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Tube Bending Techniques

To: shop-talk@autox.team.net
Subject: Tube Bending Techniques
From: "Kai M. Radicke" <kai@radiohead.net>
Date: Thu, 24 Apr 2003 14:57:53 -0400
AFAIK,

"Collapse" Bending, for lack of a better term, involves collapsing the inner
(short) radius of the tube to form the bend.  Typically the method used to
make garden furniture, school desks, and similar products.  This is what the
Harbor Freight hydraulic bender does.  Cheap and easy and you keep the same
wall thickness through the entire bend.

Sand Forming, http://locost7.info/files/tools/Sand+Bending.zip , once you
unzip the file, the images enclosed give much more detail written and visual
than I could use to describe the formation of a sand bent tube.  You end up
stretching the long radius of the bend, so along the long radius the tube
wall thickness is thinner than the straight sections of the tube.  Heat is
required for this process, which you should consider when selecting what
type of tubing you're going to use and what its characteristics are.

Hydroforming, the modern method that has replaced sand forming, this is what
Harley Davidson uses to make their V-Rod motorcycle frames.  You fill the
tube with water and a computer controlled machine whips the straight tube
into a complex array of of bends.  You still end up stretching the long
radius of the bend.  Equipment is very expensive for everything but mass
production of the same tube over and over.

Mandrel bending, is what most shops uses to bend tubes as it is fairly
economical and can be hand operated (smaller machines) or computer
controlled for mass production.  Cheaper than hydroforming so there are
probably a handful of shops in your area that can mandrel bend tubing for
you.  Process involves inserting a mandrel / die into the tube, which sort
of looks like a bunch golf balls or solid milk caps on a steel cable.  The
tube is then bent to the desired shape, and the die keeps the tube from
collapsing along the bend.  Size of the die segments is dependant on the ID
of tube, and the spacing in-between each segment of the die is dependant on
how tight you want your bends to be.  You still stretch the long radius of
the bend, so that the wall thickness gets thinner along this section.

For all but collapse bending, the tighter you make the bends, the thinner
the wall thickness of the long radius becomes.

If anyone has anything to add, please do, or corrections even...

Kai

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