Liquid nitrogen instead of cutting fluid

2023-10-31

Liquid nitrogen instead of cutting fluid, machining and cutting in the environment of more than 100 degrees below zero!

In the tool wear mechanism, "wear" itself is not the only cause of tool failure, "high temperature" is the real enemy of the tool.

Due to the influence of high temperature, the slight wear of the tool may "self-magnify". That is to say, when the cutting edge of the tool becomes slightly dull, due to the increase in friction during cutting, it may lead to an increase in cutting heat, so that the tool temperature increases, the tool material softs - this will promote the cutting edge of the tool to further passivation at a faster speed, and the more serious the tool passivation, the greater the cutting friction, the higher the tool temperature, so forming a vicious circle. Eventually, the tool will fail in a "self-accelerating curve" driven by the cutting heat.

The cutting fluid we call "coolant" is not entirely successful in providing cooling functions. Although castable cooling can eliminate part of the heat generated during the processing process, its cooling action is separated by a certain distance and cannot reach the real heating zone (that is, the cutting zone where the tool cuts the workpiece material under the chip). The method of conveying coolant through the inner channel of the spindle tries to bring the coolant closer to the cutting zone, while the new technology of conveying coolant through the inside of the blade does bring the coolant very close to the cutting zone.

In conventional wet cutting, the main role of the cutting fluid is to lubricate and/or spray on the tool and workpiece to take away heat. In low-temperature cutting processing, the role of cutting fluid (liquid nitrogen) is refrigeration. The temperature of the pouring coolant may be +20 ° C, while the temperature of the liquid nitrogen is -196 ° C, a difference of nearly 220 ° C. Such a large temperature difference is enough to turn the knife into a heat absorber. Due to its very low temperature, the tool can suck the cutting heat from the cutting edge into the tool body like a sponge, so as to ensure that the cutting performance and service life of the tool will not decline too quickly due to high cutting temperature.

In order to quantitatively analyze the performance advantages of liquid nitrogen cooling, the effects of different cooling methods on the machining properties of titanium alloys were compared by cutting tests. As shown in the figure below, under the cooling condition of the coolant pouring, the tool processed at a surface cutting speed of 300sfm (90m/min) (which is high-speed cutting for titanium alloy) is blunted after 1 minute; Under low temperature cutting conditions, the tool can be reliably processed for 10 minutes before being blunted. When the cutting speed is increased to 400sfm (120m/min), the service life of the same tool is 5 times different. The area between the two performance curves in the figure reflects the productivity difference between the two cooling methods. When machining in this zone, if the user uses a low-temperature cutting method instead of pouring cooling method, it can choose a higher cutting speed while maintaining the same tool life, or obtain a longer tool life at the same cutting speed.

Comparison of tool life when cutting titanium alloys with two cooling methods

These two performance curves will eventually converge. When the cutting speed is low, the performance difference between low temperature cutting and conventional cast cooling cutting is the largest. When the cutting speed is increased, this difference will gradually decrease. When the cutting speed reaches some very high value, this difference will completely disappear. In the stainless steel cutting test shown in the following figure, when the cutting speed is less than 300sfm (90m/min), the tool life of low-temperature cutting is 10 times that of poured cooling cutting; When the cutting speed is increased to 400sfm (120m/min), this difference is gradually reduced to 4 times; When the cutting speed is increased to about 650sfm (200m/min), this difference no longer exists.

Comparison of tool life when cutting stainless steel with two cooling methods

In addition to the advantages of cutting performance, there are other benefits to using low temperature cutting technology. That is the potential benefits of low-temperature cutting technology in ensuring the personal safety of employees. After low-temperature machining, there is no slippery cutting fluid left on the machine surface, so the risk of slip injury can be greatly reduced in processing where some operators sometimes need to walk around on large machine tool workbenches.

Other benefits have to do with environmental protection. Instead of using artificial coolant, cold cutting uses nitrogen from the air and eventually returns to the air. Therefore, there is no need to dispose of the waste liquid. Nitrogen does not contaminate the air or processed medical devices or other sensitive workpieces.

The use of liquid nitrogen cooling low temperature (ultra-low temperature) processing, to solve some difficult to process metal materials, non-metallic materials and composite materials difficult to process problems.

When using liquid nitrogen cooling cutting grinding, it is necessary to use some lubricants to solve the problem of poor lubrication in the process of chip formation. At the same time, the new metal surface on the workpiece has a strong chemical activity, and will rust quickly when exposed to the air, therefore, it is necessary to use some anti-rust agents to prevent the workpiece and the machine tool from rusting.

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