Re: [ATM] pitch lap

[Mark Cowan <toolontop@yahoo.com>]

Hi David:

There was a huge long discussion on pitch laps just
now, you might check the archives at atmlist.net for
that if you missed it...

But briefly - plaster of paris won't coexist with
polishing - Hydrostone(tm) or cement will but must be
coated with a sealant (like waterproof epoxy) where
the pitch contacts it.

Secondly, no you don't want a full-size lap for
parabolizing a large thin mirror - you can do it with
a series of sub-diameter (60% to start) laps working
tool-on-top with good support and rotation for the
mirror which is always face up.  As the parabolization
completes from the outside in you will switch to
smaller sub-diameter laps appropriate to the diameter
of the remaining work until you're done.

The method I use for lap making is the one John Dobson
teaches where you pour the lap on the tool, press with
CeOx slurried mirror, use a wet dowel or similar tool
to press in channels, and rework with mirror until you
have good contact over the full surface.  It takes
longer to describe than to do, but the pitch, tool,
and mirror must all be at good temps for this to work
because it has to be finished while the pitch still is
flowable.

Cheers,
Mark Cowan
Salem, OR

--- David  Williams <dwilliams1128@earthlink.net>
wrote:
> Hello ATM list
> 
> I am making a 20" mirror 1" thick and I have used a
> 12" sub diameter pitch lab tool solid pitch tool. 
> The way I made the tool was I took a compass, set it
> to the maximum setting (12" diameter) and then cut
> out a piece of 3/4" plywood with a saber saw.  Then
> I took masking tape and made a dam around the edge
> of the wood, then I mixed some plaster of Paris (I
> found out that some of this stuff dries as fast as
> you mix it, but some of it dries in about 15--20
> minutes: it's this latter one that you want).  The
> prices are somewhat different too.  Then I take the
> mirror and turn it curved side up and wet it and put
> soap on it, then put aluminum foil down on it. 
> After that you take the wooden tool with the
> slightly thickened plaster and invert it over the
> mirror and slightly press it down.  Then check to
> see if the plaster runs out.  Press until the
> plaster just starts to run out around the edges.  
> 
> After the plaster has dried then you work with the
> pitch, in a similar maner that you did with the
> plaster, but you have to add heat to the pitch in
> order to get it to melt and run.  I tried to cut the
> wooden strips and nail and lay them over aluminum
> foil and then pour the hot pitch in the channels
> that way.  But that didn't work.  The pitch stuck to
> the aluminum foil and made a big mess.
> 
> I talked to a friend of mine named Jerry Martensen
> who has made telescope mirrors before and he said
> that I should have painted a heavy layer of CEO
> (cerium oxide)  and water onto the aluminum to
> prevent the sticking.  
> 
> He said that a sub diameter lap is good for
> polishing, but for parabolozing, you need to have a
> full diameter lap.  So I'll have to order more pitch
> from Willmann Bell.  I got 5 lb. for about $25.  
> 
> How many of you out there would recommend the method
> of pouring the lap all in one piece and then cutting
> the channels with a saw?  Or the other way where you
> cast the pitch out into squares?
> 
> Does anybody know of a good deal on a cheap foucaolt
> tester?
> 
> Thank you for your help.
> dwilliams1128@earthlink.net
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 



	
		
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