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Original length of Bent Tube

Original length of Bent Tube

Original length of Bent Tube

We received a fab print (.pdf) of a finished part with minimal dimensions and information. It is a 2" square tube bent part at 90 degrees. I calculated out the finished part length based on thier dimensions, but was wondering how to figure backwards to pre-bent length. We typically dont do bending but want to supply the customer with the exact length needed to bend out thier finished part. Would it be equal to the nuetral axis of the finished part?

Thanks in advance!

RE: Original length of Bent Tube

Take a piece of tube, measure, bend, measure again.

Allowances for bending vary with material and process.  There is only one way to know for certain.

RE: Original length of Bent Tube

hi rfuse2

I would calculate the developed length as your suggesting, then as TheTick said bend re-adjust and bend again.


RE: Original length of Bent Tube

pesonally, i think you're trying too hard ... to give the customer exactly the right length of tube ... they're going to be bending it, do you know exactly how ? (since i think that'll be a factor; tho' probably more a factor of how good a job they do).  also if you give them exactly the right length, they're going to have position the bend at exactly the right place.  it'd seem much easier for all concerned to give them a bit to work with, so that they trim to final dimensions after bending.

RE: Original length of Bent Tube

You don't need an exact length to determine bend allownace.  Just long enough that the ends are no longer affected by the bend.  From there, you can determine bend allowance for use with any length.

Customer should order either a bent tube or a piece of specified length.  They are being sloppy.  (good luck telling them, though.)

RE: Original length of Bent Tube

Thanks for the replies! We are an aluminum extrusion company that does not have the capability of bending 2" square tubing at this point. Maybe exact wasn't the right word. We are just in the quoting stages and need to know a length to cut off the end of the press. Either way, they will be charged for every pound of aluminum pushed through thier die, we are just trying to help them out by optimizing as close as possible because they don't know what they need yet.


RE: Original length of Bent Tube

If your not after an exact lenght, then just measure along the center of the square tubing. One side will shrink, one will lenghten and the centre will be (roughly) the same.

RE: Original length of Bent Tube


Please don't scrimp by sending them the exact length: They won't be able to use any of the tubing delivered.

Consider the physics (or mechanics of the machine doing the bending the square tubing.  (Pipe is similar, but round.  8<)   

One:  First, an inlet straight part of the tube is clamped in the machine by either a vise, or forced against an anvil sized to the outside of the tube steel.  Length of the inlet straight is determined by the shop's machine.  For my wrought iron bending, I only need 3-6 inches.  Commercial shops need 4-10 inches.

Two: The inside radius of the bend is determined by a mandrel at the centerline of the bend: OD/2 of the mandrel = IR of the bend.   A second straight part of the tube must continue past the end tangent of the bend.    

Three: The machine forces this excessive outlet (straight) tube around the mandrel by either rollers on the outside of the tube or a sliding second mandrel, levering the tube around the mandrel.  

As pointed out above, the material in the tube itself yields both on the outside and inside of the tube, but NOT equally.   Neural axis is about 5/16 (1/3) across the tube: you can see paint or mill scale flexing off of the inside surface as it compresses, and can see the outside wall of the tube fail and "bend" inward if the IR is too small.  Cheap fabricators don't care about the outside failure, and sometimes do it deliberately to try to get as tight a radius as possible.    

So even if you could exactly calculate the theoretical length, you'd need to trim both the inlet and outlet ends of the unbent tubing at an angle to make up for the different stretching distances across from inside to outside.   If that were done, then they could not bend the tubing because the shop could not use either the inlet or outlet clamps on the tube bender to work as a lever to oppose the bending force.  (Hot work by squeezing soft red-hot metal into a die is different, but then they still need a way to clamp the metal being forced into the die.)  

In practice, sometimes the shop makes the bend on the first piece with a little bit of wastage, but then trims the bend so the excess at the inlet of part one become the outlet lever of part two, etc....

Call your customer and see how they can supply you with their scrap for recycling, and what lengths are most efficient to go through the production line.  Perhaps you can save them money with longer (or shorter!) raw material lengths.  

RE: Original length of Bent Tube

In addition to what has been suggested. :>/

in the old days before cnc
mockup would be for tubes to be cut with stock.
length recorded
then measured
then trimed.
if the the tubes are fitted to lets say a flange
then excess is required to trim to for fit up in a trim & weld assy fixt.
and if it's welded allow for weld shrink.

RE: Original length of Bent Tube

Sorry, I forgot to address the bend angle.

Remember, we are deep inside the plastic deformation of the tubing now.  So you have to squeeze the tube walls past their 3% "flexible" point and into the metal's yield zone.  (Al or Cu are a bit different than iron or steel, I don't their curves since I only have to roll steel tubing and steel solids.  Same principle though: force the metal past its flex point into yield, then go to the bend angle you want, then go past that angle by a little bit so when the leverage force gets relaxed, the metal "goes back" to the final angle you want.

Then check it against a template, and cut both ends to fit if it meets your spec for deformed walls, angle, etc.  

A long piece going into a machine can be rapidly be processed into a lot of short bent pieces - but ONLY after the operator has tested the first bunch and set his machine's mechanical "stops" at the correct position.  

For any job requiring more than 2-3 pieces, you should demand your shop (though I know you are the material supplier and not the actual fabricator of these parts) use the stop blocks on their roller and check pieces regularly by full-sized templates to maintain accuracy.  You can't do it by "eye."  

RE: Original length of Bent Tube

Good points have been raised. Wastage of last piece on a length of tube, allowance for cutting blade thickness are to be considered .

 Also please check the unit of length for the stock pipe and bent tube. If the units vary,then again there can be wastage. I overlooked this recently for an order,where the customer's drawing was in inches length,while I sourced the pipe in  metric units. It was a huge loss.

Learn the rules,so you know how to break them properly.
Dalai Lama


RE: Original length of Bent Tube

Wow ... great info here. Thanks for all the replies!


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