1. PCBN tool material and its cutting performance 1.1 The types of PCBN tool materials are divided into two categories: PCBN directly sintered by CBN single crystal and PCBN sintered body with a certain proportion of binder; Points: There are two types of integral PCBN agglomerates and PCBN composite sheets sintered with cemented carbide. At present, PCBN composite sheets with adhesives are widely used. The hardness of PCBN is different according to the ratio of binders added. The more the binder content, the lower the hardness and the better the toughness; the types of binders are different. The use of PCBN is also different.
1.2 Cutting performance of PCBN tool Since both CBN crystal and diamond crystal are of zinc blende type, and the lattice constant is similar, the chemical bond type is the same, so CBN has the hardness and compressive strength close to diamond, and because it is made of N and B atoms. It is composed and therefore has higher thermal stability and chemical inertness than diamond. The main properties of PCBN tool materials are as follows:
1) High hardness and wear resistance. The microhardness of CBN single crystal is HV8000～9000, which is the second high hardness material known as Hanyang Technology. The hardness of PCBN composite sheet is generally HV3000~5000. Therefore, it has higher wear resistance than hard alloys and ceramics when processing high-hardness materials, and can reduce dimensional deviation or dimensional dispersion in processing of large parts, especially suitable for equipment with high degree of automation, and can reduce tool change. Adjust the knife assist time to make full use of its performance.
2) High thermal stability and high temperature hardness. The heat resistance of CBN can reach 1400-1500 °C, and the hardness at 800 °C is the normal temperature hardness of Al2O3/TiC ceramic. Therefore, when the cutting temperature is high, the material to be processed will be softened, and the hardness difference between the tool and the tool will increase. It is beneficial to the cutting process and has little effect on the tool life.
3) It has high chemical stability. CBN has high oxidation resistance and does not produce oxidation at 1000 °C. It does not react with iron-based materials at 1200-1300 °C, but it will hydrolyze with water at 1000 °C, resulting in a large amount of CBN is worn out, so care must be taken to select the type of cutting fluid when wet cutting with PCBN tools. Under normal circumstances, the wet cutting does not significantly improve the life of the PCBN tool, so the dry cutting method is often used when using the PCBN tool.
4) Has good thermal conductivity. The thermal conductivity of CBN material is lower than that of diamond but much higher than that of hard alloy. With the increase of cutting temperature, the thermal conductivity of PCBN tool increases continuously, so the heat at the tip of the tool can be quickly transmitted, which is beneficial to the machining accuracy of the workpiece. Improvement.
5) Has a lower coefficient of friction. The coefficient of friction between CBN and different materials is between 0.1 and 0.3, which is much lower than the friction coefficient of cemented carbide (0.4 to 0.6), and slightly decreases with the increase of friction speed and positive pressure. Therefore, the low friction coefficient and excellent anti-adhesive ability make it difficult for CBN tools to form a retention layer or built-up edge when cutting, which is beneficial to the improvement of the surface quality.
2. PCBN tool is applied to advanced cutting process 2.1 Suitable for high speed and ultra high speed cutting technology PCBN tool is most suitable for high speed machining of cast iron, hardened steel and other materials. Figure 1 shows the relationship between the flank wear and the cutting distance of the PCBN tool when cutting cast iron and hardened steel. It can be seen that when the cutting speed exceeds a certain limit, the cutting speed is higher, and the wear rate of the flank of the PCBN tool is worse. Small, that is, the life of the tool under high-speed cutting is rather high, which is especially suitable for modern high-speed machining.
2.2 The best tool material for hard cutting technology The finishing of quenching hardware (hardness HRC55 or higher) is usually done by grinding method. However, with the development of tool materials and the improvement of machining precision of lathes (especially CNC lathes) The use of hard cutting instead of grinding to complete the final machining of parts has become a new finishing route. This car-grinding process has the following advantages:
1) It can improve the processing flexibility, break through the limitation of grinding wheel grinding, and can process workpieces with different geometric shapes by changing the cutting edge and the cutting method;
2) The environmental problems in cutting processing are becoming more and more serious. The waste liquid and waste generated by grinding process are more and more difficult to handle and remove, and it is harmful to the human body. Hard cutting does not require the addition of coolant, which is of great significance;
3) High cutting efficiency, short processing time, low equipment investment cost, and reduced processing cost;
4) The energy consumed for cutting the same volume is only 20% of the grinding, so the cutting heat generated 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 properties of the workpiece;
5) In the case of the same metal removal rate, hard cutting saves energy compared to grinding.
Commonly used for hard cutting tools are ceramic, TiC coated inserts and PCBN tool materials, but for hard turning at higher speeds (above 1000 m/min), PCBN is the best tool material. For example, in the gearbox of the automobile gearbox synchronizer (the material is 20CrMnTi, the hardness is HRC58~62), the efficiency is increased by more than 4 times, and the processing cost is reduced to 1/3~1/ of the original grinding process. 2.
2.3 Ideal tool materials for dry cutting process Due to economic and environmental reasons, dry cutting processing methods have become a key research topic in the field of machine manufacturing in recent years. Let us first look at the problems caused by the use of cutting fluid in the wet cutting process:
1) The long-term exposure to the air or the cutting fluid caused by the cutting heat during the cutting process is misty, which is easy to pollute the environment and endanger the health of the operator. The additives such as sulfur and chlorine in the cutting fluid are more harmful and affect the quality of the processed 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 only accounts for 3% to 4%;
3) Leakage and overflow of cutting fluid will pollute the environment and cause safety and quality accidents;
4) The cutting fluid transmission, recovery, filtration and other devices and their maintenance costs are high, increasing production costs.
Due to the above reasons, the dry cutting process has been applied quite well in western industrialized countries. The "Red Crescent" dry-cutting process recommended by Makino Corporation of the United States not only maximizes the cutting performance of the tool, but also greatly increases productivity compared to wet cutting. The mechanism is that the cutting speed is high, and the generated heat is concentrated in the front part of the tool, so that the material near the cutting zone reaches a red hot state, and the yield strength is lowered, thereby achieving the effect of improving the cutting efficiency. The premise of using the red crescent dry cutting process is that at higher cutting temperatures, the strength of the material to be cut is significantly reduced, making it easy to cut, and the strength of the tool material should have better red hardness and thermal stability under the same conditions. Also have better wear resistance and anti-adhesion.
Tool materials suitable for the dry cutting process are ceramics, cermets, coated cemented carbides and PCBN tool materials, etc., but in terms of red hardness and thermal stability, PCBN materials are the most suitable tool materials for dry cutting processes, and Since 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 is dry at high speed. Under cutting conditions, it has a higher tool life than wet cutting.
2.4 Adapted to automated processing and difficult processing of materials PCBN knife has high hardness and wear resistance, can process high-precision parts for a long time at high cutting speed (small size dispersion), greatly reducing the number of tool changes and tools The time it takes for wear to compensate for downtime. Therefore, it is very suitable for CNC machine tools and processing equipment with a high degree of automation, and can make the high efficiency of the equipment fully utilized.
In the application of difficult-to-machine materials, PCBN tools also show excellent performance, such as surface spray-welding (coating) materials, machining with other materials, tool life is extremely low, and can not be processed by grinding, and PCBN is The only suitable tool material; for example, the high-alloy wear-resistant cast iron used in petroleum power plant equipment uses PCBN tools to improve cutting efficiency by more than 4 times compared with cemented carbide tools, and the cost per piece of tool is reduced by 1/5. In addition, PCBN tools also show good cutting performance in the machining of sintered materials such as cemented carbide.
2.5 Application examples of PCBN tools Because PCBN has high hardness and wear resistance, it is not chemically inert with iron metal at high temperature, so it is mainly used for cutting of high hardness materials and difficult materials, such as hardening. Cutting of difficult-to-machine materials such as steel, high-alloy wear-resistant cast iron, high-temperature alloy, high-speed steel, surface spray-welded materials, and sintered metal materials.
1) Machining hardened steel can achieve the effect of grinding by car. Since the cutting depth is more than ten times larger than the grinding depth, the machining efficiency is high and the surface does not burn. If the shifting sliding gear (20CrMnTi, hardness HRC 58-62) is machined by car, the cutting efficiency is more than 4 times higher than that of the original grinding.
2) Processing high-alloy (18% tungsten or chromium) wear-resistant cast iron, the cutting speed is more than 10 times higher than that of hard alloy cutting tools, and the cutting efficiency is increased by more than 4 times.
3) Processing high cobalt chromium molybdenum corrosion resistant alloy, PCBN cutting speed is 160m / min, 8 times that of cemented carbide tools.
4) Processing thermal spray (spray welding) materials, surface spray weldments can not be ground, and the cutting efficiency of carbide tools is extremely low. After switching to PCBN tools, the machining efficiency can be improved, and the processing cost can be saved by more than 50%. PCBN tools can also be used for precision cutting of non-ferrous metals and machining of sintered metals.
If the cutting amount, tool geometry and other processing conditions are correctly selected, PCBN is a high-efficiency long-life tool making network that can create high economic benefits, but if used improperly, it will cause a lot of waste.
1) Reasonable choice of cutting amount: The cutting speed should be selected according to the material to be processed, which is generally much higher than that of cemented carbide tools. The cutting speed of the hardened steel with a cutting hardness of HRC 55-65 is 80-120 m/min. Since the PCBN cutting hard material is softened by the softening of the metal in the small area of ​​the cutting area, the PCBN cannot be used when the cutting speed is too low. Cutting performance of the tool. The feed rate is generally not more than 0.2mm/r, and the amount of back-feeding is generally less than 0.3mm.
2) Reasonable selection of tool geometry parameters: Since the toughness of PCBN tools is higher than that of ceramics and lower than that of hard alloys, the choice of tool geometry parameters is mainly to ensure the edge strength, and the tool rake angle is generally selected (0° to -10°). ), the back angle is chosen to be small, about 10°. In addition to special requirements, in order to ensure the strength of the tool tip, the tip angle should not be less than 90°; the cutting edge should be ground negatively, generally 0.2mm × (-15 ° ~ -30 °), 30 before use. ～50 times magnifying glass to check the tool to ensure that the cutting edge is not chipped.
3) Cooling and lubrication: For other material tools, the use of cooling lubricants is beneficial to improve the machining surface and extend the life of the tool. However, in the special case of PCBN tools, the ideal machining quality and long-term can be achieved without adding coolant. Tool life is not afraid of softness for PCBN tools. If a coolant is used, a water-soluble coolant cannot be used because CBN tends to hydrolyze at 1000 ° C, causing severe tool wear.
4) Process system for machine tools: Since PCBN tools are mostly used for the machining of difficult-to-machine materials such as hardened steel and wear-resistant cast iron, and the cutter has a negative chamfer, the radial force is large. This requires the rigidity and precision of the machine to be good, the vibration of the system is small, the cantilever is small when the tool is installed, and the PCBN tool is not suitable for the surface processing.
At present, although the price of PCBN tool materials is relatively high compared to the price of cemented carbide and ceramic tools, the cost of tools distributed to each workpiece is lower than that of other materials. When advanced cutting technology is used, if the machine tool is to be ground When equipment investment is integrated into production costs, the use of PCBN tools will bring greater economic benefits. For the general SMEs, the grinding process of the finishing process is always the bottleneck of the manufacturing process. If you purchase a good lathe, use PCBN tools, and apply advanced cutting processes such as car grinding, you can save equipment investment and improve. Productivity can greatly increase the flexibility of the process. In addition, due to the increase in personnel costs and environmental protection requirements, it is also of great significance to promote the use of PCBN tools, to fully utilize their potential performance, and to improve the level of cutting technology.
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