On Tue, 22 Mar 2005 21:45:00 GMT, Christopher Graham wrote:

Thanks for the reference. This: http://www.medfordtools.com/mb3.html
bender looks like it would work.

So, I assume I also need a bunch of dies for the sizes and angles I want to
bend.

Just the sizes of different tubing. The inner mandrel doesn't move;
it stay stationary compared to the inner side of the curve while
bending. The angle is dictated by where you stop pushing the arm,
that's what the stop pins are for. The outer mandrel applies force
to the tubing because there's a roller on the arm pushing it
against the inner mandrel (with the tubing between). I'm not going to
attempt an ASCII-art drawing but I hope you know what I'm saying.

As the arm goes around, the roller on the arm rolls against the back of
the outer form (straight form), pushing it into the inner form
(round form), making the tubing match that curve.

I have these questions:

- How close does the tubing diameter have to be to what the die was designed
for?

Dead nuts on. New dies for each diameter. 3/8" will not work for
1/2", and so on. Hopefully you're not running a taper through your
u-bends, if you are you may want to reconsider that if that's an
accoustically feasable change. I'd be surprised if it wasn't.

- Does the die radius exactly determine the bend radius?

Different materials will spring back a different amount. Not sure how
bad brass will be. Technique matters as well - a smooth pull of the
handle is absolutely necessary.

- Can the angle of bend be anything up to the angle the die was designed
for?

Yes, but beyond 180 degrees it gets messy. You can notch the
inner mandrel so you can rotate the finished piece out, but I can't
think of how that would help your particular need.

I have a 4 axis cnc milling machine, so what I may do is order one die set
and use this as a guide to making others.

Not a bad idea. It wouldn't be too tough to build one of these, really.
I've needed one often enough that it'd be a fun project that would
see enough use to justify. Maybe _next_ winter.

Dave Hinz