Re: [ATM] artificial convex curve for pitch lap

[Mike Lockwood <melockwo@uiuc.edu>]

Jeff,

jeff newsom wrote:
> Howdy folks- This one's probably over the top but......... So,I've
> got a 14.5" piece of glass ,with an f/5 curve(hypothetically,soon
> to be a reality). This is approx as I'm not allowing for the bevel
> . I'm making the pitch lap base ahead of time out of layers of
> baltic birch(the good stuff! shite!) and the diameter is 10.5" .

Whoa.....  I don't recommend using wood unless you absolutely cannot 
get Hydrostone, dental stone, some other plaster/cememt product, or 
thick metal disk.  (I know Mel has used wood, but he has made a lot of 
mirrors.  Last I corresponded with you, you were trying to set up your 
first Foucault tester.)

Wood will warp and move with humidity and with the pressure you apply 
to it (and you will have to apply pressure), no matter what quality it 
is, and even if it is sealed.  Also, a plaster/cement/metal disk will 
weight more and that will help you keep it in contact.  The weight of 
the disk is an advantage that many don't realize.

> I'm using layers of poster board to simulate the convex curve. I've
> figured the sag roughly to be .181 and with a micrometer figured
> the number of pieces of poster board I'd need to  be about ten.I've
> cut smaller and smaller paper discs all roughly 1/4 smaller than
> the previous one. But this is obviously building up the central
> region too much.Now I'm guessing it probably won't matter that much
> after I poured pitch over it and pressed it, but how could I
> determine more accurately the different disc diameters??????

That poster board, as I understand it, will be under the pitch?  If 
so, it will compress under polishing pressure and the lap will act in 
a bizarre manner that will probably cause it to be unusable.  The lap 
substrate must be a SOLID material that DOES NOT FLEX.  This is why 
glass, plaster, and thick metal are the best choices for it.

Actually, if you pour the pitch thick enough (and have enough pitch), 
you don't need to worry about having the lap substrate curved!

I use flat pieces of glass all the time.  Make sure the pitch is at 
least 3/8" thick at the edge of the lap (it will be thicker in the 
center) and you will be fine.

I strongly urge you to rethink your lap substrate.

There are many ways to pour a subdiameter lap.  We can advise on that 
after you have a proper substrate.

	Mike Lockwood

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