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Metal tube joint basics

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JohnBouyea

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Interesting how the ends of the tubes are flattened to leave ribs on the sides for strength.
View attachment 103335
Ok, getting back on topic RE: the flattened tube end connection [smiley]
The referenced article analysis is above my head though the conclusion about leaving the outer edges as a stiffener seems like it would be an improvement. Got it and especially if the tube is in compression like the geodesic done example cited.
How about if the tube is in tension overall between the two end holes? Envision a wing lift strut for example. Taking this a bit further, would there be merit of inserting an additional plate-or-sheet material and welding it between the collapsed tube prior to drilling the hole bearing the connection point?
My question isn't theoretical. Case in point: lift struts on the M-19 Flying Squirrel. M-19 Flying Squirrel homebuilt Aircraft Project
 

Little Scrapper

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Here’s a new wheel and my favorite one that’s wore right out. The old one makes the sweetest little finesse tool for coping those saddle joints because it’s slower. Give it a shot. Always wear gloves and glasses when doing this. I know you know that but never hurts to be reminded.

797DD421-56CB-4BF4-826C-5180D8DA2004.jpeg6CEB7B48-E0BD-4CB7-A81C-8B281FC3A97E.jpeg
 
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TFF

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I agree with Scrapper. Dulling a flapper wheel keeps you from making short parts. They work way better than you think they should. They will make a nub piece of metal in no time. The cool part is they will make a curve. They look clumsy but are not.
 

proppastie

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I believe looking at the Sandlin Goat drawings there is a caution about smashing the ends too sharply flat at the transition to the round portion of the tube.....also his tubes are flattened equally to the C/L of the tube not to one side as the paper shows......

seems to me the test and conclusion was that the tube failed in buckling in the center of the span and not at the flattened end.....however the added piece of flat bar and the nut and bolt the tube was bolted with would add much weight vs a coped joint. All in all the joints would be much heavier if flat plates were used.....and you still would have to weld the plates on.

a different design with just bolted flattened tubes for cross members would be easier than welding but still would be heavier perhaps,.... because of the bolts. And you probably would need gussets to prevent racking of a single bolt connection....and now we have the riveted gusset method.....properly done works.
 
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pictsidhe

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I agree with Scrapper. Dulling a flapper wheel keeps you from making short parts. They work way better than you think they should. They will make a nub piece of metal in no time. The cool part is they will make a curve. They look clumsy but are not.
Everyone hates them at work, I fish them out of the trash to dispose of properly at home...

If you squish a tube completely flat, you get a 180 degree zero radius bend on the edges. Many alloys will crack when you do that.
 

cheapracer

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Actually YOU need to buy a hole-saw jig with an electric drill (which are or were not expensive) then you too will have perfect fits EVERY time. <g>

Brian W
Well I have been doing it MY WAY for some 40+ years and it works just fine. It is with some distance the fastest way to fishmouth.


As Scrapper says, it only takes a few moments with a flappy disc to adjust it if required.

... and the times I have used a hole saw is to bore a hole into a tube and simply slip the smaller tube into the hole, with a bit of force you can get almost angle desired as it squashes the tube on the inside and spreads it on the outside of the angled joint. No fishmouthing involved.
 

120mm

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I was taught by an old school fuselage builder/repair guy that so called "perfect" fitting fish mouth joins were undesirable, and a little bit of slop is what you want. He demonstrated how tight joins caused stresses that distorted the piece while loose joins showed less distortion. As a result, my give a darn is pretty low for pretty fish mouths
 

Geraldc

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I was taught by an old school fuselage builder/repair guy that so called "perfect" fitting fish mouth joins were undesirable, and a little bit of slop is what you want. He demonstrated how tight joins caused stresses that distorted the piece while loose joins showed less distortion. As a result, my give a darn is pretty low for pretty fish mouths
Loose for oxy acetylene no gap for tig.
Taking this a bit further, would there be merit of inserting an additional plate-or-sheet material and welding it between the collapsed tube prior to drilling the hole bearing the connection point?
This was the way I normally did it to sop cracking on sides of tube.
 

120mm

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Quite so! And why are YOU buying acetylene when you could be making it, like one machine shop in Shamrock Tx did, within living memory?
My grandpa had a nice supply of fixins for making his own acetylene.
 

BBerson

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He would need and might have an exact length from software. Most homebuilders mark and cut it 1/4" long (chop saw for me) then whittle away in seconds with snips or grinder till it slips in. Most clusters have 5-6 weird fit ups and each need a custom fit on the spot like scrapper said. Because you never know the length.
A hole saw cannot be used a second time to trim. A hole saw might be nice for Patey big tubes. 3/8" tube is mostly just cuff off. Everything depends, as always.
 
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TFF

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When welding you know. If you are fighting the weld and burning holes, there is issues. If you are good, you just work around the problem. If you are not, it’s called learning experience. The only difference between a good and bad welder is the good welder learns to get past the bad welding.
 

Little Scrapper

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I was taught by an old school fuselage builder/repair guy that so called "perfect" fitting fish mouth joins were undesirable, and a little bit of slop is what you want. He demonstrated how tight joins caused stresses that distorted the piece while loose joins showed less distortion. As a result, my give a darn is pretty low for pretty fish mouths
that applies to gas welding, you want a slight gap because of the massive heat.TIG is completely the opposite. Very small concentration of heat and the filler is much more precise.

“Giving a darn” has nothing to do with creating a gap for heat expansion. You can still create a beautiful saddle joint and actually give a darn while simultaneously making a slightly lose fit.

A loose fit has nothing to do with perfection. A loose fit is created with intention, not giving a darn is a character trait, usual a undesirable trait anywhere around a airplane.

You should always want to do the best you can because it spills in to other areas of building. Again, if you read what was written it’s still a matter of a about 2 minutes on a joint which isn’t exactly demanding on a person. My joints are tight on TIG and slightly loose on Gas Welded fuselages. All my fuselages have been accurate anywhere you measure them to probably within a 1/32” of Inch front to back. Not because of trying hard but because I give a darn. It takes no more energy to care about quality.
 

pictsidhe

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The gap is only wrong if I can't weld it nicely. There is an optimum and some latitude. I have mostly gas or stick welded fish mouths.
 

BBerson

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Ron Alexander advised 1/16" gap in his TIG Sport Aviation articles. With no gap a weld can lay on top and look nice with insufficient penetration. Traditional weld practice is to grind a V groove for penetration. On thin metal a gap is an alternative to V grind. MIG tolerates a wider gap because of excess filler.
 

Little Scrapper

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Ron Alexander advised 1/16" gap in his TIG Sport Aviation articles. With no gap a weld can lay on top and look nice with insufficient penetration. Traditional weld practice is to grind a V groove for penetration. On thin metal a gap is an alternative to V grind. MIG tolerates a wider gap because of excess filler.
In the welding world which is thousands of percent larger than home built aircraft building a 1/16” gap isn’t discussed much at all. I’m formally trained and extremely experienced and welds on thin wall tubing is never something that lays on top. If anything the trick is to keep it from overheating. Penetration issues are caused from lack of experience, not because of a gap issue.

Anyone welding a airplane fuselage needs to be good enough to understand the difference between a good weld and a bad weld and that takes experience and time.

I’m not saying this to be a jerk but a 1/16” gap tig welding has nothing to do with penetration on .035” material or any other material for that matter.

When I was trained in school (multiple schools) we were trained to dial in the appropriate settings for the thickness and type of weld. It’s really that simple. Practice is what gets you consistency.
 

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