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Re: [ATM] Tell me about scratches

Rich,

Rich Ball wrote:
> Alas, now three hours into polishing my 8" f4.5 Pyrex mirror I find a couple
> scratches.  One is not deep enough to feel with a fingernail.  One is.  It's
> about 1/4" long.

I feel for you.  For those that don't know, Rich had a nearly finished 
mirror fall and break not too long ago.  He's sticking with it, and 
this is mirror #2.

Rich, where is your Cerox from?  I checked my saved messages and you 
had problems with scratches on the previous mirror.  If you're 
cleaning up thoroughly between grits I suspect either your polishing 
compound or your water.

Heed all of Michael Lindner's advice, especially the part about 
particles in your water.

Additionally, I always run the side of my clean hand across the lap 
and mirror before I start pressing.  If both are dry, you can feel any 
contaminants.  Usually there is just dust and maybe some little bits 
of paper towel.  Wipe until it feels perfectly clean.  If you still 
have scratches after doing this, chance are the particles are in the 
polishing compound or water.

> It's all about pride I suppose. They won't damage the image.  But here's the
> problem.  I'd go back to fine grinding and take them out.  I saved the tool
> and made new concrete pitch-lap base.   But how do I protect against more
> scratches?  I have taken what I thought to be reasonable care.

Fine grind away the scratches if you want to.  25 mircon will probably 
do it.

> I'd like to hear from some glass pushers who manage to get through polishing
> and figuring without these annoying and pride-damping intrusions.  What do
> you do to guard against them?  Is it possible to avoid them without access
> to an industrial clean room?

Since I have switched to Cerox 1670 (Salem Distributing) and MicroGrit 
aluminum oxide, I haven't had any scratch problems.  (The story was 
different with my Willmann-Bell grinding kit....)

	Mike Lockwood


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