Grinding is generally used as the final process of workpiece machining. Its task is to ensure that the product parts can meet the precision and surface quality required on the drawings. Grinding surface roughness is closely related to the accuracy of the part, and certain accuracy should have a 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 effect of the grinding surface roughness on the part performance is: the smaller the surface roughness value, the smaller The better the wear resistance, corrosion resistance, and fatigue resistance. The reverse is true. Therefore, in the grinding process, care must be taken to reduce the surface roughness. The main process factors affecting the surface roughness of the grinding process have a significant effect on the grain size of the grinding wheel. The finer the grain size of the grinding wheel, the more abrasive grains that participate in grinding, and the lower the grinding surface roughness. General grinding, take the grinding wheel of 46 to 80-grain size when grinding, 150 to 240-grain size grinding wheel should be selected. When grinding the mirror surface, the resin graphite grinding wheel of W10 to W7 grain size should be used to obtain better workpiece surface roughness.
In recent years, with the development and application of new technologies and the development of high-precision grinding technology, the grinding size has reached 0.1 to 0.3 μm, and the surface roughness has reached 0.2 to 0.05 μm. The surface of the grinding layer and the residual stress are very small. Improved processing quality. Forming grinding, especially high-precision forming grinding, is often the key issue in life. Grinding has two problems: The first is the quality of the grinding wheel. The main thing is that the grinding wheel must have good self-protrusion and shape retention at the same time, and the two are often contradictory. The second is the wheel dressing technology, which is to obtain the required wheel profile and sharpness efficiently and economically. Therefore, in order to improve the efficiency and precision of grinding, especially for high-efficiency finishing of difficult-to-machine materials, CBN grinding wheels are used for high-efficiency and strong grinding, which makes the strong grinding machine break through the limitations of traditional grinding, and the productivity is doubled, and some parts are rough. Without rough, it can be directly ground into a finished product, which not only improves the processing efficiency but also improves the processing quality. Such as SG abrasive. It is a new type of ceramic alumina abrasive. It uses pure corundum as a raw material and combines it with water, such as magnesium oxide, to produce a blocky gel, which forms a brittle object after drying. It is then crushed to the desired particle size and sintered at a temperature of 1300°C to 1400°C. Its hardness is much higher than that of common alumina, and its toughness is good. Therefore, it can be operated under higher speed and larger load conditions. The metal removal rate is more than three times higher than that of ordinary alumina. Its greatest advantage is that the grinding zone has a low temperature, the grinding wheel always has sharp grinding edges, the shape of the grinding wheel is good and the time is long. Cubic boron nitride grinding. It is a hard, wear-resistant abrasive and has excellent properties such as high thermal conductivity and chemical resistance. The latest generation of abrasives is characterized by its sharpness, high strength, and its ability to be used for unsupported cutting. These features reduce grinding forces during grinding operations, thereby reducing damage to the workpiece.
In fact, the strict control of the abrasive synthesis conditions will directly affect the final crystal form of the abrasive particles and the physical properties including strength, thermal stability, and fracture characteristics, thereby affecting its performance. For example, four ABN products produced by De Beers, each of which has its own different characteristics. ABN200 is a brittle, black abrasive used mainly for ceramic bonded grinding tools and metal coated resin bonded grinding tools. The ABN300 has similar strength to the ABN200 but has an amber color and is commonly used in metal and epoxy bonded grinding tools. ABN600 is black and is a typical high-strength cubic boron oxide abrasive with a specific crystal face. It is also used in metal and epoxy bonded abrasives. ABN800 is the latest generation of high-strength single crystal cubic boron oxide material. The ABN800 has similar strength as the ABN600, but it can be seen that there are significant differences between the two, the former having sharp corners and high thermal stability.
In the grinding process, block or round abrasive particles may be considered for grinding with a relatively large negative rake angle to the workpiece. For sharp, multi-angled particles, in most cases, a large positive rake angle is used for grinding. Therefore, when grinding high-carbon steels (such as some high-speed steels), it is best to choose abrasive particles with negative rake angles. Sharper polygonal abrasive grains with larger positive rake angles have potential advantages when processing tougher materials such as certain high speed steels and case hardened steels.
The effect of slow-feed grinding on the characteristics of abrasives. The type and size of the load applied to a single abrasive particle during the grinding process will affect the cutting performance of the abrasive particle, and thus affect the choice of the best abrasive. In order to illustrate this point, a sub-stage test was conducted: Grinding M2 high-speed steel using a 0.5 mmr90°V type ceramic bonded grinding wheel with a tip radius to test the two ABN 600 and ABN 800 abrasives and the normal grinding force, Power and wear of the grinding wheel are monitored. As can be seen, the ABN800 normal grinding force is lower. When the feed rate increases, the cutting rate increases and the grinding force also increases. However, for the ABN800 grinding wheel, the increase in grinding force is relatively small. The trend of increasing grinding power with increasing feed rate is basically the same as that of the ABN600. Although the grinding force and energy measured by the grinding wheel made of ABN800 abrasive are low, the wear of the face of the grinding wheel with a curvature of 0.5 mm is also reduced at the same time. When the feed rate is increased from 200 mm/min to 300 mm/min, the relative percentage of increase in the three parameters, that is, the cutting power is increased by 50%. With the worsening of the grinding conditions, the advantages of abrasive particles with sharp-angled features are more pronounced.
Nickel-chromium steel is a more difficult material to grind. For the sharp-edged abrasive with high chip removal rate, a nickel-chromium steel grade 718 was ground with a ceramic bonded wheel. For the two high-strength abrasives, the ABN600 and ABN800 were tested as before. It can be seen that compared to the ABN600, the ABN800 still maintains its advantages when the table speed is 150mm/min and 200mm/min. When the table speed was further increased to 300 mm/min, both types of grinding wheels exhibited high wheel wear rates. However, the test results show that when using the grinding wheel made of sharp abrasives to grind the same material, reasonable grinding wheel life can be obtained at both speeds of 150 mm/min and 200 mm/min.
The resin-bonded grinding wheel was used for the surface grinding test of M2 high-speed steel. In the experiment, the size of the small grinding wheel made of ABN600 and ABN800 abrasives was 125 mm×6 mm. The experimental results show that the grinding wheel with sharp abrasive has a long life and low grinding power.
As we all know, in the grinding process, instantaneous interface high temperature occurs between the abrasive and the workpiece grinding surface. Experiments have shown that cubic boron oxide produces a lower interface grinding temperature compared to commonly used abrasives. The key reason is that the specific grinding energy of cubic boron oxide grinding wheels is lower than that of abrasive grinding wheels. Through experiments, it can be seen that with the increase of cutting depth, the reason why cubic boron oxide grinding wheels have 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 proper selection of grinding wheels can reduce the grinding surface accuracy, improve the surface quality of grinding, can double the grinding efficiency, and achieve low-cost processing The effect is that the grinding wheel has a long life, the dressing frequency is low, the metal removal rate is high, and the grinding force has a small cooling effect.
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