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Metal tube joint basics

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BrianW

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Joining square tubes to be welded takes one straight cut.
Jointing round tubes to be welded takes a hole saw on a drill stand.
Joining tubes to be bolted can use a sawn slot, and a flat profile end fitting to be welded on a long tube axis. But a method which always seemed like an eye-sore was the pinch and drill method which collapses the tube end, which is then drilled. This has been used on some aircraft - but it always seemed weak & fatigue prone to me. But I know better than to trust gut-feelings in these matters. Is there any design analysis on flattened tube joints?
 

BJC

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Jointing round tubes to be welded takes a hole saw on a drill stand.
Lots of airplanes have been built without a hole saw to cope the ends. An open, rounded profile, grinding wheel and a good set of snips to start the notch can produce a good fit in less time (after the first three or four) than it takes to set up a hole saw. If you need it to look good before welding, a few strokes with a round file works.


BJC
 

cheapracer

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Jointing round tubes to be welded takes a hole saw on a drill stand.
I have never used anything but 2 straight cuts with a dropsaw to make a fishmouth.

Starting at the most basic, 2 x 45 degree dropsaw cuts will make you a 90 degree join.

You need to either grind or cut of the pointy bits is all. Grab some old tube and practice.

fishy.jpg
 

WonderousMountain

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We built a bolted Geodesic dome this way
Not much for joint strength, but failure mode
will always be buckling - in that structure.
No wear was reported. Mostly holds up drapes.
 

Pops

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I make patterns for the most used joints. Next size tube in .058 wall. You can also make from flat sheet metal as in picture.
Picture of the 4 seat Bearhawk landing gear legs jig. Several sets made from this jig. Cut to a little longer length , mark with a silver pen, rough cut with a 3" or 4" electric cut off wheel, ( the one from HF works good). Finish with half round files for a perfect fit. Quick and easy.
 

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pictsidhe

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I've always just made two cuts with a hacksaw or angle grinder. Three on thick wall stuff. You get an eye for it pretty quickly. Not perfect, but you'd never know that looking at my gas welded joints.
 

Little Scrapper

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People tend to overthink this stuff. Make a straight cut just make it slightly oversized in length. Then trim it up and put the curve on it with a bench grinder or hand grinder to make it fit. The rounded saddle part of the end that sits on the tube is just slightly rounded from straight. It’s not much.

Many parts of a fuselage are smaller diameter tubes going in to a longeron that’s larger in diameter so that makes it even easier. Example. A 1/2 inch tube going in to a 3/4” longeron is dam near a straight cut.

Don’t overthink it. Just don’t make large gaps because filling those in when welding probably adds 400% more time to the job compared to a good fit.
 

BrianW

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I have never used anything but 2 straight cuts with a dropsaw to make a fishmouth.

Starting at the most basic, 2 x 45 degree dropsaw cuts will make you a 90 degree join.

You need to either grind or cut of the pointy bits is all. Grab some old tube and practice.
...
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
 

Little Scrapper

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5D9768FF-8DB3-492C-B1F6-EA44E94E112D.jpeg
This angle gusset on my Baby Ace was cut straight then fine tuned with a 4-1/2” angle grinder with a worn out flap wheel. Flawless fit every time.

worn out flap wheels are literally the perfect tool. I recommend taking a new one and pushing it intensely hard against steel or cement to prematurely ware it out because it’s better that way.

I’m headed to my shop shortly I’ll take a photo of it.

People feel comfortable buying fancy tools because they lack confidence and feel like they need that stuff. You don’t. Nobody does. It’s a luxury is all not a solution for skill. I own expensive tools for saddle joints because my time is extremely valuable and I have so little of it but to be honest I always prefer simple hand tools accompanied with a dram of whiskey or scotch and some silence.

A tim snips and a simple cut with a Whiskey is like a little slice of heaven, a mini vacation. Expensive tools and blowing money on this stuff sucks the fun right out of it. Keep it simple.
 

Kyle Boatright

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People feel comfortable buying fancy tools because they lack confidence and feel like they need that stuff. You don’t. Nobody does. It’s a luxury is all not a solution for skill.
I agree 100%. Having built an old RV kit (which required some fabrication skills, but nothing like scratch building), I'm amazed at the tool collection that so many new (prepunched, match drilled kit) RV builders are convinced they need. $800 rivet squeezer, $400 bandsaw, industrial air compressor, etc.

On the modern RV kits, you'd <net> save time by using your $12 hacksaw to cut the angles you need, as opposed to taking 2 hours to buy, load, unload, and setup a bandsaw. And you'd have $400 in your pocket.

Not that bandsaws aren't great tools, but (like drill jigs for steel tubing) you don't need one to build the airplane...
 

pictsidhe

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Good luck with that hole saw.....
Piece of tubing, file/grinder, pliers. Blowtorch and a bucket of water if you have heat treatable material and aren't drilling something soft. I made one a month ago at work. I usually just buy them if not in a hurry or a weird size. But this was on the weekend and was needed ASAP for our fastest money printing machine. I have to request any purchases for stuff we don't have in the stores and they only get ordered mon-fri. That made it $50,000 worth of hole saw... On weekdays, I can also get the toolroom to make custom or urgent stuff with much better machinery than I have access to.
A bad workman blames his tools, a good one makes them! I often have fun there on the weekends...

Some of you would like some of the tools I've made for work. They aren't very useful for building aeroplanes, though.
 
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BrianW

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View attachment 103368
This angle gusset on my Baby Ace was cut straight then fine tuned with a 4-1/2” angle grinder with a worn out flap wheel. Flawless fit every time.
///

People feel comfortable buying fancy tools because they lack confidence
////

A tim snips and a simple cut with a Whiskey is like a little slice of heaven, a mini vacation. Expensive tools and blowing money on this stuff sucks the fun right out of it. Keep it simple.
Why restrict your little slice of Heaven to snipping and flap-grinding?
REAL men use a hammer and a block, a hacksaw and a hand drill, with several snips and files to turn out BEAUTIFUL ships in ten or fifteen years! Yes, I AM joking.
 
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