Model Forum / General / Models / May 2008

The hardness refers to the strength of the bond holding the grit
together, rather than the grit itself.

A hard material will dull or blunt the grit, and if it doesn't break
off then the wheel will glaze and not cut, so a softer wheel allows
the grit to break away easier and then effectively presents a sharp
new cutting edge.

Softer materials don't wear the wheel so much, so can use a harder
bond.

Peter

It's because hard materials like hardened steel or tungsten carbide blunt
the grains of abrasive before they have time to be released by the bonding
agent to expose fresh sharp grains. The wheel then just rubs and builds up
heat. The rate of grain release, or wheel breakdown, must therefore be
inversely proportional to the hardness of the material being ground. Whether
a wheel is 'hard' or 'soft' refers to how fast it breaks down and exposes
new grit and not to the actual hardness of the grit itself which is what
confuses people new to grinding.

A hard wheel breaks down slowly and a soft wheel breaks down fast. However
the harder the material being ground the harder the grit must also be. In
fact you can't effectively grind a material with a grit that is softer than
the material being ground. Silicon carbide grit is harder and has sharper
cutting edges than aluminium oxide grit so it is needed more on very hard
dense materials like tungsten carbide but it is also weaker and breaks down
quickly so perversely it's used on soft materials like non ferrous metals
and stone. On hard steels it can give a very good surface finish and is
sometimes used as a finishing wheel in a very fine grit rather than the more
commonly used aluminium oxide grit for general purpose grinding and
roughing.

I went into all this in a big way some years back when I was trying to find
a suitable replacement wheel for the original one on my old Black and Decker
valve refacer. It's a custom shaped dished 5" wheel that's no longer made so
to put it mildly you're pretty much f***ed for anything suitable off the
shelf. I even had a spare wheel when I bought the machine but not knowing
they were no longer made and how rare it was I made a mandrel to put it on
the mill for general grinding and doing something stupid one day I broke the
bloody thing so when the one on the valve refacer wore out I was screwed.

I made an adaptor for the valve refacer so it could take a straight 5" wheel
which then gave me some choice but nothing like what you'd have in a more
common 6" wheel. Of course everyone said I wanted aluminium oxide for
grinding steel. I tried all sorts of things. White aluminium oxide which
gives good stock removal without too much heat buildup but a poor surface
finish. A brownish wheel of a type I forget which was nigh on useless.
Pink/gray aluminium oxide which is the norm for steel but still tended to
chatter. I solved most of that with a finer grit, a new diamond and better
dressing techniques but never quite matched what the OE wheel would do. That
always used to give a mirror finish, never chattered and wasn't even that
fussy about how it was dressed. When I tried to duplicate its performance
with off the shelf wheels I became convinced it must have been made out of
something magical.

Eventually I discovered that the original wheels were actually a special
grade of silicon carbide although they were more blue than green. Happily
someone has now started making them again and I'll get one once my current
wheel is worn out but it does me for now.