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.
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.
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
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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)
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";
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Reading this guide will help you to understand the science of tube bending:
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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.
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.
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.
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.
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.
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.
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