Cubic Boron Nitride (CBN) was first synthesized by General Electric (GE) in the United States in the 1950s using artificial methods under high temperature and high pressure. Its hardness is second only to diamond but much higher than other materials, so diamond and CBN are collectively referred to as superhard materials.
CBN has high hardness, chemical inertness and thermal stability under high temperature, so it is widely used in the grinding process as an abrasive CBN grinding wheel. Because CBN has better characteristics than other tool materials, it has been successfully developed as a sintered CBN body for cutting tools—Polycrystalline Cubic Boron Nitride (PCBN). Since then, PCBN has been used in various fields of cutting with its superior cutting performance, especially in the cutting of high-hardness materials and difficult-to-machine materials. After more than 30 years of development and application of abrasive powder types, PCBN tools for processing different materials have now come to the market.
1. Types of PCBN tool materials
According to the added components, it can be divided into PCBN directly sintered from CBN single crystal and PCBN sintered body with a certain proportion of binder. According to the manufacturing compound method, it can ve divided into the integral PCBN sintered block and PCBN composite sheets composite sintered with cemented carbide. At present, PCBN composite sheets with adhesives are widely used. The hardness of pcbn material varies according to the proportion of the added adhesive. The more the adhesive content, the lower the hardness and the better the toughness. The types of adhesives are different and the use of PCBN is also different.
2. Cutting performance of PCBN tools
Because CBN crystals and diamond crystals belong to zinc blende type, and their lattice constants are similar, and the chemical bond types are the same, CBN has hardness and compressive strength close to the diamond, and because it is composed of N and B atoms, it has a ratio of Diamond has higher thermal stability and chemical inertness. The main properties of PCBN tool materials are as follows:
(1) Has high hardness and wear resistance. The microhardness of CBN single crystal is HV8000～9000, which is the second highest hardness material currently known. The hardness of PCBN composite sheet is generally HV3000～5000. Therefore, when used for processing high-hardness materials, it has higher wear resistance than cemented carbide and ceramics, which can reduce the size deviation or size dispersion in the processing of large parts. It is especially suitable for equipment with a high degree of automation, which can reduce tool changes. Adjust the auxiliary time of the knife to give full play to its effectiveness.
(2) It 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 higher, it will soften the processed material and increase the hardness difference between the tool and the tool, which is conducive to the cutting process, but 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 chemically react with iron-based materials at 1200-1300°C, but it will hydrolyze with water at about 1000°C, causing a lot of CBN is worn out, so when using PCBN tools for wet cutting, you need to pay attention to the type of cutting fluid. Under normal circumstances, wet cutting does not significantly improve the life of PCBN tools, so dry cutting methods are often used when PCBN tools are used.
(4) It has good thermal conductivity. The thermal conductivity of CBN material is lower than that of diamond but much higher than that of cemented carbide, and with the increase of cutting temperature, the thermal conductivity of PCBN tools continues to increase, so the heat at the tip of the tool can be quickly transferred, which is conducive to the improvement of the accuracy of workpiece processing.
(5) It has a low coefficient of friction. The coefficient of friction between cbn powder and different materials is between 0.1 and 0.3, which is much lower than that of cemented carbide (0.4-0.6), and it slightly decreases with the increase of friction speed and positive pressure. Therefore, the low coefficient of friction and excellent anti-adhesion ability make it difficult for CBN tools to form a stagnant layer or built-up edge when cutting, which is conducive to improving the quality of the processed surface.
1. PCBN tools are suitable for high-speed and ultra-high-speed cutting processing technology
PCBN tools are most suitable for high-speed cutting of materials such as cast iron and hardened steel. When the cutting speed exceeds a certain limit, the higher the cutting speed, the lower the wear speed of the PCBN tool flank surface, that is, the tool life under high-speed cutting is higher. This feature is especially suitable for modern high-speed cutting.
The finishing of hardened hardware (hardness above HRC55) is usually done by grinding. However, with the development of tool materials and the improvement of machining accuracy of lathes (especially CNC lathes), hard cutting is used instead of grinding to complete parts. The final processing has become a new way of finishing. This hard cutting method 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 cutting method;
(2) The environmental protection problem in cutting is becoming more and more serious. The waste liquid and waste produced by grinding are difficult to handle and remove, and they are harmful to the human body. Hard cutting does not need to add coolant, which is of great significance;
(3) The cutting efficiency is high, the processing time is short, and the equipment investment cost is small, such as our polycrystalline diamond cutters, which can reduce the processing cost;
(4) The energy consumed to cut the same volume is only 20% of grinding, so less cutting heat is generated, 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 more energy compared with grinding.
The tool materials usually used for hard cutting include ceramics, TiC coated inserts and PCBN tool materials, but for hard turning at higher speeds (above 1000m/min), PCBN is the best tool material variety. For example, when the shift fork (material is 20CrMnTi, hardness is HRC58～62) of the automobile gearbox synchronizer sleeve is processed by car instead of grinding, 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.
2. Ideal tool material for dry cutting process
Due to economic and environmental reasons, dry cutting processing methods have become a key research topic in the field of machinery manufacturing in recent years. Let's first look at the problems caused by the use of cutting fluid in the wet cutting process:
(1) Long-term exposure to the air or cutting fluid mist-like volatilization caused by cutting heat during cutting can easily pollute the environment and endanger the health of the operator. 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 processing costs. According to statistics, cutting fluid accounts for 15% of production costs, while tool costs only account for 3% to 4%;
(3) The leakage and overflow of cutting fluid will pollute the environment, and it is prone to safety and quality accidents;
(4) The cutting fluid transmission, recovery, filtration and other devices and their maintenance costs are high, which increases the production cost.
PCBN cutters have high hardness and wear resistance, and can produce high-precision parts (small size dispersion) at high cutting speeds for a long time, greatly reducing the number of tool changes and the time it takes for tool wear compensation to stop. Therefore, it is very suitable for CNC machine tools and processing equipment with a high degree of automation, and can make the equipment's high efficiency fully exerted.
3. Applications of PCBN tools
Because PCBN has high hardness and wear resistance, it does not react with iron group metals at high temperature and is chemically inert, so it is mainly used for cutting high-hardness materials and difficult-to-machine materials, such as hardened steel and high alloy wear-resistant cast iron, high-temperature alloys, high-speed steel, surface spray welding materials, artificial hard material and sintered metal materials.
(1) Processing hardened steel can achieve the effect of turning instead of grinding. Since the cutting depth is more than ten times larger than the grinding depth, the processing efficiency is high and the surface does not burn. For example, when machining variable speed sliding gears (20CrMnTi, hardness of HRC 58～62) by turning on behalf of grinding, the cutting efficiency is more than 4 times higher than that of the original grinding.
(2) When processing high alloy (containing 18% tungsten or chromium) wear-resistant cast iron, the cutting speed is 10 times higher than that of cemented carbide tools, and the cutting efficiency is increased by more than 4 times.
(3) For processing high-cobalt-chromium-molybdenum corrosion-resistant and heat-resistant alloys, the cutting speed of PCBN tools is 160m/min, which is 8 times that of cemented carbide tools.
(4) When processing thermal spraying (spray welding) materials, surface spray welding parts cannot be processed by grinding, and the cutting efficiency with cemented carbide tools is extremely low. After switching to PCBN tools, the processing 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 cutting of sintered metals, etc.
If the cutting amount, tool geometry parameters and other processing conditions are correctly selected, PCBN is an efficient and long-life tool that can bring high economic benefits. But if it is used improperly, it will cause a lot of waste.
1. Reasonable selection 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 selectable cutting speed for hardened steel with cutting hardness HRC55～65 is 80～120m/min. Since PCBN cutting hard materials is performed by softening the metal in the small area of the cutting area, when the cutting speed is too low, the cutting of PCBN tools cannot be performed. The feed rate is generally not greater than 0.2mm/r, and the back-grab amount is generally selected to be less than 0.3mm.
2. Reasonable selection of tool geometric parameters: Since the toughness of PCBN tools is higher than that of ceramics and lower than that of cemented carbide, the main consideration for the selection of tool geometric parameters is to ensure the strength of the cutting edge. The rake angle of the tool is generally selected (0°～-10° ), the relief angle should be smaller, about 10°. Except for special requirements, in order to ensure the strength of the tool tip, the tool tip angle should generally not be less than 90°; the cutting edge should be ground with a negative chamfer, generally 0.2mm×(-15°～-30°), 30° before use ～50 times magnifying glass to inspect the tool to ensure that there is no chipping at the cutting edge.
3. Cooling and lubrication: For other tool's materials, the use of cooling lubricating fluid is beneficial to improve the processing surface and extend the tool life, but for the pcbn tool except for special circumstances, the ideal processing quality and longer life can be achieved without coolant. If coolant is used, then the water-soluble coolant cannot be used, because CBN is prone to hydrolysis at 1000°C, causing severe tool wear.
4. Process system for machine tools: PCBN tools are mostly used for cutting hard-to-machine materials such as hardened steel and wear-resistant cast iron, and the tools have negative chamfers, so the radial force is relatively large. This requires the rigidity and precision of the machine tool to be good, the vibration of the system to be small, the cantilever to be small when the tool is installed, and the PCBN tool is not suitable for processing waste surfaces.
At present, although the price of PCBN tool materials is relatively high compared to that of cemented carbide and ceramic tools, the cost of the tool that is evenly spread on each workpiece is lower than that of other materials. When using advanced cutting processes, if the equipment investment is spread into the production cost, the use of PCBN tools will bring greater economic benefits. For general small and medium-sized enterprises, the grinding process of the finishing process is always the bottleneck of the manufacturing process. If you buy a lathe with good performance, use PCBN tools, and apply advanced cutting processes such as turning instead of grinding, you can save equipment investment cost and improve productivity to increase the flexibility of the processing process. In addition, due to the increase in personnel costs and the requirements of environmental protection, it is also of great significance to promote the use of PCBN tools to give full play to their potential efficiency and improve the level of cutting processing technology.
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