If you’re trying to optimize metalcutting speeds, especially in difficult-to-machine materials, “Heat transfer is the name of the game.” That’s the summary offered up by Ranajit Ghosh, manager of applications R&D and growth for Air Products (Allentown, PA). “Most heat from a machining operation exits through the chip, but some transfers to the cutting tool, toolholder, machine spindle and workpiece. Heat buildup in the cutting tool causes thermal softening of the edge, which is why we often have to employ coolant.”
But what if you can’t use conventional coolant? Or it doesn’t do enough? Or you need to harden the part? Cryogenic cooling may be the answer.
Ghosh explained that removing heat from machining operations occurs through two primary transfer modes: convective and conductive cooling. “Convective cooling occurs through the ambient air or as a result of fluid coolant, which could be compressed air, aqueous or oil-based coolants, or cryogenic gases. Heat removal occurs [due to] the temperature difference between the coolant temperature and that of the hot cutting zone.
“Conductive cooling occurs through the tool as a result of the temperature difference between the hot cutting zone and the cooler back end of the tool,” he continued. “The back end acts as a heat sink to remove heat from the cutting area.” With conventional coolant, this effect is relatively small. But applying cryogenic coolant can reduce the bulk tool temperature to −200°F (−129°C) and lower, and the resulting temperature gradient between the cutting edge and the body of the tool is huge.
As stated above, the primary benefit of improved cooling is the increase in cutting speeds and feeds. Cryogenic cooling can greatly improve cycle times when cutting hardened steels and irons (over RC 50), powdered metals, hard-metal matrix composites, medical alloys such as cobalt-chromium, aerospace superalloys, and polymers such as silicone and PEEK. Speed improvements can be on the order of 40%–400%. Beyond that, cryogenic cooling (at least with liquid nitrogen) is environmentally friendly and reduces or eliminates the need for post-machining clean-up. It even improves the machined surface, yielding a smoother finish, better surface and subsurface hardness, improved wear resistance, and higher compressive residual stresses.
“Cryogenic machining also improves the white layer,” explained Rick Knopf, managing partner of Industrial CryoTech (Macungie, PA). “When you cut, heat is generated at the surface of the part, and the molecules realign as the part cools. Studies show that there is a better realignment in cryogenic machining, resulting in a smoother, harder surface. It may be just microns, but the aerospace industry and others should be embracing this because the improved finish will yield a significantly longer wear cycle.” (For more on this, Knopf pointed to studies conducted by I.S. Jawahir at the University of Kentucky.)
Sometimes, cryogenic machining solves more unusual problems. Knopf recounted a large aerospace shop that could not break chips formed when machining material containing niobium. “The chip grew as long as they were in the cut, up to hundreds of yards long. Nitrogen cooling enabled a small increase in feed rate—enough to break the chip every 2′ [0.61 m]. The insert would have disintegrated at that higher feed rate without cryo cooling.”
Knopf also pointed to a request from a Lockheed plant to use cryogenic cooling for machining aluminum. The goal wasn’t solving a tough metal cutting problem—it simply wanted to eliminate coolant.
Option 1: External Liquid Nitrogen
The two leading approaches to cryogenic cooling both use −320°F (−196°C) liquid nitrogen (LN2) and technology licensed from Air Products. Let’s start with an approach that introduces LN2 externally: Industrial CryoTech’s ICEFLY. Depending on the application, the ICEFLY system uses a proprietary nozzle to deliver a jet of LN2 into the cutting zone (or part) or a mixture of gaseous and liquid nitrogen. The “jetting” of the LN2 is important because, oddly enough, if LN2 were applied at low or ambient pressure, the boundary film effect (aka the Liedenfrost phenomenon) would result in extremely poor heat transfer. (Gaseous nitrogen would immediately form on the surface of the tool and part, preventing the super-cold LN2 from removing the heat.)
“As the temperature difference between the coolant and the target surface narrows, the heat transfer improves because of change from film to nucleate boiling,” said Ghosh. “The Liedenfrost effect also occurs with conventional flood cooling, but to a lesser extent, because of a smaller Delta T. As such, the heat transfer coefficient with conventional flood cooling is significantly better than with ambient pressure cryogenic cooling. The ICEFLY system counteracts the boundary film effect by jetting LN2 into the cutting zone to minimize the thickness of the boundary layer and create more turbulence for a higher rate of heat transfer. As a result, the heat transfer rate with ICEFLY is higher than with flood cooling.”
The LN2 source can be either a factory’s central supply or a standalone dewar. From there, a jacketed line carries the LN2 to the ICEFLY unit, usually mounted in the machine. “Then a vacuum jacketed line carries it to the end-of-line box,” said Knopf. “The end-of-line box has solenoids to turn the flow on and off at the point of use. Inside the machine, the lines run down the side of the toolholder to deliver coolant at the spindle nose.
“That way, our nitrogen is not going through the spindle and we don’t have to worry about freezing bearings,” he continued. “And you can retain your through-spindle coolant system and even use both on the same part.” One example is roughing using LN2 to improve material removal, then finishing with conventional flood coolant. Another option is running standard coolant on one tool and nitrogen on another.
“We sell these systems like you would a bar feeder or chip conveyer,” said Knopf. “We attach it to your machine for an upfront cost of $40,000–$50,000, with installation. We also rent units for evaluation.” Consumable costs are modest. ICEFLY uses 0.5 L of liquid nitrogen per minute, so a typical 240-L dewar would cover an eight-hour shift of continuous machining at a cost of $90–$120, depending on location.
Machining Polymers with ICEFLY
The ICEFLY system can vary the proportion of liquid vs. gaseous nitrogen in the spray to dial in any temperature between −250°F and 32°F (157°C and 0°C). The higher the proportion of gas in the mix, the higher the temperature. This ability is particularly important when machining polymers, because, according to Knopf, each has its own temperature “sweet spot” for machining, in which the material has “cold flow” properties and cuts well. “At higher temperatures the polymer would be too rubbery, producing giant burrs when machined,” he said. “At colder temperatures there is a ductile zone, and below that the material becomes glassy and cracks when machined.” (Note that in machining polymer coolant would be directed at the part, or both the part and the cutting interface.)
Besides improving polymer machinability, cryo cooling also helps remove any burrs. “A spinal cage made out of PEEK might take 45 minutes to machine in a small Swiss lathe and 45 minutes to deburr, typically done manually with a toothbrush and scalpel,” said Knopf. “Then a soda blast [produces] a burr-free part.” With cryogenic machining, secondary deburring takes one to two minutes. “It won’t get rid of all the burrs, but it will eliminate 95% of them, and the remaining burrs are less attached,” he said. “If you machine at room temperature, burrs are essentially melted back on.”
Besides maintaining the desired temperature, Industrial CryoTech’s approach also helps ensure good coolant delivery, according to Knopf. “Rather than mixing the liquid and the gas internally and then running it 3′ [0.91 m] down the delivery line to the nozzle, where it can change temperature 150°F [66°C], we do our mixing right at the tip of the nozzle,” said Knopf. “We send the liquid through the center and the gas through the outside, right around the edge of the nozzle. The gas on the outside traps the liquid on the inside, so you can fine tune your delivery and control the temperature from a greater distance.” Otherwise, when the liquid exits the nozzle it hits the surrounding air and starts to disperse.
“We recognize that sometimes, due to the complexity of the setup, you can’t get your nozzle ½” [12.7 mm] away from the cutting tool,” said Knopf. “You’ve got clamps, fixtures, and tools around the part. Our system works well in a production environment, especially on polymers. In a typical medical application with the nozzle about 5″ [127 mm] from the part, the ICEFLY cooling spray forms a cone resulting in a cooling zone between the size of a nickel and a quarter. This often encompasses the entire size of the part. So even a fixed nozzle works fine in these applications.”
Adjustments and Limitations
For turning, the ICEFLY system can deliver targeted coolant without an issue. Milling is more complicated. For machines with an automatic toolchanger, Industrial CryoTech offers SpiderCool programmable coolant nozzles modified to work with LN2. “We changed some of the components so it wouldn’t freeze,” explained Knopf. “On a machining center, the nozzle gets out of the way when changing the tool and then comes down and aims where it’s needed again.”
Still, Knopf admitted that this approach is limited to relatively small parts. “When you machine Inconel and the other aerospace superalloys, the ideal situation is to deliver the coolant right at the cutting tool. With our approach, which sprays the coolant on the outside, there are situations that don’t always meet this ideal.”
For example, in a shell mill with six inserts, hitting one insert at a time with coolant doesn’t deliver sufficient cooling, which could be done by hitting all six inserts simultaneously with through-tool coolant. The same is true if the contour of the part or the fixture or a clamp interferes with the coolant. When the stream of nitrogen stops hitting the insert, heat builds up quickly.
“As a result, we are developing a large rotating toolholder,” said Knopf. “In some applications we’ve worked on, such as hogging out titanium or other nasty metals, spraying on the outside of the insert is not as effective as delivering it through the tool.”
That’s where 5ME (Cincinnati) comes in.
Option 2: Internal Liquid Nitrogen
The 5ME Cryogenic System moves LN2 through the machine, spindle, and cutting tool to just behind the cutting edges. This approach yields the ultimate machining heat sink: a cutting tool refrigerated to −321°F (−196°C). This effectively raises the critical temperature of any given application (i.e. the point at which any further rise in temperature causes a dramatic increase in tool flank wear) to roughly 300°F (149°C). That means a tool can be run much faster and generate much more heat and still have acceptable tool life.
5ME no longer requires a specialized dewar to supply the LN2, but from there the components are a bit unusual. First, vacuum-jacketed insulated lines carry the liquid nitrogen from the source to the cutting tool. According to 5ME, this has no influence on the temperature of the machine components, no effect on spindle bearings, and produces excellent reliability and maintainability. The tube goes through the existing spindle (including high-torque and high-speed applications) using the ID of the through-coolant drawbar. That means the machine would no longer have through-coolant capability, though you can leave flood coolant capability on the machine to increase versatility.
The patented solid-carbide cutting tool technology, which 5ME brands as BlueZone, incorporates channels that radiate out from the center, following the flute pattern internally to a venting port. 5ME has licensed Fullerton Tool Co. (Saginaw, MI) and Star SU LLC (Farmington Hills, MI) to provide BlueZone solid-carbide round tools. Additional global cutting tool OEMs have been licensed to provide both round tools and indexable inserts and will be announced over the next several months.
5ME’s system also includes a sub-cooler that removes pressure-generated heat from the line, returning the liquid nitrogen flow back to -321°F and condensing dual phase nitrogen (liquid and gas) back to 100% liquid. This component ensures that there is always optimal LN2 flow going to the cutting tool, regardless of the LN2 source or its distance from the machine tool.
Finally, the system comes with a programmable control that regulates the flow rate of the LN2 to the tool, with a user-friendly interface directly on the CNC. Will Gruber, 5ME’s manager of marketing and channel sales, cryogenics, said that “the user controls the flow of the liquid nitrogen in the same way he would control feed rate, with programmable flow rates or manual flow rate override. Likewise, it’s the flow that determines how much heat the system can pull from the machining operation, with average flow rates of 0.25–1 L/min and the capability to increase flow as high as 6 L/min. This minimal consumption rate results in consumable costs per an eight-hour shift as low as $30–$50.”
Big Payoff for US Taxpayers
The 5ME system can be retrofitted to virtually any machining center and several OEMs have joined with 5ME to offer it as a new machine option. That group includes Okuma America Corp. (Charlotte, NC), DMG Mori USA (Hoffman Estates, IL), Mazak Corp. (Florence, KY), Fives Machining Systems Inc. (Hebron, KY), and FFG (Hoffman Estates, IL).
Doosan Machine Tools America (Pine Brook, NJ) is the latest partner. Andy McNamara, director of sales, said Doosan adopted the technology to “address the need for lower tool costs and faster speeds in tough materials, not to mention the downtime associated with worn out tooling and constant chasing of tight tolerances. Combined with our ability to offer horizontal machining centers with either linear or box ways, it gives us an advantage in titanium and hard metals.”
Most users cannot publicize their results due to non-disclosure agreements or ITAR regulations. However, since the US Air Force backed one of the first applications, some details are available. Cryogenic machining of large titanium airframe components for the F35 Joint Strike Fighter decreased cycle time by 52%, improved surface integrity and part quality, and lowered overall costs an estimated 30%. An Air Force report on its 2016 Small Business Innovation Research and Small Business Technology Transfer Program said cryogenic machining of “titanium parts across the F-35 supplier base is expected to save the program more than $260 million, according to Lockheed Martin’s own conservative estimates.”
Gruber said the 5ME system includes “all the mechanical components, installation, CNC interface and training. Typical ROI for a system is 6–12 months, based on increasing material removal rates, increasing tool life, sustainability gains, and post processing reductions due to part quality improvements.”