Darn you guys. I have a big Miller Stick welder, a Smith Oxy-Acetylene and
Miller TIG. Now I'm going to have to get a MIG also??
I'll use your posts to convince my wife.

Stu
"Bob Hoover" wrote in message
...
On Mar 16, 3:15 pm, wright1902glider wrote:


Now, here's the question: Assume I'm about to weld a commonly-
occurring cluster in the side of a Pratt-truss.

Where should I tack the joints? Does it matter?
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Of course it matters, Harry. EVERYTHING matters!

It sounds as if you've got things laid-out pretty well. So which part
of the cluster is going to take the MOST heat?

Come on; put on your thinking cap and let's figure it out. LEAST heat
is always going to be some place where the two parts come together
almost flat yet over-lapping. Where do you get that kind of a fit on
your cluster?

It SHOULD be where the vertical member intersects the top & bottom
longerons. So that's where you do the tack weld. But the tack-weld
is eventually going to become part of your FINISH weld so you keep
that in the back of your mind as you light-off and create that little
golden nugget.

Does that make sense? It should. Because you haven't gotten to the
diagonals as yet.

So let's get to them. One of them is going to be tough to fit because
the vertical is already tacked... or is it? Odds are, it is, which
means you're going to have to leave one of your diagonals sorta open.
That is, one SIDE is going to be a big HOLE because you've snipped off
the corner so as to get the diagonal to LAY FLAT.

Same story: Where is it going to take the most heat?
Same Answer: Where you have to build-up the thickest weld-ment.

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The 'problem' becomes a no-brainer when you drop back to the basics.
Welding the tips of your cluster FIRST takes the least amount of
heat. Welding down into those angles not only takes more metal, it
also takes MORE HEAT, maybe even two passes, allowing it to cool down
between passes.

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I'd like to see some other BETTER welders jump in here. John; the
John doing the fuselages for the Legal Eagle & the Double Eagle would
be a good source. Leonard's design has a couple of five-legged
clusters that can make a grown many cry but John is doing them as if
they were easy. Trouble is, John doesn't read this Newsgroup and is
probably too busy to join in.

Tubing-wise, I try to avoid those real complicated clusters but some
designs seem to forget that not all weldors were created equal :-)

I think I've already mentioned that I use MIG to tack and gas to
finish. The main reason for doing so is because the MIG'er always
gives you SOME amount of filler where as O-A does not, but when doing
a cluster there are a couple of places where you want to build-up your
weld... meaning you'll need more heat... but you want to build it up
WITHOUT adding all that much metal. Gas welding doesn't expect you to
build up metal at every pass whereas MIG does. So I use MIG to tack..
but try to keep it in the deep V's that are harder to get to with O-
A.

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-Bob

PS -- Don't blame that acid-etching on me! Blame it on the guy who
insists on getting that zinc all over the place.

PPS -- Harry, what we really need here is some pictures.