19.5.1 The Grinding Wheel
Centerless grinding wheels tend to be larger in diameter and wider than in center grinding. This means there are many more cutting edges available for grinding. Since the cutting action is shared between many more abrasive grains, wheel life between dressing operations tends to be greatly increased. This makes for economic production with little need to interrupt the process for redressing.
Most often, the type of abrasive used for the grinding wheel is vitrified aluminum oxide or silicon carbide. Other types of abrasive may be used, but the vast majority of operations are undertaken with one or other of these two conventional abrasives. These abrasives are inexpensive compared to superabrasives and it is, therefore, reasonable to experiment with different wheels until an optimum wheel specification is achieved. There are many variations of wheel specification and abrasive types used in centerless grinding. The selection of a particular wheel will depend strongly on compatibility with the work material, work-material hardness, the roughness limit, and factors such as size tolerance and risk of thermal damage.
Recent developments in conventional abrasives include monocrystalline grains for greater fracture resistance, sol gel grains that are designed to wear by microfracture while remaining sharp, and high-aspect-ratio grains designed to remain sharp even with heavy wear.
For special operations where extremely high accuracy and low workpiece roughness are required, alternative types of wheel can be used in keeping with modern grinding technology. For example, it is possible to grind ceramics using metal-bond diamond wheels with electrolytic inprocess dressing (ELID). For low-temperature grinding of ferrous materials, vitrified cubic boron nitride (CBN) abrasive is increasing in use for center and surface grinding. Vitrified CBN wheels can also be applied in centerless grinding. When using CBN, significant advantages can be obtained but it is important to pay careful attention to CBN grinding technology [Marinescu et al. 2004]. CBN is notable for its natural hardness and high thermal conductivity. Both properties are ideal for grinding.