Grinding processing is generally used as the final process of workpiece processing, and its task is to ensure that the product parts can achieve the accuracy and surface quality required on the drawings. Grinding surface roughness is closely related to the precision of parts, and a certain precision should have corresponding surface roughness. Under normal circumstances, to effectively control the size, the roughness Ra value should not exceed one-eighth of the dimensional tolerance, the impact of grinding surface roughness on the performance of the parts is: the smaller the surface roughness value, the better the wear resistance, corrosion resistance and fatigue resistance of the parts. The reverse is true.
Therefore, in the grinding process, attention must be paid to reducing the surface roughness. Among the main process factors affecting the surface roughness of grinding, the grinding wheel particle size has a significant influence on it. The finer the grinding wheel particle size, the more abrasive particles involved in grinding at the same time, the lower the grinding surface roughness. Generally, the grinding wheel of 46 ~ 80 particle size should be used in grinding, the grinding wheel of 150 ~ 240 particle size should be used in fine grinding, and the resin graphite grinding wheel of W10 ~ W7 particle size should be used in mirror grinding, so that better workpiece surface roughness can be obtained.
In recent years, with the development of new technology and the development of high-precision grinding technology, the grinding size reaches 0.1 ~ 0.3μm, the surface roughness reaches 0.2 ~ 0.05μm, the grinding surface metamorphic layer and residual stress are very small, and the processing quality is significantly improved.
Form grinding, especially high-precision form grinding, is often a key problem in life. There are two problems with form grinding:
The first is the quality of the grinding wheel, mainly that the grinding wheel must have good self-sharpening and shape retention at the same time, and the two are often contradictory.
The second is the grinding wheel dressing technology, that is, to obtain the required grinding wheel profile and sharpness efficiently and economically. Therefore, in order to improve the efficiency and accuracy of grinding, especially for the efficient finishing of difficult-to-process materials, efficient and powerful grinding uses CBN grinding wheel, which makes the powerful grinding break through the limitations of traditional grinding, and the productivity is doubled. Some parts of the blank do not need to be roughed, and can be directly ground into finished products.
This not only improves the processing efficiency, but also improves the processing quality. Such as SG abrasive. It is a novel ceramic alumina abrasive, pure corundum as a raw material, it is combined with a medium such as magnesium oxide in water, to produce a lump gel, after drying to form a brittle object. It is then crushed to the desired particle size at a temperature of 1300 ° C to 1400 ° C
Made by sintering. Its hardness is much higher than ordinary alumina, and the toughness is good, so it can operate under higher speed and larger load conditions, and the metal grinding rate is more than three times higher than ordinary alumina. Its biggest advantage is that the grinding area temperature is low, the grinding wheel always has a sharp grinding edge, and the shape of the grinding wheel is good and the time is long. Cubic boron nitride grinding. It is a hard and wear-resistant abrasive, and has excellent properties such as high thermal conductivity and chemical resistance. The latest generation of abrasives are characterized by sharp, high strength and can be used for unsupported cutting, which reduces the grinding force during the grinding process and thus reduces the damage to the workpiece.
In fact, whether the abrasive synthesis conditions are strictly controlled or not will directly affect the final crystal shape and physical properties including strength, thermal stability and fracture characteristics of the abrasive particle, thus affecting its performance. For example, the four ABN products produced by De Beers company, each of these four products has its own different characteristics. ABN200 is a brittle black abrasive, mainly used in ceramic bonded grinding tools and metal coated resin bonded grinding tools. ABN300 has a similar strength to ABN200 but is amber colored and is commonly used in grinding tools bonded with metal and epoxy resins. ABN600, which is black, is a typical high-strength cubic boron oxide abrasive with a specific crystal face, and is also used in metal and epoxy resin bonding abrasives. ABN800 is the latest generation of high strength single crystal cubic boron oxide material. ABN800 has a similar strength to ABN600, however, it can be seen that there are significant differences between the two, the former has sharp edges and high thermal stability.
In the grinding process, block or round abrasive particles can be considered for grinding with a large negative front Angle with the workpiece. For sharp polygonal particles, large positive front Angle is used in most cases for grinding. Therefore, when grinding steels with high carbide content (such as some high-speed steels), it is best to choose abrasive particles with negative front angles. Sharp polygonal abrasive particles with large positive front angles have potential advantages when machining harder and stronger materials, such as certain high speed steels and surface hardened steels.
Influence of abrasive characteristics in slow feed grinding. In the grinding process, the type and size of the load placed on a single abrasive particle will affect the cutting performance of the abrasive particle, and then affect the choice of the best abrasive. To illustrate this, understage tests were carried out: M2 high speed steel was ground with a ceramic bonded wheel with an end radius of 0.5mmr90°V, two ABN600 and ABN800 abrasiades were tested, and normal grinding forces, power and grinding wheel wear were monitored. It can be seen that the ABN800 normal grinding force is low. When the feed rate increases, the cutting rate increases, and the grinding force also increases. However, for the ABN800 grinding wheel, the improvement of grinding force is relatively small. The increasing trend of grinding power with the increase of feed rate is basically the same with ABN600. Although the grinding force and energy measured for the grinding wheel made of ABN800 abrasive are lower, the wear on the end face of the wheel with a curvature of 0.5mm is also reduced. When the feed rate is increased from 200mm/min to 300mm/min, the relative percentage of the three parameters is increased, that is, the cutting power is increased by 50%. With the aggravation of grinding conditions, the advantages of abrasive particles with sharp angular characteristics become more prominent.
Nickel-chromium steel is a difficult material to grind. In view of the high chip removal rate of abrasive with sharp edges, the grinding test of 718 nickel-chromium steel was carried out with ceramic bonded grinding wheel. Two kinds of high strength abrasives, ABN600 and ABN800, were tested as before. As you can see, compared to the ABN600, the ABN800 retains its advantages at 150mm/min and 200mm/min table speeds. When the table speed is further increased to 300mm/min, both grinding wheels show high grinding wheel wear rates. However, the test results show that when grinding the same material with a wheel made of sharp abrasives, reasonable wheel life can be obtained at both 150mm/min and 200mm/min speeds.
The surface grinding test of M2 high speed steel was carried out with resin-bonded grinding wheel. In the experiment, the size of the small grinding wheel made of ABN600 and ABN800 abrasives is 125mm×6mm. The experimental results show that the grinding wheel with sharp abrasives has long life and low grinding power. It is well known that during grinding, a transient interface high temperature occurs between the abrasive and the workpiece grinding surface. The experimental results show that the interfacial grinding temperature of cubic boron oxide is lower than that of common abrasives. The key reason is that the specific grinding energy of the cubic borax oxide wheel is lower than that of the commonly used abrasive wheel. Through the test, it can be seen that with the increase of cutting depth, the reason why the cubic borax oxide wheel has low grinding energy is mainly due to the high thermal conductivity of cubic boron oxide.
In summary, in the grinding of different materials and process conditions, the reasonable selection of grinding wheels can reduce the grinding surface accuracy, improve the grinding surface quality, improve the grinding efficiency exponentially, and achieve the effect of low-cost processing, and the grinding wheel has a long life, low dressing frequency, high metal grinding rate, and good grinding force cooling effect.
