The hottest dry cutting process and its measures

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Dry cutting and its measures

the use of cutting fluid must meet the requirements of environmental protection. Therefore, people have carried out a lot of exploration and research, such as developing new cutting fluid, strengthening the management of cutting fluid, adopting cutting waste liquid treatment technology and improving cooling methods, trying to reduce the adverse effects of using cutting fluid, but they have not achieved satisfactory results. In order to fundamentally eliminate the adverse factors brought by traditional cutting fluid, dry cutting (dry cutting fashionable and beautiful BASF motor vehicle solution also helps to reduce vehicle weight) is a feasible method. Based on the experience of dry cutting technology at home and abroad, this paper analyzes the factors affecting dry cutting, discusses some measures to realize dry cutting, and finally briefly introduces the research and application of dry cutting at home and abroad

1 Factors Affecting dry cutting

dry cutting refers to the process method that does not use cutting fluid in cutting. It eliminates a series of negative environmental effects brought by cutting fluid from the source, and has the following characteristics: (1) the formed chips are clean, pollution-free, easy to recycle and treat; (2) It saves the purchase and maintenance costs of devices related to the supply, recovery and filtration of cutting fluid, as well as the costs related to chip and cutting fluid treatment, which not only simplifies the production system, but also saves the production cost; (3) Without polluting the environment, the factory does not need to bear the pollution of cutting waste liquid, and there will be no safety and quality accidents related to cutting fluid

however, if cutting fluid is not used in machining, the cutting state will be further deteriorated, that is, the cutting temperature will rise, the friction between the tool/chip and the tool/workpiece surface will further intensify, the chip removal will be impeded, the tool life will be reduced, and the quality of the machined surface will become worse. Therefore, in order to make the dry cutting process smooth, we must seek technical solutions to make up for the main functions of cutting fluid. This requires careful analysis of various specific boundary conditions and complex factors affecting dry cutting (attached figure)

2 measures to realize dry cutting

1) developing tool materials suitable for dry cutting

developing tool materials suitable for dry cutting is one of the keys to its successful application. In dry cutting, the cutting speed is generally higher than that of ordinary machining, which can accelerate chip removal and heat dissipation, so as to improve the tool life and increase the heat resistance of the tool. However, in high-speed cutting, a large amount of heat will be generated in the cutting area. The larger the cutting amount, the higher the temperature in the cutting area. The traditional methods generally need to add coolant for cooling. However, the use of new tool materials and the selection of reasonable cutting parameters can not only withstand the high temperature in the cutting process for high-speed cutting, but also transfer most of the heat generated to the chips, so that dry cutting can be carried out

dry cutting tool materials should have excellent thermal properties. Commonly used are bonded cemented carbide with high red hardness and good wear resistance, coated cemented carbide, ceramics, coated HSS, diamond and cubic boron nitride (CBN). In recent years, 20% - 30% SiC whiskers have been added to Al2O3 matrix to form "whisker toughened ceramics". The function of SiC whisker is like the reinforcement in reinforced concrete. It can become an obstacle to block or change the direction of crack development, and greatly improve the toughness of cutting tools. It is a promising dry cutting tool material

2) develop tool coating technology

appropriate tool coating can not only enhance the heat resistance of tools, reduce the friction coefficient between tools/chips and tool/workpiece surface, reduce tool wear, but also reduce cutting force and cutting temperature, improve tool life, and improve the quality of machined surface. Therefore, the development of tool coating technology is also one of the keys to achieve dry cutting

at present, the main development trends of dry cutting tool coating technology are as follows: first, the use of "soft" coating, such as the "movic" soft coating process developed by German guhring company, is to coat the tool surface with a layer of solid lubricant -mos2. This coating has lubrication function on the tool surface, which can reduce the friction between chips and tools during processing, avoid chip buildup, and improve tool life. The test shows that when dry tapping is carried out on aluminum alloy containing 9%si, uncoated taps can only process 20 holes, compared with PA66, LN coated taps can process 1000 holes, while movic coated taps can process 4000 holes. The second is to use soft/hard composite coating, that is, first coat the tool with "hard" coating (such as tin), and then coat it with "soft" coating (such as MoS2). For example, if a (Ti, Al) n+mos2 coated drill is used to dry drill deep holes in the cylinder block of gray cast iron engine, the service life of the tool is up to 1600min, while the service life of the drill with only tin or TiCN coating is 19.6min and 44min respectively

3) optimize tool geometry

in dry cutting, the optimization of tool geometry is very important. Dry cutting is not suitable for standard tools. Therefore, the optimized tool geometry should be selected for dry cutting to reduce the friction between tools and chips in processing. The optimization of tool geometry should be done as follows: first, reduce the contact area between the tool/workpiece surface with grading ring, for example, the inverted cone and spiral angle of the drill should be reduced; Second, consider the maximum lubricity of the tool surface to prevent the formation of chip buildup

in addition, during dry cutting, the cutting edges of fine-grained high-quality cemented carbide tools and diamond tools can be slightly passivated to maintain the sharpness of the edges with the strength of their own matrix, so as to reduce the cutting temperature. This can not only maintain the high performance of the tool, but also maintain the best service life of the tool

4) using MQL technology

when dry cutting cannot be fully realized, the "minimum quantities of lubricant (MQL)" technology can be used to minimize the use of cutting fluid. MQL is also known as near dry cutting. The "minimum amount of cutting fluid" here refers to a small amount of cutting fluid supplied to the cutting process (when the machine tool works under the best conditions, the consumption of cutting fluid should be less than 50ml/h, while the consumption of cutting fluid when using normal jet cooling may exceed 6L/min)

mql technology mainly includes two methods: external lubrication and internal cooling by mist. In recent years, the external lubrication method of mist is to send the coolant into the spray cooling system and mix it with the gas. Under high pressure, a multi head nozzle will continuously spray the mist atomized to the surface of the tool to cool and lubricate the tool; The cooling method in the mist is to directly send the cooling mist to the cutting area through the spindle and tool for cooling and lubrication. According to the processing needs, the internal and external air mist cooling and lubrication methods can be used together, and the effect will be better

The biggest advantage of MQL technology is that if used properly, the cutting tools, workpieces and chips can be kept dry, which avoids the trouble of waste disposal. Therefore, it is reasonable to call this technology "dry cutting". At present, MQL technology is mainly used for drilling, reaming and tapping on cast iron, steel and aluminum alloys, as well as deep hole drilling and end milling of aluminum alloys

5) adopt a reasonable processing method

dry cutting processing because cutting fluid is not used, hot cutting chips are easy to stay in the cutting area, increasing the heat of workpiece, tool and machine tool. If the heat is too large, it will also cause the workpiece to harden and the cutter to deform, causing cracks on the surface of the workpiece. Therefore, chip removal has become a major problem. However, with the development of processing technology, adopting reasonable processing methods can also solve the problem of chip removal. For example, instead of the traditional top-down drilling method, bottom-up drilling can use gravity chip removal, and there is no need for a certain pressure of cutting fluid to assist chip removal. In addition, the lubricating mist, gas and chips in the cutting area can be sucked by the suction system; The conditions of chip removal can also be improved by using the air injection system. In some cases where complete chip removal is not possible, a combination of automatic and manual chip removal methods can also be used

in addition to the above methods, other measures can be taken: such as selecting reasonable cutting parameters; Improve the design and manufacturing capacity of the tool, because the complex tool geometry can solve the chip removal problem in the enclosed space and reduce the cutting force; The use of new machine tools with fast and effective chip removal, as well as the development of machine tools specially used for dry cutting processing

3 application of dry cutting processing technology

dry cutting was once a controversial processing method, but with the development of tool materials, coating technology, tool structure and process equipment, as well as the requirements of laws and markets, the United States, Germany, Japan and other countries have carried out a lot of research and applied it to actual production, and achieved obvious economic and social benefits

the "Red Crescent" cast iron dry cutting technology researched and developed by lebond Makino company in the United States is to use ceramics or CBN tools for high-speed machining. Due to the high cutting speed and feed rate, the heat generated will soon gather at the front end of the tool, making the workpiece material there reach a red hot state, and its yield strength will decrease, which can greatly improve the cutting efficiency. Generally, the metal removal rate (turning) of cast iron is 16cm3/min, but it can be increased to 149cm3/min by using red crescent dry cutting. The company also invented a patented process, which can use cemented carbide tools to carry out dry turning of quenched steel (>50hrc) and titanium alloy when the cutting speed reaches 305m/min. The cutting heat is taken away by the high-pressure cooling gas passing through the spindle, the cutting temperature can be reduced to the minimum, and the contact between the tool and the workpiece can be limited to 25% per revolution

syun Ichi Yamagata of Japan has researched and developed the dry extrusion tap of steel, which can significantly improve the tap life compared with the ordinary wet extrusion tap. For example, M4 on cooled rolled steel plate is processed with ordinary extrusion tap × 0.7 through-hole thread, when processing about 7000 holes, the cutting part of the tap will be worn and bonded, making the cutting impossible; However, processing more than 50000 holes with dry extrusion Taps will not produce bonding and significant wear, and the taps can still be used. The application results show that compared with ordinary extrusion Taps, the service life of dry extrusion taps can be increased by several or ten times

German guhring company has carried out research on high-speed dry drilling processing of aluminum parts in engines, achieved remarkable economic benefits, and established a good corporate environmental protection image. In addition, Germany, Finland and Britain have also studied the processing methods of high-speed gears, such as dry hobbing and dry boring, which have been applied in production and achieved good results

at present, the dry cutting in China is mainly the traditional milling of cast iron, while the research and application of other dry machining methods are still very small. (end)

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