Lefever called their ribs Engine Turned. Bill has acurately described a method by which they could be made, I don't truly know for certain just how Lefever did them. This would of course be on a flat one. Some of their ribs were concave & also had the engine turning. In this case the cutter would have to be tipped at an angle & run down the rib thus cutting on one side only. The mill would then have to be tipped the other way & the opposite arcs cut.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra