The key to the role of dry cutting in gear machining lies in finding a way to replace cooling and lubrication. At present, there are two kinds of successful dry cutting methods: high speed dry cutting and low temperature cold air cutting.
First, high-speed dry cutting and cold wind cutting
High-speed dry cutting
The processing method is to use a high cutting speed to perform cutting processing under the action of no cooling and lubricating agent. Dry cutting must use proper cutting conditions. First of all, with a high cutting speed, the contact time between the tool and the workpiece is shortened as much as possible, and then the chips are removed by compressed air or the like to control the temperature of the work area. With the wide use of numerical control technology, the rigidity and dynamic performance of the machine tool are continuously improved, and it is not difficult to increase the cutting speed of the machine tool. Practice has proved that when the cutting parameters are set correctly, 80% of the heat generated by the cutting can be taken away by the chips.
The high-speed dry cutting method has strict requirements for the knife: 1 The tool should have excellent high-temperature resistance and can work without cutting fluid. Cutting materials such as new cemented carbide, polycrystalline ceramics, and CBN are the materials of choice for dry cutting tools; 2 that the friction coefficient between the chip and the tool is as small as possible (the most effective method is the tool surface coating), supplemented by Well-discharged tool structure reduces heat build-up; 3 Dry cutting tools should also have higher strength and impact toughness than wet cutting tools.
Cold wind cutting
The cutting method is a processing method that uses cool air of -10 to -100°C and a very small amount of vegetable oil instead of cooling and lubricating agent cooling. It was first put forward by Yokokawa Yoshihiko of Meiji University in Japan. The study found that in the metal cutting process, if only a very small amount of vegetable oil is supplied to the processing site with good lubricating effect and is not oxidized, the processing point will lose its lubricity due to high temperature. If cold air (-10 to -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.
Cutting performance is greatly improved during cold cutting. Tests have shown that cold air cutting and grinding are more than 2 times more efficient than oil cutting and grinding. Comparison of cutting performance with and without vegetable oil cutting agents and cold air. It can be seen that the use of cold wind alone is better than the use of plants, while the cold wind is used with trace amounts of vegetable oil, the cutting performance of the tool is further enhanced. Cutting conditions during the test: workpiece diameter: f92 ~ f98mm, cutting speed: 45.1 ~ 48.0m/min, feed: 0.5mm, cutting tool: nose radius R0.4, equivalent to SKH4 high-speed steel, do not regrind blade.
â–² No precision lubricant, no cold air processing; â– Precision lubricants, no cold air;
â—There are precision lubricants, there are cold air; â—†No precision lubricant, there is -16.7 °C cold wind, no vegetable oil cutting agent and cold cutting performance when used in a cold blast of very low temperature, in order to prevent the oil from freezing and lose lubrication Sexuality, can apply a trace of vegetable oil on the tool surface before processing, and then supply strong cold air to the processing point. That is, the strong cold air and oil agent will not be mixed together, and the effect will be better. The use of trace vegetable oils not only reduces the cutting energy consumption, but also eliminates the workpiece rust.
Second, dry cutting in the field of gear processing applications
New carbide coating methods and the use of CNC machine tools have led to a new trend in cylindrical gear manufacturing: high-speed cutting of uncooled carbide tools. If the process parameters are optimized, the machining time can be shortened and the tool life can be longer. Mitsubishi Corporation of Japan and Gleason Corporation of the United States have conducted fruitful research in this area.
Mitsubishi Corporation of Japan introduced the world's first dry hobbing system. It uses a cutting speed that is twice the speed of traditional hobbing and can reach 200m/min. Dry hobbing has special requirements for hobs. The special dry hob designed by Mitsubishi Corporation uses MACH7 high-speed steel with a special coating on the surface to help heat dissipation and reduce tool wear. Its life can be extended to wet Cut 5 times. This system is ideal for processing automotive final transmission gears, large load-bearing gears, automotive pinions, and planetary gears. Production costs are reduced by at least 40%.
Gleason Corporation uses a carbide hob to machine bevel gears on a Phoenix machine using a dry cutting method. Compared to the conventional HSS tool wet cutting method, the cutting time is reduced by 50%, and the surface roughness of gears is significantly reduced. And geometric accuracy is also greatly improved, processing accuracy up to AGMA12-13. The company's GP series hobbing machine, with its unique design, makes its dry cutting quality comparable to wet cutting quality. Its bed is designed with a large angle slope for chip flow. The internal circulation of the bed is cooled to help maintain heat balance. In addition, the machine is equipped with a vacuum dust removal system. This series of hobbing machines, rolling speed up to 3000r/min.
The high-speed dry cutting method has the following advantages: First, because it eliminates the separation process of the oil dust, there is no cooling oil tank and the separation device for the oil dust, and the corresponding electrical equipment, therefore, the machine tool has a compact structure. Second, this method greatly improves the processing environment; processing costs are also greatly reduced. In order to further prolong the life of the tool and improve the quality of the workpiece, it is possible to use 10 to 1000 ml of lubricating oil per hour for micro-lubrication during gear dry cutting. Chips produced by this method can be considered as dry chips. The precision, surface quality and internal stress of the workpiece are not affected negatively by trace amounts of lubricants. Process monitoring can also be performed using automatic control equipment.
LMT-Fette used dry cutting gears with carbide tools to reduce the processing time and cost of one-piece gears. The KC250H dry hobbing machine developed by Japan's Kento Iron Works adopts a carbide hob, cold air cooling, and a small amount of lubrication to perform high-speed hobbing. Since the cold air is supplied at a stable temperature, the workpiece undergoes minimal thermal deformation. Compared with the traditional high speed steel hob KA220 wet hobbing machine, the machining speed is increased by 3.2 times, and the gear precision is also significantly improved.
Third, the summary
Compared with wet cutting, dry cutting not only greatly improves the productivity of machine tools, reduces the cost of workpiece processing, but also helps to protect the environment and save natural resources. In foreign countries, the research and application of dry cutting are relatively extensive. According to relevant department statistics, 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 JIS 14000 series of standards were established in 1996 and they 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, and remain invincible in the international market competition, it is necessary and urgent to carry out the research and development of dry cutting processing technology.