Knowledge of cutting fluid lubrication
Lubrication in metal cutting technology (lubrication in metal processing II)
(2) Problems and countermeasures in the use of cutting fluid.
1, the problem of wave-cutting metamorphism and odor
The main reason for the deterioration and odor of cutting fluid is that it contains a large number of bacteria, and the bacteria in cutting fluid mainly include aerobic bacteria and anaerobic bacteria. Aerobic bacteria live in mineral environment, such as water, cutting fluid concentrate, machine tool oil leakage and so on. Under aerobic conditions, they split into two every 20 ~ 30 minutes. Anaerobic bacteria live in an anaerobic environment, divide twice every hour, release SO2 by metabolism, give off the smell of rotten eggs, and the cutting fluid turns black. When the bacteria in the cutting fluid is greater than 106, the cutting fluid will stink.
(1) Bacteria enter the cutting fluid mainly through the following ways:
1) There are bacteria invading in the preparation process, such as bacteria in the water for preparing cutting fluid.
2) Bacteria in the air enter the cutting fluid.
3) Cutting fluid infection is caused by the transfer between work procedures.
4) Bad habits of operators, such as littering.
5) The cleanliness of machine tools and workshops is poor.
(2) Methods to control the growth of bacteria
1) Use cutting fluid with high quality and good stability.
2) Using pure water to prepare concentrated solution is not only convenient to prepare, but also can improve the lubricity of cutting fluid, reduce the amount of chips taken away and prevent bacterial erosion.
3) When using, the proportion of concentrated solution in cutting fluid should be controlled not to be too low, otherwise bacteria will easily breed.
4) Because the oil used by the machine tool contains bacteria, it is necessary to minimize the mixing of oil leaked from the machine tool with cutting fluid.
5) When the pH value of cutting fluid is 8.3 ~ 9.2, it is difficult for bacteria to survive, so new cutting fluid should be added in time to improve the pH value.
6) Keep the cutting fluid clean, and don't let the cutting fluid come into contact with pollutants such as dirty oil, food and tobacco.
7) Frequently use fungicides.
8) Keep the workshop and machine tools clean.
9) If the equipment has no filtering device, oil slick should be skimmed regularly to remove dirt.
2. Corrosion of cutting fluid
(1) Corrosion reason
1) The proportion of concentrated solution in cutting fluid is low.
2) The pH value of cutting fluid is too high or too low. For example, when pH > 9.2, aluminum will be corroded. Therefore, the appropriate pH value should be selected according to the metal material.
3) Contact of dissimilar metal materials.
4) Pad the workpiece with paper or wood.
5) Stacking of parts.
6) The number of bacteria in cutting fluid exceeds the standard.
7) The humidity in the working environment is too high.
(2) Methods of preventing and treating corrosion
1) Prepare cutting fluid with pure water, and the proportion of cutting fluid should be used according to the recommended value in the cutting fluid manual.
2) If necessary, use antirust liquid.
3) Control the number of bacteria to avoid the generation of bacteria.
4) Check the humidity, and pay attention to control the humidity of the working environment within a suitable range.
5) Avoid cutting fluid pollution.
6) Avoid contact with different materials, such as aluminum and steel, cast iron (containing magnesium) and copper.
3, the problem of bubbles.
When cutting fluid is used, sometimes a lot of foam will be produced on the surface of cutting fluid.
The main reason of (1) foam.
1) The cutting fluid level is too low.
2) The cutting fluid flow rate is too fast, so there is no time for bubbles to overflow, which accumulates more and more and produces a large number of bubbles.
3) There are too many right angles in the tank design, or the cutting fluid nozzle angle is too straight.
(2) Methods to avoid foaming
1) In the centralized cooling system, the pipelines are connected in series in stages, and the pipeline pressure near the cooling box is lower.
2) Ensure that the cutting fluid level is not too low, check the fluid level in time and add cutting fluid in time.
3) Control the cutting fluid flow rate not to be too fast.
4) When designing the sink, be careful not to have too many right angles.
5) When using cutting fluid, it should be noted that the cutting fluid nozzle angle should not be too straight.
4, the operator's skin allergy problem
(1) The main cause of skin allergy of operators.
1) The pH value is too high.
2) Composition of cutting fluid.
3) Insoluble metals and oils used in machine tools.
4) The proportion of concentrated solution is too high.
5) Protective suspension layer on the surface of cutting fluid, such as odor sealing layer and anti-foam layer. Bactericide and unclean cutting fluid.
(2) At work, in order to avoid the skin allergy of the operator, the following points should be noted.
1) The operator should apply protective oil, wear work clothes and gloves, and pay attention to avoid direct contact between skin and cutting fluid.
2) The proportion of concentrated solution in cutting fluid must be used according to the recommended value of cutting fluid.
3) Use bactericide according to the dosage in the instruction.
In addition, fluororubber and grease rubber are less deformed by cutting fluid, so they can be given priority when used as machine tool seals. In order to prevent deformation, the oil content of rubber used in machine tool seals should generally be greater than 35%. In addition, in order to effectively prevent cutting fluid from causing machine tool paint to fall off, epoxy resin paint or polyurethane paint can be selected.
In a word, if we can pay attention to the above problems when using cutting fluid in normal production, we can avoid unnecessary economic losses and effectively improve production efficiency.
1 development trend of cutting fluid technology
As we all know, cutting fluid has the functions of lubrication, cooling, cleaning and rust prevention, which has obvious effects on improving cutting quality and efficiency and reducing tool wear. In recent ten years, China's cutting fluid technology has developed rapidly, and new varieties of cutting fluid have appeared constantly, and its performance has been continuously improved and improved. Especially, the popularization and application of water-based synthetic cutting fluid produced in the late 1970s and semi-synthetic cutting fluid (micro-emulsified cutting fluid) developed in recent years have opened up a new way for the development of mechanical processing in the direction of energy saving, reducing environmental pollution and reducing industrial production costs. To sum up, cutting fluid technology mainly has the following characteristics:
1) minimum lubrication technology
Dry machining is machining without any cutting fluid, which can fundamentally eliminate the disadvantages of traditional wet machining which is easy to pollute the environment, and is a profound revolution in cutting technology. Dry cutting is a promising new cutting technology, because it meets the requirements of developing green manufacturing technology today. However, dry cutting has the characteristics of high temperature and large cutting force, and there are a series of problems that wet machining does not have. At present, dry cutting technology is still immature and its application scope is limited. However, the traditional wet cutting has many disadvantages. Therefore, some people put forward the quasi-dry cutting technology. Quasi-dry cutting is a cutting method between wet cutting and pure dry cutting, also known as minimum lubrication technology. Quasi-dry cutting adopts spray cooling method, which can play a better cooling and lubricating role with little cutting fluid.
2) Liquid nitrogen cooling technology
Low-temperature (ultra-low-temperature) cutting with liquid nitrogen is a method to cool the workpiece, tool or cutting area at low temperature by using liquid nitrogen. Nitrogen is the most abundant component in the atmosphere, and liquid nitrogen, as a by-product of oxygen industry, has a wide range of sources. Using liquid nitrogen as cutting fluid can directly volatilize into gas and return to the atmosphere without any pollutants. From the point of view of environmental protection, it is a promising cutting substitute.
Liquid nitrogen cooling has direct and indirect application methods.
A. direct application. That is, liquid nitrogen is directly sprayed into the cutting area as cutting fluid. Generally speaking, diamond tools can't be used to process ferrous metals due to serious tool wear. However, an American scholar used a liquid nitrogen cooling system to replace diamond tools with stainless steel. Because the low temperature inhibits the diffusion and graphitization of carbon atoms, the tool wear is greatly reduced, and good machining quality is obtained, and its surface roughness reaches Ra 25 μ m. During grinding, the high temperature in the grinding area often causes thermal damage to the workpiece surface, such as burns and microcracks. In order to solve these problems effectively, S.Paul of Indian Institute of Technology grinds five kinds of common steels with liquid nitrogen ultra-low temperature cooling. The results show that the proper and reasonable use of liquid nitrogen cooling can effectively control the temperature in the grinding zone and keep the grinding temperature below the phase transition temperature of the material without grinding burn. Moreover, this effect is more obvious when the plasticity of the material increases and the feed speed is higher. Liquid nitrogen cooling processing of nonmetallic materials and composite materials has also been widely studied abroad. For example, FRP (fiber reinforced plastic) is a composite material with high strength/weight ratio and fatigue resistance. It is difficult to process by traditional cutting methods, which limits the use of this material. New Zealand scholars have carried out ultra-low temperature cooling processing, using liquid nitrogen for continuous cooling (0.4-0.5L/min), which greatly improves the machinability of this material, not only obtains satisfactory machined surface quality, but also greatly prolongs the tool life. It also shows good cutting performance when cutting thermosetting plastics, synthetic resin, graphite, rubber and glass fiber at low temperature.
B. indirect use. Mainly the tool cooling method, that is, the tool is continuously cooled during machining, so that the cutting heat is quickly taken away from the tool, especially at the tip of the tool, which always works at low temperature. Scholars at Lincoln University in the United States used a PCBN tool equipped with a new cooling system for experimental research. This tool stores liquid nitrogen in a square box at the upper part of the turning tool, inputs it from the inlet and flows out from the outlet. The test shows that when cooled by liquid nitrogen, the service life of turning tool can be doubled, the wear can be reduced by 1/4, and a lower surface roughness can be obtained.
Another special indirect utilization method is jet cooling. Some Japanese scholars have developed a jet cooling system. The cooling gas used in the system is cooled by liquid nitrogen in the heat exchanger, and its temperature is lower than -50℃, and the cooling gas is directly injected at the grinding point. The experimental results show that the residual compressive stress of workpiece material after grinding is larger than that when grinding with grinding fluid, and the distribution area of residual compressive stress is wider. Residual compressive stress can improve the fatigue life of parts, which is very important for some parts, such as aircraft parts. When machining, CBN grinding wheel treated with solid lubricant or adding a very small amount of superfine vegetable oil (10 1) can play a good lubrication role.
It can be seen that with the increasingly strict environmental protection laws and regulations and the increasing cost of cutting fluid use and treatment, liquid nitrogen may become a substitute for cutting fluid in a certain range.
3) Centralized cooling lubrication system
Centralized cooling and lubrication system is to combine independent cooling and lubrication devices of multiple processing equipment into a cooling and lubrication system. The main advantages of this system are: (1) prolonging the service life of cutting fluid; (2) It is easy to realize the automatic control of cutting fluid performance index and ensure the cutting fluid quality; (3) less waste liquid, convenient for centralized treatment, beneficial to protecting the ecological environment, (4) convenient for maintenance, maintenance and management; (5) It is convenient for chip transportation and centralized processing.
4) Realize the automation of cutting fluid quality management.
Strict and scientific daily management is the key to ensure that cutting fluid can meet various quality requirements for a long time and give full play to economic benefits. With the continuous improvement of machining automation and the emergence of unmanned chemical plants (workshops), it is imperative to realize automatic detection and control of cutting fluid in cooling and lubrication system. The detection and control system can automatically detect and control the working temperature, concentration, PH value, microbial quantity and odor of cutting fluid, and can predict the failure of cutting fluid.
5) Cutting fluid with high performance, long service life and low pollution and its waste liquid treatment technology.
With the development of the whole technology of machinery industry, the cutting speed of machine tools is faster, the cutting load is greater and the cutting temperature is higher. At the same time, new technologies to adapt to the processing of new materials are constantly emerging, which requires new high-performance cutting fluids to meet the processing requirements. At the same time, according to the requirements of labor hygiene and environmental protection, cutting fluid should try not to contain substances harmful to human health and ecological environment. In recent years, more and more advanced equipment such as CNC machine tools and machining centers have been imported from China. If the cutting fluid used is imported from abroad, the transportation is inconvenient and the cost is high. Therefore, it is urgent to develop high-performance cutting fluid to replace imported products. In China, the application of water-based cutting fluid is more and more extensive, and it has begun to transition from emulsion to synthetic cutting fluid and microemulsion with good performance and long life. Microemulsion has been widely used in developed countries, and environmental protection cutting fluid is developing vigorously.
Develop long-life cutting fluid, study the methods to prolong the service life of cutting fluid and reduce the discharge of cutting fluid waste liquid; It is an important content of cutting fluid research at home and abroad to study more effective and economical treatment methods of waste liquid and reduce the accumulation of harmful pollutants in the environment.
2 cutting fluid additives
The performance of cutting fluid, the difficulty of waste liquid treatment, the harm to operators and the degree of environmental pollution all depend on the types and properties of these additives. As a future cutting fluid additive, it must have excellent performance and be harmless to human body and environment. A large number of application practices show that the following additives should be strictly restricted: nitrite and similar compounds, phosphate compounds, chlorides, formaldehyde and similar compounds. The effective way to improve the quality of cutting fluid is to research and develop new efficient and non-toxic additives. This has attracted the attention of scholars at home and abroad, and a lot of work has been done to develop some new additives:
1) boric acid cooling additive
Boric acid vinegar is a multifunctional environmental protection additive. ① Borate vinegar has high oil film strength, low friction coefficient and excellent antifriction and wear resistance; ② It has good antirust performance; ③ It has antibacterial, bactericidal and non-toxic effects.
2) Molybdate inhibitor
Molybdate is a corrosion inhibitor. Adding it into cutting fluid can form a passive film of Fe-MoO4-Fe2O3 on the metal surface, which has good corrosion inhibition effect. Generally speaking, key acid salt, as an antirust agent, is non-toxic and does not pollute the environment, but it is expensive, which affects its wide use. In order to promote its application, people have studied the synergistic effect of key acid salts and other organic and inorganic corrosion inhibitors.
3) New preservatives and fungicides
Cutting fluid itself has the conditions for microorganisms and fungi to breed and reproduce, and it is easy to rot and deteriorate. The function of antiseptic and bactericide is to inhibit the growth of bacteria and mold and kill the bacteria in the liquid, thus prolonging the service life of cutting fluid. In recent years, due to the promulgation and implementation of environmental protection laws and regulations, many countries are restricting the use of toxic antiseptic fungicides and developing new antiseptic fungicides. According to Japanese patent, copper salts made of oleic acid, stearic acid and other fusulinic acids have a corrosion resistance of over 1 year. According to American data, copper citrate also has a good antibacterial effect, and the addition amount is 300PPM. An emulsified oil named MobilmentAquaRho was developed by American Mobil Chemical Company, and the composition containing phosphorus, nitrogen and boron was added. The service life of the emulsion reached more than 4 months.
3 Conclusion
In recent years, scholars at home and abroad have focused on cutting technology without cutting fluid, but for most materials and processing types, it is far away to completely replace wet cutting and wet grinding, if possible. We can't avoid the reality that cutting fluid will be used for a long time in the future. To solve the cleaning problem in the process of mechanical manufacturing, we should take two measures, that is, while studying the technology of no cutting fluid, we should pay attention to the transformation and innovation of cutting fluid itself, so that dry machining and wet machining can complement each other. Therefore, we put forward the research policy of "developing new processing technology without cutting fluid from the point of view of environmental harmlessness of cutting fluid itself" The research work should focus on the following aspects:
Develop green cutting fluid that is completely harmless to people and the environment;
Optimize the liquid supply mode and parameters to reduce the consumption of cutting fluid;
Study the scientific management technology of cutting fluid use, try to prolong its service life and reduce waste liquid discharge;
Study the recovery and harmless treatment technology of cutting fluid waste cone;
Study the comprehensive evaluation method of cutting fluid considering environmental impact, and provide technical support for enterprises to choose cutting fluid correctly;
Research various machining methods without cutting fluid, especially dry cutting technology. Study the best process parameters of tools, workpieces and machine tools, and expand the application scope of dry cutting technology;
Research and development of new efficient and nontoxic additives.
Metal cutting fluid plays an important role in metal cutting and grinding. Practice has proved that choosing proper metal cutting fluid can reduce the cutting temperature by 60~ 150℃, the surface roughness by 12, the cutting resistance by 15~30%, and the service life of tools and grinding wheels can be doubled. It can also wash away the iron filings and ash powder in the cutting area, thus improving the production efficiency and product quality. Therefore, it is widely used in machining.
Cutting fluid shall have the following functions:
In the process of workpiece cutting, cooling can reduce the temperature of cutting area in time and quickly, that is, reduce the temperature rise usually caused by friction. Cooling also affects cutting efficiency, cutting quality and tool life.
Lubrication can reduce the friction between the tool and the workpiece. Lubricating liquid can penetrate between the tool and the workpiece and its chips, reducing friction and adhesion, reducing cutting resistance, ensuring cutting quality and prolonging tool life.
The cleaning action causes chips or abrasive particles to be washed away from the machining area of the tool and workpiece to prevent them from adhering to each other and to the workpiece, tool and machine tool.
Anti-rust function should have certain anti-rust performance to prevent the workpiece and machine tool from rusting. If the antirust performance is improved, it can also partially replace the inter-process antirust.
The above four functions of cooling, lubrication, cleaning and rust prevention are not completely isolated, they have unified aspects and opposing aspects. For example, cutting oil has good lubrication and rust prevention performance, but poor cooling and cleaning performance; The cooling and cleaning performance of aqueous solution is good, but the lubricating and antirust performance is poor. Therefore, the advantages and disadvantages should be comprehensively weighed when choosing cutting fluid.
The performance of cutting fluid shall meet the following requirements:
Large heat capacity, good thermal conductivity and good cooling effect.
It has high oiliness or adsorption on metal surface. The formed adsorption film has high strength, can be firmly adsorbed on the metal surface, and plays a good role in lubrication.
Good antirust performance, no corrosion to metals, and no damage to the accuracy and surface roughness of machine tools and workpieces due to corrosion.
Low surface tension, easy to spread evenly, beneficial to cooling and washing, and good lubricity.
It is convenient to use, low in price and easy to configure, and is most suitable for various metal materials and various processing methods (such as turning, grinding, planing, etc.). ). It has certain transparency and does not smoke when the cutting speed is increased.
Harmless to human body, non-toxic and tasteless. No harm to skin and nasal mucosa, no irritation to eyes and so on.
Good stability and long service life. During long-term use and storage, it will not be layered, precipitated, mildewed or deteriorated.
The amount of cutting waste liquid is large, so waste liquid treatment should be considered to avoid environmental pollution.