In the tool wear mechanism, "wear" is not the sole cause of tool failure; rather, "high temperature" poses the greatest threat to the tool.
High temperatures can lead to a phenomenon known as "self-amplifying wear." When the cutting edge of a tool becomes slightly dull, the friction during cutting increases. This, in turn, generates more cutting heat, which raises the tool's temperature and can soften the tool material. As a result, the cutting edge becomes even more passive at an accelerated rate. The more significant the tool's passivation, the greater the cutting friction and the higher the tool temperature, resulting in a vicious cycle. Ultimately, this leads to tool failure, following what could be described as a "self-acceleration curve" driven by cutting heat.
To counteract this, low-temperature cutting using liquid nitrogen is an effective method. This technique cools the workpiece, tool, or cutting area by applying ultra-low temperatures. Nitrogen, the most abundant element in the atmosphere, is readily available as a by-product of the oxygen production industry. When used as a cutting fluid, liquid nitrogen evaporates into gas and returns to the atmosphere after use, leaving no pollutants behind. From an environmental perspective, it presents a promising alternative to traditional cutting fluids. This method significantly extends the service life of the tool.
Cutting fluids, often referred to as "coolants," are not entirely effective in providing sufficient cooling during metal CNC machining processes. While traditional pour-in cooling can help remove some heat generated during the cutting process, it only cools the surface from a distance and cannot effectively reach the actual hot zone — the area below the chip where the tool interacts with the workpiece material. To address this, coolant delivery through internal channels in the spindle has been developed, and newer technologies even allow coolant to be delivered directly inside the cutting insert, bringing it very close to the cutting zone.
In conventional wet cutting, the primary role of cutting fluid is to provide lubrication and to wash away heat from the tool and workpiece. In contrast, cryogenic cutting uses liquid nitrogen as its cutting fluid, where the main function is to cool the tool. For instance, while the temperature of a typical pour-in coolant may be +20°C, liquid nitrogen reaches a drastically lower temperature of -196°C, resulting in a temperature difference of nearly 220°C. This significant temperature disparity allows the tool to act as a heat sink, effectively absorbing heat from the cutting edge into the body of the tool, much like a sponge absorbs water. This ensures that the cutting performance and longevity of the tool are not rapidly compromised by high cutting temperatures.
A comparative cutting test was conducted to assess the performance benefits of liquid nitrogen cooling and examine how different cooling methods affect the CNC machining processing of titanium alloys. As illustrated in the figure below, when using pour-in coolant, a tool cutting at a surface speed of 300 surface feet per minute (90 meters per minute) quickly became dull after just 1 minute of operation. However, under cryogenic cutting conditions, the same tool was able to process for 10 minutes before losing its cutting edge.
When the cutting speed was increased to 400 surface feet per minute (120 meters per minute), the tool's service life varied by a factor of five between the two cooling methods. The area between the two performance curves displayed in the figure indicates the difference in productivity between these cooling strategies. Therefore, when machining within this parameter range, adopting low-temperature cooling instead of pour-in methods allows users to either achieve higher cutting speeds without reducing tool life or extend the tool's life at the same cutting speed.
Tool life comparison when cutting titanium alloy with two cooling methods
The two performance curves shown in the figure are expected to converge eventually. At low cutting speeds, the performance difference between cryogenic cutting and conventional pouring cooling cutting is at its highest. As the cutting speed increases, this difference gradually diminishes. Once the cutting speed reaches a very high level, the difference disappears entirely.
In the stainless steel cutting test illustrated in the figure below, when the cutting speed is less than 300 surface feet per minute (sfm) or 90 meters per minute (m/min), the tool life for cryogenic cutting is ten times longer than that of pouring cooling cutting. As the cutting speed increases to 400 sfm (120 m/min), this difference decreases to four times. Finally, when the cutting speed reaches approximately 650 sfm (200 m/min), the difference is no longer noticeable.
Tool life comparison when cutting stainless steel with two cooling methods
Cryogenic cutting technology offers several advantages beyond improved cutting performance. For the MAG R&D team, which primarily focuses on enhancing cutting efficiency, one surprising benefit is the increased safety for employees. After the cryogenic cutting process, there is no residual slippery cutting fluid on the machine's surface, significantly reducing the risk of slip-related injuries, especially in operations where operators may need to walk on the worktable of large machine tools.
Additionally, this technology has positive implications for environmental protection. Instead of relying on synthetic coolants, cryogenic cutting utilizes nitrogen, which is extracted from the air and ultimately returned to the atmosphere. This process eliminates waste liquid disposal issues and ensures that nitrogen does not contaminate the air around sensitive medical devices or other workpieces being processed.
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Post time: Jan-21-2025