The hardness of a grinding disc is the resistance that the binding of the disc exerts against grain shedding.

The bond hardness of a cutting-off wheel or grinding disc can be influenced and adjusted by the resin used as well as by the added filler materials. According to DIN ISO 525 the bond hardness of cutting-off wheels and grinding discs is identified by letters of the alphabet that are added to the type designation. The identifiers range from "A" (extremely soft) to "Z" (extremely hard), i.e. the harder the bond, the higher the position of the corresponding letter in the alphabet.

As a general rule, the selection of a cutting-off wheel / grinding disc that is ideal for a particular application requires that the softness of the bond of the abrasive used increase with the hardness of the material to be processed.

If the bond selected by the user is too hard, worn and dull grains will not be shed from the backing during the work process, resulting in a rapid decrease in aggressiveness of the disc. This may cause the cutting edge to overheat, resulting in the "vitrification" of the disc and the total loss of its cutting performance.

If the bond selected by the user is extremely soft, the abrasive tool affords the user a high level of aggressiveness as the grain used is always fresh, resulting in shorter processing times. At the same time, an extremely soft bond will result in increased wear of the tool and, consequently, decreased service life.

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