Re: [ATM] Ultrathin Project update

["matt" <electro_optic@bellsouth.net>]

rigid backing is the way to go.
However,  instead of the proposed mold release, use pitch .
This should be a 2 step process, step one is to make the concave hole in the 
plaster that matches the mirror convex back. Then remove the mirror from the 
plaster . Step two is add a thin layer of pitch to the concave hole and 
place the mirror on that . Maybe have to play with heat and cold to set it 
completely stress free and then to make it stay there.
Polish and figure the mirror and when you're done, heat the thing and the 
pitch will release .

Best regards,
Matt

----- Original Message ----- 
From: "Richard" <cnc@cncservo.co.uk>
To: "vorblesnak" <atm@atmlist.net>
Sent: Wednesday, May 16, 2007 2:17 PM
Subject: Re: [ATM] Ultrathin Project update


> Hi vorblesnak,
>
> Wednesday, May 16, 2007, 3:00:11 PM, you wrote:
>
> vpo> I have exhausted the ideas and rigs to make a sub-diameter lap work.
>
> Just before you give up David...
>
> I've been giving this some thought. If the convex side sat in
> something infinitely rigid that conformed exactly to the convex surface
> then there wouldn't be a problem, agreed?
>
> How can we approach that?. The one thing that came to mind was setting
> the convex side in fine plaster in such a way that there is no stress
> on the mirror once the plaster sets. Once it has set, the mirror is
> then pretty well supported against polishing pressure. It may be that
> you would have to fill the concave side of the mirror with plaster at
> the same time to keep mass balance and thus avoid stress during
> setting. Maybe a fine tripod stops the mirror sinking totally and a
> fine spray of mold release on the mirror stops the plaster sticking to
> the mirror. If you could put the whole assembly in a vacuum chamber
> after pouring it would help the structure of the plaster a lot. Zero
> bubbles are left in the set plaster. Ask a jeweler what type of
> plaster they use for centrifugal casting of gold rings - that gets the
> vacuum treatment so the fine detail isn't lost.
>
> You'd end up with a near perfect support for you mirror for polishing
> which must spread the load about as well as possible. Heck, maybe you
> can even lap it in with a few wets if it needs it. Maybe the balancing
> lump that forms in the concave side can be used for your lap.
>
> Lot of maybes there, but it could be food for thought, if you haven't
> already tasted it.
>
> -- 
>
> Best regards,
>  Richard in the UK
>
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