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)