Ed Huntress wrote:
"Karl Townsend" wrote in message
anews.com...

I'd be wary of that because it could interfere with getting a good
scratch-in coating on the parts. But whatever works.


What do you mean here? Don't know the term scratch in.

Clean the steel conventionally as well as you can. Then mix up a small
amount of epoxy and apply a glob to a piece of sandpaper. Sand the
spot you're bonding to, "wet" with the epoxy, until you've scratched
into the entire surface. Keep the spot wet with epoxy; don't let air
touch the metal again once you've started.

If you want to be neat about it, apply another glob of mixed epoxy to
a piece of rag, and wipe the spot to wipe away the grit, oxide, etc.
Again, keep it wet; air should never touch metal.

Then do the same to the other piece you're bonding. Apply another
small glob of epoxy to the spot and stick the two pieces together.
Let it cure.
This "scratch-in" method produces similar results to the chemical
treatments used in production -- phosphoric-acid anodizing on
aluminum, and some acid etches used on steel. It's used in high-grade
repair work and some custom assembly. Obviously, it's too labor
intensive to use in production. It works on most metals but it's
particularly useful on stainless and aluminum. The mechanical bond
you get from the rough surface is secondary. The primary objective is
to produce a clean, high-energy surface to which the epoxy gets a
true adhesive bond. Scratching also produces more surface area for
the adhesive bonding.
The higher the strength of epoxy you're using, the more critical it
is to have an oxide-free surface to bond to. The difference is really
large. Also, keep in mind that you do not want a really tight fit in
metal-to-metal bonding with epoxy. You get the strongest joint with a
gap of 0.002" to 0.005", depending on the epoxy. For that reason some
people leave the
sanding grit in the epoxy and don't clean it off; it serves as a
spacer. It's easy to starve an epoxy joint and ruin its strength.

Doing the scratch-in thing to a hole or other internal shape is a
little tricky, but it works.

BTW, this is for a high-strength joint. If the torque load is light
enough that green Loctite would do the job, then this is all a waste
of time.

I thought I knew all there is to know about epoxy. How wrong can one be?
This is another one for the library. Thanks, Ed!

--
Michael Koblic
Campbell River, BC