[ATM] reducing mirror weight by sandblasting

[Dan Feldkhun <glasspusher@mindswell.com>]

Hi all,

I am thinking about reducing the weight of a 12.5" pyrex mirror blank by 
sandblasting pockets on the back side.  I've never done this before and 
web-based information on sandblasting mirrors seems to be scant, so I'd 
appreciate any advice.

My goal is to remove 1/4" - 5/16" deep pockets (the mirror is 1/2" thin to 
start out with).  I haven't figured out the best pocket shape yet, but I'm 
thinking the pockets would be delineated by ridges between PLOP-generated 
support points (I am using a 27-point cell design, so there will be plenty 
of them ;).

My current plan of action involves:

- using a 5.5HP 12CFM 20gal air compressor to siphon grit out of a bucket 
and blast it onto the mirror.
- using a stainless-steel air gun nozzle with ~1/8" output aperture fed by 
a 1/4" plastic air-hose
- using as sandblasting grit: "black beauty" or #60 or #40 carborundum?
- patterning a 12x12" vinyl tile to serve as the sandblasting mask
- placing mirror on a turntable and slowly moving the nozzle radially
- constantly measuring pocket depth with a ruler or gauge as sandblasting 
progresses
- acid-etching after sandblasting to relieve stress and smooth the edges
    with HF-based etching compound under a fume-hood (I have access to a 
clean room)
- doing any grinding and polishing only after sandblasting/etching is completed

Some questions that come to mind are:

- Is the compressor/hose/nozzle combination to wimpy for the job?  What 
kind of hogging-out rate is to be expected?
- What do people use for masking the glass?  Vinyl seems to be popular in 
the industry, but how fast does it disappear?
- Is it more effective to do local sandblasting with an narrow concentrated 
stream, or to cover a larger area?
- What would be a good cavity pattern to maximize stiffness-to-weight 
ratio.  Has anyone used FEM software to figure this out?
- Is residual strain in the glass an issue after sand-blasting and etching?
- Is undercut of the ridge walls a common problem while sandblasting?
- How small are the glass chips produced? Is a dust mask sufficient to keep 
glass out of my lungs?

Thanks for any help.

Clear skies,

:Dan

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