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.

--
Ed Huntress



I don't know about loctite these days but I was told by a race engineer
that years ago they used a loctite product that seemed to be exactly
what they wanted to hold hubs on a race car so would result in a lighter
hub assembly. They failed with regularity, it turned out that on
examination the loctite, while it had the tensile and compressive
properties, had **** poor fatigue properties, so failed due to the
cyclical nature of the drive shaft.