Blake Mantel wrote:

 Oh yes you can Paul!!!

 I had to figure out how to do this on a 4" OD rod of 6061-T6 which
needed a 5" adapter
 plate to mount an antenna at a local airport. I own a Miller
EconoTig (I want a
 SincroWave 180 :) which will get up to 160 amps when I push it.

 Get some standard building fiberglass insulation and wrap it
around ALL the other
 areas not being welded. Start with DCEN and heat it until the area
gets to weld temp.
 Switch to AC and go for it!

 Be careful because you have a large thermal mass and watch out
because you will be
 heating for a while and then it will suddenly slump.  Just wrap
your plate and it
 should work .

 Good Luck,
 Blake

Good tips. Thanks.

I have noticed that when welding even .125 tubing. It takes a while to heat
the part overall and then it starts to melt faster. I have to back off on the
foot pedal immediately after that happens. I guess if you heat it to 500 or so
degrees in an oven it starts melting right away so you don't have to
change the foot
pedal so much at first. I never tried 1/2 inch but I blew the circuit breaker
trying to weld on 1/4 inch. I did not heat it in an oven first.

Things like this are what makes welding aluminum so difficult.
Every different kind of joint has different heat transfer characteristics.
Aluminum has high heat conductivity so something as obscure as a
draft in the welding
area can make a big difference. I find I have to mock up a joint to
practice on
before I weld the final part.

Steel on the other hand has low heat conductivity compared to aluminum
so it is much easier to weld.

Paul Lamar

Big plates are BIG heat sinks.   When it is possible,  just preheat
everything by putting it on a hot plate.  Let it heat for a long
time.   If that is not possible, then play your torch (big rosebud)
over the plate until it is hot all over.  I think you will be able to
weld 1/2" aluminum with l75 amps.  I welded 1" plate recently with
the welder set at 200 amps.  This was after the hot plate pre heat.
Without the preheat  I could not start the weld  with 250 amps.
Using just acty. put a layer of soot on the plate.  Then heat the
plate.  When the soot burns off, you are at the right pre-heat
temperature.   Jerry

 
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