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.

Fortunat1 wrote:

"Maxwell" wrote in
:


edge of the weld touching the beam is .01 inch)
· Undercut shall be no more than 1/32 inch deep for all other cases


Thanks, that's what i needed to know OK. But from a practical
standpoint, is it normal to have such an undercut on a weld? I would
imagine undercut would be a stress riser, for starters.


I was hoping someone would come along with some solid data on this
one, but if you are talking about an aircraft weld, I hesitate to
think ANY undercut should be acceptable.

Are you welding tubing or plate? Aluminum or steel? Gas, stick, mig or
tig?

Usually, undercut is caused by a combination of factors. Weld speed,
deposit rate and weld position, are common factors.

Yeah, for the most part, I can control the puddle quite well and my
welds are tidy enough. It's Gas, I'm talking steel tubing mostly and
it's in clusters, mostly in the "armpit" where there's an acute angle
between the tubes. I've been practicing in anticipation of finish
welding my already tacked fuselage.
If the undercutting is a problem, I think it must be in the way I'm
viewing the puddle. that is to say I'm consciously getting just a bit
too much penetration in those ares where the bead is more concave. I'm
afraid if I don't do this I'll end up with a cold weld.I'm feeding rod
in at a good rate, I think, and I also think I have the tip size and
pressures about right (it's a little difficult to get the bead started
in the tighter angles) and the weld runs fairly quicly once I'm out of
the tight area. The bead is fine once I'm running up the outside of the
longeron.. I've tried varying the angle of the tip in relation to the
work, everything from near vertical to angled well towards the progress
of the weld, but this only seems to give fine control. Less heat than
I'm using makes for a very globby, cold start. Having said all that, I
picked up my torch for the first time in a year and a half last month
and have made huge progress in welding in that space of time, so maybe
I'll get a handle on this through practice. OTOH, i don't want to
continue practicing a mistake!

--
Bruce A. Frank, Editor "Ford 3.8/4.2L Engine and V-6 STOL
Homebuilt Aircraft Newsletter"
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| on all aspects of alternative |
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0 0 TIG welding

While trying to find the time to finish mine.