Assuming you don't have a massive rotary table the method used for expansion links is to mount the work on a long piece of flat bar. Make a pivot at one end and fix to the mill table, the other end of the bar has a pin that can slide in the tee slot of a rotary table. By turning the rotary table the end of the bar will move in an arc, set your work 18" from the pivot plus 1/2 x tool dia and it will but your radius.
If that does not make sense then pm me your e-mail, I think I can find a sketch or photo.
Jason