I'm developing a product (electronic musical instrument) to make in my home
shop where I will need to repeatably bend brass, aluminum or stainless steel
tubing with diameters ranging from say 1/4" up to 1". Typical tubing pieces
will be up to say 24" long and may have three bends of up to 120 degrees and
maybe 3" radius (tighter bends for smaller tubing.) I may have to make
batches of up to 50 pieces at a time, and don't wait it to take forever. It
would also be useful, but not essential to be able to do spiral bends.

The commercially available machines/tools I've Googled are for larger
tubing, or smaller tubing. I'm prepared to invest (no more than necessary)
in buying a good machine if necessary, but have limited space.

Any suggestions?

Thanks in advance.

- Chris Graham

On Tue, 22 Mar 2005 15:46:32 GMT, Christopher Graham wrote:

The commercially available machines/tools I've Googled are for larger
tubing, or smaller tubing. I'm prepared to invest (no more than necessary)
in buying a good machine if necessary, but have limited space.

When I worked in an aircraft instrumentation shop, we had a mandrel
bender for tubing, which did 1/2" up to 1" or so. Round base maybe
18" in diameter, vertical shaft on bearings in the center (coming up),
with an arm attached to that shaft. Mandrels went on the shaft, and
between that and the arm. Stop pins for starting and stopping position
were inserted into holes in the table.

It was painted green, but I can't google up a picture of one. Does
anyone know the machine I mean? The one that I used has gone to
places unknown, so I can't call anyone up and ask for details.
Worked great for making repeatable batches of precisely bent tubing,
which sounds just like what Christopher wants to do here.

Dave Hinz

On 22 Mar 2005 16:27:18 GMT, Dave Hinz wrote:
On Tue, 22 Mar 2005 15:46:32 GMT, Christopher Graham wrote:

The commercially available machines/tools I've Googled are for larger
tubing, or smaller tubing. I'm prepared to invest (no more than necessary)
in buying a good machine if necessary, but have limited space.

When I worked in an aircraft instrumentation shop, we had a mandrel
bender for tubing, which did 1/2" up to 1" or so. Round base maybe
18" in diameter, vertical shaft on bearings in the center (coming up),
with an arm attached to that shaft. Mandrels went on the shaft, and
between that and the arm. Stop pins for starting and stopping position
were inserted into holes in the table.

Following up to myself...
http://www.medfordtools.com/benders/mb3.html is a different machine
but along the lines of what I'm talking about.

There are lots of ways of repeatably bending tubing using shaped rollers
and stop blocks. The big problem is keeping the inner dimensions from
crushing. Look over in the Musicians and Instrument Makers Forum
(www.mimf.com) archives for several discussions. Brass wind makers deal
with this all the time. They basically reduce to:
1) fill the tube to keep it rigid (using Woods metal, shot, sand, high
pressure water - your call).
2) build specialized tooling for each diameter tube (no, you aren't
going to find a generalized tubing bender for under $10,000 or so and
most of them are designed for diameters like boat railing). These are
usually some variation of a large diameter circle with a smaller
diameter roller on a handle attached to the middle of the large circle
that allows you to bend the tube to match the larger circle. Often look
pretty crude, but they work well and are cheap to make since they're
mostly plywood.
3) experiment.

You're going to be building a lot of benders and learn a lot about
making rollers with concave faces (use a wood router and a half round
bit), adjustable stops, clamping, work hardening and metal fatigue. But
on the plus side, once you have one for each diameter, you can bend any
shape you want (spirals, pentagons, whatever)

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.

I have these questions:

- How close does the tubing diameter have to be to what the die was designed
for?
- Does the die radius exactly determine the bend radius?
- Can the angle of bend be anything up to the angle the die was designed
for?

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.

- Chris

"Christopher Graham" wrote in message
news:wl00e.759956$8l.580013@pd7tw1no...

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.

I have these questions:

- How close does the tubing diameter have to be to what the die was
designed for?
- Does the die radius exactly determine the bend radius?
- Can the angle of bend be anything up to the angle the die was designed
for?

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.

- Chris



Chris

I will try to answer some of your questions. I'm not an expert, but have
learned enough to know that "simply bending a tube" often ends up not being
that simple. I bend tube for my copper creations and have run into alot of
information trying to get my bends right and using the right equipment.

Each die is made for a specific OD of tubing. So you'll need one set of dies
for each OD of tubing, also for each radius of bend.

There is springback to account for, how much depends on the type of metal
and it's hardness. For example soft copper tubing will have less springback
than hard drawn copper tubing. Yes the angle of bend can be anything up to
the andle the die was designed for.

As far as making your own dies, yes I've done it. However, I know that some
dies for mandrel bending and other specialty bending are not the usuall have
round shape. Don't know if this fact would apply to you or not.

Who ever you buy your machine and initial die from should be able to tell
you what you need to know. Most manufacturers are pretty good at making the
customer happy and selling you a machiine that does what you want it to do.
Just make sure you give them all the information so that you purchase the
right product. Why buy something that doesn't do what you want it so.

Tube bending can be a complicated science. There are things to calculate
like "D of bend" "Wall factor" "bend difficulty factor";
http://www.bendtooling.com/bending_formulas.htm

Reading this guide will help you to understand the science of tube bending:
http://www.hinesbending.com/BASICTUBEBENDINGGUIDE.pdf Granted not all of it
may apply to you, but it will help you to understand exactly what is going
on when you are bending.

Hope this helps.
Lane

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