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Exotic Woods

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Stan Caldwell writes:

"I am writing on behalf of my nephew, Jake, who is in the eighth grade.
He is participating in a science olympiad and needs to fabricate a
laminated wood beam and load test it.  According to Jake, the beam must
span 350 mm (13.78") and be no higher than 40 mm (1.57") and no wider
than 20 mm (0.79").  It must be a straight, solid rectangular section
fabricated from strips of wood no thicker than 3.175 mm (0.125"), glue,
and nothing else.  Finally, it must support a center load of 15 Kg
(33.1#) with a deflection less than 20 mm (0.79").
=20
I am thinking that the winning beam will be fabricated from wood that is
dense and very fine-grained, without knots or other irregularities.  In
other words, an exotic hardwood from South America or Asia, like Pau
Lope or Teak, rather than good old Southern Pine or Douglas Fir.  The
wood must be available in Dallas, or available online with delivery
within the next two weeks.  Of course, such woods do not appear in NDS,
so I have very limited information on them.  Also, I am thinking that
the laminations should be vertical, rather than horizontal.  This should
make a stronger beam, and make the glue choice less important.=20
=20
1]    What wood species do you recommend?  What are its mechanical
properties?
=20
2]    Do you concur with vertical laminations?
=20
Thanks in advance for your thoughts.
=20
=20
Stan R. Caldwell, P.E."


Stan,
A couple of thoughts.

I calculate the stress in the beam you describe to be about 350 psi.
Even basswood will make that.  If the contest is to test these things to
destruction, well, that's different.

While some of the tropical hardwoods are indeed hard, you can get a
tremendous amount of strength out of oak, ash, maple, or hickory.
Straight-grained samples of these woods should easily go above 6000 psi
at ultimate--probably well above; so we're in the vicinity of 20 times
your nephew's design load.  (On a side note, I once saw a
turn-of-the-century design aid from a timber supplier from Seattle which
recommended 10,000 psi for permanent piling and 12,000 for temporary.
That's over 10 times what we would use today.  Still amazes me.)  I
wouldn't bother with a glue more exotic than Elmer's Woodworker's glue,
or maybe a polyurethane like Gorilla glue.  (By the way, if you use a
polyurethane, wear gloves--it will turn your skin black.)

The whole point of laminating is to randomize the defects in the wood,
and allow you to start with dried pieces of wood with any warping
machined back to square.  Either horizontal or vertical laminations will
accomplish that.  The important thing is to pick straight-grained pieces
from which to cut the laminates, make the gluing surfaces as perfect as
possible, and get a good glue-up by squeegeeing the glue onto each
surface (I use old credit cards) and clamping firmly but not too hard.

If you do decide to use the exotics, be aware that many of them are
oily.  Teak is the worst offender.  Best practice is to wipe the gluing
surfaces with acetone or something similar, then glue with epoxy.
Goggles and ventilation are a must; frankly, I've never wanted to glue
anything that much.

Speaking of epoxy, something you should be aware of:  it is highly
exothermic.  I made a canoe paddle for my son a few months back.
Laminated the blade out of basswood, walnut, cypress, and maple (using
polyurethane glue), then fiberglassed it using West epoxy.  I mixed up
what I thought I'd need (which turned out to be about four times too
much) in a plastic beer glass and started wetting out the fiberglass
cloth.  I was nearly done with the second side when I noticed a thin
haze of smoke coming from the vicinity of the beer glass.  As I looked
up, the glass melted and slumped over, spilling the scalding-hot epoxy
on the thin piece of poly I had laid over my workbench.  I grabbed the
stir-stick, now flash-set in the upper (hotter) half of the epoxy, and
dragged the liquid lower half out over the poly as thinly as possible to
cool off.  Thankfully, the poly only melted through in a few small
places.  More excitement than I really wanted.  Next time I'll use a tin
can.

HTH,
Mike Hemstad
St. Paul, Minnesota 

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