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 Post subject: Square Tube Joints
PostPosted: May 28, 2018, 11:43 pm 
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Hi, I'm doing a BEC design and I was planning on using square tube for most of the frame.

Due to the corner radius on the tubes (and I don't have any in hand so I don't know the radius) there are small gaps that appear at some joints (assuming planer cuts). Is this important to avoid? There's probably a term for this that I don't know, "overhang" doesn't quite seem right.

One picture is looking down into the cockpit area, the other picture is just zoomed in. At the joint with 4 tubes notice there are 5 gaps (one hidden) all at the radius of the main frame tube. "Red" is the inside surface, 'Yellow" is the saw cut mating surface and "Blue" is a chamfer. I don't like the gaps. In CAD it only measures about .015".

Thoughts?


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 Post subject: Re: Square Tube Joints
PostPosted: May 29, 2018, 12:21 am 
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Not a problem, and in fact, leads to stronger welds because the tube shape forms natural "Vs" that aid weld penetration.

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 Post subject: Re: Square Tube Joints
PostPosted: May 29, 2018, 12:42 am 
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Plus One what Kurt said. It'll save you some pre-weld grinding--in those pics, you wouldn't even have to chamfer where the ends meet the curves (though you still ought to where the ends meet flat surfaces on the adjoining tubes...which you can't see in those pics, but they're there).

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 Post subject: Re: Square Tube Joints
PostPosted: May 29, 2018, 1:32 am 
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On the other hand, think about how you're going to weld inside those shallow angle joints.

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 Post subject: Re: Square Tube Joints
PostPosted: May 29, 2018, 3:48 am 
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OK, I feel better about the gaps.

I was looking at those tight angles wondering if I can get in there. Maybe I should experiment some more (with welding). What's a typical minimum angle?


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 Post subject: Re: Square Tube Joints
PostPosted: May 29, 2018, 7:59 am 
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Good luck cutting them that accurately in real life.

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 Post subject: Re: Square Tube Joints
PostPosted: May 29, 2018, 8:54 am 
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Driven5 wrote:
On the other hand, think about how you're going to weld inside those shallow angle joints.


I've found that flux-core wire works noticeably better than gas shielding for working in the narrow vees.


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 Post subject: Re: Square Tube Joints
PostPosted: May 29, 2018, 9:25 am 
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As has been said by others (above) the gaps can actually work for you and a enable stronger joint.

As for welding the shallow angles, I had several very similar to the 3D CAD image you have uploaded. I had to experiment quite a bit with my MIG setup to get it to work well. I did take notes about the settings on my Miller 140 MIG welder. They may not be ideal for your welder, but might help with a starting point when you get to that point.

Long story short, it was a combination of the right voltage, wire feed speed, shielding gas flow and starting stick-out on the torch that worked. There won't be a lot of maneuvering room for your torch to do a fancy "stack of dimes" weld as your torch will likely short out hitting one of the tubes trying that. I found starting a nice flow of filler wire and keeping it flowing in a molten state to both tubes simultaneously did the trick. A gentle back and forth on the torch while welding gives decent weld quality. "Practice makes perfect."

Cheers,

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 Post subject: Re: Square Tube Joints
PostPosted: May 29, 2018, 5:06 pm 
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FieroReinke wrote:
Good luck cutting them that accurately in real life.


Yea, I would guess I'd be lucky to get them within 1/32". :oops:


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 Post subject: Re: Square Tube Joints
PostPosted: May 30, 2018, 7:14 am 
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I used square for straight sections and round for anything that needed to be bent, was out of plane and diagonals - the round is a lot easier to weld when space is tight. Used it on the trans tunnel too, but that is for sheet metal clearance.


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 Post subject: Re: Square Tube Joints
PostPosted: May 30, 2018, 6:02 pm 
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You're going to need to weld the hidden welds before you hide them. When you chamfer the other tubes to clear the weld bead you're going to have a small gap regardless.

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 Post subject: Re: Square Tube Joints
PostPosted: May 31, 2018, 4:11 am 
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Thanks for the tips. I was starting to guess that round tubes are partially selected to ease tighter angle welds.

I tested on a piece of 1" today with a 30 degree angle, pretty tight, doable, but I wouldn't want to do alot of them.

If I weld in the cross tube first (90 degree) then fit the angled tube in, can I cut another piece of square tube like a pie slice, fit it into the tight corner so there's a wider space to weld in? It would basically cover the unwelded tight angle. I'll maybe try it tomorrow.

I'll look for a thread with pros and cons of square vs round tube for frames.

Thanks guys!


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 Post subject: Re: Square Tube Joints
PostPosted: May 31, 2018, 6:18 am 
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When you are literally "in a pinch", another option is to switch to flux core and remove the gas collet from the end of the nozzle. The flux core weld carries the shield so it can arc pretty far from the nozzle. Messy, harder to see, and difficult to clean though.

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