Re: ATM Pitch lap problem

[astro <astro@cfl.rr.com>]

Mike

	I would agree with Dan on this.  There are some time honored traditions
that just can't be beat, and cold and warm pressing are two of them.  In
our large (14 Active Members) ATM group we adhere to the art of
reforming the channels using a heat lamp played over the pitch lap and
then press the channel with a 1" X 1" Aluminum bar on edge.  This gives
us a very nice channel, and it's very clean.  No dripping pitch, no
burnt fingers - just nice channels.  We use the hot water method for
warming the lap and glass for warm press and stand on the mirror backer
(wood) after we invert the mirror and insert the screen or mesh.  Cold
pressing is usually done with a full 5 gal bucket on a backer plate or
if available someone will borrow the 100 lb. vise off the milling
machine.  In the end - nothing beats good contact.  What we do during
the ATM class is borrow someone's mirror at 500 grit and flash polish
and do a foucault with it so we can look at the surface finish and
form.  We then use the Sony Camcorder to feed to a TV or just let
everyone look at the zoomed image and see the ripple and orange peel. 
It gives them a feel for how much polishing they are in for.  When we do
final polishing, they stand in line for the Tim Allen mirror grinder
(brute horsepower overcomes all) so the machine can impart the
appropriate punishment on the poor finish gremlins.

Frank Sperl
Mitty Observatory Machine Shop
ATM Group Mentor - CFAS ATM Group


Dan Cassaro wrote:
> 
> Mike,
> 
> >         There is some relationship between unchannelled laps and difficulty of
> > initial forming. This is why I channel at the beginning and just let the
> > channels close as I polish. I don't rechannel unless I modify the lap
> > for some reason.
> 
> This is what I've been doing lately, and it seems to work great.
> 
> >         Even after hot pressing, cold pressing and all the initial forming of
> > the lap, nothing gets the lap and mirror into good contact like actually
> > polishing with the thing. When I originally started working mirrors, I
> > would try to bring the two into perfect contact. I wasted a lot of time.
> > Now I just get a basic fit, and then start polishing.
> 
> This is surprising.  On a few occasions, I've tried polishing with a lap that
> wasn't quite in contact, following your logic that it would "polish into contact".
> 
> Without exception, I've been rewarded with a very ugly surface, with lots of
> plainly visible texture.  Now, I favor a harder lap than most, so maybe that is in
> play here.   I'd be curious to learn more about your technique before I again
> abandon one of my cardinal rules for polishing glass.
> Dan
> http://www.jacksonville.net/~dcass/PlateGlassforSale.html