Make the grinding stone contacted with the workpiece surface
that the actual grinding process occurs.
The bond wears a lot by reciprocating and rotating
the grinding stone or the workpiece.
The grains fall off before grinding workpieces
due to a lack of bond's strength to hold the grains.
This process that the grinding stone can not grind the workpiece
due to a large amount of the grinding stone wear is called
Grain falling off.