multiple tube joints

If you are welding a joint where several tubes come together, as is common on race car chassis and airplane frames, do you fit all of the tubes and
then start welding, or do you fully weld the first set, then add the second, then add the third? I normally do the latter, as I like getting full beads around every tube if at all possible, of you fitted up all of the tubes, some of each tube would be obscured.
Brian
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Brian wrote:

Standard practice on airplanes is to do all the fitting first and then weld the visible intersections without worrying about what is beneath. For practical purposes, the strength comes from the design more than the welds, and most airplanes--excluding aerobatic mounts and anything designed by Steve Wittman--are so overbuilt that they'd probably hold together in normal use if they were just tacked well.
Owen Davies
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Pardon my ignorance but who is Steve Wittman that warrents such special praise??
Owen Davies wrote:

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RoyJ wrote:

http://www.totalracing.com/wittman /
--

Tim Wescott
Wescott Design Services
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: Standard practice on airplanes is to do all the fitting first and then : weld the visible intersections without worrying about what is beneath. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^     --Does this mean that where tubes intersect at such an acute angle that the tig torch can't get in there, you just leave it un-welded?
--
"Steamboat Ed" Haas : My shop is open to
Hacking the Trailing Edge! : visiting dog-nitaries...
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steamer wrote:

I'll pretend that was a serious question.
Most homebuilders use an oxyacetylene torch, and the flame seems to reach into the tightest clusters just fine. As for tig, a method I really want to learn one of these days, I assume the weldors use a gas lens to extend their reach. In any case, they manage to weld all the intersections one way or another. It's hard to imagine that an airplane with a tube intersection unwelded would ever get certified for flight, no matter what the builder claimed had been done to the internal junctions.
Owen Davies
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He referring to the joints UNDER the final top joint.
Owen Davies wrote:

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: He referring to the joints UNDER the final top joint.     --So what you're saying is you've got a ball of tubes intersecting and the welds are only exterior; i.e. that somewhere in the middle are tube ends that are unwelded? Sounds weird.
--
"Steamboat Ed" Haas : My shop is open to
Hacking the Trailing Edge! : visiting dog-nitaries...
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: I'll pretend that was a serious question.     --Silly clueless me.
: Most homebuilders use an oxyacetylene torch, and the flame seems to : reach into the tightest clusters just fine. As for tig, a method I     (SNIP)     --Yeah, I was thinking it would be a good place for brazing joints because the stuff flows so well. But would this not be considered adequate, strength-wise, for this application?
-- "Steamboat Ed" Haas : My shop is open to Hacking the Trailing Edge! : visiting dog-nitaries... http://www.nmpproducts.com/intro.htm ---Decks a-wash in a sea of words---
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after you ruin a few joints, you find that you can Tig into pretty tight intersections. What I do now is, after tacking, start the bead at the tightest, most obnoxious spot. That way I am working with cooler metal and have a little more control over the puddle as I get the bead to fuse on both tubes. Plus I am fresh and not as tired and I seem to have more motor control over what I am trying to do.
Brian
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steamer wrote:

My apologies. You have provided so much interesting information to this group over the years I've been visiting it that I have considerable respect for your knowledge and experience. Under the circumstances, I thought you were twitting me about some perceived, but unspecified, defect in my statement. There are so few areas in which I know more than someone else that this is not an uncommon experience--particularly here on the Net.

Given that some tube-frame racing cars used to be brazed, or so I've heard, it probably would be plenty strong enough. However, I have not heard of anyone brazing structural parts of airplanes since the 1920s, and my impression is that it was an amateur technique even then.
Owen
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| Given that some tube-frame racing cars used to be brazed, or so I've | heard, it probably would be plenty strong enough. However, I have not | heard of anyone brazing structural parts of airplanes since the 1920s, | and my impression is that it was an amateur technique even then. | | Owen
In the 20's, I'm sure brazing with a torch was the shit and a highly skilled career. You didn't get gas in bottles back then I don't think; everything was from gas generators. Considering the technology, I don't think there was many other choices, so the welders of the day did the very best they could with what they had. IIRC, the only electric welding back then was bare stick with damp or slightly rusty rods, and the equipment must have been expensive as hell. That said, I don't think that stick welding was quite as strong as today. For what it's worth, when I first learned to weld I had to do a few rods with the flux stripped off just to get a better idea of how the arc worked. That's how demanding it was back then for stick, I suppose.
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must
rods
worked.
Actually electric (carbon torch, arc, etc.) welding was more common than most people would think. All it required were the leads, and some batteries. The preferred power sources used by welding shops were large (wet cell) batteries. The batteries were usually being charged overnight and when not actually being used for welding. Electric welding started to gain in popularity around the start of the 20th century.
As a side note: Farmers made good use of carbon torches powered by batteries, and also of carbide generators. Many farms had carbide generators that provided gas for lights, gas for heat for brood chickens etc. On of my grandfathers used a carbide generator for lights etc, up until 1960 when they finally got electrical power. My other grandfather quit using a carbide generator shortly after WWII when rural electrification started big time. In my front lawn there is a small rise. Under that rise in the ground is an acetylene generator tank. (probably isn't any good today). This was left over from a previous house that had been here. It was too much trouble to dig it up when we built here. Oh yeah, the previous house that set here also got electricity in 1960.
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Aircraft structures don't use brazing. Its not considered safe, strengthwise. I don't recall the exact reasons, but it has something to do with the metallurgy of the fill and the 4130N being brittle or something.
John
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Brazing can be done on tube structures but the fit-up is critical. in the order of one thousands of an inch tolerence. If done properly the strength of the joint is a couple times greater than the base metal.
Add a couple extra thousands of an inch gap and the joint will have only a small fraction of the strength of the base metal.
There used to be a couple of people on this group from England that mentioned it was required in some racing classes for the roll cage. Guess I should have taken notes.
One thing I do remember is that the roll cages were not from 4130 just a generic DOM tubing.
John Noon
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