Bending Square Tubing

I'm looking to make a square tubing bender, I need to make a 6' dia ring, with 1/2"x1" tubing is there a book on how to build one if not can you give some info on one. Thank You.

The Tinman Respondeth:

A 3 roll slip roll is designed to roll up wire and sheet forms. BUT.... Many craftsmen make up 3 roll machines to roll up hat sections, square tube, and round tube. The rolls should be such that one is flat across. Another is grooved to accept the tube fully into the groove. And the 3rd is grooved also. This way the two carry the tube whilst the middle thrusts downward between them to make the bend.