In the field of mechanical manufacturing, cutting fluids used in cutting (grinding) are important for improving machining efficiency and processing quality, but cutting fluids cause serious pollution to the environment during various periods of manufacture, use, handling and discharge. After the completion of the machining process, so-called "secondary effect" contamination occurs when the cutting fluid deposits on the surface of the part are removed.
According to statistics, the use cost of cutting fluid is quite expensive, adding up the cost of cutting fluid and related equipment, energy consumption, handling, labor, maintenance and materials to 7% to 17% of the total manufacturing cost. All tool costs are only 2% to 4% of the total manufacturing cost.
In 1996, the International Standards Association issued the ISO14000 series of standards for environmental management. Countries such as Germany, the United States, Canada and Japan have also developed more stringent industrial emission standards, further limiting the use of cutting fluids.
Faced with lower production costs, improved production conditions and sustainable development requirements for environmental protection, and more pressure from relevant laws, rational use of manufacturing resources, less waste, less environmental pollution, and a virtuous cycle of "green manufacturing" technology It will surely become the main development trend in the 21st century manufacturing field.

1 Principles and characteristics of green processing technology


In the process of searching for green processing technology to replace the original cutting fluid casting method, many beneficial attempts have been made to create a variety of representative cutting lubrication cooling technologies. Biostable cutting fluid, spray cooling, liquid nitrogen cooling , low temperature air cooling cutting, MQL and dry cutting. From the perspective of quality, efficiency, economy and environmental protection, each of these technologies has its own characteristics and superiority.
  1. Biostable cutting fluid <br> Biostable cutting fluids rely on the specific ingredients contained in their formulations to extend their service life. When the content of this specific component is reduced (such as after discharge), it does not have this antibacterial function. It is easily decomposed by living things and is basically harmless to the human body and the environment. Bio-stable cutting fluids have been formed into a series of products abroad, and have been introduced to the Chinese market, such as Mobilcut series of water-soluble cutting fluids, German ARAL series cutting fluids, and water-soluble cutting fluids of British company.
  2. Spray cooling <br> Spray cooling is performed by a mixture of gas-liquid two-phase fluids sprayed into the processing zone by high-pressure (3 to 6 atmospheres) atomization. The atomized particle particles undergo phase transformation under the high temperature of the cutting zone. (Vaporization), absorbing a large amount of cutting heat. The fine mist carried in the high-speed airflow easily penetrates into the cutting contact area, improves the permeability of the cutting fluid, reduces the friction and friction heat, and the lubrication function is fully exerted. Chip removal problems associated with chips are also eliminated to varying degrees. In addition, the processing cost of the cutting fluid is eliminated, and environmental pollution is reduced. The medium commonly used for spray cooling is water, emulsion, vegetable oil and animal oil, among which the emulsion is the most widely used.
  3. Low-temperature air-cooled cutting <br> Low-temperature air-cooled cutting is a method of strongly scouring the processing area with a cold air of -10 to -100 °C. It can significantly reduce the temperature of the processing area, tool and workpiece, effectively suppress tool wear, improve tool durability and improve the processing quality of the machined surface. If a small amount of lubricating oil is supplied in the cold air, the improvement effect is more pronounced. Currently commonly used media are air, nitrogen (there are two ways to heat the liquid nitrogen to the desired temperature and normal temperature to cool to the desired temperature) and carbon dioxide (dry ice evaporation).
  4. Liquid nitrogen cooling <br> The liquid nitrogen cooling method uses liquid nitrogen to cut the workpiece, tool or cutting zone in a low-temperature state, and uses the low-temperature brittleness of the material to complete the chip and the toughness and plasticity. Separation of the workpiece.
  5. Dry Cutting <br> Dry cutting is a process that does not use cutting fluid during machining. It eliminates a series of environmental negative effects caused by cutting fluid from the source.
    Dry cutting technology originated in Europe and is currently the most popular in Western European countries. LeBlond Makino Company of the United States has also developed a "red crescent" process for high-speed dry cutting of cast iron with ceramic tools and CBN tools and put it into actual production, which has achieved good economic benefits.
    The implementation of dry cutting does not simply stop the use of cutting fluid. It is necessary to consider the entire process system including machine tools, workpieces, and especially tools. The new types of tools that are currently available (including tools with advanced materials, tools for coating treatment, and tools that optimize geometry and structure) provide a strong guarantee for dry cutting. Research on the properties of tool materials is also necessary. In fact, for ceramic tools, since dry cutting reduces the possibility of thermal shock and thermal fatigue, the processing effect is often better than ordinary wet cutting.
    Dry cutting applications have been successful in cast iron materials, but their range is not yet extensive. Dry cutting of steel and difficult-to-machine materials is also being studied.
  6. MQL
    MQL technology combines compressed gas with a very small amount of lubricating fluid and then ejects it into the processing area to effectively lubricate the processed part between the tool and the workpiece. In Germany, there are still differences of opinion on whether it has a cooling function at the same time, and it is called micro-lubrication and micro-lubrication cooling, respectively.
    MQL can greatly reduce the friction between “tool-workpiece” and “tool-chip”, which can suppress temperature rise, reduce tool wear, prevent sticking and improve the quality of workpiece processing. The lubricating fluid used is rare, but the effect is small. It is very remarkable, which not only improves the work efficiency, but also does not pollute the environment.
    The amount of lubricating fluid used in the MQL method is very small, typically 0.03 to 0.2 L/h, and the amount of cutting fluid used in a typical machining center is as high as 20 to 100 L/min. Moreover, as long as the MQL technology is used properly, the processed tools, workpieces and chips are dry, avoiding the later processing. The clean and clean chips can be recycled after being compressed and completely polluted, so it is called quasi-dry. Cutting.

2 Research Status of MQL


Experiments show that MQL technology has different processing performance under different types and amounts of cutting fluid, different processing materials, different tools, different processing methods and different processing parameters.
  1. MQL cutting of aluminum alloys <br> When using high-speed milling of aluminum alloys with MQL technology, the tool life can be comparable to that of cast cutting. Moreover, in the case of oil mist lubrication, the material on the tool adheres little.
    The experimental results of drilling aluminum alloy with the amount of vegetable oil of 20ml/h show that under low speed and low feed, MQL has less cutting effect than casting; at higher cutting speed and higher feed, the effect of MQL It is better than casting.
  2. MQL cutting of ordinary steel parts <br> When milling S45C steel with 30ml/h vegetable oil, MQL is very effective in suppressing tool wear compared to conventional casting cutting at a cutting speed of 750m/min or less. At 750m/min, gradually increase the amount of vegetable oil to 30ml/h, and found that the tool wear condition improved with the increase of the dosage. If the amount of vegetable oil is continuously increased, it will not help the tool wear. In addition, when the cutting speed is above 750m/min, the tool wear is rapidly intensified, and the effect of MQL is not satisfactory.
    When milling ASSAB 718 HH steel with 8.5ml/h BP CILORA 128 (high viscosity) cutting fluid, MQL technology greatly reduces cutting force at low speed, low feed, and low depth of cut compared to conventional cast cutting. The flank wear is reduced, the quality of the machined surface is improved, the concentrated thermal stress in the chip is avoided, and the weight and length of the burr are reduced.
    When the S45C steel is turned at a normal speed of 9.6 ml/h of a certain water-soluble oil, the MQL achieves the same level of conventional cast cutting in suppressing tool wear, improving surface roughness, and controlling built-up edge formation. If a cutting fluid containing an extreme pressure agent is used, the tool wear can be more effectively extended.
    When the medium carbon steel was turned at a normal speed of 200 ml/h, the results showed that MQL was better than cast cutting at low speed and high feed. It not only reduces the cutting force and feed force, but also reduces the variation of the cutting force, thus reducing the wear of the tool due to vibration during cutting. The quality of the machined surface has been improved and the thickness of the chips is also greater than the thickness of the chips under cast cutting.
    The combination of MQL technology and coated tools enables the best results. When X90GrMoV18 alloy steel was processed using a high speed steel coated drill, a through hole having a diameter of 8.5 mm and a length of 25 mm was machined at a cutting speed of 30 m/min and a feed rate of 0.1 mm/r. When using pure TiAlN coated high speed steel Drill Bits for pure dry drilling, the drill bit is damaged after drilling a cutting length of 3.5 m; with a (TiAlN+MoS 2 ) composite coated drill bit and minimal lubrication, the drilling length is increased to 115 m.
  3. MQL cutting of difficult-to-machine materials such as titanium alloy, high-silicon aluminum alloy and stainless steel <br> In the high-speed cutting of titanium alloy by MQL technology, the tool is severely peeled off and chip discoloration. It shows that there is still a certain gap between the titanium alloy with low-lubrication and high-speed machining of difficult-to-machine materials compared with traditional casting.
    The experimental results of cutting high-silicon aluminum alloy parts and stainless steel by MQL combined with low-temperature air-cooling technology show that this method can prolong the service life of the tool, inhibit the formation of built-up edge, improve the surface precision, and eliminate waste liquid and Waste treatment system to reduce production costs and prevent environmental pollution.
    The experimental results of continuous turning stainless steel with MQL combined with water mist (cooling) composite spray lubrication cooling method show that this method can effectively play a role in the continuous cutting of difficult lubrication in the cutting zone, which can improve the quality of the machined surface and reduce tool wear. Moreover, the lubrication and cooling effects can be adjusted at any time by changing the amount of oil mist and water mist in the spray.

3 Practical application of MQL


At present, MQL technology is mainly used for drilling, reaming and tapping on cast iron, steel and aluminum alloy, as well as deep hole drilling and face milling of aluminum alloy.
Tbyssen in the United States integrates the lubrication system into the main shaft. The flow rate is controlled by the CNC program. The unit can drill 10 holes with a diameter of 8mm and a center distance of 20mm in 6.5s, using a cup of lubricating oil per hour. Part of it is evaporated, and the amount of cutting fluid in the chips is greatly reduced, so the processing cost is greatly reduced.

a. automobile manufacturing b. auto parts industry c. machine tool manufacturing d. handicraft industry e. other industries f. high speed machining milling machine g. machining center h. special plane i. universal machine tool k. other machine tools
Figure 1 Application status of micro-lubrication

In Germany, the MQL plant has 15,000 sets of markets per year in recent years (see Figure 1) and will increase further. The combination of MQL and new tools is also in the ascendant. It can be predicted that in the next two or three years, 5% of the German-made machining centers will use MQL combined with lubricated coated tools to replace cast cooling.

4 MQL environmental and economic performance evaluation


In many metal processing, oil lubricants are strongly agitated to form contaminated oil and gas mist in the air of the workshop. The additives, microorganisms and other components in the lubricant may also be inhaled by the operator after atomization. Respiratory system, digestive system cause harm, and may even cause cancer; in addition, oil mist will form a layer of adhesive on the upper part of the plant, causing trouble for daily maintenance. Therefore, it is necessary to study the concentration of air oil mist particles caused by different processing methods.
The traditional pouring method is different from the concentration of oil mist particles in the air caused by MQL. A joint research project between the University of Cicinati and Techsolve. Inc. in the United States conducted a comparison of the concentration of airborne oil mist particles produced by MQL and cast cutting, at 11 ml/min and 6.5 l/min respectively at the Tongil TNV-80CNC vertical machining center. The amount of AISI/SAE 4340 steel was drilled and milled. The results show that the oil mist particle generation rate (the amount of particles produced per minute) caused by the MQL method at lower cutting speed and metal removal rate is 340 to 3300 times that of the conventional casting method at the time of drilling; It is 100 to 140 times. At higher speeds and metal removal rates, this ratio will be greater.
In addition, the oil mist particle formation rate of different cutting fluids is also different. Under the same conditions, the production rate of oil mist particles of pure synthetic liquid is much higher than that of water-soluble cutting fluid, and even the concentration of airborne oil mist in the workshop exceeds the current OSHA (Occupational Safety and Health Administration) and NIOSH (National Institute for Occupational Safety and Health) regulations. Standard.
Compared with the traditional casting cutting method, MQL can greatly reduce the amount of cutting fluid, avoid the disposal of waste liquid, reduce the processing cost (including the purchase cost of cutting fluid, storage cost and waste disposal cost, etc.), and under certain conditions. Can achieve equal or even better processing performance. It marks a crucial step in getting rid of traditional cast machining. However, it also faces its own inherent challenge of eliminating the odor, bacteria and enzymes caused by traditional cutting fluids, while at the same time causing a rapid increase in the concentration of oil mist particles in the processing area. The environment and health have adversely affected. In addition, due to the high amount of oil mist particles generated, the machine tool using MQL processing method must have closed, suction ventilation and air freshening facilities, which brings additional cost to MQL processing.

5 MQL issues to be further studied

  1. Research on the mechanism of action of MQL <br> The current experimental research is mainly aimed at the research of certain materials and several different processing methods. The conclusions are only partial conclusions. It is necessary to further explore the uniformity of the oil film during MQL cutting. The law of permeability and lubrication cooling performance and the inherent law of the essential changes of workpieces, tools and chips under different processing methods and cutting amounts.
  2. Optimization of MQL lubricant delivery direction, outlet pressure and oil mist usage <br> Generally speaking, the internal spray method can quickly and effectively lubricate the processing area compared to the external spray method, which is especially suitable for closed machining, but the tool structure More complicated. For the external spray method, the amount of oil output by the oil pump and the amount of oil sprayed from the nozzle are different from the amount of oil actually delivered to the tool for lubrication. The effect of different nozzle orientations on tool wear is significant. The most effective nozzle orientation in milling is that the oil mist outlet is facing the tool cut-out point. The combination of different parameters of these three factors has an optimization problem for improving the processing performance, reducing the production cost and benefiting the overall effect of environmental protection, and further research is needed.
  3. MQL combined with new tools, cryogenic air cooling technology and water mist cooling technology for high-speed machining of difficult-to-machine materials The MQL method can provide comparable or even better lubrication performance than cast cutting, but its cooling effect is not satisfactory. The large amount of heat generated during high-speed cutting of difficult-to-machine materials cannot be dissipated in time, causing the cutting temperature to rise rapidly, resulting in a drastic decrease in lubricant lubrication performance and poor processing performance. Therefore, the in-depth study of MQL technology combined with new tools and low-temperature air-cooling and water-cooling technology with strong cooling performance is of great significance for high-speed machining of difficult-to-machine materials.
  4. Effect of processing parameters on the generation rate of oil mist particles <br> Processing parameters such as cutting speed, depth of cut, tool material, tool geometry, workpiece material, etc. all have an effect on the oil mist generation rate. For example, at higher cutting speeds, higher cutting temperatures are produced, which accelerates the evaporation of the cutting fluid, resulting in higher air oil mist concentrations.
  5. Design and Research of MQL Oil Mist Generation Device <br> The measurement results of oil mist concentration in different orientations of processing area show that: An important source of oil mist particles in air is not the processing process, but the process of conveying oil mist to the processing area. At present, most of the MQL devices use compressed air as the carrier of the oil mist, so the focus of the next research will focus on the design of the trace oil mist transfer unit. In addition, the size and distribution of the oil mist particles generated by the device also have a certain influence on the processing performance.
  6. Study on the safety and protection technology of oil mist particles <br> The smaller the oil mist particles generated during processing, the easier it is to float into the air and be inhaled by the human body. One of the existing methods for controlling the particle size is to add a trace amount of a polymer such as acrylate or polyisobutylene to form a larger, heavier particle with an oil mist droplet, which is less likely to diffuse into the air. For the MQL method, in-depth study of the size and distribution of oil mist particles, measures to control its size and concentration, and development of protective technology for oil mist equipment have great significance for its development on green and economic processing roads. .

6 Summary


Many of the technologies emerging during the green cutting research process are a major innovation in traditional production methods. As one of the important branches of these technologies, MQL can greatly reduce friction, suppress temperature rise, reduce tool wear, prevent adhesion and improve the quality of workpiece processing. The use of lubricating fluid is rare, the effect is very significant, which not only improves the work efficiency, but also It will pollute the environment. MQL will be more widely valued and promoted due to its own superiority.

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