Welding; How much undercut?

"Bruce A. Frank" wrote in
:

In a clusters there is a lot of heat build up necessary because so
much metal to draw the heat away. It is a good idea to switch to the
next larger diameter rod when welding clusters than when welding
single tube to single tube junctions. The larger filler rod helps to
suck a little of the heat away from the tube so that the tube doesn't
get so hot that undercut forms as you weld.

Undercut is a stress raiser, but in a standard truss-frame tube
fuselage built with 4130 there is a major overkill situation. The
tubes individually just aren't stress enough to lead to cracks where
there is minor undercut.

That said, I do not think you can allow any undercut in the firewall
structure which is holding the weight of the engine, or in the engine
mount itself. I would also avoid it in the landing gear structure.

If you have the tube up to the red heat level (gets that slight "wet"
look) then concentrating the heat on the filler letting the puddle
expand outward to incorporate the tube, you will accomplish a solid
weld with no cold lap. For the record, as much heat as is put into the
base metal when using an oxy/acet torch, cold lap is virtually
impossible. Not saying it can't be done, just not likely.


Thank you Bruce! As always, you are the man..

I went out and did a bit more. I had my welds pretty good a few years
ago and I went out determined to recapture that. Increasing the feed of
filler worked wonders and all signs of undercut are gone..
I think I was afraid of not getting the penetration I needed and was a
little too agressive with the puddle as well. At the moment,I'm getting
a nice bright glow from the parent edges just before the puddle runs
over them so I'm confident there's good penetration (confirmed with the
hacksaw) wheras before I was looking for a virtual lava river which was
obviously causing the undercut.

Welds are pretty good, but I am having one further problem; After a
minute or tow, my torch (a meco) often tends to go oxy, even though I
had it set right in the first place. Not a big problem once I spot it
early, but annoying al the same. It seems to stabilise after a while.
Could be that my acetylene tank is just running down, but I thought that
the regulators would cover that almost until the gas ran out.. Maybe I
need new regulators..