"Existential Angst" wrote in message
...
Awl --

First, is "two-part epoxy" redundant? Does "epoxy" nec. imply two parts?

Not necessarily. See 3M Scotch-Weld.


Next, does the strength of a two-part epoxy vary with its setting time?
Intuitively, I would think the longer the stronger.

In very general terms, yes.


The classic is "5-minute epoxy", but I believe I have seen 60 sec epoxy,
and I have 90 minute epoxy.
I find 15 minutes to allow for a few parts without rushing, and without
waiting forever to cure. Altho I have also found that the epoxy is not
really hard in that time.

Of the consumer-grade polyamine-cure epoxies (most of the stuff you're
likely to encounter), expect a 48-hour time at room temperature
for full cure, and about a week to get really hard.


The stuff I have now is Permatex (a 30 min epoxy), but DevCon and Loctite
are big players, with big-time industrial uses, such as epoxying
bearings, instead of press-fitting them into their housings. I believe
DevCon stated that epoxying bearings is in fact the preferred method,
being stronger than press fit.

The shear strengths are enormous, but require at least .010 on the radius
for good bonding. So a slip-fit for epoxy would be counterproductive.

You need at least 0.005" for a high-strength bond, metal-to-metal. A little
more is better.


In some of my proposed assembly methods, I could use set screws, pins,
epoxied pins, or just epoxy -- typically nominal 7/8 alum round in 16 ga
SS tubing, or some such.

I thought epoxying would be slam dunk, and indeed it saves machining, but
goddamm, you gotta pick yer poison, it seems. I could see arguments for
actually using a mechanical method AND epoxy, in some applications.

That's how many aircraft wing skins are assembled. The epoxy provides sheer
strength, and rivets resist peel failure. It's called rivet-bonding.


A feature of epoxy, which could be both good and bad, is its instability
to heat. I think boiling water, or at least not that much hotter, can
undo epoxied parts.

Ordinary amine-cure epoxies lose strength in the mid-200s F. Some of the
industrial products are good up to 350 F or more.


Iny thoughts?

Using epoxy for assembling metal parts is not a trivial thing, if you want
decent performance. It's time to get on the phone with Devcon, or 3M, etc.,
engineers.

--
Ed Huntress