Performance and application of the hottest PCBN to

2022-08-24
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The performance and application of PCBN cutting tools

cubic boron nitride (CBN) was first synthesized by General Electric (GE) Company of the United States under high temperature and high pressure in the 1950s. Its hardness is second only to diamond and far higher than other materials. Therefore, it and diamond are collectively referred to as superhard materials

cbn has high hardness, chemical inertia and thermal stability at high temperature. Therefore, as an abrasive, CBN grinding wheel is widely used in grinding. Because CBN has the characteristics superior to other tool materials, people tried to apply it to cutting processing at the beginning, but the particles of single crystal CBN are small, so it is difficult to make tools, and the sintering property of CBN is very poor, so it is difficult to make large CBN sintered bodies. Until the 1970s, the former Soviet Union, China, the United States It is only in Britain and other countries that CBN sintered body, polycrystalline cubic boron nitride (PCBN), as a cutting tool, has been successfully developed. Since then, PCBN has been applied in various fields of machining with its superior cutting performance, especially in the machining of high hardness materials and difficult to machine materials. After more than 30 years of development and application, PCBN tool materials for processing different materials have emerged

1 PCBN tool material and its cutting performance

1.1 the types of PCBN tool materials

are divided into two categories according to the added components: PCBN sintered directly from CBN single crystal and PCBN sintered body added with a certain proportion of binder; According to the manufacturing method, there are two types: the whole PCBN sintered block and the PCBN composite sheet sintered with cemented carbide. At present, PCBN composite with binder is widely used. According to the different proportion of binder added, the hardness of PCBN is also different. The more binder content, the lower the hardness and the better the toughness; Different kinds of binders lead to different uses of PCBN. Table 1 shows PCBN cutters with common binders and their uses. Table 1 PCBN tools with common binders and their uses

cbn content (%). Types of binders. When the main uses need to be restarted or reset parameters ~ 60tin quenched steel ~ 70tic cast iron ~ 70al2o3 cast iron ~ 90aln high strength cast iron ~ 80Co heat resistant alloy steel cast iron

1.2 cutting performance of PCBN tools

because CBN crystals and diamond crystals are sphalerite type, and their lattice constants are similar, and the types of chemical bonds are the same, Therefore, CBN has the hardness and compressive strength close to diamond, and because it is composed of N and B atoms, it has higher thermal stability and chemical inertia than diamond. The main properties of PCBN tool material are as follows:

1) it has a high hardness and wear resistance of 13.07 billion yuan. The microhardness of CBN single crystal is hv8000 ~ 9000, which is the second highest hardness material known at present. The hardness of PCBN composite is generally hv3000~5000. Therefore, when it is used to process high hardness materials, it has higher wear resistance than cemented carbide and ceramics, which can reduce the dimensional deviation or dimensional dispersion in the processing of large parts. It is especially suitable for high automation equipment, which can reduce the auxiliary time of tool change and tool adjustment, and give full play to its efficiency

2) has high thermal stability and high temperature hardness. The heat resistance of CBN can reach 1400 ~ 1500 ℃, and the hardness at 800 ℃ is the normal temperature hardness of al2o3/tic ceramics. Therefore, when the cutting temperature is high, the processed material will soften, and the hardness difference between CBN and the tool will increase, which is conducive to cutting, but has little impact on the tool life

3) has high chemical stability. CBN has high antioxidant capacity, and it does not produce oxidation at 1000 ℃, nor does it have chemical reaction with ferrous materials at 1200 ~ 1300 ℃, but it will hydrolyze with water at about 1000 ℃, causing a large number of CBN to be worn. Therefore, when wet cutting with PCBN tools, attention should be paid to the selection of cutting fluid types. Generally, wet cutting does not significantly improve the service life of PCBN tools, so dry cutting is often used when using PCBN tools

4) has good thermal conductivity. The thermal conductivity of CBN material is lower than that of diamond, but much higher than that of cemented carbide. With the increase of cutting temperature, the thermal conductivity of PCBN tool continues to increase. Therefore, the heat at the tool tip can be quickly transmitted, which is conducive to the improvement of workpiece machining accuracy

5) has a low friction coefficient. The friction coefficient between CBN and different materials is between 0.1 ~ 0.3, which is much lower than that of cemented carbide (0.4 ~ 0.6), and decreases slightly with the increase of friction speed and positive pressure. Therefore, the low friction coefficient and excellent anti bonding ability make it difficult for CBN tools to form retention layers or chip nodules during cutting, which is conducive to the improvement of the machined surface quality

2 PCBN tools are used in advanced machining processes

2.1 suitable for high-speed and ultra-high-speed machining technology

pcbn tools are most suitable for high-speed machining of cast iron, hardened steel and other materials. Figure 1 shows the relationship between the flank wear of the PCBN tool and the cutting distance when cutting cast iron and hardened steel. It can be seen that when the cutting speed exceeds a certain limit, the higher the cutting speed, the smaller the flank wear speed of the PCBN tool, that is, the service life of the tool under high-speed cutting is higher, which is especially suitable for modern high-speed cutting

Figure 1 wear of PCBN tools at different speeds

2.2 the best tool material for hard cutting technology

the finishing of hardened hardware (hardness above HRC55) is usually completed by grinding. However, with the development of tool materials and the improvement of machining accuracy of lathes (especially CNC lathes 3. Nylon materials are used in the six dominant beds of oil pan), Using hard cutting instead of grinding to complete the final machining of parts has become a new way of finishing machining. This process method of turning instead of grinding has the following advantages:

1) it can improve the machining flexibility, break through the limitation of grinding wheel grinding, and process workpieces with different geometric shapes by changing the cutting edge and cutting mode

2) the environmental protection problem in cutting is becoming more and more serious. The waste liquid and waste produced by grinding are becoming more and more difficult to treat and remove, and are harmful to human body. However, it is of great significance for hard cutting without adding coolant

3) high cutting efficiency, short processing time and low equipment investment cost can reduce the processing cost

4) the energy consumed by cutting the same volume is only 20% of that of grinding, so the cutting heat is less, the machined surface is not easy to cause burns and micro cracks, and it is easy to maintain the integrity of the surface performance of the workpiece

5) under the same metal removal rate, hard cutting saves energy compared with grinding

the tool materials commonly used for hard cutting include ceramics, tic coated blades and PCBN tool materials, but PCBN is the best tool material in the industry to carry out hard turning at a higher speed (above 1000m/min). For example, the efficiency is increased by more than 4 times and the processing cost is reduced to 1/3 ~ 1/2 of the original grinding process after the machining of the shift fork of the synchronizer sleeve of the automobile gearbox (the material is 20CrMnTi, and the hardness is hrc58 ~ 62)

2.3 ideal tool material for dry cutting process

due to economic and environmental reasons, dry cutting method has become a key research topic in the field of mechanical manufacturing in recent years. Let's first look at the problems caused by the use of cutting fluid in the wet cutting process:

1) the mist volatilization of cutting fluid caused by long-term exposure to the air or cutting heat in the cutting process is easy to pollute the environment and endanger the health of operators. Sulfur, chlorine and other additives in the cutting fluid are more harmful and affect the quality of the machined surface

2) the use of cutting fluid affects the processing cost. According to statistics, cutting fluid accounts for 15% of the production cost, while the tool cost accounts for only 3% - 4%

3) leakage and overflow of cutting fluid will pollute the environment and are prone to safety and quality accidents

4) the transmission, recovery, filtration and other devices of cutting fluid and their maintenance costs are high, which increases the production cost

due to the above reasons, dry cutting technology has been widely used in western industrial developed countries. The "Red Crescent" dry cutting process recommended by American Makino company can not only give full play to the cutting performance of the tool, but also greatly improve the productivity compared with wet cutting. The mechanism is that due to the high cutting speed, the heat generated is concentrated in the front of the tool, so that the material near the cutting area reaches a red hot state, and the yield strength decreases, so as to improve the cutting efficiency. The prerequisite for adopting the Red Crescent dry cutting process is that the strength of the cut material decreases significantly and becomes easy to cut at a higher cutting temperature, while the strength of the tool material should have better red hardness and thermal stability, as well as better wear resistance and adhesion resistance under the same state

tool materials suitable for dry cutting process include ceramics, cermets, coated cemented carbide, PCBN tool materials, etc., but in terms of red hardness and thermal stability, PCBN material is the most suitable tool material for dry cutting process, and because PCBN tool material has the above advantages, it is more suitable for dry cutting under high-speed conditions. Figure 2 shows the comparison between dry cutting and wet cutting of gray cast iron with PCBN tool, It can be seen that PCBN has a higher tool life than wet cutting under the condition of high-speed dry cutting

Figure 2 Comparison of PCBN tool life when dry cutting and wet cutting cast iron

2.4 it is suitable for automatic machining and machining of difficult to machine materials

pcbn tools have high hardness and wear resistance, and can process high-precision parts for a long time at high cutting speed (small size dispersion), greatly reducing the number of tool changes and the time spent in tool wear compensation downtime. Therefore, it is very suitable for CNC machine tools and processing equipment with a high degree of automation, and can give full play to the efficiency of the equipment

in the application of difficult to machine materials, PCBN tools also show their excellent performance, such as the processing of surface spray welding (coating) materials, processing with other materials tools, the tool life is very low, and the grinding method cannot be used for processing, while PCBN is the only suitable tool material; For another example, for high alloy wear-resistant cast iron used in oil power station equipment, the cutting efficiency of PCBN tool is more than 4 times higher than that of cemented carbide tool, and the cost of a single tool is reduced to 1/5 of the original. In addition, PCBN tools also show good cutting performance in the cutting of sintered materials such as cemented carbide

2.5 application examples of PCBN tools

because PCBN has high hardness and wear resistance, and does not react with ferrous metals at high temperatures, it is mainly used for the cutting of high hardness materials and difficult to machine materials, such as hardened steel, high alloy wear-resistant cast iron, high temperature alloy, high-speed steel, surface spray welding materials, sintered metal materials and other difficult to machine materials

1) machining hardened steel can achieve the effect of turning instead of grinding. Because the cutting depth is more than ten times greater than the grinding depth, the machining efficiency is high and the surface does not burn. For example, when the variable-speed sliding gear (20CrMnTi, hardness HRC 58 ~ 62) is machined by turning instead of grinding, the cutting efficiency is more than 4 times higher than that of the original grinding

2) machining high alloy (containing 18% tungsten or chromium) wear-resistant cast iron, the cutting speed is more than 10 times higher than that of cemented carbide tools, and the cutting efficiency is more than 4 times higher

3) when machining high cobalt chromium molybdenum corrosion-resistant and heat-resistant alloy, the cutting speed of PCBN tool is 160m/min, which is 8 times that of cemented carbide tool

4) when processing thermal spraying (spray welding) materials, the surface spray welding parts cannot be ground, but the cutting efficiency of cemented carbide tools is very low. After using PCBN tools, the processing efficiency can be improved and the cost can be saved

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