112275,
One method that we use a lot in carbon steel is to brake the flange on a press brake and then run the sheet through a pyramid(slip) roll with grooves cut in the rolls for the flange.
At a 48" radius, this should not be difficult.
Another option might be the use of a flanging tool on a Pullmax machine. Sounds like it may be similar to what you are doing now. Basically, the Pullmax is a circle shear using a nibbling action but it can be adapted to straight, curving or combination nibbling, flanging or ribbing.
A third option is a machine built by Blue Valley for flanging tube. We use one of these on a production part. It is more of a spin forming process and more pricy than either of the other methods but if you have a number to do, it might work.
You never said whether the flange is inside the curve or outside. It do make a difference!
Good luck.
Griffy