This is what machining looks like when the setup includes a flow of liquid nitrogen through the tooling and an infrared thermograph is used to take the picture. The darker areas are cooler. In this case, the hottest area (white) is 82°C (179.6°F); the cutting tool body is a comparatively frigid -32°C (-25.6°F).
As this chart shows, the performance of machining compacted-graphite iron (CGI) with carbide tools. Note the improved performance that can be realized by using liquid nitrogen cooling along with minimum-quantity lubrication (MQL).
Can the use of liquid nitrogen—nitrogen that's at -321°F—improve machining operations?
The answer to that, while still in the process of becoming definitively determined, seems to be "yes." That's the indication coming out of work that's being done at MAG Industrial Automation Systems (mag-ias.com) under a U.S. Navy-sponsored development program.
According to MAG's George Georgiou, the process is being tested on materials including titanium, nickel-based alloys, stainless steel, compacted-graphite iron (CGI), and metal-matrix composites. While most of those materials are more widely deployed in aerospace applications than in automotive, CGI is finding increased use in powertrain, so this development is something that should be kept in mind (think, for example, of face milling deck surfaces on blocks).
In tests conducted on CGI, milling speed increases of 60% were achieved using carbide tools with the cryogenic process compared with non-hyper-chilled cutting. In tests where polycrystalline diamond (PCD) tooling was used, there was a speed increase of 4X compared to traditional milling.
MAG has developed an expertise in minimum quantity lubrication (MQL) for cutting applica-tions, so it was used in conjunction with cyro machining in tests. The use of the lubrication helps reduce tool friction and workpiece material adhesion. While there was no improvement to the performance of the PCD tools when MQL was deployed, carbide tools, which are more susceptible to abrasive wear than PCD, were able to work at speeds 3X conventional.
The liquid nitrogen is run through the spindle and through the insert. Running the ultra-cold liquid through the inserts provides two benefits: one is that it does the best job of cooling the insert (as opposed, say, to flooding the cutting zone with the liquid) and the other is that it minimizes the amount of liquid nitrogen used during machining. MAG researchers have determined that milling and boring operations can be performed with cryogenic cooling with a consumption of 0.04 liters of nitrogen per minute per cutting edge.
What's more, Georgiou points out that the process, which doesn't require the use of cutting oils, is comparatively "green," particularly in cases when machining is done without any other fluids. For example, there's no fluid collection required, no mist collection, no chip wringers, and no disposal cost for the nitrogen. In addition to which, if there is a considerable machining throughput advantage, it would be possible to reduce the number of machine tools necessary to achieve required production rates.—GSV