Grinding Wheel Selection
The primary considerations in choosing grinding wheel specifications for a particular machining task are the grindability of the material and the output requirements. From grinding wheel specifications, information regarding their appropriateness for a specific machining task can be derived.
Grain type is selected especially with a view to hardness, toughness and proneness to react with the workpiece material. The common abrasives corundum and silicon carbide exhibit considerably lower hardness values than boron nitride and diamond. Their thermal resistance is however significantly higher than that of diamond.
From the chemical affinity of various grain types to certain workpiece materials, we can designate different areas of application as represented in table 6-2. Due to the high price of such novel abrasives, the decision in favour of boron nitride instead of corundum or diamond instead of silicon carbide should only be made after a calculation of profitability.
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Table 6-2. Chemical affinity between abrasives and workpiece materials
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General predictions concerning the effects of grain types on the process and output are impossible, since the influence of further boundary conditions like the workpiece material must also be considered. Comparisons between silicon carbide and pure corundum grinding wheels with various workpiece materials have shown that, depending on the predominant wear mechanism, the cutting forces, roughness and thermal influence also behave differently. The key rule is that silicon carbide form more cutting edges per grain, so smaller chip thicknesses appear at each individual cutting edge as well as larger total cutting forces in general.
Grain size selection follows the criteria of
• target surface quality and
• desired material removal rate.
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Table 6-3. Guidelines for selecting grain sizes
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Table 6-3 provides guidelines for choosing a suitable grain size. With increasing grain size, the number of cutting edges decreases. This leads in turn to larger chip thicknesses. The attainable surface quality becomes worse, but the possible material removal rate higher. Coarse granulations are therefore used for pregrinding, fine granulations for finish-grinding. This allocation shows that a comparison of grain sizes is always made with respect to the material removal rate. Every grain size is thus assigned to a certain task.
The assignment of a bond to the respective grinding task is performed in a very general way in the literature. Ceramic bonds are brittle and fracture-sensitive, synthetic resin bonds on the other hand are tough, elastic and insensitive to impact. The type and amount of bond, together with the grain size, has an effect on the structure and hardness of the grinding wheel [HAGE69, LEIC75].
Hard grinding wheels generally result in improved grinding conditions and accurate form and dimension. However, the cutting force and thermal stress on the workpiece also increase with grinding wheel hardness.