Low-temperature machining under liquid nitrogen cooling
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Shared from the internet: In the mechanism of tool wear, \"wear\" itself is not the only cause of tool failure; \"high temperature\" is the real enemy of tools. Due to the effects of high temperatures, slight wear on the cutting tool can \"self-amplify\". In other words, when the cutting edge of the tool becomes slightly duller, the friction during cutting increases, which can lead to an increase in cutting heat and thus a rise in the tool’s temperature. This causes the tool material to soften, which in turn accelerates the dulling of the cutting edge. The more dull the tool becomes, the greater the cutting friction and the higher the tool’s temperature, creating a vicious cycle. Ultimately, the cutting tool will fail following a \"self-accelerating curve\" driven by the cutting heat. Low-temperature (ultra-low temperature) cutting using liquid nitrogen is a method in which liquid nitrogen is employed to keep the workpiece, tool, or cutting area in a low-temperature cooled state during cutting. Nitrogen is the most abundant component in the atmosphere, and liquid nitrogen is available in large quantities as a by-product of the oxygen production industry. Using liquid nitrogen as a cutting fluid, it evaporates directly into gas and returns to the atmosphere after use, leaving no pollutants; from an environmental perspective, it is a promising alternative to conventional cutting fluids. This processing method significantly extends the service life of the cutting tool.
? The cutting fluid, which we call “coolant,” is not entirely successful at providing cooling. Although poured cooling can eliminate some of the heat generated during processing, its cooling effect is limited to a certain distance and cannot reach the actual heat-generating area (i.e., the cutting zone where the tool cuts the workpiece material beneath the chips). The method of delivering coolant through internal channels in the spindle attempts to bring the coolant closer to the cutting area, while the new technology of delivering coolant through the interior of the blade indeed brings the coolant very close to the cutting area. In conventional wet cutting processes, the main function of the cutting fluid is to lubricate and/or spray onto the tool and workpiece to carry away heat. In low-temperature cutting, the function of the cutting fluid (liquid nitrogen) is to provide cooling. The temperature of the cooling fluid used for pouring might be +20°C, while the temperature of liquid nitrogen is -196°C; the difference between the two is nearly 220°C. Such a large temperature difference is sufficient to turn the tool into a heat absorber. Due to its extremely low temperature, the tool can draw cutting heat away from the cutting edge into the tool body, just as a sponge absorbs water, thereby ensuring that the tool’s cutting performance and service life do not decline rapidly as a result of high cutting temperatures.
? The cutting fluid, which we call “coolant,” is not entirely successful at providing cooling. Although poured cooling can eliminate some of the heat generated during processing, its cooling effect is limited to a certain distance and cannot reach the actual heat-generating area (i.e., the cutting zone where the tool cuts the workpiece material beneath the chips). The method of delivering coolant through internal channels in the spindle attempts to bring the coolant closer to the cutting area, while the new technology of delivering coolant through the interior of the blade indeed brings the coolant very close to the cutting area. In conventional wet cutting processes, the main function of the cutting fluid is to lubricate and/or spray onto the tool and workpiece to carry away heat. In low-temperature cutting, the function of the cutting fluid (liquid nitrogen) is to provide cooling. The temperature of the cooling fluid used for pouring might be +20°C, while the temperature of liquid nitrogen is -196°C; the difference between the two is nearly 220°C. Such a large temperature difference is sufficient to turn the tool into a heat absorber. Due to its extremely low temperature, the tool can draw cutting heat away from the cutting edge into the tool body, just as a sponge absorbs water, thereby ensuring that the tool’s cutting performance and service life do not decline rapidly as a result of high cutting temperatures.
To quantitatively analyze the performance advantages of liquid nitrogen cooling, MAG Company conducted comparative cutting tests to examine the impact of different cooling methods on the machining properties of titanium alloys. As shown in the figure, under cooling fluid-fed cooling conditions, the tool used for machining at a surface cutting speed of 300 sfm (90 m/min) – which is considered a high-speed cutting speed for titanium alloys – became dull after just 1 minute ； Under low-temperature cutting conditions, the tool was able to perform reliable cutting for 10 minutes before becoming dull. When the cutting speed is increased to 400 sfm (120 m/min), the service life of the same tool varies by a factor of 5. Georgiou explained that the area between the two performance curves in the graph reflects the difference in productivity between the two cooling methods. When machining within this range, if the user replaces pouring cooling with low-temperature cutting, it is possible to use a higher cutting speed while maintaining the tool life, or achieve a longer tool life at the same cutting speed.
When the cutting speed is low, the performance difference between low-temperature cutting and conventional pouring-cooled cutting is the greatest. As the cutting speed increases, this difference gradually decreases. When the cutting speed reaches a very high value, this difference disappears completely. In the stainless steel cutting test shown in the figure below, when the cutting speed is below 300 sfm (90 m/min), the tool life under low-temperature cutting is 10 times that of cutting with cast cooling ； When the cutting speed is increased to 400 sfm (120 m/min), this difference gradually decreases to 4 times ； And when the cutting speed is increased to about 650 sfm (200 m/min), this difference disappears.
In addition to the advantages in cutting performance, using low-temperature cutting technology also has other benefits. For the MAG R&D team, whose main focus is on improving cutting performance, a pleasant surprise was the potential benefits of low-temperature cutting technology in ensuring the safety of employees. After low-temperature cutting, no slippery cutting fluid remains on the surface of the machine tool; therefore, in operations where operators sometimes need to move around on the worktable of large machine tools, this can **reduce the risk of slipping and injury**. Other benefits are related to environmental protection. Low-temperature cutting does not require synthetic coolants; instead, it uses nitrogen taken from the air, which is then returned to the air. Therefore, there is no need to treat the waste liquid. Nitrogen does not pollute the air, nor does it contaminate the medical devices or other sensitive components that are being processed.
This might be used in some machining centers, but it’s not feasible on a large scale – for example, for the machining of large-scale equipment