Re: (ATM) Graphite/foam tubes - help

[Andrew Moore <alm@netcom.com>]

I have only been playing with composites for the last few months, so I don't
claim to be an expert.  But one book that has been immensely helpful
is Andrew C Marshall's "Composite Basics".  This doesn't have an ISBN
number.  I found it in the Wick's Aircraft Supply catalouge.  Would be
in the Spruce catalouge too, I think, since it is oriented towards aircraft 
construction.

If you are not familiar with the so-called "vacuum bagging" technique,
you will find this interesting.   In a sentence, it involves putting
your layup in a bag and applying a vacuum.  This serves several
purposes: it sucks up excess resin and produces a very strong, thin,
light and clean part (providing the mold was clean to start with).
As the author points out, even if you achieve only a 5 psi vacuum
(14.7 psi at sea level being the max), that is still 700 pounds/sq. foot.
The burden of finishing is then moved from filling and sanding to
preparing a clean mold.

There is at least one company that makes fabrics for vacuum bagging
(Taconic - Process Material Division, in Santa Maria, CA).  But I
think that unless you're building this tube for DOD, it wouldn't pay.
Instead, I had a lot of success with tin foil and (lots of) paper towels
and 10 gallon trash bags (or construction-grade plastic sheets).  For
a vacuum pump, I rigged a car gas-line to the intake of an air-compressor.
To attach the gas-line to the vacuum (i.e., trash) bag, I used rubber
gaskets (cut from an old bicycle intertube) sandwiched between hex nuts
on a threaded pipe joiner (to which the gas-line was clamped).

For of a vacuum pump, the author suggests one might get by
with a vacuum cleaner.  But I don't recommend that - an air-compressor
is actually cheaper and less likely to burn out. And for a "release ply"
(the teflon-coated, perforated film that lays down directly on top of the
wet glass/epoxy - and through which the excess epoxy permeates)
he suggests plastic wrap - but the stuff I used melted from the heat of
the expoxy and left a wrinkled finish.  I had better luck with aluminum
foil with lots of holes punched in it.

Okay, so much for a long-winded reply to a question that wasn't asked :(
Regarding foam materials for your core: actually balsa wood is probably
your best bet.  After that, a stiff urethane (vs. PVC) foam.  The
reason for balsa is that it has a high compressive strength compared
to other cores.   Without that, a core doesn't really add much in my opinion
except for superfluous strength, since  damping can be achieved with stiff 
rings at the ends of the tube and a snug attachment to a sturdy mount.
Anyhow, if you must use a core, balsa also has better sheer strength
than most foams.

Finally, you asked about the number of layers of glass - I would use at
least two or three on each side of the core.  BTW, you hear a lot about
"stippling" to force the epoxy down through the glass.  A better technique
is to paint the epoxy on first and then lay the glass over it.  This 
eliminates a lot of air bubbles.

Hope this helps...  experiment, experiment, experiment!
-Andrew Moore <alm@netcom.com>