In the tube bending world, bend radius of a round tube is ALWAYS
described as the centerline of the tube. Bend radius of a square tube is
ALWAYS described as the inside radius of the tube. Go figure. But using
the proper nomenclature makes it easier to get assistance with your
project.

On the hbobbist level, we have tried the sand route with a hydraulic ram
bender with marginal success.
http://www.harborfreight.com/cpi/cta...emnumber=32888
bends over about 40 degrees would kink. And this is for a tube with
heavier wall than yours. I think your idea of gypsum has merit. You want
something that will clump and not flow under pressure. You will need to
pack it very tight for a distance of at least 2 to 4 diameters past the
bend area. You will need to have some sort of drill powered spring
cutter to get the material out of there.

If you decide that you want to build a bender like the one you
mentioned, I think I would try and figure out some sort of mandrel
inside the tube. The point where the bending occurs is well defined,
about 1" from the point of contact with the straight die. Use a plug
mandrel about .005" to .010" (.1mm to .3mm) smaller than the inside of
the tube. It needs to be relieved a bit on the outside of the bend to
conform to the newly formed bend. With this design, the mandrel does not
move so it can be just a solid plug screwed into the end of a solid rod.
You will need to have it easily adjustable for exact placement as well
as well some sort of cam to pull it the first inch or two when you are
done with the bend.

Lukasz Kucharski wrote:
"Wall factor" is the diameter divided by the wall thickness. The higher
the number,the more support you will need to keep the outer wall from
collapsing and the inner wall from crumpling. 10 and less is pretty much
self supporting, you are up at 24 to one, this will require substantial
internal support. You can do it with a mandrel, cerobend (low temp
metal) or sand. Sand is not very useful, it must be packed VERY hard.



Thanks for your reply Roy. Thanks for lots of adequate information. The
website you recommended is very helpful. I decided to build 5 x OD radius
die just to be on the safe side. Unfortunately mandrel and cerobend are
beyond my reach. I wonder if it's possible to fill the tube up with dry
cement or maybe gypsum instead of sand. What do you think about this idea?
Small grain substance like gypsum or cement wouldn't be hard to pack tightly
or would it?.



Regards



Lukasz