3 axis is enough... to make a 3d surface as a fin
draw the fin in a 3d cad program, then you transfer it to CAM, which then in turn will give you XYZ the cordinates for the machine to follow,
A good ingineer would not need much time for this, with a drawing as the profiles are not that many shaped...
the rough milling would take a matter of minuttes where the finishing would take somewhat longer depending on the machine. If the machine is fast enough, you should be able to go f10000-f30000,again depending on the machine.
I plastics there is almost no upper limit to how fast you can go if the machine is good and acurate enough.
Where I used to work we would make the positive molds for windmills wings. only the outer 1.5meter of the wing.. Those programs would run 500.000 lines all with xyz cordinates. and that could be run in about 8hours.. So I would think for a fin you can do it, depending on the surface in less than 10minutes for on side..
Dennis