On 20 Oct 2005 15:30:16 -0700, "
wrote:

Hello Wayne,

Thanks for the suggestions. I really am using 100% copper (Alloy 110).

I already figured out the insulating bit when I made the hinges. When I
was welding the hinges on the metal table, I couldn't get a puddle.
When I put the hinge in a little cave of fire bricks, I was able to get
a puddle.


I can try welding two 1' sections of the bar. However, considering that
copper seems to just suck up the heat, I'm not sure it would be a good
test considering that I'll actually be joining a 4' length to a 6'
length.

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. With heat input, the bar will continue to rise in
temperature until heat loss rate = heat input rate. If you
insulate for enough length, then *once preheated* it shouldn't be
appreciably different than welding a same-size bar of steel --
perhaps easier, given the lower melting temp. Once adjacent copper
is hot, its ability to "suck heat" is limited. However, it may
take a while to pour enough heat in to get the part of the bar inside
the blanket hot enough to proceed with welding.

The foregoing assumes that the insulation contains heat considerably
better (25 times better) than a bar exposed to free air. I don't
know if that's a reasonable assumption for an inch or two of ceramic
blanket, but it might be worth a try. As said before, I'd preheat
with O/A or oxypropane (lower temp but more heat) so as not to run
into the dutycycle limit of your welder.

I like the end cap idea, but not sure how I would fabricate those. I
can always screw on gussetts to the bottom since there will be a copper
panel on the lower portion of the gate that would hide the gussets.
However, gussets on top would be ugly.

When/if I finish this, is this the kind of thing people would like to
see in the drop box?

By all means, please do!