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The technical problems of diamond/CBN grinding wheels are all here!

Use a suitable grinding wheel to select the appropriate grinding wheel 


      for different workpiece materials and different processing requirements. Different grinding wheels should be used. Even if the same workpiece material is ground, the grinding ratio of different grades of grinding wheel may differ by 2 to 5 times, so the appropriate grinding wheel should be used. 


      The diamond grinding wheel is a grinding tool for grinding high-hardness and brittle non-metallic materials such as cemented carbide, optical glass, ceramics, gemstones and stone. However, since diamond is easily carbonized at 700 ° C to 800 ° C, it is not suitable for grinding steel. CBN is slightly inferior to diamond in hardness, but it has good thermal stability (can withstand temperatures from 1300 ° C to 1400 ° C). It does not decompose at high temperatures like diamond, ie oxidation and graphitization. It is chemically inert with iron elements. The thermal conductivity is high (46 times that of corundum), and the grinding wheel made of it has a long service life (up to several times to hundreds of times of corundum grinding wheel), which is an alternative to corundum grinding wheel for hardened steel, high speed tool steel and bearing steel. Abrasives of high-hardness and toughness metal materials such as stainless steel, heat-resistant steel and titanium alloy. 


      The binder of the superabrasive grinding wheel is resin, metal, ceramic, electroplating and brazing. Resin bond grinding wheel is mainly used for sharpening tool measuring tools, grinding holes, cylindrical grinding and surface grinding; metal bond grinding wheel is used for cutting grinding, shaping curve grinding machine, single-edged tool for grinding, grinding small drill bit The spiral groove is also suitable for electrolytic grinding; the ceramic bond grinding wheel has high grinding efficiency, good shape retention, high durability, easy dressing, long service life of the grinding wheel, and good chemical stability due to the ceramic bond itself. Heat-resistant, oil-resistant, acid-base resistant, adaptable to various grinding fluids, low grinding cost, so it has become an effective and high-precision grinding tool; electroplated grinding wheel is suitable for high-speed fine grinding and forming grinding, also Suitable for grinding of holes (wet grinding) and for use on coordinate grinding machines; brazing wheels are suitable for grinding. 


      It is pointed out that different grades of abrasives have different crystal shapes and particle shapes due to different manufacturing processes, and have different strength, thermal stability and fracture characteristics. They should be selected according to the type of bonding agent, the workpiece material to be ground and the grinding method. Different abrasives. For ceramic bond grinding wheels for efficient and high-precision grinding, high-strength and sharp-grained abrasives such as PDA (synthetic diamond) and ABN800 (CBN) from DeBeers can be used. Brand.



      The super-hard grinding surface is also coated with metal coating (metal plating can strengthen and toughen, slow down thermal shock and combine bridge between abrasive particles and binder to improve the performance of grinding wheel) and non-plated metal Species should be selected according to different types of binders, workpiece materials, dry grinding and wet grinding. Dry grinding generally uses copper clothes, such as RVD-Cu, CBN-Cu; wet grinding uses nickel clothes, such as RVD-Ni, CBN-Ni. At present, the plated metal has been developed from nickel and copper to titanium, tungsten alloy, non-metal ceramics, etc., from a single coating to a composite coating, and has been developed from abrasive coating to micro-powder plating. 


      The concentration of hard abrasive grinding wheel can not be selected too low, high concentration can bring high grinding ratio, 200% concentration super abrasive abrasive wheel is actually a kind of ordinary grinding wheel with super hard abrasive as auxiliary material, generally not recommended Named. At present, high-speed effective grinding uses higher concentrations. The concentration of ceramic bond CBN grinding wheel is effectively higher. The concentration of ceramic bond CBN grinding wheel is generally more than 125%, such as imported ceramic CBN grinding wheel. The concentration is generally 175% to 200%. 


      The hardness level of the grinding wheel indicates the amount of binding force of the bonding agent to the abrasive. The reasonable choice of the hardness and stability of the grinding wheel and the hardness is an important prerequisite for ensuring the quality of the grinding. Foreign super-hard abrasive grinding wheels generally have 3~7 hardness grades to choose from, but the super-hard grinding wheels produced in China do not have the hardness grade. The corresponding standard is not specified in GB6409, so the quality is not easy to guarantee, which brings inconvenience to the users. Supplement and perfection. 


      Choose the right amount of grinding


      The grinding speed of the superabrasive grinding wheel should be selected according to the specific conditions of the grinding wheel type, grinding method and cooling. Increasing the working speed of the grinding wheel can significantly improve the grinding efficiency and grinding ratio (for example, the grinding speed of the grinding wheel is increased by 40%, the grinding ratio can be doubled), and the grinding cost is reduced. Diamond grinding wheel with metal and resin bond, the grinding speed is usually 15~35m/s, the grinding speed of ceramic bond CBN grinding wheel can be increased to 45~160m/s, and the grinding speed of single layer CBN grinding wheel can be 125~ 250m/s. The line speed of 80~125m/s has become the mainstream of the current world CBN grinding wheel grinding. The grinding depth is generally 0.002~0.010mm, the workpiece speed is usually 10~30m/min, and the longitudinal feed speed can be 0.3~1.5m/min. Take a large value for rough grinding and a small value for fine grinding. 


      Super-hard grinding wheel grinding requires 


      super-hard grinding wheel grinding, especially high-speed grinding. It requires good processing stability, low vibration and high line speed. For example, the PLANOMAT series of flat and forming grinder wheels exhibited at CIMT 2001 have a working speed of 170 m/s. Therefore, there is a requirement for the grinding machine: 


      (1) The grinding wheel spindle rotation precision is high. 


      (2) The rigidity of the grinding machine is good, and the general requirement is about 50% higher than that of the ordinary grinding machine, and usually should not be less than the order of 100N/μm. 


      (3) The precision of the grinding machine feeding system should be high, the feeding speed should be uniform and accurate, the longitudinal feeding small speed should reach 0.3m/min, and the horizontal feeding (cutting depth) should reach 0.001~0.002mm/single stroke to ensure The dimensional accuracy, geometric accuracy and surface roughness of the grinding. 


      (4) The moving parts of the grinding machine, such as the headstock main shaft and bearing parts, the feed motion guide parts, etc. should have a reliable seal to prevent the entry of super-hard abrasives and accelerate the wear of the parts. 


      (5) To have a relatively complete grinding fluid filtration and supply system, the filtration accuracy should be less than 0.5μm. (6) All parts of the grinding machine should have corresponding anti-vibration and vibration isolation measures, and the grinding wheel should be finely balanced. 


      Finishing of superabrasive grinding wheel


      The dressing of super-abrasive grinding wheels is usually divided into two processes of shaping and sharpening. Shaping is to carry out micro-cutting on the grinding wheel to achieve the required geometry and precision, and to make the abrasive finely crushed to form a micro-blade; sharpening is to remove the bonding agent between the abrasive grains, so that there is a certain space between the abrasive grains. The cutting edge is formed in addition to the abrasive particles protruding from the bonding agent. 


      For multi-porous ceramic bond super-abrasive grinding wheels (including diamond and CBN grinding wheels), because the bonding agent is loose, usually shaping and sharpening can be done at one time; for the bonding agent is a dense grinding wheel (such as resin, metal bonding agent) , shaping and sharpening must be carried out separately. 


      Ultra-abrasive grinding wheel effective shaping tools are the first to push diamond finishing tools. Trimming with single-grain diamonds often affects the accuracy of the profile due to wear of the dressing tool. If it is trimmed with a diamond roller, the dressing tool can be prevented from being worn too fast and a good grinding wheel shape can be obtained. The feed rate of the dressing device is high, and the feed rate should be controlled at the micron level. Excessive dressing affects both the quality of the grinding and the life of the grinding wheel. 


      There are many methods for sharpening. Generally, mechanical methods (such as cutting into a sharpened block with a corundum block, hydraulic jet repair and sharpening) or electric machining can be used. The latter is mostly used for the grinding wheel of the metal bond, and the grinding wheel is used as the anode to remove the metal bond by electric spark or electrolysis; if the graphite powder is added to the bonding agent, the method is also applicable to the resin and the ceramic bond grinding wheel. 



      In recent years, the University of Tokyo has developed a rotating diamond cup wheel dressing tool. The mechanism is to use the abrasive falling off the cup wheel to impact the bridge between the super-hard (diamond or CBN) abrasive particles and the bonding agent, and the crushing effect between the falling abrasive grains and the super-hard grinding wheel. And to achieve the purpose of finishing. By adopting this method, whether it is a multi-porous ceramic bond super-hard grinding wheel or a compact grinding wheel, the shaping and sharpening can be completed at one time, and at the same time, since the cup-shaped grinding wheel rotates spontaneously, the wear is small, and the shaping force can be kept almost during the trimming process. Variable, and the value is much smaller than when trimming with a single-grain diamond dressing tool. 


      Correct use of grinding fluid


      For grinding of superabrasive grinding wheels, dry or wet grinding can be used. However, the use of wet grinding not only extends the life of the grinding wheel, but also prevents the workpiece from being burnt. Different grinding fluids will make the grinding ratio of the grinding wheel several times or even ten times different. The degree of mechanical wear, chemical attack and thermal damage of the grinding wheel during grinding is closely related to the cooling effect. However, when using CBN grinding wheel, it is necessary to pay attention to the fact that CBN reacts with water vapor and oxygen in the air at high temperature to form ammonia and boric acid (BN+3H2O→H3BO3+NH3). This reaction is called hydrolysis and will accelerate. The grinding wheel is worn. Therefore, water-soluble oils or aqueous solutions with extreme pressure additives are often used in use to attenuate the hydrolysis. When wet grinding with different grinding fluids, the relative life of CBN grinding wheels is different. The life of the grinding wheel cooled by pure oil is long, the life of water-cooled grinding wheel is second, and the life of water-added anti-rust additive cooling wheel is low. 


      The unconventional use of superabrasive grinding wheels is 


      generally considered to be due to the fact that diamonds and iron elements are chemically inert, so diamond wheels are not suitable for grinding workpieces such as gray cast iron. However, this occurs only at high temperatures. For example, when grinding with a diamond wheel, the diamond will oxidize in the air when the temperature reaches 650 ° C, and it will be graphitized when it reaches 700 ° C ~ 800 ° C. At low temperatures, these materials do not react. Therefore, as long as the appropriate grinding speed is selected and the wet grinding method is used, and the sufficient grinding fluid is supplied at the same time, the diamond grinding wheel can completely grind the casting, which has been confirmed in production practice. When wet grinding, the grinding ratio of the grinding wheel of the diamond wheel (9600) is much higher than that of the grinding of the gray cast iron with the CBN wheel (2700). 



      It is generally believed that CBN grinding wheels are not suitable for grinding soft materials such as mild steel, because the grinding debris generated at this time tends to block the grinding wheel. However, the current research believes that the ceramic bond CBN grinding wheel can be used to increase the linear speed of the grinding wheel to form fine grinding debris, and then to remove the grinding debris by means of high-pressure jet grinding fluid, and to supply sufficient grinding fluid to prevent the grinding wheel from clogging. Therefore, the CBN grinding wheel is a kind of soft material that can grind low carbon steel (hardness greater than 30HRC). However, CBN grinding wheels are not effective in grinding any material. Usually it is not suitable for grinding cemented carbide and the combination of cemented carbide and steel, in which case the diamond wheel has a better effect.


      Conclusion 


      Although the production of artificial super-hard abrasives in China started late, it has developed rapidly. In 1997, the annual output of synthetic diamond in China has reached 500 million carats, and the annual output of CBN is 8 million carats. At present, a complete technical system such as super-abrasive abrasives, special grinding fluids, special dressing technology and equipment and specialized grinding machines has been formed. At present, China has been able to produce RVD, CBN and metal-coated high-grade abrasive products, such as "metal base ceramic CBN grinding wheel", "65m / s ceramic bonding agent CBN grinding wheel", etc., the specifications of super-hard abrasive grinding wheel have formed a series, super The diameter of the hard abrasive grinding wheel can reach φ750mm (the φ1100mm grinding wheel is being trial-produced), the linear speed can reach 80m/s, and the high-speed grinding wheel of 125m/s is developed, which creates conditions for the promotion and use of the super-hard abrasive grinding wheel. 


      It is foreseeable that with the widespread use of superabrasive grinding wheels, the revolution and development of the grinding machine manufacturing industry will be promoted, and the development of effective high-performance grinding machines will further promote the use of super-hard abrasive grinding wheels.


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