Enough tangent: this is a fly cutter when not in motion:
The cutting tool spins around in a circle, taking a thin slice at a time. Why use this instead of an end mill? Because the diameter of the cut in this case is about three inches which this mill could not turn well. It is also claimed fly cutters produce a nicer finish. I happen to like the look of a milled surface; it looks mechanical and sophisticated.
I was trying to verify that my program for slicing away material with an end mill worked with a fly cutter as well:
And it does, because in a sense a fly cutter is just a 3" diameter end mill that only cuts at the edges. Downside is that you have to take a pretty thin slice to avoid problems, like .05" as I was doing here. But it takes fewer passes than an end mill. Does this come out ahead on time? Not sure. An experiment for another day.
I got started on this because I tried to run a 4" long piece of acetal through the planer. Instead of parallel sides, I ended up with gross valleys and peaks; don't put short pieces into a planer. Rather the scrap this victim, I ran the mill on it to make all surfaces parallel.