There are no shortcuts on this one, you need to use the proper size dies with the tubing you are trying to bend. You wouldn't think an 1/8 inch would make any difference but it does, the tubing won't bend smoothly, as you found out. If you are building a full cage you probably are thinking of racing it, so as Astroracer mentioned most sanctioning bodies have standards that you have got to adhere to.

Don