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  #1  
Old 10-02-2011, 09:58 PM
DDaut DDaut is offline
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Bending Aluminum Rectangular Tubing

Can anyone tell me how to bend the rectangular tubing like in this picture by Loewman. I sent Loeman a request but he hasn't posted since 2009. So I'm not sure he's still around. I'm thinking about building me a boat similar to this. Thanks
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Bending Aluminum Rectangular Tubing-aluminum-bracing.jpg  
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  #2  
Old 10-02-2011, 10:26 PM
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viking north viking north is offline
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Shouldn't be a problem for a machine shop to do this, as a matter of fact one of those in expensive hydrolic jack benders should do the job however you might have to modify the bending dies normally used for pipe. Another method without using a bender is to do a series of saw cuts say 1 in. apart at the sharpest part of the bend radius (further apart as you distance from it)( saw cuts are made thru the top and down each side to the bottom but not into the bottom of the rectanguar cross section.) Then simply bend by hand as desired , rewelding the cuts to hold the shape. I use a regular wood working miter(chop) saw and while a proper alum. cutting blade is recommended i use a regular 60 tooth carbide blade to cut alum. extrusions and flatbar. (Don't do as I do but Be sure to clamp, run the saw at high speed and feed very slow)) --Geo.

A yacht is not defined by the vessel but by the care and love of her owner--

Last edited by viking north : 10-02-2011 at 10:42 PM. Reason: structure
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Old 10-02-2011, 10:28 PM
Ad Hoc Ad Hoc is offline
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If it is a proper RHS, then you can't really, it is too stiff. You can put in a lazy gentle curve, but that is it, at a push!. Otherwise you'll yield the alloy at the outer surface and it'll crack either as you bend it, or shortly after in servce. Most will ignore this, saying...looks ok to me as once the boat is built, unless a home build, wont know about it!!

What most do, is cut slits into the RHS to allow the bend to be of an "open section" rather than the 'closed section'. The slots being on the compression side. But, once welded, you've lost your unwelded strength!..and the curve is like a ployline...lots of small straight sections...not pretty either.

There is no reason why one would wish to bend proper RHS, by that i mean a square drawn closed cell tube of appreciable thickness. Can't be done as it is too stiff, that is why open sections are used, such as bulb bars etc.
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Old 10-02-2011, 10:59 PM
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viking north viking north is offline
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There you go bending not a good idea haven't tried tighter bends with rectangular stock but had no problem with using the saw cut method. Wouldn't worry about the looks and the skin will form as a smooth gradual bend, simply weld it on the small straight sections to provide a filler in those voids. If you're worried about strength(I wouldn't) weld an additional flat bar on top of the welded cut sections over the tightest bend rad. area. It will be stronger than the origional unbent extrusion. After all this is a sort of working boat not the Queens barge-- Geo.
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Old 10-02-2011, 11:31 PM
Ad Hoc Ad Hoc is offline
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Originally Posted by viking north View Post
If you're worried about strength(I wouldn't) weld an additional flat bar on top of the welded cut sections over the tightest bend rad. area. It will be stronger than the origional unbent extrusion....
Sorry but that is incorrect. The unwelded strength and as-welded strength of these extrusions is roughly a difference of some 50%.

So you need a substantial increase in stiffness to overcome the 50% loss of strength.

For example, a 75x75x5mm RHS has a modulus of 30.6cm^3
If you wled a 75x5mm FB ontop, the modulus increases to 32.2cm^3.

The stiffness has increased just 5%, compared to a loss of 50% when welded.

Extrusions work best where the greatest stress in unwelded.
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Old 10-02-2011, 11:55 PM
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thudpucker thudpucker is offline
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Long ago I read something on packing the tubing with sand before you bend.
Supposedly the sand will keep the tubing from Kinking.
I never tried that though....
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Old 10-03-2011, 12:40 AM
Poida Poida is offline
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To qualify myself and the amount of experience I have in bending the RHS in question. I have none.

However I have bent aluminium and large tubing.

Aluminium is best annealed before bending. To do this you use oxy/acet with a carbonizing flame ie black soot coming out. Make the surface of the ali black. Then revert to a heating flame and heat the ali, carefully until the black soot on the ali has gone.

I would looking at the job at hand use a roller bender. This is a bender that has rollers you put the RHS through that are out of line so the tube bends as it goes through. The rollers themselves have the shape of the RHS to hold the tube from going out of shape and the bend may have to be done through multiple passes.

Mr Thudpucker. I have bent large copper pipes where filling with sand was neccessary but hopefully these appear to be able to be bent without doing that.

Of course a roller bender for this job would be a large capital outlay and for a one off job sourcing a bending specialist would be the best option.

Trusting I have been no help at all.

Poida
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Old 10-03-2011, 01:01 AM
Ad Hoc Ad Hoc is offline
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Originally Posted by Poida View Post
To do this you use oxy/acet with a carbonizing flame ie black soot coming out.
You should never use heat to bend/form aluminium.

You may get the shape you want, but you have destroyed its properties and introduced cites of crack initiation by such "manual" methods.
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Old 10-03-2011, 02:03 AM
aranda1984 aranda1984 is offline
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How to bend rectangular aluminum tubes

You can bend certain extruded aluminum tubes, U channels and square and T shapes. However, there is a limit to the curvature before the etreme fibre will fail.
Normally you need a piramid type three roller "roll forming" machine.

With hollow or unequal shapes you need to have the appropriate shape cut into the rollers to prevent the side of the material from collapsing outward.

The other method is to have a the proper radius jig made and fit half the profile into a groove in this jig. Now you must clamp one end of the profile and with a roller that has the other half of the profile (as a groove,) bend, force the tube into the shape you need, by rolling it along the perifery of the jig! (Must have allowance for spring-back.)

Sand might be OK for certain hollow shapes, however the tubes must be totally filled, vibrator settled and packed and the ends sealed, but you get better results with molten rosin fill. Using rosin, you must take care with the heating or the strength of the aluminum will be lost!
These last two methods are used on a large table (like a giant steel pegboard) where you don't need any jigs. You just use a comelongs and pegs and clamps.

This is best left to people who know what they are doing.

There are many other methods, to succesfully bend tubing. It all depends on the quantity, the quality and the size of the radius.

The cut and bend method can produce fine shapes, but the strenght is out the window because of the heat from the welding.

See a fabricating shop that has the setup for tube bending.
If they have the right equipment, it is a very simple and inexpensive process.

Regards, Stephen I. M.
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Old 10-03-2011, 03:21 AM
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thudpucker thudpucker is offline
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OK, I'm glad the Sand statement was explained.

As I recall a Roller for bending square tubes involved a lot of Disipline to keep the Tubing in shape.

Could you clamp two or three pieces of Metal or Hardwood to the sides of the bend.

Make a Top and Bottom of the bend with some hard wood.
Then force the Top and bottom down onto your tubing.
It's be interesting to buy a piece and see?

How about taking the pieces to be bent and a pattern to the Machine shop?
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Old 10-03-2011, 05:18 AM
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dinoa dinoa is offline
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Annealing the aluminum will destroy its temper and revert it to the 0 condition where it can be formed more easily. The roller and radius jig would be the best way to go but I have rolled 2"X 2" X .125" T6-6061 even though it may have been a larger radius than in the picture in a Farman roll. In the annealed (T0) condition I have little doubt that it could be formed to your curve.

After the rolling the aluminum would have to be retempered to regain its original properties at a shop. Google aluminum tempering.

Dino
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Old 10-03-2011, 06:15 AM
Poida Poida is offline
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Ad Hoc I don't think you understand the process involved. For example I did not say "you need heat to bend it"
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  #13  
Old 10-03-2011, 07:40 AM
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viking north viking north is offline
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Boys-Boys Boys-- don't get so technical-- this is bending a few pieces of alum. framing for a flat bottom barge not an ice breaker--DDaut go with the saw cut method, reweld the cuts and stitch on your skin, it will be 150% ok. If it makes you feel better weld on a backing flat bar. The strength of your bow section should be looked at as the strength of the overall structure including the sides and the welded in deck section. Even at 50% welded strength if that is the case,( the outside faces of the frames are not welded) it will be more than sufficient. Think of it as an aircrafts wing -- overall structure strength--Take it from this old boatbuilders experience i've formed rectangular tube radii in this method a hundred times ( notice I didn't enhanse by a 1000 factor) --Have fun --Geo.
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Old 10-03-2011, 08:53 AM
cyclops2 cyclops2 is offline
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Ice breaker or swamp air boat design?

Flat bottomed with a curved up bow design

DO as all other real companies do. Use a SOFTER Aluminum.

H beam CAN & WILL bend very easily in a commerical roll bender. No kinks or cuts needed.
Flat surfaces with no kinks will always be available to weld very easily & neatly.

Piece of cake to do.
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Old 10-03-2011, 08:58 AM
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viking north viking north is offline
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Exactly the other alternative-- H--Beam --
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