"Iain Hibbert" wrote in message
news
On Fri, 13 Jan 2006 00:19:34 +0000, ChrisR wrote:

Cutting tools do not polish unless they are dull and rub. Wire brushes
can
be kept sharp

who said anything about cutting tools?

a grinder with a grinding disc is not a cutting tool

a grinder with a cup wire brush is not a cutting tool

Without getting too involved with technical explanations both are cutting
tools.

A proper hardened wire cup brush on an angle grinder can be sharpened and
fettled on a grindstone by an experienced tradesman if it has been allowed
to dull by rubbing. If you examine the end of an individual wire on an
effective brush there will be a sharp corner at the end of each wire on an
ineffective brush the end will be rounded and or bent over so there is no
sharp edge. One problem with "as cast" finishes, cast iron keels for
example, is that there will be a chilled (iron carbide) skin that is very
hard and will dull a hardened brush very quickly reulting in rubbing. The
brush is only really effective at removing material on the cast iron skin in
this case. Rubbing the iron skin itself will not be terribly effective as it
will merely glaze and polish. Depending on the grade of iron below the skin
a soft grey iron is relative soft and machjineable, quite a different of
course.

A grindstone or grinding disk consists of hard grit embadded in a softer
usually polymeric matrix. The grit will be hard with randomly disposed sharp
cutting edges. As the exposed sharp edges of the individual pieces of grit
dull they are shed exposing new grits with sharp cuting edges. The
composition of the wheel or disk must be matched to the material being
ground if the wheel is not to glaze or clog. Lubricants canbe very effective
to aid cutting and cooling some materials.
A blunt brush or grinder is just as useless as a blunt knife.