# Bending alunimum tube



## mrkyak (Jul 11, 2005)

The usual 6061 alunimum frame tubes are anodized, like from nrs.
Is it correct that anodizing only addresses the exterior finish and provides no structural component?



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## BCJ (Mar 3, 2008)

I've researched this and come up with "it depends" on the grade (T6 or something else?), the method used (mandrel bender vs something else) and the radius. Down River advertises that it uses 6063, and most of the research I've seen suggests that 6063 is more "bendable" over a wide range of conditions.


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## bigscottone (Jan 4, 2011)

anodizing only affects the outside surface. 
I'm no metallurgist, but aluminum loves to crack, and to micro-crack. 6061 is different on a molecular scale than 6063, and is more prone to failure when bent or stretched. You are probably fine, but I would bet that bend would fail under a stress test, and at minimum is more likely to crack over time. that said, i would probably do it anyways if I had a bender and 6061 lying around.


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## Avatard (Apr 29, 2011)

BCJ said:


> I've researched this and come up with "it depends" on the grade (T6 or something else?), the method used (mandrel bender vs something else) and the radius. Down River advertises that it uses 6063, and most of the research I've seen suggests that 6063 is more "bendable" over a wide range of conditions.


I've bent 6063 using a h/f pipe bender and a lot of effort to prevent kinking. It bends pretty easily. It also bends when you torque down on the pipe fitting screws, fwiw. I'm pretty sure if I made a frame from it that the crossbars would bend ever so slightly from loading especially if you like to jump around from drybox to cooler on a loaded raft etc.


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## mrkyak (Jul 11, 2005)

Alunimum pipe not tube. 6" radius.


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## B4otter (Apr 20, 2009)

Take a cheap magnifying glass and look over both inside and outside of your bend for cracks, splits, or the harder to describe "impact craters" that come when you stress pipe beyond its tolerance. But you should be good - I think that stuff has like 0.14 wall thickness, and provided you did the bend cautiously and not in one swell foop it should be plenty strong. 
6063 does bend better, and it's what just about every fabricator uses for outside rails on double rail diamond plate frames, but if you got 'er done without hairline or worse fractures I'd bet you won't ever have a problem with it. 
Anodizing only weakens the underlying surface/structure it's applied to under certain conditions. It has no structural component otherwise. 
For those of you under 40, until Bruce Helin (PRO) built his first aluminum frames back in the 70's - and got us all to thinking - we built our frames out of wood, sometimes with metal plate corners, and welded our own oarlock stands... look how far we've come, and how many more folk are out on the rivers we run...


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## orto11 (Mar 8, 2013)

Take a picture of that pipe with a carpenters square on it, my eye is not that bad but from your photo that is no where near 90 degrees. May be your photo though?


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## jakebrown98 (Mar 4, 2008)

I'm trying to work out similar issues myself. NRS says they use 6063 T6 for their bent stuff because it doesn't lose as much strength as the 6061 T6 they use for everything straight. It also bends easier. Their website is not necessarily accurate when it lists a particular alloy for a particular part and getting specific information might take a few transfers through their customer service system. 

The temper, indicated by the T or otherwise, has a lot to do with the properties of the metal. Make sure you're comparing apples to apples and check the temper. AAA Inflatables supposedly uses 6063 T6 in the 1 1/4 IPS (1 5/8 OD) for most of their frames. It must be durable enough for them, but I intend to call and find out. I hope to use some for a first and last crossbar in a custom frame with rounded corners and LoPros.


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## caverdan (Aug 27, 2004)

If you are worried about the strength in the corners or you end up with hairline cracks, you can always have gussets welded in to strengthen them. I worked in the Continental Airlines sheet metal shop for a short time. For complex bending projects or any bent parts that needed to be a hardness of T-6 or greater, we would start with softer aluminum and harden the end results in special digital ovens. Not that practical or cost efficient for building raft frames though.


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## elkhaven (Sep 11, 2013)

I recently tried to get 6061 t6 bent locally and failed misserably. Everyone that tried got to 20 degrees and pop... didn't sheer the tube but stress fractured it. I ended up talking a a fellow buzzard into doing it with 6063... Thanks by the way, everyone that sees the rear tubes wants to ditch their NRS and get some...

Any ways back on topic. MRkyak, you seem to be well equipped and at least fairly internet savvy... I researched all this and if I had access to a mandrel bender as you appear to I'd be making all kinds of cool shit. If you want to use pre-annodized NRS tubing you can anneal the tubing back to T0 bend it and supposedly re-temper it (roughly) according to some web surfing I did. The re-temper is all about max temperature and rate of cool but I bet you could figure out how to do it. 

I think we largely underestimate the strength of this tubing! I might be worried about oar towers or something hyper critical to functionality but asside from dumping my home bent and tempered frame (If I had gone that route) into the ledge hole in lava with 3k lbs of gear I'm pretty sure no part of it would ever fail under raft loads...

The difference between pipe and tube is simply dimensional and terminology. They each follow a different naming and sizing convention based on the industry they're used in. 

Annodization is basically controlled oxidation of the AL. It creates a coating on any exposed surface that penetrates into the substrate a bit and adds a little material to the surface. It can be ground/sanded off and worked with. It does not alter the internal structure of the material (past it's penetration, which is measured in mils)... so yes, to your second post, it's true it does not strengthen the material, just makes it more resistant to wear. 

You may see cracks in the annodization if looking with a glass/microscope as it is much harder and stiffer than the substrate, so you may want to sand that off before you judge whether your tubing/pipe is still sound.


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## jakebrown98 (Mar 4, 2008)

elkhaven said:


> I think we largely underestimate the strength of this tubing! I might be worried about oar towers or something hyper critical to functionality but asside from dumping my home bent and tempered frame (If I had gone that route) into the ledge hole in lava with 3k lbs of gear I'm pretty sure no part of it would ever fail under raft loads...


I'm thinking you're right about this. My crafty idea is to take two NRS drop bars w/o the LoPros and use them as the end cross bars of a raft frame. I'd join them together with 6061 T-6 and NRS's take down splices. Then add a few more crossbars. I'm worried that the 6063 they use in the drop rails won't be stiff enough to hold up half a cooler and my bum. But man I love the look and functionality of bent corners on a frame. And I just don't trust the hand rail fittings those frames usually come with... I like the LoPros.

I asked them the other day if the Foot Bar they make is 6061 or 6063, because I've been slinging coolers, and cans, and floors off of that thing for years. They haven't responded yet.


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## elkhaven (Sep 11, 2013)

jakebrown98 said:


> I'm thinking you're right about this. My crafty idea is to take two NRS drop bars w/o the LoPros and use them as the end cross bars of a raft frame. I'd join them together with 6061 T-6 and NRS's take down splices. Then add a few more crossbars.


I originally thought about doing this with their drop bars for a double rail addition to my existing frame, even started a thread about it. I like the idea of rounded corners as well, it's really the only thing I don't like about my current frame.


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## mrkyak (Jul 11, 2005)

Thanks for all your info. The bender I used did a full 90 degrees. I'll keep on playing with it. If anybody local wants to work on a cool frame I'd love to help. I do tend to overload my boat (25 days on the grand, 1raft, 1 kayak, 1sup, with river sauna) so the 3000 pound lava ledge hole avoidance recommendation is seconded.


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