Metallic bonds exist in multiple varieties. All metallic bonds exhibit, in contrast to grinding wheels bonded with resins or vitrified mixtures, an increased level of heat conductivity. The properties of metallic bonds can be summarised along these lines:
• high resistance to wear,
• difficult to dress, or, in the case of single-layered grinding wheels, not dress — able,
• high heat conductivity,
• increase in frictional heat.
Metallic bonds are of particular importance for grinding wheels with superabrasive materials. This will be further explored in chapter 3.4.2.
The rubber bond consists mostly of synthetic caoutchouc, sulphur and zinc oxide to speed up vulcanisation. Grinding wheels bonded with rubber are used, for example, in abrasive cutting discs and in control wheels for centerless grinding. Rubber bonds are comparatively cool-grinding bonds. This can be attributed to the fact that grits that have become dull due to low grinding wheel hardness break off early and only a small amount of frictional heat is generated.
Binders in the form of highly viscous, foamless natural glues serve primarily as the bond material for non-moisture-proof abrasive material as a foundation. These are almost exclusively used in wood treatment, maintaining a large market share in this area. In addition, in many branches of industry, binder-coated grinding wheels are used for surface finishing.
The method is designated as either crude or fine finishing. The tools used for this consist of felt, leather or sometimes wood discs, which are coated with tough, hard, yet temperature-sensitive warm binder or with cold binder which is insensitive to heat but brittle (mostly liquid glass). After several hours of drying, a second binder coating is applied. Immediately, the coated surface is rolled in grinding powder of predetermined grit size in a tank until the binder dries and no more grits are attaching to the leather. After drying, the surface once again is coated with binder and rolled in grinding powder. In this way, abrasives receive an elastic backing material and can assimilate to the form of the tool [HADE66].
One seldom-used bond type is the magnesite bond. It is manufactured with magnesite or burned magnesium (MgO), which converts to magnesium hydroxide with water.
Magnesite bonds produce a soft grinding wheel, exactly like silicate bonds. Such grinding wheels are utilised in dry-grinding thin, heat-sensitive tools, primarily cutlery. Although they dull easily, this disadvantage is accepted for the sake of being able to work at lower grinding temperatures [HADE66].
Shellac, dissolved in alcohol or water mixed with alkalines (borax, ammonia etc.), functions nowadays only rarely as a binder for abrasive grits, although it is completely water-repellent after drying. However, its use has incited the artificial resin industry to replace it with more efficient synthetic resins.