The size of individual
abrasive grits influences the stock removal rate, chip clearance in the wheel,
and corner holding ability. Abrasive grain size is determined by the size
of the screen through which it passes. The number of the nominal size indicates
the size of the opening in the screen.
A lower number of grain size indicates a large grain, and a higher number denotes a fine grain.
As though the problem of screening grain size is complicated because of the unsymmetrical shapes of the grits, some idea of the grain sizes in inches can be gained by knowing the size of the square opening between the wires through which the grain will not pass:
30 grit | .0232" square holes
60 grit | .0098" square holes
120 grit ..0035" square holes
The grit size number may also indicate a combination of two or more adjacent sizes. Combinations are often found to provide a more efficient grinding action than single sizes. Grain size varies from removal rate, surface finish desired and workpiece material.
The coarsest grit sizes (8 through 20) are used for billet conditioning and snagging, where the purpose is to rapidly remove large amounts of excess or impure material with little concern about surface roughness or geometry of the part.
Sizes 24 through 46 are also normally used in high stock removal operations where geometry is important and some control of surface finish is required on soft materials.
Finish depends upon several wheel characteristics. Fine surface finishes may often be attained with the use of fine grain wheel. Other variables in producing fine surface finishes are bond type, wheel hardness and stock removal rate.
Finer grit sizes do help significantly in grinding hardened work. Here there is less tendency for the wheel to load (metal in the wheel face) and grain has better penetrating ability on the hard workpiece. On stock removal operations fine grain generates mere heat, so steps should be taken to quench the additional heat.
On softer materials, where the tendency to load is greater (larger ductile chips), the coarser grain size are more efficient ( more pieces per dress or higher metal removal rate) because of the greater chip clearance between the grains.