it's a compromise of hardness, bond , grit size and density.
hard , dense wheels last
a long time in some situations , but can load easily and must be dressed often. for dry
freehanded grinding of hss , they can burn the tool pretty fast.
the super soft toolgrinding wheels have an open structure = big spaces between the grits.
less bond.
they pretty much dress themselves. not the best choice for grinding weld undercuts
or sharpening that old mattock. fast rate of atrition , so expensive for the wrong purpose.
they are really nice for grinding drills and lathe tools . bench grinders are cheap when
compared to the cost of nice stones, so i have 2 . one for dirty ,ugly jobs , and 1 with
a soft wheel on one side and a s/c on the other. so i have 2 $20 wheels on a $60
jet grinder
you usually have to go to the right store to get the choice of wheel . if you go to
sears or the local hardware store , they MIGHT have 6" 8" and 80,120 grit stones.
msc or rutland will have every grade imaginable..so for tool grinding you might get
an A-80 grit with an H or i bond, or an A-60 with an L or M- for plow bolts and such.
there's a whole science to abrasives , grinding media, and such ,but to me
its mostly personal preference and habits. i prefer coarse wheels,
using a non-woven scotchbrite or beartex wheel for the final polish. a 120 grit wheel takes
forever and creates lots of heat.
a 4x36" stationary belt sander makes for swell
touch-ups on drills and for rounding the corners of endmills. it takes a very predictable
bite with each pass ,making easy results.
its not the color , but the bond that makes it cool cutting .some just happen to be white
or pink. a blue wheel with a similar composition should cut the same.