The Case for Cryogenic Machining
Automotive components could see significant process improvements.
Cryogenic machining, or the supercooling of specifically designed cutting tools by liquid nitrogen supplied by jacketed tubes fitted into a machining center, has been getting well-documented success in speeding the processing of traditionally hard-to-machine alloys such as those based on titanium. As such, much of the activity is occurring in the aerospace sector. But talk to the patent holders 5ME (5me.com) at their tech center in Warren, Michigan, and it becomes quickly clear that the real room for opportunity is in automotive.
“What we have found is when you get some materials cold, primarily hexagonal-lattice materials such as titaniums, cobalt, stainless steels and alloyed steels, they get tougher and harder to machine,” says George Georgiou, 5ME Lead Cryogenic Engineer. “Therefore, our cryogenic machining patents focus on the internal cooling of the tool, whether an indexable milling cutter or round carbide tool, and back-venting the cryo cooling away from the material to affect it as little as possible.”
Cryogenic machining also focuses on the machine tool. The Warren Tech Center showcases four cryogenically enabled machines, including a MU-5000VL 5-axis machining center from Okuma America Corporation (okuma.com). To use the process, it is necessary to store liquid nitrogen (LN2) either in a small tank next to the machine (a “dewar”) or in a central storage location that allows for setting up individual drops to each machine. As the system is self-pressurizing, there is no need for pumps and other power-consuming assets.
Vacuum-jacketed insulated lines feed the LN2 from the machine source system to the spindle, ram or turret, depending on the machine type. The LN2 feed system is critical to seal out ambient heat and feed a cool -321°F liquid to its point of use. Conversely, it also protects critical machine components from exposure to the extreme cold temperature on LN2 .
The patented 5ME sub-cooler takes pressure-generated heat out of the system, which returns the LN2 flow back to -321°F and condenses dual-phase liquid and gas back to 100 percent liquid. 5ME deems sub-cooling critical to help prevent the formation of nitrogen gas from downstream heat leaks and pressure drops. It delivers accurate liquid metering for the flow-control valves and ensures liquid nitrogen is delivered to the cutting edge, where the optimum heat extraction and extreme performance gains happen.
Moreover, this is a programmable NC control-based system that allows for operators to program in the automatic control that signals to the feed system how much flow is appropriate, or allows for auto-override to emergency shut off or overflow the system.
Works on Existing Equipment
5ME adds that its cryogenic tooling system can easily be retrofitted into almost any OEM spindle, turret or ram. A vacuum-insulated tube rotates within the spindle allowing use in high-torque or high-speed applications. Tools are also available in HSK or ANSI interfaces for 4- and 5-axis machining centers. The jacketed tubes transfer the cryogen through the spindle, turret or ram without influencing functional component temperatures. In more than three years of continuous testing, the company has reported no thermal cycling issues, no effect on spindle bearings, and long seal life.
For titanium-based materials 5ME is reporting twice the cutting speed with equivalent tool life compared to traditional milling with flood coolants. In compacted graphite iron (CGI), which is becoming widely used for blocks and heads, the results are even better—five times the increase in cutting speed with equivalent tool life—making more parts faster.
Cryogenic machining also offers valuable ancillary benefits. Eliminating flood coolant from a cell or a shop eliminates many added costs and makes the plant safer and healthier. And while the machines feature redundant sensors and seals, any escaping liquid nitrogen simply evaporates into the atmosphere.
But efficiency gains form the heart of investigating cryogenic machining. Part of 5ME’s testing on rough-milling CGI shows speeds of 500 m/min with virtually no measurable wear on the wiper portion of the cutting tool. Manufacturers could go from five machines to two, meaning significant capacity increases for high-volume programs. In stainless steel alloys, the company reports cryogenic machining gains 10.8 hours for every 40 hours of traditional machining.
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