On Fri, 22 Oct 2010 22:17:12 -0700, "Califbill"
wrote:

Just build a reinforced metal top with a small window or two in it. You do
not need to see what is happening except for grins. Could grind a lens on
the end of a plastic slug to see all over the cavity.

Im still wondering how the OP is going to close the bags of food when
the vacuum has been achieved if he cant reach inside.




"DoN. Nichols" wrote in message
...

On 2010-10-21, RS at work wrote:


DoN. Nichols wrote:


Yes, it does. But ...

[ ... my post snipped ... ]

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.

O.K.


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.

I think that the centers of some of the bumpers are support
columns distributed over the top, so I'm not sure what this proves
without knowing precisely where the weight was applied, and where the
support columns were relative to that.

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.

That is somewhat more meaningful.

I also noted that people build big aquariums out of glass
and acrylic glued together and supporting holding tons of water.

We don't know what other support members may be present hidden
from view.

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.

O.K. They will certainly err on the conservative side.

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.

Hmm ... I don't know the ID of your "6?" (which I presume to be
6 inch) pipe, so calculating from the OD, I come up with 28.27 square
inches, or 145.6 pounds total force.

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.

Again -- I don't trust the bonded principle. There are a lot of
things which can go wrong in the bonding. And -- if you are going to
try solvent bonding over a large area -- expect the solvent to be
trapped in the center areas, weakening them.

And -- add to that the fact that acryllic tends to form cracks
from exposure to oil (from the machine tool which fabricated the parts,
unless you worked with totally clean new tools dipped in acetone to wash
off the oil before cutting. Also -- all clamping surfaces will
similarly need to be cleaned of oil totally.

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?.

Until I see evidence to the contrary, I am reluctant to consider
two layers of Plexiglas bonded together to be any stronger than the
thicker of the two individual layers.

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.

This looks a lot smaller than what you originally wanted in
volume.

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.?

Maybe -- but consider that the walls are undergong lateral force
which could weaken them. Consider a pipe 20' tall supporting a load
something like 80% of the calculated safe load. Then apply 20% of that
force sideways to the center of the pipe. What would happen?

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

This -- I don't trust. Especially with the surface area being
solvent glued together. How long would it take the solvent to dry in
the center of the glued surface?

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.

Well ... I tend to be rather conservative when it comes to
vacuum containers.

And for the tempered glass -- what happens with that if a
scratch is applied to the surface -- especially on the center of the
inside, so it becomes a stress riser.

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.

I would feel more comfortable with Lexan. It is not as brittle
as Plexiglas. (Have you ever used the scribe and bend method of cutting
Plexiglas?)

Thanks again for your criticism.

You're welcome.

Best of luck,
DoN.


I am the Sword of my Family
and the Shield of my Nation.
If sent, I will crush everything you have built,
burn everything you love,
and kill every one of you.
(Hebrew quote)