed out that a major toolmaker purchased an ICEFLY system to conduct research on developing an insert grade specifically for cryogenic machining.

An Internal Approach

Liquid nitrogen can also be delivered through a machine tool spindle and a cutting tool directly to the cutting edge. This is the method Cincinnati-based 5ME LLC employs with its Cryo technology. The cryogenic machining system uses tube-in-tube, vacuum-jacketed feed lines to deliver LN2 from an external bulk storage tank, or dewar (vacuum flask), to the cutting zone while protecting the integral machine components from being exposed to the low temperatures, said Pete Tecos, executive vice president of marketing and product strategy.

Installation of the Cryo kit doesn’t involve removing the spindle. Instead, a lance, which also has the tube-in-tube, vacuum-jacketed design to keep the OD at ambient temperature, runs through the drawbar ID, noted Michael Judge, executive vice president of business development. “It’s very noninvasive.”

A vacuum-jacketed line, however, doesn’t run through a cutter body, so 5ME designed a line of indexable and solid-carbide cutting tools insulated with PTFE (polytetrafluoroethylene) to use with its cryogenic machining technology. “You cannot run the 5ME cryogenic machining technology system with non-5ME cryogenic tools,” Judge said. The company sells end users about 60 families of cutting tools, including endmills, facemills, high-feed mills, thread mills, drills and turning tools.

Cryo·tec tools from Walter have two cooling channels: one for CO₂ and one for an aerosol dry lubricant or air. The tools have an optimized diameter and CO₂ channel directions, as well as a tool shape designed to transport aerosol and air from the spindle through the tool body and to the insert.

Although 5ME is machine tool agnostic and willing to work with any builder, Judge noted the targeted machines are medium-sized horizontal machining centers and large vertical machining centers. This means an HMC with at least a 630mm (24.8 “) pallet size and a VMC with a large spindle, such as CAT 50, CAT 60 or HSK 100A. Targeting these machines helps ensure a reasonable payback because the Cryo hardware kit is about 30 percent of the cost for a new machine tool that size and larger.