Summary of Information: Polycrystalline Cubic Boron Nitride (PCBN) is a polycrystalline crystal sintered from CBN micropowder and a small amount of binder. Since the successful development of PCBN in 1973, through the efforts of many materials experts and tool experts, PCBN materials and their tools It has fully entered the practical stage. In industrialized countries, PCBN tools have been used in automotive, heavy machinery and other mechanical processing fields. Cubic boron nitride (CBN) is a pure synthetic material, after artificial diamond, the United States GE company in 1957 First, another new superhard material synthesized using a high-temperature ultra-high pressure device was announced. Polycrystalline cubic boron nitride (PCBN) is a polycrystalline crystal sintered from CBN micropowder and a small amount of binder. Since the successful development of PCBN in 1973, PCBN materials and their tools have been fully entered through the efforts of many materials experts and tool experts. In the practical stage, PCBN tools have been used in the machining industry of automobiles and heavy machinery in industrialized countries. According to the data, the sales of PCBN tools in the world reached 135 million US dollars in 1995, and the research on CBN materials and their application technology in China. Not enough, the annual output of PCBN tools is only several million yuan, and the annual consumption is only about 10 million yuan, and most PCBN tool blanks or tools are imported from abroad. First, the performance of PCBN materials 1. The main characteristics of CBN Boron nitride has a variety of isomers. CBN is a dense phase of boron nitride with a high hardness and a Knoop hardness of 47,000 N/mm2, which is second only to diamond. At the same time, CBN has good thermal conductivity, and its thermal conductivity is 13 times that of cemented carbide and 3 times that of copper. In addition, CBN has much better thermal stability and chemical stability than diamond, can withstand high temperatures of 1300 to 1500 ° C, and is highly chemically inert with Fe elements. It can be seen that CBN is an ideal tool material for cutting black metal. 2. Characteristics of PCBN Due to the limitations of CBN's own characteristics and its manufacturing technology, it is still difficult to produce large-grain CBN single crystals directly used for cutting tools. For this reason, PCBN polycrystalline materials obtained by sintering CBN by a bonding agent are rapidly developed, and their sizes are basically unlimited. The micro-grains in the PCBN structure are disorderly ordered, uniform in hardness, non-directional, consistent wear resistance and impact resistance, and overcome the deficiencies and anisotropy of single crystal CBN. PCBN belongs to the aggregate of CBN. In addition to the characteristics of CBN, PCBN is also related to the content of CBN, the size of the particle and the type of binder. The CBN content mainly affects the hardness and thermal conductivity of PCBN, and the content is high, and the hardness and thermal conductivity of PCBN are high. The particle size of CBN is an important factor affecting the toughness of PCBN. The larger the particle size, the weaker the resistance to breakage, and the sharpness of the cutting edge of the tool produced by this tool is poor. The binder in PCBN mainly has a metal type represented by Co and Ni, and a ceramic type and a cermet mixed type mainly composed of TiC, TiN, and Al2O3. When metal materials are used as bonding agents, PCBN has better toughness and electrical conductivity; ceramic materials as bonding agents have better thermal stability. In addition to the above excellent performance, in order to further improve the strength of PCBN, various PCBN manufacturers currently directly sinter or weld the PCBN layer of about 0.5mm on the cemented carbide substrate to form a composite whole (also called PCBN composite). sheet). The high hardness, high thermal stability and high chemical stability of the PCBN material together with the excellent strength and weldability of the cemented carbide substrate make the PCBN tool not only capable of cutting workpieces of various hardnesses, but also easy to manufacture. Table 1 compares the performance of PCBN materials with other tool materials. Table 1 Comparison of PCBN and other tool materials performance II. PCBN tool cutting characteristics——hard cutting Hard cutting refers to the cutting process after hardening heat treatment of iron parts, and past grinding is still the only processing method. With the successful development of ceramic tools, especially PCBN tools (PCBN materials have excellent red hardness. The hardness at 1000 °C is still higher than the hardness of cemented carbide at room temperature), hard cutting has been rapidly flying. When the PCBN tool is hard-cut, the PCBN is not sticky and has little wear, so that high machining accuracy and surface quality (Ra 0.2-2.0 μm) can be obtained, and the car can be worn. According to a survey conducted by the Japan Cutting Association on 53 large companies in the country, 55% of the Japanese PCBN tools are used to replace the original grinding and honing. PCBN tools are mainly suitable for cutting hardened steel above 45HRC, wear-resistant cast iron, heat-resistant alloys above 35HRC or special cast irons that are difficult to machine with other tools. If the hardness of the material being processed is too low, the advantages of the PCBN tool will be less obvious. III. Types and Models of Major PCBN Tool Blanks in Foreign Countries The types and models of major PCBN tool blanks in foreign countries can be roughly divided into two categories, namely high-content PCBN and low-content ceramic bond PCBN (also called ceramic CBN). High content of PCBN (general CBN content 80%～90%) is based on direct bonding between CBN, with high hardness, high thermal conductivity and good toughness. This kind of PCBN cutter is suitable for processing high hardness alloys and high hardness in tissues. Material of the material, such as chilled cast iron, cemented carbide, heat resistant alloy, etc. At present, such PCBN blanks mainly include GE's BZN6000, De Beers' Amborite, DBC80, and Sumitomo Electric's BN100 and BN600. When cutting hardened steel (alloy steel, bearing steel, die steel, carbon steel, etc.), PCBN tools are required to have high hardness and resistance to diffusion and wear due to high cutting temperatures. For this, a low-content PCBN blank for ceramic bonding should be used. Although the hardness, thermal conductivity and toughness of PCBN with higher content of PCBN are slightly lower, it is because of the good heat resistance and poor thermal conductivity of ceramic materials, which causes the heat retention temperature of the cutting zone to rise and the workpiece to form rapidly. It is plastically deformed and easy to cut, and a good effect (also called a softening effect) can be obtained. At present, such tool blanks mainly include GE's BZN8100, BZN8200, De Beers' DBC50, and Sumitomo Electric's BN300 and BN220. Table 2 shows the models and characteristics of major PCBN blanks abroad. Table 2 Models and characteristics of major PCBNs abroad IV. Application range of PCBN tools · According to industry, the automobile industry accounts for 50.37%, heavy machinery ~ 20.74%, pump industry 8.89%, bearing gears 5.93%, rolling rolls 5.19%, aviation Aerospace 2.90%, other 5.93%. · According to the processing materials, hardened steel accounts for 65%, cast iron 28%, and heat-resistant steel 7%. · According to the processing steps, turning accounts for 60%, boring 32.5%, and end milling for 7.5%. · In place of the original processing, 50% is alternative grinding, 30% replaces carbide tools, 10% replaces cermet tools, 5% replaces ceramic tools, and 5% replaces honing. V. PCBN tool application technology In hard cutting, the radial force is very large, and the rigidity of the machine tool and system must be good and the power is large enough. When clamping a PCBN tool, the tool's overhang should be as short as possible to prevent chattering and deformation of the toolholder and keep the PCBN tool in good condition. PCBN tools have poorer toughness than cemented carbide tools, so when cutting, generally use a negative rake angle (0 ° ~ -10 °), a small relief angle (6 ° ~ 8 °) and a negative chamfer [[0.1 ~ 0.2) × (10 ° ~ 30 °)]. It is better to further passivate the cutting edge further. In addition, where possible, use a small lead angle and a large tool nose radius to help protect the cutting edge and extend tool life. Proper use of PCBN tools is the key to accurately determining the durability of PCBN tools. It is recommended that the flank wear standard of the PCBN tool be 0.3 to 0.5 mm (small value when finishing the car), and it is better to perform regular re-grinding. However, re-grinding requires certain equipment and skills. Most factory machines do not have suitable conditions. PCBN tool re-grinding should be done by professional manufacturers. Determine the cutting speed, feed rate and depth of cut to consider production efficiency and processing costs. Normally, the cutting speed can be about 2 times higher than that of the cemented carbide tool. The high cutting speed produces a large cutting heat, which increases the plasticity of the material to be processed and helps to reduce the cutting force. Table 3 shows the economic cutting speed of PCBN tools. Table 3 Economical cutting speed of PCBN tools PCBN tools use dry cutting as well. If cutting fluid is required, it must be cooled sufficiently to disable water-soluble cutting fluid. With the advancement of technology, more and more PCBN blanks are available, and the speciality is getting stronger and stronger. For different materials and requirements, the corresponding PCBN materials should be selected to achieve satisfactory processing results.
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