I. Introduction <br> <br> cutting fluid is generally one of the most factors of production machining indispensable, it plays an important role in ensuring the machining accuracy, surface quality and improve productivity. As global environmental awareness increases and environmental regulations become more stringent, the negative effects of cutting fluids on the environment are becoming more apparent. According to statistics, 20 years ago, the cost of cutting fluid was less than 3% of the cost of the workpiece. At present, in high-productivity production enterprises, the cost of cutting fluid supply, maintenance and recycling costs together account for the manufacturing cost of the workpiece. 13%-17%, while tooling costs only 2%-5%. About 22% of the total cost associated with cutting fluids is the processing cost of cutting fluid. It is estimated that if 20% of the cutting process uses dry machining, the total manufacturing cost can be reduced by 1.6%. Green manufacturing that is environmentally friendly is considered a modern manufacturing model for sustainable development. Dry cutting without any cutting fluid in the process is a green manufacturing process to control the source of environmental pollution. It can obtain clean, non-polluting chips, eliminating the need for cutting fluid and its processing, etc., which can be further reduced. Production cost 3. Therefore, the direction of future machining is to use or use as little cutting fluid as possible. With the development of high temperature tool materials and coating technology, dry machining has become possible in the field of machine building. DryCutting technology came into being under such a historical background and has developed rapidly since the mid-1990s. Its development history is only ten years, and it is a frontier research topic of advanced manufacturing technology.
Second, dry cutting technology
Dry cutting is a processing method that does not use cutting fluid during processing. Dry cutting is not simply to stop using the cutting fluid, but to ensure high efficiency, high product quality, high tool durability and reliability of the cutting process while stopping the use of cutting fluid. This requires dry cutting with excellent performance. Tools, machine tools, and ancillary equipment replace the role of cutting fluid in traditional cutting to achieve true dry cutting. First of all, to understand the role of cutting fluid in the traditional cutting process, the general cutting fluid has three main functions: 1) cooling, taking away the heat generated by cutting, reducing tool wear and preventing surface oxidation of the workpiece; 2) lubricating, reducing Friction, reduce cutting force, ensure smooth cutting; 3) Chip removal, quickly take away the chips on the surface of the workpiece, and prevent the chip from scratching the surface of the workpiece. However, from the perspective of environmental protection, the negative effects of cutting fluid are more and more obvious, mainly in the following aspects: 1) The high temperature generated during processing causes the cutting fluid to form a misty volatilization, pollute the environment and threaten the health of the operator; Some cutting fluids and adhesive tapes must be treated as toxic and hazardous materials, and the treatment cost is very high; 3) leakage and overflow of cutting fluid have a great impact on safe production; 4) additives for cutting fluids (such as Sulfur, chlorine, etc.) can cause harm to the operator's health and affect the quality of processing. In addition, a large number of studies on the cutting process have also shown that the traditional cooling, lubrication and chip evacuation of cutting fluids are not fully and effectively utilized in many machining processes, especially in high-speed cutting. Therefore, attempts have been made to change this condition with or without cutting fluids to accommodate clean production processes and reduced production costs. Dry machining technology is an advanced processing method produced in this case. The dry cutting technology not only reduces the environmental pollution of the cutting fluid, improves the working conditions of the operator, but also saves the cost associated with the cutting fluid and reduces the cost of the chip recycling process. Dry cutting technology places higher demands on machine tools and tooling technology. In recent years, industrialized countries have paid great attention to dry cutting research. Dry cutting This new processing method is one of the development trends of metal cutting in the future.
Third, the status quo of dry cutting technology research
Dry cutting technology is a green cutting technology developed to adapt to the world's rising environmental requirements and sustainable development strategies. The scientific significance of dry cutting was officially established in 1995. At the 1997 International Conference on Production Engineering (CIRP), Professor F. Klocke of the Aachen University of Technology in Germany gave a keynote report on “dry cuttingâ€; in January 1999, At the meeting of the recipients of the National Science Foundation's “Design and Manufacturing Disciplineâ€, Dr. BPErdel, President of the internationally renowned tool manufacturer MAPAL, also gave a keynote report on the development of dry cutting in the United States. Dry cutting technology has been in various industries and academics in various countries. The community has caused widespread concern. At present, industrial developed countries including Europe, the United States and Japan attach great importance to the research of dry cutting. Dry cutting technology has been successfully applied to the production field, which has a certain relationship with the strong industrial base and strict environmental regulations of these countries. Among them, German companies are particularly popular. In mass production, 10%-15% of the processing uses dry cutting technology and has achieved good economic benefits. Many well-known machine tool manufacturers in the world have dry cutting in their catalogue. Machine tool machining center. Japan has also conducted extensive research on dry cutting, and recently they have developed several dry machining centers that do not use cutting fluids. On one of the machines, a dry cutting system with liquid nitrogen cooling is used to extract high purity nitrogen from the air. The liquid nitrogen is sent to the cutting zone at a normal temperature of 5-6 atmospheres, and dry cutting can be smoothly performed.
Research on dry cutting technology in China has also started. Chengdu Tool Research Institute, Shandong University of Technology and Tsinghua University have systematically studied superhard tool materials (such as ceramics, cubic boron nitride, diamond, etc.) and tool coating technology, and have achieved many research results. China's ceramic tools have formed a certain production capacity, which provides a preliminary technical basis for the research and application of dry cutting technology. The KT series machining center developed by Beijing Machine Tool Research Institute can realize high-speed dry cutting. However, in general, China still has a large gap in the research of dry cutting theory and abroad. The scale of application in industry is smaller, and we need to accelerate research and popularization and application in the future.
Fourth, dry cutting tool technology
Dry cutting technology is a huge system engineering that requires a series of measures from the aspects of tool technology, machine structure and process. This article will focus on the cutting technology of dry cutting.
When designing a dry cutting tool, it is necessary to select not only the applicable tool material and the coating used, but also the consideration and optimization of the tool material, the tool coating and the tool geometry. Different cutting methods have different requirements for tool design. Dry cutting tools must meet the following conditions: 1) The tool material should have good heat resistance and wear resistance; 2) The friction coefficient between the chip and the tool should be It may be small; 3) The groove shape of the tool should ensure smooth and easy heat dissipation; 4) The tool should have high strength and impact toughness. The following are discussed in terms of tool material, coating and geometry.
1. With the new tool material for more than ten years, the emergence of high hardness materials has made it possible for dry cutting. Dry cutting requires not only high hardness and thermal toughness of the tool material, but also good wear resistance, thermal shock resistance and adhesion resistance. The tool materials currently used for dry cutting are mainly ultra-hard materials such as ultra-fine cemented carbide, ceramics, cubic boron nitride and polycrystalline diamond. Figure 1 shows the hardness versus temperature for several tool materials. Ultra-fine cemented carbide can improve the toughness of ordinary cemented carbide, has good wear resistance and high temperature resistance, and can produce deep hole drills and inserts with large rake angles for dry machining of milling and drilling. Ceramic knives (Al203, Si3N4, cermet (Cennet) and other materials have little hardness at high temperatures, that is, they have good red hardness, so they are suitable for general purpose dry cutting without coolant [1131. But this The materials are generally brittle, that is, the thermal toughness is not good, so it is not suitable for interrupted cutting. That is to say, ceramic tools are more suitable for dry turning and not for dry milling. Cubic boron nitride (CBN) materials High hardness, up to HV3200 ~ HV4000, second only to diamond, good thermal conductivity, up to 1300W / MK, with good high temperature chemical stability, good thermal stability at 1200 ° C. Using CBN tool to process cast iron, can greatly improve Cutting speed, used for machining hardened steel, can be replaced by turning. Polycrystalline diamond (PCD) tool hardness is very high, up to HV7000 ~ HV8000, thermal conductivity up to 2100 W / MK, linear expansion coefficient is small. PCD tool The heat generated during cutting can be quickly transferred from the tool tip to the tool body, thus reducing the machining error caused by the thermal deformation of the tool. PCD tools are more suitable for dry machining of copper, aluminum and aluminum alloy workpieces.
Figure 1 Relationship between hardness and temperature of different materials
2. Coating technology with coating technology is an important way to improve tool performance. In the past decade, tool coating technology has developed very rapidly, with up to 15 coating materials and some tools with up to 13 layers on the body. The coating process is also becoming more and more mature. With the development of technology, the technical problem of low bonding strength between the coating and the base material has been solved. There are two main types of coated tools: one is “hard†coated tools such as TiN, TIC and Al203. These tools have high surface hardness and good wear resistance. Among them, the TIC coated tool is particularly resistant to flank wear, while the TiN coated tool has a higher resistance to "crater" wear. The other type is "soft" coated tools such as MOS2, WS and other coated tools. This kind of coated tool is also called “self-lubricating toolâ€, and its friction coefficient with the workpiece material is very low, only about 0.01, which can effectively reduce the cutting force and reduce the cutting temperature. For example, the "MOVIC" coated tap developed in Switzerland is coated with a layer of MOS2. Cutting experiments show that uncoated taps can only process 20 tapped holes; 1000 tapped holes can be machined with TiAlN coated taps, while MoS2 coated taps can process 4000 tapped holes. High speed steel and cemented carbide can be used for dry cutting after PVD coating. CBN tools that were originally only suitable for dry cast iron castings can also be used to process steel, aluminum alloys and other superhard alloys after coating.
In fact, the coating has a function similar to a coolant, which creates a protective layer that isolates the tool from the heat of cutting so that heat is rarely transferred to the tool, thus keeping the tip hard for a longer period of time. And sharp. Smooth surface coatings also reduce friction to reduce cutting heat and keep tool materials from chemical reactions, as high temperatures have a large catalytic effect on chemical reactions in most high-speed dry cutting. TiAlN coating and Mo2 soft coating can also be alternately coated to form a multi-coated tool, which has the characteristics of high hardness and good wear resistance, small friction coefficient and easy chip flow, and excellent alternative cooling. The function of the liquid. Tool coating plays a very important role in dry cutting technology.
3. Tool geometry design Dry cutting tools usually have crater wear as the main cause of failure, because there is no cutting fluid in the machining, and the temperature of the tool and chip contact area rises. Therefore, it is usually necessary to have a large rake angle and a blade inclination angle, but the blade edge strength is affected after the rake angle is increased. In this case, a suitable negative chamfer or rake face reinforcement unit should be provided, so that the tool tip and The cutting edge will have sufficient volume of material and a reasonable way to withstand cutting heat and cutting forces, while reducing the adverse effects of impact and crater expansion on the tool, allowing the tool tip and cutting edge to remain adequate for longer cutting times. Structural strength.
In recent years, many large-angle turning inserts have been developed abroad (such as the front angle of a ME-13 new carbide insert introduced by Carboloy in the United States up to 34°) and a spiral-edged milling insert with a positive rake angle ( The blade has a nearly constant rake angle along the cutting edge, and the back rake angle or side rake angle can be changed from negative to small or from small to large), aiming to reduce the driving power of the machine tool and reduce the cutting force by reducing the cutting force. Temperature to meet the tool requirements for dry cutting.
Mitsubishi Metal Corporation of Japan has developed a "slewing turning tool" for dry cutting. The tool uses a circular super-hard blade. The supporting part of the blade is equipped with a bearing. During machining, the blade can automatically rotate, so that the cutting edge is always maintained. Sharp, with high work efficiency, good processing quality, long tool life and so on.
Figure 2 Heat pipe cutter structure
There is also a heat pipe cutter that also achieves an ideal dry cut. Figure 2 shows a heat pipe turning tool. Their structure is basically the same as that of a conventional turning tool, except that a heat pipe is formed inside the shank body. The working medium in the heat pipe is generally acetone, ethanol and distilled water. The heat pipe is a highly efficient heat transfer element that utilizes the two strongest heat transfer mechanisms, boiling heat absorption and condensation heat release. The heat conductivity of the heat pipe is several hundred times that of silver and copper rods. The heat pipe tool is a self-cooling tool, so there is no need to cast the cutting fluid from the outside, especially for CNC machine tools, machining centers and automatic production lines.
V. Summary and outlook
Compared with wet processing, dry processing not only improves production efficiency, reduces production costs, but also contributes to environmental protection. It is an ideal and clean metal cutting method. Dry cutting technology has been in existence for less than a decade, and it is an emerging green manufacturing technology. In the 21st century, the manufacturing industry is increasingly demanding green environmental protection. Dry cutting technology as a green manufacturing process is of great significance for saving resources, protecting the environment and reducing costs. With the rapid development of machine tool technology and various super-hard and high-temperature tool materials and coating technology and related research, dry cutting technology will be widely used in the field of metal cutting.
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