[ATM] Pitch laps!!!

[Mike Lockwood]

Norm,

Norm Prince wrote:
> Okay I spent an hour and change searching the archives on the subject of 
> pitch laps and learned a bunch of stuff and found some great ideas to help 
> me make one for this 12" project.  Alas i didn't find the answer to the 
> question that's been sloshing around in my head.  I'm about to move on to 
> 5AO so as usual i'm trying to think ahead.

Thanks for searching before posting!

> Here is what i've been thinking:  I'd rather not make the lap out of the 
> tile tool I've been using to grind on the chance that I might have to go 
> back to grinding if I've made any horrible mistakes that will take forever 
> to polish out.  So I was going to pour another plaster disk to use as a base 
> for the pitch lap, but i'm really freaked out about scratching things and 
> don't want to get plaster anywhere near the mirror.

OK, first of all, relax about the scratches.  Small scratches come out 
quickly with fine grinding.  Yes, it is a good idea to keep the 
grinding tool available.  Take reasonable cleaning precautions and 
check the disks before starting work, and think about where 
contamination might come from.  Then move on.

> The question(s) is then will a flat plaster disk be acceptable...or should 
> it have the same curve as the mirror to become a pitch lap?  Should I just 
> go ahead and pour the lap on the tile tool i've been using?

Pouring pitch on a flat tool is fine, so long as it is fairly thick. 
I like a 1/4" minimum pitch thickness at the edge of a convex lap. 
Yes, this may have some effect on how spherical things are after 
polishing, but odds are you won't have a sphere anyway and you'll have 
to adjust your technique.  If that is true, it won't matter.  I've 
made pitch laps on flat pieces of Pyrex up to 22" in diameter and they 
work fine.

Pouring pitch on the tool is just fine too, so long as it is quite 
clean.  If you do this, just adopt the attitude that you won't get any 
scratches.  That will start you thinking positively, and then you are 
more likely to adopt habits that will avoid scratches.

Store the pitch lap in a tupperware container between uses, and cover 
it immediately after use.  That cuts down on the time for dust and 
other stuff to fall on it.

> I also came across an opinion in the archives that plaster would be bad to 
> use for a pitch lap...anybody have any info on that?...is a sealed plaster 
> disk okay?

I use Hydrostone and I have never had to seal it.  I have never had a 
scratch that I could attribute to Hydrostone.  I can't speak about any 
other plasters.

	Mike L.