Gary Wolf wrote:

Bill,
   Selective annealing of a section will help but it is not how someone like
Burns would do it. This is too slow for a bender that would do ~100 bends
per hour. It has to be done cold with tooling that controls the flow of
metal so that the results will be consistent.
   A roller will work with stiff material like 4130 but on aluminum it will
produce a dent in the outside surface of the tube, where there is point
contact. On soft material the tube needs a follow block that fits against
the outside of the tube and spreads the load.

Bulent,
   They are in fact dragging a series of balls inside the tube. They are
strung together like beads on a lamp chain. The lubricant is supposed to
prevent the drag marks.

   Material like aluminum is difficult to bend because it will not stretch
very far before it begins to thin, neck down, and part. Think of a tube made
of cheddar cheese and you are getting close to how it performs. The trick is
to minimise the stretching on the outside of the bend and to encourage the
inside to gather and compress. The easiest way is to allow it to corrugate.
The harder way is to force it to thicken.
   Chrome moly tube is actually easier to bend cold that is aluminum. It
just takes a lot of horsepower but the tube doesn't neck down and part, and
once it has been formed over the first ball, the work-hardening tends to
help support the outside of the tube against collapsing.

                                                        Gary Wolf

 
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