DoN. Nichols wrote:


Yes, it does. But ...

1) It is not *glued* up. There is a single sheet of what looks like
1" thick or better Lexan (not Plexiglas) as the door only.
Lexan is not as brittle as Plexiglas.

2) It is sealed by an O-ring to a flat machined metal surface.

3) The enclosure is welded full length seams on the inside -- and
(I believe) also on the outside.

4) The enclosure is *not* Plexiglas. I don't know the metal
involved. Perhaps stainless steel, perhaps aluminum. If the
latter I would expect metal fatigue over the years as the number
of pressure/vacuum cycles builds up.

5) It appears to be designed to mount in a 19" relay rack. (The
trick is getting that rear crossbar past the front rails. :-)

I kept reading (earlier in the thread) plans for gluing up a box
of Plexiglas -- and *that* I do not trust -- especially in normal
thicknesses.

Enjoy,
DoN.




DoN,

I appreciate your criticism.

I find that the process of designing something even more satisfying
than building the thing it self. I have set an arbitrary goal of $200
for building this project, which may or may not be doable; nothing is
cast in stone yet.


For the top of the chamber I was originally thinking about some 1”
Lexan, then I priced the stuff, not cheap.

I then asked myself how strong does it have to be? And, is there
something else that can do the job cheaper?


Tempered glass is pretty strong stuff. I have seen a 200# person
stand on the glass of a pinball machine. And that is only 1/4”
thick. In checking a bit, I noted that one glass company shows a guy
jumping up and down on a sheet of 3/8 supported on wood blocks about 5
feet apart. I also noted that people build big aquariums out of glass
and acrylic glued together and supporting holding tons of water.

While I am not sure, I suspect that a clear span of 14” X 16” X 3/8 or
1/2 might support the 3,300 lbs of pressure and still give an
acceptable safety factor. I have a call into the glass company to
check with the engineers that know.

Assuming I will get a satisfactory reply to the glass inquiry, the
plastic sidewall I described admittedly sounds like a failure waiting
to happen, but here is my thinking.

Assume that you have a nice thick chunk of 6” pipe sealed on one end,
and on the open end you have a sheet of Plexiglas and a rubber
gasket. If a vacuum were drawn out of the pipe would a 1/4 sheet of
acrylic have the ability to resist fracture if you got near a full
vacuum? I think it would, as it is about 300 lbs. evenly applied over
about 19 square inches.

Now if you can accept that proposition, think about a 1/4” sheet of
Plexiglas that is 6 “ by 4” and has a 1/4” X 1/2 bonded to each one of
the 4” ends and between and along the 6 inch sides you have two 3/8
thick sides that are 1 inch high and are tee shaped with the top of
the tee being 1/2” high 6 1/2” wide and the bottom being 5 1/2” wide
and bonded to the ends with the tabs sticking out.

What you would have would be a Plexiglas box with 1/2” sidewalls and 4
tabs that are 3/8” X 1/2” X 1/2”.

If you were then to suspend this box on the tabs between two rails and
loaded the box with 360 lbs., would it hold? I think it would.

Now if I haven’t lost you yet, the top and bottom of the chamber are
going to be exerting a total force against the sidewalls of about
6,720 lbs. This will be supported by about 15 square inches of
Plexiglas that is 6 inches high. Would a block of Plexiglas 3 3/4
inches square and 6 inches high support 6,720 lbs.?

As far as the glue joints go, the glue joints should be stronger than
the plastic itself.

If my thinking is off here I would like to know where. The regular
RCMer’s tend to be a bunch that is pretty damn smart, which is why I
tossed the concept out there.

The cost between Plexiglas and Lexan is not much, so I might opt to go
that route, also I might increase the number of side support ribs to
spread out the load a bit more to increase the safety factor, it kind
of depends on the yield I can get from a stock sheet of plastic.

Thanks again for your criticism.

Roger Shoaf