RE: [PSUBS-MAILIST] acrylic viewport thickness

[Phil Nuytten <pnuytten@compuserve.com>]

Hi, Alec:
        You made the point to Rick L (. . . .Hi,Rick! . .) that it is the
minimum thickness in a blown dome that counts. That isn't perzacly true. As
always, it's a matter of area versus thickness. If you take a 1" thick
sheet of acrylic - Plexi G - or whatever and blow it through a 24 inch
opening, it will wind up quite thin at the apex. No suprise, there. Lets
say, for sake of argument that it comes out at a little less than 1/2" at
the apex on a full 12" deep hemisphereical dome. You would then run the
stress analysis for a 24" X say, 7/16ths dome - as you suggest. But - and
it is a big 'but' - only the very apex is that thin. It's footprint may be
less than a foot in diameter - much less area than the full dome - and
supported by thicker material - which is, itself, a small area supported by
thicker materail. If you actually work the numbers for a series of ring
areas by their increasing thickness as they approach the 1" base, you will
be pleasantly surprised at how deep a thin apex/thick based blowing will
go! You can easily develop a predictable curve for such domes - just as you
can work out with a fair degree of accuracy what thickness the apex will be
for a given start thickness, diameter, and percentage of 180 degrees. Just
another trick of the trade!
Regards
Phil Nuytten





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