On Fri, 20 Sep 2013 14:03:50 -0500, Tim Wescott
wrote:

This is out of curiosity:

How good of a bond can one achieve with epoxy to aluminum? How good can
one achieve with epoxy that ordinary folks can mail-order, vs. what folks
who have the ear of a Locktite sales rep and applications engineer can
achieve? How good can one achieve with JB Weld?

An Internet acquaintance has a scale model airplane that needs a
driveshaft driven by an 049 engine (that's a bitty one that swings a 5"
prop, not a humongous one that swings a 12" prop -- 1/20th cubic inch,
not 1/2). His current driveshaft uses a solid aluminum rod, and is
heavier than he'd like.

Because of the way the plane balances with the current shaft, every gram
he takes out of the shaft takes out more than two grams from the plane,
because he's currently got a buttload of weight in the nose.

I'm thinking that one could machine ends out of aluminum, then plug them
into a thin-wall (.035"), 1/2" or 5/8" diameter tube made of 2024. I'm
also thinking that bonding everything with epoxy would be a valid way to
go -- but I'm not so sure about the epoxy in a part that's going to be
subject to lots of vibration in at least eight of the six available
degrees of freedom.

Threading such thin wall tubing makes me think that it'll just break.

Welding makes me think it'll never happen.

Soldering???

I'm sorry I won't be around to discuss this further, but here are a
few points to consider:

1) You're talking about shear loads, at which epoxy is very good.

2) The amine curing agents for epoxy, which are the ones we can get at
hardware stores and marine supplies, give you the lowest temperature
tolerance and rarely exceed 5,000 psi in cured epoxy. You can achieve
10,000 psi with other curing agents, but you'll have to Google "epoxy
curing agents" and spend some time looking around. Amine cures are
good for around 180 deg. F. You can get another 40 deg. out of other
agents.

3) With aluminum, the surface treatment is everything. They glue
airplane wings together by applying a phosphoric acid anodize. You can
get about the same strength by doing a good scratch-in job. Maybe
someone here can help.

4) With a high performance epoxy, you want a 0.003" - 0.004"
clearance, or your strength will be a fraction of what it can be.

Good luck!

Ed Huntress