1 Introduction With the rapid development of high-speed processing technology, the amount of cutting fluid used in the processing process is increasing, and its flow rate is sometimes as high as 80 to 100 L/min. However, the use of a large amount of cutting fluid has caused a very prominent negative impact: (1) The production cost of parts has been greatly increased. In the total cost of parts processing, cutting fluid costs account for about 16%, while the cost of cutting tools only accounts for the total cost. 4%. (2) Severe pollution to the environment, such as discharge of untreated cutting fluids into rivers, lakes and seas, will pollute land, water and air, seriously affecting the growth of animals and plants and destroying the ecological environment. (3) Directly endanger the physical health of shop workers. The water-based cutting fluids that are widely used in production currently contain chemical components that are harmful to the human body. In the cutting (grinding) process, the cutting fluid is volatilized by heat to form smoke, and the unpleasant odor is often filled in the workshop, which can cause many diseases in the lungs and respiratory tract of the operating workers. The direct contact between the human hand and the cutting fluid can induce various Skin diseases directly affect the health of workers. The above negative impact has become a major obstacle to the development of the machinery industry. This leads people to ask such a question: Can we use or use less cutting fluid in machining? The Dry Cutting technology came into being in this historical context and it has developed rapidly since the mid-1990s. Dry cutting technology originated in Europe and is currently the most prevalent in Western European countries. According to statistics, about 8% of German companies have adopted dry cutting technology. By 2003, more than 20% of the German manufacturing industry will use dry cutting technology. In the field of dry cutting research and application, Germany is currently in an internationally leading position. Japan has successfully developed a dry machining center that does not use cutting fluid. The dry cutting system equipped with liquid nitrogen cooling extracts high-purity nitrogen from the air, and sends liquid nitrogen to the cutting zone at a pressure of 5 to 6 atmospheres at room temperature to achieve dry cutting smoothly. China Chengdu Tools Research Institute, Shandong University of Technology and Tsinghua University have conducted systematic research on superhard cutting tool materials and tool coating technologies. Ceramic cutting tools currently have a certain production capacity in China. These are all dry cutting technologies. Research and application laid a preliminary technical foundation. Beijing Machine Tool Research Institute has recently developed a KT series machining center that can achieve high-speed dry cutting. 2 The function of cutting fluid and the main difficulty in achieving dry cutting Dry cutting is not simply stopped using cutting fluid. Must analyze what happens after the cutting fluid is deactivated? What measures should be taken to eliminate these adverse effects? To this end, first of all, there is a comprehensive and in-depth understanding of the role and function of cutting fluid in machining. In machining, the cutting fluid has three main functions: Lubrication function - When the cutting fluid enters the cutting area, it will penetrate into the contact surface between the tool, workpiece and chip, forming a layer of lubricating film. This layer of lubricating film can reduce the friction during the cutting process, reduce the cutting force, reduce the adhesion phenomenon between the chip and the tool, and suppress the generation of the built-up edge, which is beneficial to improving the technical quality of the machined surface. Cooling function - more than 90% of the energy consumed in the cutting process is converted into cutting heat. The cutting fluid can effectively remove the cutting heat from the machining area of ​​the machine tool, thereby greatly reducing the temperature rise in the cutting area of ​​the machine tool and improving the durability of the tool and the machining accuracy of the workpiece. Assisting chip evacuation and chip breaking: The cutting fluid acts as a high-pressure and large-flow rinsing agent, which can spur small swarf away from the workpiece or tool, and quickly discharge the swarf from the machine tool, so as to improve the machining accuracy of the parts and the service life of the tools. Strip chip is mainly based on the geometry of the tool to achieve chip breaking, but the high pressure cutting fluid also plays an auxiliary role in chip breaking and chip removal. When performing dry cutting, the lack of the above-mentioned lubrication of the cutting fluid, cooling and auxiliary chip removal and chip breaking, etc., the cutting heat will increase sharply, the temperature in the processing area of ​​the machine tool will rise significantly, and the tool life will be greatly reduced. For dry cutting to go smoothly, to reach or exceed the processing quality, productivity, and tool durability in wet machining, a series of measures must be taken from the aspects of the tool, machine tool, and workpiece. Therefore, dry cutting technology is a huge system engineering, and the biggest difficulty lies in how to improve the performance of the tool in dry cutting. At the same time, it also puts forward new requirements for the structure of the machine tool, workpiece materials and process. 3 Dry cutting tool technology The ability of the tool to withstand the enormous thermal energy during dry cutting is the key to achieving dry cutting. The main measures are:
Fig. 1 Relationship between hardness and temperature of different materials
The use of new tool material for dry cutting not only requires high red hardness and thermal toughness of the tool material, but also must have good wear resistance, thermal shock resistance and anti-adhesion. Figure 1 shows the hardness versus temperature for several tool materials. As can be seen from the figure, the hardness of ceramic cutting tools (Al2O3, Si3N4), Cermet, etc. is rarely reduced at high temperatures, ie, it has good red hardness and is therefore suitable for general purpose dry cutting. However, these materials are relatively brittle and have poor heat toughness and are not suitable for interrupted cutting. Lithium nitride (CBN), polycrystalline diamond (PCD), ultra-fine carbide, and other superhard cutting tool materials are widely used for dry cutting. The use of coating techniques to coat the tool is an important way to improve tool performance. There are two broad categories of coated tools: one is "hard" coated tools such as TiN, TiC, and Al2O3. This type of tool has a high surface hardness and good wear resistance. The other is "soft" coated tools, such as: MoS2, WS and other coated tools, such coated tools are also known as "self-lubricating tool", and its friction coefficient with the workpiece material is very low, only about 0.01, can Reduce cutting force and reduce cutting temperature. Cutting experiments show that the uncoated tap can only be machined with 20 tapped holes; 1000 tapped holes can be machined with TiAlN tapped taps, and 4000 tapped holes can be machined with MoS2 coated taps. After high-speed steel and cemented carbide are treated with PVD coating, they can be used for dry cutting. Originally suitable only for CBN tools for dry cast iron, it can also be used for processing steel, aluminum alloys and other superhard joints after coating. In fact, the coating has a function similar to the coolant. It creates a thermal barrier so that heat will not or rarely pass into the tool, which will keep the sharpness and sharpness of the tool tip longer and longer. The coating also protects the tool material from chemical reactions during high-speed dry cutting. In the development of dry cutting technology, special attention should be paid to the effective application of coated tools. Optimizing Tool Parameters and Cutting Capacity The geometry and structural design of the tool must meet the requirements of dry cutting for chip breaking and chip removal. Chipbreakers play a key role in chip breaking in the processing of ductile materials. At present, the design and manufacturing technology of the three-dimensional surface chipbreaker of the lathe has been relatively mature, and the corresponding chipbreaker structure and size can be quickly designed for different workpiece materials and cutting amounts, and the chip breaking capacity can be greatly improved. Chip flow direction control capability. High-speed machining has the advantages of small cutting force, heat dissipation, and good process stability. The organic combination of high-speed cutting technology and dry cutting technology will gain multiple benefits such as high production efficiency, good processing quality, and no environmental pollution. 4 Dry-cutting machine technology There are two special issues to consider when designing a dry-cutting machine tool: one is the distribution of cutting heat; the other is the discharge of chips and dust. When dry cutting, the heat generated in the processing area of ​​the machine tool is relatively large. If it is not discharged from the main structure of the machine tool in time, it will cause thermal deformation of the machine tool, affecting the accuracy of the workpiece and the reliability of the machine tool. For some heat that cannot be discharged, the relevant components should be insulated. In order to facilitate chip removal. Dry-cutting machine tools should use vertical spindles and inclined bed bodies whenever possible. The inclined cover plate on the worktable can be made of thermal insulation material. A large amount of hot chips are fed directly into the spiral flutes. The suction system prevents the accumulation of hot chips on the table and other supporting components The built-in circulating cooling system is used to improve the thermal stability of the machine process system. Temperature sensors are installed in certain key areas of the processing area to monitor changes in the temperature field of the machine tool and, if necessary, to perform accurate error compensation through the numerical control system. The filter system filters out the dust particles generated during the dry cutting process and is sucked away by the exhaust system. The dust-producing processing area should be strictly isolated from the spindle components and hydraulic and electrical systems of the machine tool. In addition, micro-pressure can be applied to these components to prevent the intrusion of dust. For dry cutting of reinforced plastic such as aluminum alloy or fiber, high-speed machining center or other high-speed CNC machine tools must be used. The spindle speed is generally as high as 25000 to 60000r/min, and the main motor power is 25 to 60kW. Usually, the "electric spindle" is used. Transmission structure method; feed speed up to 60 ~ 100m/min, acceleration 2 ~ 8g (g = 9.81m/s2). It is more than 10 times that of ordinary CNC machine tools. Now it has gradually replaced the ball screw with a linear servo motor to achieve high-speed feed motion. 5 Dry cutting process technology The workpiece material largely determines the possibility of dry cutting. The combination of dry cutting "workpiece material/machining method" is shown in the table below. As can be seen from the table, super hard alloys and steels are the most difficult to dry cut. Difficult to dry-cut workpiece material and machining method combination table Workpiece Material Machining Method Turning Milling Reaming Tapping Drilling Cast Iron Steel × × × Aluminum Alloy × × Cemented carbide × × × × × Composite Note: X means difficult to dry The cutting aluminum alloy has a high heat transfer coefficient, and absorbs a large amount of cutting heat during the addition process; the thermal expansion coefficient is large, and the workpiece undergoes thermal deformation; the hardness and the melting point are both low, and the chips can easily come into contact with the tool during processing. Welding or adhesion, which is the biggest problem encountered in the dry cutting of aluminum alloys. The best way to solve this problem is to use high speed dry cutting. In high-speed cutting, 95% to 98% of the cutting heat is transmitted to the chips, and the chips are partially melted at the interface with the tool rake face. An extremely thin liquid film is formed, so that the chips can be easily cut off from the workpiece in an instant, which greatly reduces the cutting force and the possibility of generating built-up edge. The workpiece can be kept at normal temperature, which not only improves the production efficiency of the plant, but also improves the machining accuracy and surface quality of the aluminum alloy workpiece. In order to reduce the diffusion and adhesion of the material between the tool and the workpiece at high temperature, special attention should be paid to the proper match between the tool material and the workpiece. For example, diamond (carbon element C) has a strong chemical affinity with iron. Therefore, although diamond tools are very hard, they are not suitable for processing steel workpieces. Titanium alloys and certain high-temperature alloys contain titanium, so they cannot. Dry cutting is performed with a coated tool containing titanium. Another example is that PCBN tools can dry-cut hardened steels, chilled cast irons, and hard workpieces that have been surface-sprayed. However, when machining low-hardness workpieces, the tool life is less than that of ordinary hard alloys. .
Figure 2 Laser-assisted dry cutting
Fig. 3 Cooling tool with liquid nitrogen
Fig. 4 Tool wear of PCBN tool after turning RBSN material
The hard car is a new process called “car instead of grindingâ€, which is used for the processing of some slewing parts that are not suitable for grinding, and is an efficient dry cutting technology. In the hard car of silicon nitride (Si3N4) workpieces. Due to the material's extremely high tensile strength, any tool is quickly damaged. Laser-assisted cutting can be used to preheat the cutting zone of the workpiece with a laser beam (see Figure 2), so that the workpiece material is partially softened (its tensile strength is reduced from 750 MPa to 400 MPa), cutting resistance can be reduced by 30% to 70% Tool wear can be reduced by about 80%, and vibration during dry cutting is also greatly reduced. Greatly improved the material removal rate. Make dry cutting work smoothly. Titanium-aluminum vanadium alloy (Ti6Al4V) and reactive sintered silicon nitride (RBSN) are typical hard-to-process materials, their heat transfer coefficient is very small, a large amount of heat is generated in the dry processing, the tool material is chemically decomposed, and the tool is absolutely worn out. Figure 3 shows a new method for processing such materials using liquid nitrogen cooled tools. A metal cap is flipped over the rake face of the turning tool. The inner cavity and the upper surface of the blade together form a closed chamber. The cap has liquid nitrogen inlet and outlet. In the dry cutting process, liquid nitrogen constantly flows in the closed chamber, absorbing the cutting heat on the blade, so that the tool does not generate excessive temperature rise. Always maintain good cutting performance and achieve dry cutting smoothly. Figure 4 shows the experimental results of tool wear when machining RBSN workpiece material with PCBN tool. When the tool is not cooled with liquid nitrogen, the cutting length of the PCBN tool is only 40 mm, and the flank wear is as high as 3 mm, cutting can no longer be carried out. After the above liquid nitrogen device was used, the wear of the tool was greatly improved. After the turning length of 160 mm, the flank wear was only 0.4 mm. The roundness error of the workpiece was also reduced from 20 μm to 3.2 μm. Liquid nitrogen is a readily available raw material that can be used inexpensively and repeatedly. 6 Quasi-dry cutting Pure dry cutting is sometimes difficult to perform. In this case, Minimal Quantity Lubrication (MQL) can be used. In this method, compressed air is mixed with a small amount of lubricating fluid and then injected into the processing area to lubricate the processing area between the tool and the workpiece. MQL technology can greatly reduce the friction between "tool-workpiece" and "tool-chip", and it can suppress temperature rise, reduce tool wear, prevent adhesion, and improve the quality of the workpiece. It uses very little lubricating fluid. The effect is very significant, which not only improves the work efficiency but also does not cause pollution to the environment. For example, when the (Ti, Al)N+MoS2 layer tool is used for pure dry cutting of aluminum alloy workpieces. After the tool drills 16 holes, the chips stick in the chip flutes, making the tool completely useless. After using MQL technology. There are up to 320 boreholes. The drill bit has not yet undergone significant wear or adhesion, and all the holes have been added to meet the requirements. The amount of lubricating fluid used in the MQL method is generally 0.03 to 0.2 L/h, which is about 60,000 parts of wet cutting. Clean and clean chips can also be recycled after being compressed, completely free from environmental pollution. Therefore, the MQL method is also called "Near-Dry Cutting". The combination of quasi-dry cutting technology and coated cutting tools can achieve the best results. For example, X90CrMoV18 alloy steel is machined with a high speed steel coated drill. When pure dry drilling is performed with TiAlN coated high-speed steel drill bits, drilling losses after drilling a 3.5m boring length are damaged; using (TiAlN+MoS2) composite coated drill bit and minimum lubrication method, the drilling length is increased to 115m. 7 Summary and Outlook Dry cutting technology is a major innovation in traditional production methods and is a new kind of clean manufacturing technology. The increasingly stringent environmental regulations in countries around the world are conducive to accelerating the promotion and application of dry cutting technology; the development of various super-hard, high-temperature tool materials and their coating technologies has created extremely favorable conditions for dry cutting technology; minimum amount of lubrication The effective application of the device and the appearance of standard cutters for hole machining in various center holes have made the quasi-dry cutting more and more applications in hole machining of aluminum alloys and various difficult-to-machine materials. Dry cutting technology has only a short history of 10 years from now. It is a new green manufacturing technology. It is of great significance to the implementation of the sustainable human development strategy and is the cutting-edge manufacturing technology in the new century.