Listers and Brent,
After about 15 years working as a Body Man and 2 years fabricating race cars
roll cages and pieces for my buddies I developed a strategy for welding
upside down. (Just try to weld a cage just once and not get some serious
buzz burns standing on your head) It involved spotting panels together
rather than using a continuous weld. In Autobody uni-body construction a
continuous weld along the edge of a panel is weaker than a series of < inch
spot welds place on = inch centers < in from the edge of the panel. This is
the reason most manufacturers use spot welding rather than stitch welding.
Because you are constantly starting and stopping with welder the power is
increased compared to the wire speed to ensure a good burn and penetration
of the first < inch of wire protruding from the tip. If you insist on
continuous welding upside down and a significant amount of spatter is
falling it could be; dirty surface, to much power and wire speed for the
thickness of the metal, inappropriate welding gas, insufficient welding gas
flow, Insufficient metal overlap, too thick of wire. Other than for plate
welding 1/16 inch thick or thicker sheet metal, Mig Welders are not best
suited to butt welding sheet metal. They are exceptionally good at spot and
lap welding. If you want a high quality sheet metal butt weld use a TIG.
If you are welding sheet metal the best wire diameter to use is .023-.030
with the thinner wire producing better results with a smaller diameter weld.
I like a 75%argon / 25% CO2 mix which really cuts back on the spatter
significantly. I used to use 100% argon but the mix seems to work really
well. The straight CO2 seems to just create too much spatter and it seems
that it requires a lot of cleanup of the adjacent surfaces.
There was talk earlier about which welders to use. The welder I have is a
version of Miller sold by a local distributor (AK 1200 with 200 Amps at 60%
duty cycle) but has sufficient amperage and wire speed to "spray arc" which
leaves an unbelievably smooth weld with excellent penetration on plate steel
3/8 inch and thicker provided that it is chamfered. This permits me to weld
up to = inch thick steel with good results. My welder also has the ability
to accept a spool-matic gun for aluminum Welding. I also have a sheet metal
gun for the .023 wire and a heavy gun for the .030-.040 wire.
If you do have to do an overhead weld that is a butt weld one method I use
is the spot stitch method. This requires that you spot on one edge until a
buildup of steel occurs and then allow it to cool until you can then spot to
it constantly using the thicker weld as a way "sink" the heat away from the
panel you are welding. This will get you started and you continue to weld
using the last spot as your starting point allowing it to cool between
welds. Always tack both ends and then keep splitting the difference between
until the entire seam is welded. This may require several quick "on trigger"
"off trigger" shots - if you will - but it works very well. I have used
this technique to tub the rear end sheet metal of uni-body cars as well as
for rust repair. The On/Off trigger method is especially helpful when you
"blow a hole" due to the power being set too high for the metal thickness
you are welding. Understand that a spotted butt weld will be weaker than a
continuous weld because typically the spot weld will not penetrate as well
to the adjacent weld. This is because the weld is thicker than the
surrounding metal that the welder is set up to weld and requires a little
more power to adequately "penetrate" the welded spot. So, when spotting you
typically step up the power from what you would use if you were continuously
welding.
Occasionally you will still get the odd "spark in the boot under the laces"
or even the old "roll down the neck to your chest hair" spatter.......just
grin and bear it 'cause once you fuse metal with electricity you are now
initiated into the manly man club.
I hope that helps.
Best Regards,
Tim Ronak
Amateur Welder Extraordinaire
|