Re: [ATM] pitch lap

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

> --- David  Williams <dwilliams1128@earthlink.net> wrote:
>>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.  

This will be a plywood tool with a thin plaster layer to make it 
convex.  A plywood tool of this size and thickness will probably FLEX 
during use, making polishing unpredictable at best.  (Additionally, 
the plywood may change shape with humidity, unless it's very well 
sealed.)  It's best to make a 1.5" thick plaster disk (preferably from 
hydrostone) for polishing.

If possible, I'd recommend a larger disk, say 14" or 16" for 
polishing, since it's more likely to get you closer to a sphere at the 
end of polishing.  My personal choice would be to polish with a 16" 
tool and parabolize with a 10" tool.


Mark Cowan wrote:
 > 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.

I've had no problems using pitch directly on Hydrostone.  A coating 
might make stripping the pitch off easier after you're done, though.

 > 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 12", with care, will get the job done, and will be good for 
figuring, too.  Assuming the F-ratio is 4 or higher, you can probably 
do all the parabolizing with that size lap, as Mark describes above. 
Heed his advice about good mirror support!  (It's nice to be able to 
test for astigmatism regularly, if possible.)

	Mike Lockwood


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