Re: [ATM] reducing mirror weight by sandblasting

[Dan Feldkhun <glasspusher@mindswell.com>]

>Dan, Your calendar still shows got another week before April fools, yes?
>
>You said it's 1/2" in thick now right?  Just one man's opinion but to 
>start sand-blasting a completed, polished, mirror that's already 
>considered light weight seems to be folly.  Possibilities for blasting too 
>thin or through, induced strain, or accidently scratching the mirror 
>surface are the risk I'd be worried about.


I know it seems crazy to lighten such a thin mirror, but I am working on a 
portable scope design where an extra 2-3 pounds makes a big difference.  I 
should also clarify my previous email --  I am still in the design stage 
and haven't ground or polished (or purchased) the mirror blank yet.   Any 
sandblasting/etching would be done before polishing.  I am also not 
committed to 1/2" yet -- with material removed from the back, a thicker 
blank may be necessary to sufficiently stiffen the mirror even with a 
27point cell.  Ideally, I'd like to simulate the mirror with FEA software 
and see what kind of deflections I get with different cavity and support 
patterns.


>If you really really wanted to cut pockets like this I would think a 
>diamond bit cutting CNC machine would give more precise control for depth, 
>etc.  But I still can't imagine it's worth the effort.


A milling machine might be the way to go.  I have a little Sherline, but I 
never thought of trying to cut glass with it.  I am worried that the 
process would stress the glass too much, that the shards of glass produced 
would be very difficult to deal with in my workshop, and that the diamond 
bit would run me a fortune.

:Dan


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