On 20 Oct 2005 14:01:56 -0700, wrote:

I will be making a gate for the entrance to a house and will need to
join some thick copper rectangular bar. The 0.5" x 1" 110 copper bar is
for the gate frame and the infill of the gate will mostly be .5" x .5"
square copper rod.

I've successfully fabricated copper grillworks for other gates using my
Thermal Arc 185TSW. The grillworks were a mix .75" x .1875" flat bar
and .1875" round bar and I've been very pleased with how the Thermal
Arc works on copper.

However, I made some copper hinges out of .1875" x 2" sheet and found
that my 185 amp welder is at the very limit of what it can do.

So knowing that my Thermal Arc won't be anywhere near able to handle
the .5" x 1" bar, I was wondering what would be the best way to make
the gate frame?

I have an O/A welding setup with #2 and #3 tips (and a 175 CuFt
Acetylene tank) and was considering pre-heating the metal until it was
red before trying to tig it. I guess another person holding the O/A
torch would be useful?. But even preheating it, I'm afraid that won't
be enough and I doubt I could get close with the tig torch to that much
red hot copper.

I've been experimenting with tig brazing 3/16" copper using silicon
bronze and haven't had very much luck. The bronze has been balling up
-- I didn't have that problem when tig brazing steel.

So would it be best to mitre the joints on the thick bar stock to a 45
degree angle and braze it using O/A? Will my O/A setup be enough to
braze copper this thick? Using coarse thread screws seems problematic
since I doubt soft copper
would hold screws well.

Thanks for any insight. (Yes, I was a fool for getting copper this
thick, but that is what the homeowner wanted and I couldn't find square
copper tubing).

Copper melts at about 1984F, somewhat lower than steel. The problem
with copper is it's high thermal conductivity, which results in the
whole mass sucking heat away from the weld and conveying it by
radiation and convection to the environment.

Perhaps if you wrapped the bar in ceramic blanket, just leaving a
narrow region open for access to the actual weld site, you could
reduce the heat loss enough to get a good puddle going. It might
take a while for the TIG to heat up the thermal mass of the whole bar,
but it'll eventually get there if the blanket reduces the heatloss
rate to less than the heat output of the welder. The blanket would
also shield you from a large bar glowing within. If you run into
duty cycle limitation on the welder, you could preheat with O/A, with
blanket in place to limit heatloss rate. Might take awhile, but a
75 cu ft acetylene tank has a fairly long duty cycle...

but I figure that 4400 watts (185 amps x 24 volts) will heat up a .5"
x 1" x 10" copper bar to near melting in about 64 seconds if there is
no heat loss. The blanket won't eliminate heat loss, but it may
reduce it enough to make it almost like welding unblanketed steel. I
used 10", figuring 5" of shielded region on either side of an exposed
1/2" to 1" wide weld zone about the same as 1" of bare steel since
copper is 5X as conductive and assuming metal beyond that distance to
not be much of a factor.
..
Here is a source for small qty of Durablanket S at reasonable cost:
http://www.geocities.com/zoellerforge/flare.html



-Aaron