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Hello Jeff,
I'm no expert, but you are welcome to my opinion.
Regarding torches:
I use a Smith aircraft torch. It is small and light and has several
size tips that can be rapidly changed. The gas control is less
sensitive than an automotive type torch. Rick Hansen has built
two champions with the automotive type, but I struggle with it.
Regarding settings:
The gas pressures should be the same, 3 to 6 lbs depending on
tip size. I set the acetylene pressure by looking at the regulator,
with the acetylene valve open. (the regulator will not work unless
there is some gas flow) I then close the acetylene valve and
open the oxygen valve. Then I adjust the oxygen regulator so
it sounds like the same velocity coming out of the end of the
torch. (the gauge on the oxygen side is course at low pressures)
Regarding popping:
The pop is not coming from the puddle. It is a small explosion
taking place in the torch tip. When it happens, it blows the
molten metal in all directions. The largest glob of material
will be found on the inside of your left shoe. This is how the
ever popular Hatz Jig was invented. This handy phenomena is a
good thing. I don't use tip cleaners. I keep a piece or hard
wood handy and play the torch on it to get a little burn going.
I then rub the end of the torch in the burning wood and POP,
the
torch tip is clean and flowing. The tip must be cleaned if any
irregularity is seen in the cone.
The popping will happen
if the area being welded is getting so hot that the material
is vaporizing and emitting sparks, and the sparks are going up
the tip of the torch. The thinner the material, the more difficult
it is to maintain a uniform temperature. If things are getting
too hot, the heat reflected back on the tip can cause it to pop
at low velocity settings. This causes the gas to explode prior
to exit.
The last thing is when
closing the final weld on a tube, the air in the tube is expanding.
This is more of a poof than a pop. The puddle blows out and leaves
a nice hole. The molten material from a blowout will be found
in the right shoe. Some folks drill holes inside the intersection
of all clusters. This also allows the linseed oil to be distributed
from only a few strategic locations. (If you powder coat the
fuselage the oil should be done after, as some shops wont process
oil filled tubes)
IT IS VERY IMPORTANT TO
HAVE CHECK VALVES IN EACH OF THE GAS LINES SO THE POP CAN'T BACK
UP TO THE REGULATORS AND BOTTLES. (just in case you
don't already have them)
Regarding flame adjustment:
You have the idea. The inner blue flame should be just slightly
shrouded by the outer lighter outer flame. The gas flowing is
also cooling the tip.
Each size tip can work in a range of gas flow velocity, and the
larger the tip, the more total gas and heat (DUH-why did I say
that). For each tip, the acetylene should be set so the flame
is just about to separate from the end of the torch (before oxygen
is added). You can reduce it some, however, if it is too small
of a flow, and you are working in a corner, gas won't cool the
tip and it will pop. If it is too fast, the flame velocity will
blow the surface of the puddle around and not heat a large enough
area, if you can keep it lit.
Regarding Technique:
Controlling the temperature of the puddle is key. Good penetration
in on the verge of too hot. Temperature control is whatever combination
of torch setting, tip size, speed of bead progress, torch angle,
heat sink of adjacent material, and above all welding rod.
The welding rod is a main player in temperature. Adding more
cools, holding back allows more heat to penetrate. I prefer to
start at the point in the cluster that will require the most
heat. This allows better heat distribution during the welding,
and more can be done with fewer tip changes or torch setting
adjustments. If easier welds are done first, they sink away so
much heat from the cluster that a ton of heat is required in,
lets say, an inside corner where five tubes come together. If
two come together at a narrow angle, I start in the crotch of
the tubes by placing a tack and welding toward the center line
from there. I may stop short of completion if the cluster is
so hot that I can't prevent the material from vaporizing and
sparking. (a little sparking is normal, but the material should
not appear to boil) After the entire cluster is finished I heat
it to stress relive it at this time, and play the torch off on
the sides as needed to facilitate straightening.
Regarding the bead:
The surface tension of a properly molten puddle, that is between
the tubes, will establish a slightly concave or dish shaped bead.
The amount of bead should be the minimum required to fully fill
the puddle during penetration. Adequate filler is needed so that
the bead does not undercut the tubing E.G. create a thin wall
at the edge of the weld. My beads are on the smaller side, but
not as small as say a tig welded joint.
It is next to impossible
to weld through a thick bead and penetrate the underlying tubing,
so try to not let the rod be painted in. E.G. puddle the base
material, and then add the rod as needed. If things look to fast
or hot just get out and let it cool a little. The swirly appearance
is from progressing the puddle in rapid steps.
Regarding bending the
sides:
No one torch is fine. It will take a little time to get both
tubes up to temp., just work them all the way around evenly.
They are easy to keep hot, and they bend like butter when they
are hot enough. (have some stops to bend to and a square handy)
You know, but I will mention anyway, that the top longerons only
go to the outside of the tail to leave a gap for the elevator
horn. Doug MacBeth has a detailed note on bending the sides in
the web site. (see Bending
In Fuselage Sides)
Keep us posted on your
progress.
Best,
Dale |