Grinding Wheels

Designed to be rotated at comparatively high rotational velocities in grinding machines, grinding wheels contain a surface of rotation around a central axis. They are built by first thoroughly mixing proper-sized grains of the desired abrasive along with a bonding material. The bonding material must thoroughly coat the surfaces of all grains. This mixture is pressed, or else formed, in molds to the desired shape and size. After drawing these shapes from the mold, the bonding material must be hardened by heating or by other means. The central hole of a grinding wheel, that fits an arbor, is bushed by pouring molten lead into the hole around a removable metal core. The grinding wheel might then be mounted and rotated about its axis for truing its surface with a diamond cutting tool. Truing cuts a small amount of material from a surface of a grinding wheel so that it shall become concentric with the axis of rotation.

Grinding wheels are built with inorganic bonds (vitrified or silicate), or organic bonds (resenoid, rubber or shellac). Vitrified wheels which utilize ceramic porcelain as binder are most commonly utilized. These wheels may perform all types of grinding and microfinishing operations. Other binders include feldspar, refractory clay, and talc etc. Their high strength is attained because of firing at high temperatures. Though they are brittle and hence cannot be made thin and long. Resenoid wheels are more stronger and flexible. Thin wheels of large diameter have this bond. Resenoid bonded wheels, though, wear fast, destroy in alkaline fluids and the bonds burn at temperature above 300^oC. Rubber bonded wheels are flexible and strong and might be used for slitting operations, and centreless grinding. They deteriorate at temperature above 150^oC.

A grinding wheel must be marked to give the following information : abrasive grain size, abrasive used, structure, bonding material used, which indicates how closely the abrasive grains with their bond coatings are packed together, and grade, which denoted how strongly the abrasive grains are held in the grinding wheel. The bonding material ordinarily does not occupy all of the space among the grains, as it merely coats the grains. Therefore, grains are held together at points of contact among the bond coatings. Grinding wheels are porous to varying degrees, based upon their structure. Open spaces among the grains help to provide room for chips and to carry cutting fluid to the area where the cutting occurs. Therefore, an open structure helps to secure a cooler cutting action than a dense structure. The strength shall which the grains are held in a grinding wheel based upon the number of points of contact among grain-bond coatings, and the strengths of these connections. It means that grade based upon the structure, the kind of bonding material, and the amount of bonding material utilized. The grade of a grinding wheel is also called as its “hardness”. For example, grains might be broken loose from a soft grinding wheel more easily than from a hard grinding wheel.
Three grades of grinding wheels are Soft (A to H), Medium (T to P) & Hard (Q to Z). Soft grades are utilized for hard materials while hard grades for soft materials.