On Sat, 21 Mar 2009 12:10:37 -0700 (PDT), wrote:

On Mar 20, 6:49*pm, Bob Hoover wrote:

Of course it matters, Harry. *EVERYTHING matters!


You can say that again.

One of the things that matters is getting the finish dimensions that
you started with. I've spent hours filing and shaping a fishmouth to
get that perfect fit, only to have the end off my tube off by 1/8"
once all the warping has finished. I've learned to minimize the
warping though. You should weld the fish mouth in 4 steps. First do
each side where the mouth reaches down the side of the non-cut tube.
As the metal shrinks, it will pull the mouth tighter into the non-cut
tube. You've welded on both sides, so the warping neutralizes
itself.


make yourself a shrink warping preventer. its a good welding exercise
in itself.

when you heat up the metal to red heat and let it cool it will shrink
and unless you prevent the longeron tube from moving it will develop a
bend.

my brother in law has a simple fixture that prevents the bend (he
being a life long expert welder)

you need a little solid section that will take a threaded hole across
it for a little jack screw like the screw up part of a common old "G"
clamp.
out each side you weld some scrap tube so that you end up with a long
straight tube with the little jacking screw transversely across the
middle.

mentally place this welded up tube bit on top of a fuselage longeron
tube with the jacking screw pointing at the longeron.
ok from each end of this tube bizzo you need some arms that go out
perpendicular from each end, then downward, then back under the fuz
tube, then a little bit that goes up toward the fuz tube. at the end
of this little upward bit you place an inch or so half length of tube
so that it will cradle the fuz tube.
the tube bizzo with the little cranked arms at each end are an
interesting welding practise exercise.

now to use it you straddle the joint and clamp it very very very
lightly to the longeron you expect to warp.
when you weld the cluster and the tubes are all red hot any over
clamping will of course warp the longeron so you need just the merest
holding clamping. as the cluster cools and wants to shrink bend the
longeron the little clamp will hold the tube straight.

brother in law came up with the idea years ago and has used it ever
since on his aircraft welding. works a treat.

dont bother painting it because the flame will burn the paint off

I'll see if I can find a photo of the ones I made.

Stealth Pilot