According to clifto :
Mike Dobony wrote:
"Chris Lewis" wrote...
If you're using a grinder, water is just fine. The idea is to
keep it cool.

You are no machinist. The water does more than keep it cool. It needs a
lubricant and the water does not do a very good job of lubrication. Oil or
coolant keeps the part from becoming brittle.

I can picture how cooling would prevent brittleness. I can't imagine how
lubrication could prevent brittleness.

Mike's gotten confused. You don't use lubricants with grinding wheels.
Grinding wheels should be used on dry metal, they rely on both the
abrasion and instantaneous melting (the sparks) to cut. With hardened
steel or carbide, the melting does more of the cutting than the abrasion
does.

If you're sharpening a chisel on a grinder, you'll often need to
keep a cup of water around. The idea being that you need to keep the
edge of the blade from getting too hot ("burning") by periodically
dipping the chisel in the water. If it gets too hot, the hardened steel
of the blade detempers and gets soft - you end up softening the chisel
so it won't keep an edge. You either have to regrind the chisel
to remove the softened spot, retemper it, toss it out, or retire it
to opening paint cans and chopping thru nails.

Note with high speed grinders (eg: dremels, bench grinders) you _don't_
put the coolant on the wheel. You periodically cool the work by dipping
it.

My idea was to use a grinder to turn down part of the body of a
hex bit holder because he didn't have access to a metal lathe. Yes,
it's quite tricky, but it can be done - eg: by placing the hex
bit holder into a drill, starting the drill, and holding it against
a bench grinder wheel. The grinder will heat up the bit holder. Periodic
dipping in water will prevent it getting too hot. Softening it isn't
likely a big problem, but causing it to get too hot might loosen it
on the hex shank.

If you use a lubricant on a grinding wheel, at best you'll slow its
cutting down. More likely, you'll foul the wheel, spray gooey gunk
all over the place, and perhaps start a fire (depending on what the
lubricant is).

When you're cutting metal with edged tools (eg: lathe or milling machine),
the metal is being cut, not melted. The cutting tool is going _much_ slower.
Depending on the material, depth of cut, etc, sometimes you need a lubricant
to prevent the metal or tool bit from overheating and detempering/burning.

Lubricants do dual function - by lubricating the cutting action, they
reduce heat production. They also provide a direct cooling function by
transferring what heat is produced away from the work.

You may not be able to turn down the body of a hex bit holder on a lathe
using regular tool bits. The body is hardened at least somewhat (otherwise
it'd simply break in use), and many will simply be too hard to cut with
regular lathe bits (perhaps only with carbide bits). If a hacksaw or
file won't cut it, you'll have a real problem with a lathe.

Generally speaking, once metal is hard enough that a hacksaw or file
won't cut it, only a grinding wheel will touch it.

And BTW: different materials use different lubricant/coolants. IIRC,
cast iron works well with straight water. Steel is most often done with
Dromus, which is bought as a concentrate and diluted 10:1 with water before
use. [I'd have to pull my machinist references out to find out what's in
Dromus]. Lard (yes, lard) is good with some materials. "Sulfurous oil"
with others. Etc.

There are a few lubricants that work with almost everything - these
are usually commercially-made mixtures.
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.