On Fri, 21 Oct 2005 17:18:11 -0500, Jon Elson
wrote:


Your 185 amp TIG might be a bit light for the task at hand. But
don't get in a hurry with preheat. Even insulated, it'll "suck up
the heat" until you have all of the copper within some distance hot --
which will be never if it isn't insulated.

Bulk thermal conductivity of copper is 5X that of steel.
Resistance is resistivity * length / area so a bar of given length
and cross section will have 1/25th the thermal resistance of a
same-size bar of steel. (I missed that first go, Jon). That suggests
(as Jon did) that you'll need to insulate for some length, perhaps the
whole bar. If sufficiently insulated, then the conductivity doesn't
matter.

This has been bothering me since we started this discussion. I have
this image
in my head of Aaron K. getting this thing half built, and it is going
slower and
slower as the pieces are assembled. Finally he gets to a point where
he's going
to need every Oxy/Acetylene torch in the entire county all at once to do the
preheating! The only other alternative is to do the entire job while
sitting inside
a furnace! That's what furnace brazing is for.

Maybe I'm overstating the heat loss problems here, I don't have a clear
idea how
many cross members there are. If this thing is a square frame with vertical
bars, and no other members, maybe it can be built, but it may not support
its own weight. If this is for a person to pass through, that may not
be much of
a concern. If it is to span a driveway, I really have doubts. Either
way, if it
has a few more horizontal bars of the .5 x 1" size across the middle of
the gate,
it is going to get messy as the joints are brazed and the number of heat
paths
increase.

Jon

I agree, if the whole thing were made of heavy stock like that it
would eventually become a humongous radiator. I had the (perhaps
erronious) impression that most of the stock would be considerably
smaller, just the outer frame was the heavy stuff.