The key to the success of the dry cutting process is to find a way to replace cooling and lubrication. At present, there are two successful dry cutting methods: high speed dry cutting and low temperature cold cutting.
First, high speed dry cutting and low temperature cold air cutting
1. High speed dry cutting
The processing method is to perform cutting processing at a high cutting speed without the action of cooling or lubricating oil. Dry cutting must use the appropriate cutting conditions. First, use a high cutting speed to minimize the contact time between the tool and the workpiece, and then use compressed air or other similar methods to remove the chips to control the temperature of the working area. With the extensive use of CNC technology, the rigidity and dynamic performance of machine tools are constantly improving, and it is not difficult to increase the cutting speed of machine tools. Practice has shown that when the cutting parameters are set correctly, 80% of the heat generated by cutting can be carried away by the chips.
High-speed dry cutting has strict requirements for the knife: 1 The tool should have excellent high temperature resistance and can work without cutting fluid. New types of cutting materials such as cemented carbide, polycrystalline ceramics and CBN are the materials of choice for dry cutting tools; 2 the coefficient of friction between the chips and the tool should be as small as possible (the most effective method is the surface coating of the tool), supplemented by Good chip removal structure and reduced heat accumulation; 3 dry cutting tools should also have higher strength and impact toughness than wet cutting tools.
2. Low temperature cold air cutting
The cutting method is a processing method in which cold air of -10 ° C to -100 ° C and very small amount of vegetable oil are used instead of cooling and lubricating oil cooling. It was first proposed by Yokokawa Kazuhiko of Meiji University in Japan. It has been found that in the metal cutting process, if only a very small amount of vegetable oil with good lubricating effect and unoxidized is supplied to the processing point, the processing point will lose lubricity due to high temperature. If cold air (-10 ° C ~ -100 ° C) is supplied to the processing point, the high temperature of the processing point can be prevented and the above situation can be avoided.
The cutting performance is greatly improved during cold air cutting. Tests have shown that cold air cutting and grinding are more than twice as efficient in performance as oil cutting and grinding. Figure 1 shows the comparison of cutting performance with and without vegetable oil cutting agent and cold air. It can be seen that the use of cold air cutting alone is better than the use of vegetable oil cutting, and the cutting performance of the tool is further enhanced when the cold air is used together with the trace vegetable oil. Cutting conditions during the test: workpiece diameter: φ92mm to φ98mm, cutting speed: 45.1m/min to 48.0m/min, feed: 0.5mm, cutting tool: tool nose radius R0.4, equivalent to SKH4 high speed steel, not heavy Grinding blades.
â–²No precision lubricating oil, no cold air processing; â– Precise lubricating oil, no cold air;
◠There are precision lubricating oils and cold air; ◆ No precision lubricating oil, there is -16.7 °C cold air
Fig.1 Cutting performance with and without vegetable oil cutting agent and cold air When using strong cold air processing with very low temperature, in order to prevent oil from freezing and losing lubricity, a small amount of vegetable oil can be applied to the surface of the tool before processing, and then The processing point supplies strong cold wind. That is to say, if the strong cold air is mixed with the oil agent, the effect will be better. The use of trace vegetable oils not only reduces cutting energy, but also eliminates rusting of the workpiece.
Second, the application of dry cutting in the field of gear processing
The new carbide coating method and the use of CNC machines have led to a new trend in the manufacture of cylindrical gears: high-speed cutting of carbide-free tools without cooling. If the process parameters are optimally set, the machining time is short and the tool life is longer. Japan's Mitsubishi Corporation, the US Gleason Corporation, etc. have carried out fruitful research in this regard.
Japan's Mitsubishi Corporation introduced the world's first dry hobbing system. It uses a cutting speed that is many times the traditional hobbing speed and can reach 200m/min. Dry hobbing has special requirements for hobs. Mitsubishi's special dry hobs are made of MACH7 high-speed steel. The surface is coated with a special coating to help dissipate heat and reduce tool loss. Its life can be extended to general wetness. 5 times the cutting method. This system is ideal for processing automotive final drive gears, large load gears, automotive pinions and planetary gears, reducing production costs by at least 40%.
Gleason uses a hard-metal hob to machine bevel gears on a Phoenix machine with dry-cutting. Compared to conventional high-speed steel tool wet cutting, the cutting time is reduced by 50% and the surface roughness of the gear is significantly reduced. And the geometric precision is also greatly improved, and the machining accuracy can reach AGMA 12-13. The company's GP series gear hobbing machine, with its unique design, makes its dry cutting quality comparable to wet cutting quality. The bed is designed as a large angled slope to facilitate chip flow. The inside of the bed is circulated and cooled to help maintain heat balance. In addition, the machine is equipped with a vacuum dedusting and dust removal system. This series of hobbing machines can achieve a rolling speed of up to 3000r/min.
The high-speed dry cutting method has the following advantages: First, because it eliminates the oil chip separation process, the cooling oil tank and the oil chip separation device, and the corresponding electrical equipment, the machine tool is compact. Secondly, this method greatly improves the processing environment; processing costs are also greatly reduced. In order to further extend the tool life and improve the quality of the workpiece, 10 to 1000ml of lubricating oil per hour can be used for minor lubrication during dry gear cutting. The chips produced by this method can be considered as dry chips. The accuracy, surface quality and internal stress of the workpiece are not adversely affected by the micro-lubrication. Process monitoring can also be carried out with automatic control equipment.
LMT-Fette uses a carbide cutter to dry-roll the gears, which significantly reduces the processing time and cost of the gears. The KC250H dry hobbing machine developed by Japan's Kento Iron Works uses carbide carbide hob, cold air cooling and micro-lubrication to perform high-speed hobbing. Due to the supply of cold temperature with stable temperature, the thermal deformation of the workpiece is extremely small. Compared with the traditional high-speed steel hob KA220 wet hobbing machine, the processing speed is increased by 3.2 times and the gear precision is also significantly improved.
Third, summary
Compared with wet cutting, dry cutting not only greatly improves the machine tool production efficiency, reduces the processing cost of the workpiece, but also helps to protect the environment and save natural resources. In foreign countries, dry cutting research and application are relatively extensive. According to statistics from relevant departments, nearly half of the companies in Western Europe currently use dry cutting. Although some institutions in China are engaged in research work in this area, their application in the field of gear processing is still blank.
The ISO and JIS14000 series of standards were established in 1996, respectively, which have strict regulations on environmental management and monitoring. Their implementation is undoubtedly another challenge to the export of China's machine tool products. In order to improve the technical content and technological innovation capability of our products, it is necessary and urgent to carry out research and development of dry cutting technology in the invincible position in the international market competition.
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