[Radio Wrangler]

First you make a different sort of pipe-bending machine:

You want three identical wheels. Each about three or four times the diameter of your tubing. Make them out of almost anything. Steel or aluminium are fine, even a hardish nylon like 66 will do.

You want them wide enough to machine into them a concave profile that takes about 1/3 of the diameter of the tubing. Bore out each wheel from opposite sides to take a ball race, so you'll need six ball races.

Make axles to take the ball races. Make a pair of parallel frames to take all three axles with the races/wheels assemblies between them. Two wheels will run on the bottom side of your tubing, the third wheel is above the tubing and its axle goes through slots. A moving frame holds the ends of this axle and is pulled down by a screw adjuster.

You can set the moving wheel so the straight tube just passes through , touching all three wheels. Move the middle wheel down a little and push-pull your tubing to and fro. This will curve your tubing slightly. Move the wheel a little more and keep pushing-pulling until your tube takes on the curve you need. Concentrate and tighten up on areas with not enough curve. Take care to form the curve in one plane.

With a bit of care you can make tubing into a full, smooth circle.

Want to do a different size tube? Make three new wheels to suit the new diameter.

It's actually easier than it sounds you are not forcibly bending in one go, you are increasing bend very slightly in stages. It's a bit like snake charming. the eventual carve is sneaked up on.

David