[ATM] Dog Biscuit

[wa4guu at verizon.net (Jerry)]

I should have also mentioned that a "little" dog biscuit is not a problem in
the polishing stage. I think most people polish with a stroke speed that
will cause some dog biscuit in order to speed polishing. Then when it is
fully polished slow the speed down to smooth the surface as a part of the
figuring process. Most people try to get close to a  sphere before heading
for the parabola so this is a good time to slow down and by the time you
have the sphere pretty good the dog biscuit should be gone.

A poorly pressed lap is also a source of dog biscuit. Same action as before
about the only difference the poor contact between the lap and mirror is
present at the start of the polishing session, where the fast stroke causes
the poor contact after you begin working.

If you develop severe dog biscuit (a judgment how bad severe is) it is my
opinion that the chance of scratching is increased. My thinking here is that
the poor contact allows some gap between parts of the lap and glass that
increase the chance that if a piece of grit were in a channel of the lap
that it could more easily work its way in between a facet and the glass. If
a grain of grit fell out of your hair onto a facet of the lap as you where
stroking the mirror across it, the better the contact between the lap and
mirror the better the chance that the grit will be wiped off of the facet
and into a channel by the edge of the mirror, rather than getting in between
and scratching.

When I have seen severe dog biscuit there was also sleeks and abnormally
large number of scratches. Sleeks are usually many fine, short scratches
that polish out quick when you get the lap pressed in and slow the speed and
maybe use less polishing agent. Not much harder to get rid of than dog
biscuit. And then what I call a scratch takes a while to remove and maybe
would need a return to grinding. Someone making severe dog biscuit is
probably inexperienced and doing a number of things wrong. So maybe the dog
biscuit and scratches weren't really from the same cause and I've only seen
a few examples.

Jerry