Concave Pac Man’s Mouth
Convex “Puddle” Shape
See Detail to Right
20. Once Mr. Pac Man bites into the material, his mouth opens again and is ready to
be fed some welding rod.
21. The solid rod never touches the puddle: It is momentarily dipped into the
super-heated space between the inner cone and the puddle. If it touches the
puddle, it will instantaneously cool off the puddle and get stuck to it.
22. The rod goes in and out of the space only when Pac Man’s mouth is open and
he’s ready to be fed. Don’t get impatient. If his mouth isn’t open (the puddle
sucked into the joint forming a concavity at the front of the puddle), excess rod
will cool off the puddle.
23. The instant the rod touches the heat zone, a melted drop will fall into Pac Man’s
mouth. As soon as the drop hits the puddle, it will momentarily fill his mouth,
then form another puddle and another mouth will appear as it flows into the
melted edges and sinks into the joint.
24. The “stack of dimes” bead is the result of consistency. The more consistent you
are at holding the angle, the cone distance, feeding the puddle, and making a
tiny, gentle motion of the cone back and forth across Pac Man’s nose, the more
consistent the bead will appear.
25. We’re trying first for consistent penetration, then we’re trying for a
26. Watch the puddle closely. It should appear fairly tranquil. If it appears to be
boiling, it’s too hot and will oxidize. Back the torch away for a second, then
come back in and keep welding. If the same thing happens again, adjust to a
slightly smaller flame.
27. If welding to an edge, it will get too hot and melt away. Dance the torch in and
out while the rod is fed in. It’s usually easier to weld from the outside edge
inward, but sometimes that’s not possible.
28. As you finish the last pass on a joint, keep the torch on it and back the torch
away very slowly. Do not just take the torch away! The surrounding steel will
suck the heat out too quickly and lock in stresses.
Slightly undercut where
edges of metal are melting
and flowing into puddle.
If this isn’t happening there
is not enough penetration.
Concave leading edge.
This is what has to be controlled.
29. If it’s a cluster joint on a longeron, or a T-intersection, the weld will shrink and
bow the longeron. As the last tube is welded in place, bring the torch to the
outside of the longeron opposite the weld and heat an area 2 or 3 inches long
just short of welding temperature. Hold it for a few seconds, then let it cool
slowly. This will shrink that area and keep the longerons straight.
30. If it’s a critical joint, like a wing, landing gear, or motor mount fitting, “stress
relieve” the area while it’s still hot from the last weld: Heat the entire joint to
a dull red and let it soak for five to 10 seconds to relax any uneven contractions
inside and to even out any material changes. This is not “normalizing,” which
is a far more exacting procedure that cannot be done in the field because it
Some folks are going to argue that TIG is a better way to go for
welding 4130 than gas, but not for the first time builder. Gas is more
versatile and much more forgiving, not to mention cheaper. If I had
TIG, I’d use it, but I’d still need the Oxy-Acetylene, to stress relieve,
heat, bend, anneal, solder, braze and cook sand dabs (thanks, Kent).
One last point that can’t be over emphasized: in welding vision is
everything and whether you have 20/20 vision or not, buy some drug
store cheaters (magnifying glasses). There is simply no substitute for
being able to see exactly what is happening on the leading edge of
the puddle. Even if you already know how to weld, you’ll be amazed
how much cheaters help. As I’m constantly saing about flying in gen-
eral, “you can’t correct what you can’t see.”
I’m almost certain that we’ve missed some important points
here, but I don’t think we’ve done too badly for the space we had.
Budd Davisson is an aeronautical engineer, has flown more than 300 different types,
and has published four books and more than 4,000 articles. He is editor-in-chief
of Flight Journal magazine and a flight instructor primarily in Pitts/tailwheel aircraft.
Visit him on www.AirBum.com. For links to more information on this topic,