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If you needed a reason to consider a new machining process, one that could solve some challenging material removal operations, listen to this: “The cryogenic machining process could be the enabler for materials that would otherwise be dismissed as being too expensive to machine.”
That’s George Georgiou, Cryogenic Engineering and Product Manager, MAG IAS (mag-ias.com).
And he rattles off a number of things, like components that are now in the realm of race cars because the budgets tend to be less cost-sensitive (to put it mildly), titanium connecting rods, composite body panels, and metal-matrix composites for cylinder blocks.
But while “cryogenic machining” may sound as exotic as these Ti con rods, Georgiou knows that there are a whole lot of everyday challenges that can be readily overcome by using the process that involves a coolant that is really cold—as in -321°F (slightly warmer than the temperature on Uranus)—as in power metal valve seats and guides and high-alloy diesel cylinder heads and blocks.
What’s interesting to note about this machining process is that it allows machining to be done fast. Really fast. As in doing compacted graphite iron (CGI) cylinder boring and milling at rates 300 to 400% faster than those ordinarily achieved. An interesting implication of this, Georgiou notes, is that by achieving greater throughput, less capital equipment needs to be acquired.
The fundamental of cryogenic machining is truly basic: If it is a process where chips are produced—boring, drilling, reaming, tapping, milling, etc.—a process where heat is being generated, heat which has a deleterious effect on the cutting tool, in other words, your basic metal (or composite) cutting process, then cryogenic machining provides a significant benefit.
That’s because the liquid nitrogen used for cooling is really cold. That -321°F is the temperature at which it boils.
And lest you think that this necessitates the kind of machinery and equipment that is something out of a movie set in the 22nd century, Georgiou acknowledges that “the primary focus is putting the process on new equipment”—after all, MAG IAS is a major machine tool company—but that’s because it is such a big company, one with an array of brands in its portfolio (e.g., Cincinnati, Lamb, Cross Hüller, Giddings & Lewis, Fadal, Ex-Cell-O), “If a customer has a shop full of MAG equipment, we’re not going to leave it out to dry.” They’ll do an assessment of the applicability and economics of retrofitting that existing equipment, and if it is appropriate, make the retrofit. That said, Georgiou points out that given the various elements that are necessary for successful cryogenic machining, ranging from safety features to flow control systems for pumping the liquid nitrogen, it is often the case that new is actually more economical. Another aspect is that of chip management: cryogenic machining is a dry process (the nitrogen, which is taken from the air (what you breathe is about 78% nitrogen) and cooled, evaporates back into the air); machines designed for wet chip disposal aren’t ideal for dry cutting.
It is worth noting that MAG has been developing “MQL”—or minimum quantity lubrication—which is a near-dry process, so it has an understanding of designing machines to accommodate chip disposal. What’s more, MQL can be combined with cryogenic machining to good effect in some instances, such as in machining CGI with carbide tools. In tests of milling CGI with carbide alone, there was a 60% increase in speed with tool life equivalency to that of a conventional machining setup. When MQL was added to the mix, this resulted in tripling of that speed. They ran the same test with polycrystalline diamond (PCD) tooling. With cryogenics alone there was a 4X increase in speed; there was no additional benefit when MQL was used. Why? Because carbide tools are more affected by abrasive wear than PCD tools, so the MQL helps the carbide’s performance.
Given the foregoing about tools, know that there really isn’t anything particularly exotic about those required for cryogenic machining. For example, Georgiou says, simply, “Take a grade that works well for CGI and use that grade.” To be sure, he and his colleagues are working with people from the cutting tool industry to create some grades and coatings that are optimized for cryogenics (MAG also makes cutting tools), but conventional grades work just fine.
But the approach to using the liquid nitrogen that does the best job is one wherein the nitrogen gets as close to the cutting edge as possible (thereby extracting the heat from the cutting edge so that it doesn’t soften), so through-the-tool plumbing for the liquid nitrogen to get to the edge is necessary.
As for the amount of nitrogen needed for a given cutting operation, Georgiou says that they’ve been using about 0.04 liters/minute per cutting edge for milling and boring during developmental testing. He thinks that the amount required for drilling and tapping will be less. At about seven cents per liter, the amount of nitrogen required is actually less, he says, than the cost of the energy required to run the pump in a machine tool used to move coolant from the cutting area to the central system.
When we talk, Georgiou says that two automotive companies in North America and two in Germany are working with MAG in developing processes for cryogenic machining. (They are also working with aerospace companies, where making large components out of materials like titanium is the rule, not the exception.) Georgiou suggests that “None of them see this as a problematic process,” and that they’re anticipating launching pilot programs early in ’12.