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The selection of abrasive grit size mainly considers the grinding efficiency and the requirements for the workpiece’s surface roughness. We can select based on the following points.
①When the workpiece machining accuracy requirements are high and the surface roughness value is low, abrasives with finer grit sizes should be selected. The smaller the abrasive grain size, the more abrasive grains are involved in cutting, resulting in smaller cutting marks left on the workpiece surface, which leads to a lower surface roughness value. However, the selection of abrasive grit size must also be considered in conjunction with the grinding conditions. If the grinding amount is small and the wheel dressing is fine, selecting a coarser abrasive grit size can still achieve a lower surface roughness value on the workpiece.
②When the contact area between the grinding tool and the workpiece surface is relatively large, or the grinding depth is also large, a coarse abrasive should be used. Coarse abrasives have less friction with the workpiece surface and generate less heat. Therefore, when grinding a flat surface with the end of the grinding wheel, the abrasive grain size can be coarser than when grinding a flat surface with the periphery of the grinding wheel.
Usually, surface grinding uses a grinding wheel with a grit of 36#～46#, and the workpiece surface roughness value Ra can reach 0.8～0.4μm. If the grinding wheel speed vs is increased and the grinding depth ap is reduced, the workpiece surface roughness value Ra can reach 0.4～0.2μm. When fine grinding, abrasives with a grit of 150#～240# are used, and the workpiece surface roughness value Ra reaches 0.2μm or lower. When mirror grinding, a graphite grinding wheel with a resin bond of micro powder W10～W7 grit is selected, and the workpiece surface roughness Ra can reach 0.012μm.
③During rough grinding, the material removal and grinding depth are relatively large, so the abrasive grit size should be coarser than in fine grinding, to improve production efficiency.
④For cutting and slotting processes, abrasives with a coarse grit size, loose structure, and higher hardness should be used.
⑤When grinding ductile metals and soft metals, such as brass, copper, soft bronze, etc., the surface of the grinding tool is easily blocked by chips, so coarse abrasives should be used.
⑥For grinding materials with high hardness, such as quenched steel and alloy steel, coarser abrasives should be selected. When grinding hard metals, such as cemented carbide, due to the material’s poor thermal conductivity, which can lead to burning and cracking, it is advisable to use abrasives with a coarser grit size. For grinding thin or thin-walled workpieces, which are prone to heat deformation, coarser abrasives should also be used.
⑦For workpieces with small cutting allowances or where the contact area between the abrasive tool and the workpiece is small, finer abrasives can be selected. Compared to abrasives used for dry grinding, those used for wet grinding can have a finer grit size.
⑧When machining on a rigid grinding machine, coarser abrasives can be selected.
⑨During profile grinding, the wheel surface should maintain its shape well, so finer abrasives should be selected.
⑩During high-speed grinding, to improve grinding efficiency, the abrasive grit size should be finer by 1 to 2 grit numbers compared to normal grinding. This is because finer abrasives are sharper and can more easily cut into the workpiece. Additionally, there are more cutting abrasives per unit area of the wheel, and the force on each abrasive grain is smaller, making it less likely for the abrasives to become dull. Furthermore, even if individual abrasives fall off, their impact on the uneven wear of the wheel is less significant than with coarser grits, which helps maintain the stability of the grinding process. However, the grit size of the wheel during high-speed grinding should not be too fine, as this would worsen chip removal conditions and reduce grinding efficiency.
In general, medium-grit abrasives are the most commonly used. Fine grit abrasives are usually only used for fine grinding, lapping, and polishing. In mass production, abrasives with coarser grit sizes should be selected, as long as the surface roughness requirements of the workpiece are met, to improve production efficiency. However, in small-batch or single-piece production, where the focus is generally on the machining quality of the workpiece, selecting finer abrasives is more beneficial. The application range of abrasives with different grit sizes is shown in Table 2-9.
Table 2-9: Application Range of Abrasives with Different Grain Sizes
Conclusion
The selection of the appropriate abrasive grit size is crucial for achieving the desired balance between grinding efficiency, surface quality, and overall production goals. Finer abrasives are ideal for high-precision work, where a smooth surface finish and low surface roughness are required. Coarser abrasives, on the other hand, excel in rough grinding, cutting, and material removal, particularly in high-production environments where efficiency is prioritized.
Understanding the factors influencing grit size selection, such as the workpiece material, grinding depth, machine rigidity, and whether the grinding process is wet or dry, allows for more informed and effective decision-making. By considering these parameters, manufacturers can optimize the grinding process, enhance product quality, and improve overall operational efficiency.
In summary, selecting the right abrasive grit size is not only essential for achieving optimal results but also plays a significant role in streamlining the grinding process to meet specific production and quality requirements.
