Form tool, like garyhlucas described. A special tool, that is shaped to make a specific feature. Also,, yes, easier to turn it, then mill to finish. However,,, in aerospace, we mill perfectly round enough features like that,, with a 16 finish all day long,, on a regular old 3 axis industrial type machine. Im sure a bench top, or hobby machine can too, but accuracy and finish will not be aerospace standard!