Cooling and lubrication technology in the hottest

Cooling and lubrication technology in hard cutting

Abstract: hard cutting is a new manufacturing technology that takes into account the importance of the ecological environment and the economy of the manufacturing industry. Its cutting parameters are very different from the conventional cutting process. In this paper, a new concept of using cutting fluid in hard cutting is put forward, that is, spray cooling, low temperature cooling and minimum cooling lubrication technology. The principles of several cooling methods are given

preface

metal softening effect in hard cutting is an important factor affecting tool life, workpiece surface integrity, cutting efficiency and machining accuracy. Because hard cutting forms chips by annealing and softening the material in the shear part, the cooling efficiency is too high, which reduces the cutting effect caused by the cutting force, increases the mechanical wear, and shortens the tool life. In intermittent cutting, due to the application of cooling, the thermal cycle of the tool is intensified, which will cause thermal fatigue and premature failure of the blade material. Dry cutting provides a healthy manufacturing environment, makes work safer, avoids the harmful effects of chemicals, and comprehensively protects the ecological environment. It has become a hotspot of clean manufacturing research at present. However, it is difficult to completely implement the dry cutting technology without cutting fluid in some machining. Therefore, according to the investigation, the American Occupational Safety and Health Commission (OSHA) proposed new concepts of cutting fluid use, including low-temperature cooling, spray cooling (composite spray cooling) and minimum quantity cutting fluid application (mqcfa), which controls the amount of cutting fluid within the minimum limit

2 low temperature cooling technology

in order to effectively reduce the excessive temperature generated in the cutting area, low temperature cutting technology that directly transmits liquid gas to the cutting area and near the cutting edge of the tool is often used. Cryogenic machining uses the cooling characteristics of liquid nitrogen (-186 ℃), liquid CO2 (-76 ℃) and other low-temperature liquids to greatly improve the tool life, machining accuracy, surface quality and production efficiency. Compared with dry cutting, it can reduce the cutting force by 10% - 20% and the grinding force by about 60%. When cutting general steel, the cutting temperature decreases by 300 ~ 400 ℃

the principles of the low-temperature cutting device are as follows: use the automatic injection of bottled liquid CO2 to directly cool the cutting area. This database will help enterprises replace existing materials or when selecting new composite materials in the process of engineering research and Design: use dried air to maintain the constant pressure of the Dewar bottle, use the siphon principle to let compressed air extract liquid nitrogen from the bottle, and spray it to the cutting area through a special nozzle: use liquid nitrogen or CO2 to cool the workpiece from the outside, To achieve the purpose of reducing the temperature of the cutting area. Some use the internal cooling method of the tool, and even connect the tool directly with the freezer to cool the tool circularly, and the effect is also very obvious. Experiments show that good results can be achieved in cutting titanium alloy, stainless steel, high-strength and wear-resistant cast iron at low temperature. Figure 1 shows the low temperature cutting automatic control device using liquid nitrogen (LN) to directly cool the tool head. The device was developed by Dr. Wang Zhiyong of Nebraska Lincoln University in the United States. It successfully solved the difficulty of machining Si3N4 ceramics with PCBN tools. From the cutting temperature distribution of the finite element model in Figure 2, it can be seen that the low-temperature cooling technology can reduce the temperature of the PCBN cutter head, ensure the red hardness and metal softening effect of the PCBN cutter, and greatly reduce the wear of the cutter

Figure 1 automatic control device for low temperature cooling

Figure 2 cutting temperature distribution of dry cutting and low temperature cutting

the test also shows that the implementation of low temperature cooling technology is conducive to chip breaking. The mechanical properties of ordinary carbon steel are greatly affected by the cutting temperature, and it is easy to embrittle at low temperature, so the chips are brittle and relatively easy to break at low temperature, which provides a feasible basis for low temperature cooling to help break chips in machining

3 spray cooling technology

with the development of high-speed cutting and strong cutting, tools bear much higher pressure, friction and temperature than in the past. Although the progress of tool materials has greatly promoted the improvement of cutting efficiency, the cooling of cutting area is always an important measure to improve tool life. In view of the environmental pollution caused by waste cutting fluid, spray cooling is a better choice. Spray cooling uses the method of vaporization and heat dissipation of droplets to achieve cooling. There are two ways of vaporization: bubbly vaporization and layered vaporization. The atomization cooling cutting area belongs to bubble vaporization. When the fog drops on the surface with higher temperature, a vaporization center is formed. The separation of bubbles drives the fog drop liquid to turn violently, which makes the fog drops vaporize further, takes away the heat, and the small water drops generate phase transformation into steam. Pouring cooling makes the liquid vaporize in layers. When vaporizing, the steam becomes layered. The vaporization layer separates the heated surface from the cooling medium, so liquid pouring cooling only plays a major role in convection and heat conduction

the air flow element of the spray cooling device is mainly composed of two parts: drainage and being drained. According to the position of the drainage part, it can be divided into ejector and annular air flow amplifier. Steam ejector is the most typical type of ejector. Its structure is shown in Figure 3, which is mainly composed of nozzle, suction chamber and diffuser. The function of the nozzle is to generate high-speed turbulence, which disturbs the gas in the suction chamber and drives the gas in the suction chamber to flow into the diffuser together, resulting in airflow amplification

Fig. 3 structural diagram of steam ejector

annular flow amplifier (see Fig. 4) is characterized by the high-pressure gas flowing out of the annular gap with a small gap, forming a high-speed gas flow, flowing into the throat, and then flowing out of the expansion section. During the flow process, a large number of surrounding free air are induced to move together, resulting in a large air flow output. The annular airflow amplifier is composed of three parts: annular airflow chamber, expansion chamber and drainage chamber. It is characterized by exquisite structure. The total airflow can be adjusted by adjusting the airflow parameters in the annular airflow chamber

Figure 4 structure diagram of annular air flow amplifier

under the same cutting conditions, YT15 machine clamp indexable tool is selected to do the comparative experiment of dry cutting and spray cooling cutting at the same time. The workpiece material is j-55 steel, and the general emulsified cutting fluid is used as the spray agent. Observation with a microscope shows that the front face of the spray cooled tool is rarely worn, the tip height is slightly worn, and the negative rear face is not significantly worn, There is a certain amount of wear on the flank. The negative flank of dry cutting is seriously worn, and there are deep grooves. The front flank forms crescent depressions, and the rear flank has a certain amount of wear. During dry cutting, the chip color is dark blue, and during spray cooling, the chip color is earthy yellow. Experiments show that spray cooling is very effective in suppressing cutting heat. After a period of cutting, the tool and workpiece still feel cool when touched by hand, and the tool life and productivity are greatly improved

4 minimum cooling and lubrication technology

the cooling, lubrication, chip removal and other functions of cutting fluid cannot be fully and effectively played in the machining process, and the treatment of waste coolant has become an economic and environmental factor that cannot be ignored. In order to reduce production costs and environmental pollution, the use of cutting fluid must be limited or abolished. Cutting fluid mainly has the following negative problems: increase the manufacturing cost. In the total processing cost of the workpiece, the cost of cutting fluid accounts for 7% - 17%, while the cost of cutting tools accounts for only about 2% - 4%; Untreated cutting waste liquid is discharged into rivers and lakes, seriously damaging the ecological environment; Water based cutting fluid contains some chemical components harmful to human body, which will cause many diseases of workers' respiratory tract and lungs. Direct contact with human body will induce a variety of skin diseases. In the case that dry cutting cannot be effectively implemented at present, quasi dry cutting with minimum cooling and lubrication technology (mqcfa) is a more effective machining method

the key technologies to be mastered in adopting mqcfa are: (1) how to ensure that the lubricating fluid can reliably enter the cutting area and fully and effectively lubricate the machining; (2) How to determine the amount of lubricant required for processing. At present, there are two main methods to solve the first problem: one is "external injection method", which sprays the oil-gas mixture from the outside to the processing area. This method is simple and easy, but the consumption of lubricating fluid is large, especially in January and June 2017, the output of crude steel, pig iron and steel in China was 419.75 million tons, 362.56 million tons and 551.55 million tons respectively, which is not good for some closed processing (such as drilling, reaming, etc.). The other is the "internal spray method", which opens oil-gas channels in the tool, and the oil-gas mixture is sprayed directly into the processing area from these channels to lubricate the processing area quickly and effectively. This method has sufficient lubrication and less consumption of lubricating fluid, which is especially suitable for closed machining, but the tool structure was complex in those days. At present, the second problem is mainly solved by experimental methods. A typical machining center needs 20 ~ 100L cutting fluid per minute in wet cutting, while using mqcfa technology only needs 0.03 ~ 0.2L cutting fluid per hour. OSHA's cutting fluid standard stipulates that the maximum allowable amount of aerosol in the air is 5.0mg/m3, which is expected to reach 0.5mg/m3, and the production cost should not be too high

5 Conclusion

the economy of manufacturing industry should take into account the importance of environmental protection. The cleaner production processing method conducive to environmental protection - hard cutting will be further studied and popularized. The new concept of cutting fluid use represented by low-temperature cooling technology, spray cooling technology and minimum cooling and lubrication technology has attracted the attention of enterprises and academia in developed countries. With the machinery industry facing increasing environmental pressure, factories have to consider adopting environmentally friendly production processes, which will play a positive role in the implementation of China's sustainable development strategy and the realization of economic growth goals. (end)