Thank you Saravin, Hawkeye.
The rollers are 1 in diameter steel, with a approx 3/16 dia groove. The axles are just 5/16 bolts and the angles 3/4 x 1/8 thick.
The angles are bolted to the vise jaws using the existing holes. The 2 back rollers are spaced about 2 1/2 in.
Drive is just by the single, front roller. The ring diameter is set by the vise opening.
This configuration allows for a minimum of about 10 in diameter ring.
I've used it to form rings of 20 in dia x 3/16 brass rod and 12 in dia x 1/8 brass rod.
Wagner