Bending aluminium pipes - tent related

[Watchman]

I am planning to make a lightweight structure I can erect quickly on-site that attaches to my trailer and will cover it and my bikes. For the simple reason that the clamps I have already are for 25mm OD tubes, I will be using 25mm OD aluminium tubes.

It appears that I can get lengths of this appropriate for my plans in 18swg (1.2mm). Does anyone know anything about bending this? There are mandrel benders for hire local to me but I don't know what the tightest curve I could expect, whether I need to "bath" the inside of the corner, and whether I need to heat it up (and whether a normal blow lamp would do).

Anyone know?

[Conian]

not much help I'm sorry
but as well as bumping the thread a bit, I'll add this
prepare for a lot of creased poles!

[Watchman]

Conian said:

not much help I'm sorry
but as well as bumping the thread a bit, I'll add this
prepare for a lot of creased poles!

Well, it'll work quickly or I'll give up and either buy pre-formed joints or go "square".

A friend reckons a bit of heat from a blow lamp does wonders for ali... I'll give it a shot. I'm ordering a few extra metres to experiment on.

[Conian]

filling the tubes with sand can also help prevent creases
works on bigger pipes, cant hurt on smaller ones too?

[Watchman]

Conian said:

filling the tubes with sand can also help prevent creases
works on bigger pipes, cant hurt on smaller ones too?

That's a good idea.

I watched on TV how they make trumpets - they fill the tubing with water, seal it and freeze it overnight. The ice makes a perfect internal die, then once they've bent it, they warm it and it drops out.

I haven't a freezer long enough to take 3m of tubing in one go though. Sand could be ideal. Wet though, yes?... or it'll just pour out.

[Conian]

Block the ends n use dry?

[Watchman]

Conian said:

Block the ends n use dry?

Definitely makes more sense. Just not sure how to block-in the ends. There must be a collet arrangement I can make or buy.

Thanks for the tip. I'm going to order the tubing and mandrel-hire for the first week of Sept which I've got off work. I'll let you know how I get on.

[Watchman]

This is what I need. I should be able to make something like this:

http://www.oreillyauto.com/site/c/detail/DOR1/0260...

[Conian]

wet sand the ends

[Watchman]

Clever. You've done this before.

Thanks.

[GAjon]

1.2mm may be a bit to thin but, if you want to give it a go.

Aluminium age and work hardens, fill the tube with dry sand. Do this by blanking one end with balls of rags, fill the tube with sand then stand it on its end and bang it down on a solid floor whilst tapping the length of the tube with a mallet.
Keep doing this till you cant get any more sand in then hammer some more rags into the filling end of the tube.
You need to anneal the tube, heat it evenly with a blow torch.
Aluminium doesnt change colour when heated so get a matchstick and rub it on the pipe when it leaves a black/brown streak on the metal its at the correct temperature.
Now bend the tube araound a jig or former, anything will do but you'll only get a about 5xDR or it'll crush.
If you feel resistance, which will be quick, re anneal the metal and go again.

Good luck.

Edited by GAjon on Monday 12th August 08:44

[Watchman]

Fantastic advice. Very many thanks.

[Watchman]

GAjon said:

you'll only get a about 5xDR or it'll crush.

Sorry to ask but can I confirm this means I cannot bend it tighter than 5x its diameter? So, with a 25mm diameter tube, I cannot have a diameter or radius less than 125mm?

To be honest, 125mm radius would be fine for the application I've in mind. I might even go larger for the sake of metal integrity.

The bends will be 90 degrees.


And lastly, I can increase the wall-thickness quite easily but thought it would be easier with a thinner wall (and lighter ultimately - which is part of the point of this). Can you explain where the stress on the metal is? I guess, by filling with sand to retain the shape of the tube, I am stretching the outer part of the curve, thus making the wall-thickness erm... thinner.

I am not averse to implementing a "bath" on the inside of the curve if it meant better integrity (on first glance, it doesn't look as though it would stretch the outer curve wall much or at all) but I don't know whether it will, nor how to actually do that anyway.

[GAjon]

5DR is 5 x daimeter centreline radius, but this is just a guess on my part what you might get on that wall thickness of tube, you might be able to get it tighter? There are a lot of complicated formulas that i cant remember, diameter to wall thickness etc.

The back of the tube has to stretch and the throat will try to crumple, the sand is to reinforce the tube while your pulling it around the jig.

Its going to be trail and error and the quality of the tube may become a factor, but do keep the metal soft for working.

You may end up buying a caravan

[Watchman]

GAjon said:

You may end up buying a caravan

That is the end goal.

[bristolracer]

Have you considered using plastic electrical conduit?, you can get 25mm and all sorts of bends and tee pieces, providing your cover is not too heavy it could work.

Also you can get bending springs for it which may help you bend alluminun

[Watchman]

The structure needs to withstand high winds (often found on campsites, being so near to the sea). Aluminium poles are the choice for that sort of thing in tents so I'm going with other people's recommendations on that one.

[Odie]

Why does it need a frame? Just through a tarp over it?

[Watchman]

Odie said:

Why does it need a frame? Just through a tarp over it?

That's what I had last time. It worked OK although it flapped about in the wind a fair bit (tarp is noisy), and it was a bit of an arse to reach up and throw the tarp back over each time I put the bikes back on (they're secured to the trailer, so I store them there) which got me thinking of making something I could install once at the beginning of camp and leave up until we went home, regardless of the bike usage.

I'm going to extend it over the rear of the trailer to provide a kitchen area too.

[Watchman]

This guy is bending the exact same type of tube I am buying. Looks like the trick is annealing first:

http://www.youtube.com/watch?v=u_6JTRkhWZc

He's got a nice tight curve too.