Re: [ATM] Mirror cell ramblings re: thin plate glass3/4-inchx24-inch

["Bob May" <rmay@nethere.com>]

Actually, the triangles in a support system don't need to be
rigid!  All they need to do is to be able to support the weight
without bending permanently.  The little fun part of a cell
structure is that it conforms to the shape of the mirror back and
provides even support at each point.  If a triangle bends a
little under the stress then the positions of the other triangles
in the system just move a little bit to compensate for the motion
of the one triangle.
I'd love to make a mirror cell with spring steel just for the fun
of it with basically rigid joints just to show that it is
possible.  The compensation of the triangles won't be exactly
perfect as springs tend to have the tendency to provide more
force as they deform more but for slight movements that shouldn't
be all that bad.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net

----- Original Message -----
From: <fundo@bellsouth.net>
To: David Harbour <stainless_steel@suddenlink.net>; Ted Cohen
<tcohen@blakeglobal.com>; <atm@atmlist.net>
Sent: Wednesday, May 28, 2008 7:48 AM
Subject: Re: [ATM] Mirror cell ramblings re: thin plate
glass3/4-inchx24-inch


> --- Original message from "David Harbour"
<stainless_steel@suddenlink.net>: --------------
> > I was thinking, why not start
> > with a conventional three point system, expanded into 18
points, and then,
> > just keep adding rockers and additional points of contact to
those? Wouldn't
> > a guy eventually end up with a pretty even distribution of
weight, in one
> > plane, in theory? Of course, the rockers would have to be
constrained
> > properly (in an 18 point floatation system, traditionally,
the triangle are
> > allowed to rotate out of "ideal" position)- however, a
competent engineer
> > should be able to provide a large enough number of points of
contact on
> > rockers/sub rockers/ sub-sub rockers, etc., it seems to me,
to make the
> > thing work.
>
>
> Dave,  As you should know, every engineer has to tackle
problems like this.  The usual standard answer is to utilize the
K.I.S.S. Principle!
>
> The only problem with the above line of thinking is that you
end up with a monstrosity that out weighs the problem you are
trying to correct, i.e., an expensive, heavy piece of glass.
Each layer of triangles has to be rigid enough to not flex under
the loads it is required to support.  This will require either
thick pieces of metal or carefully engineered castings; another
problem to tackle and manufacture.  By the time you get to a 27
point support system, you have three layers of triangles which
will occupy a fair amount of space behind the mirror.
>
> Our objective is to lighten the overall scructure.  At what
point do you cross the line of mechanical complexity vs thicker
glass?  From discussions on this list over the years, and no
attempt on my part to calculate it, it seems that a 20:1 ratio is
workable with typical home brewed manufacturing capabilities (16"
x 3/4"), and maybe 25:1 or even a bit higher with experience and
more advanced fabrication abilities.  Using slumped glass may
extend these ratios, but at the cost of creating an additional
support system during fabication of the mirror.
>
> There is one advantage: engineers have to consider time, and
its cost in man hours, in every project.  Since this is a hobby
activity for us, that element can be scratched.
>
> *****-----
> Dan F.
> Memphis, TN
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>

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