Abrasive wheels are extra disks (cylinders). The cylinder is rotated at a very high speed and they are generally used for the sharpening of objects. Grinder wheels in the olden days never relied on electricity to power them like today. They were in fact run by the user working a pedal with their foot or sometimes a crank was used which was operated by hand. When the electric motor was introduced, this was the end to by hand powering of the wheels. However, they had to be designed slightly separate and had to be artificial to greater radial stress. If they were artificial the same as the by hand powered wheels, they would fly apart as the cylinder was spinning.
In some cases the structures were legitimately given extra withhold and reinforced by use of impregnated fibers. These often required the use of a lubricant or a coolant, because of the speed the Grinder spins at and the experience it makes with the workpiece, a lot of heat is produced and in turn this can severely damage the workpiece, depending on its material. The coolants main purpose was to sacrifice frictional heat build up and protect the workpiece.
Disc Sander
The lubricant was often kept in a shallow half circle bowl, so when they spin the lower half of the it passes straight through the liquid / coolant in turn preventing the workpiece from sustaining damage. Diamond wheels are also an additional one type.
In overview it is a disk that has been impregnated with an Grinder and spun at high speed for sanding and milling etc.
abrasive Wheels