Solution

It was decided to use a 4” air chuck mounted on a 4th axis (rotary table). The CNC operator machined the soft jaws to fit into the back of the workpiece, ID clamping in the 5mm hole (see photo). When the chuck is in an open position, it clamps the workpiece allowing tools access from both surfaces.

Results

The milling operation is done by one clamping, savings the extra handling (set-up) of the workpiece loading and unloading for the second operation. No need to orient the workpiece for the second operation. Needless to say that one operation is always better and more accurate than two.