There's machines that lower the feed as needed to hold a location within a tolerance. But....that sux. The reason is the rpm stays the same.
I think the reason the rpm doesn't change is mechanics, getting a spindle to ramp up or down takes a lot of energy. But has anybody heard of one that changes the rpm when it slows the feed proportionitely?
Sounds like it doesn't matter, but these things don't just race to a corner and slow down, they variably slow down on the way to the corner. And in some reall jagged toolpath they might be feeding half the feedrate of the open areas.
If you were milling 1000 parts out of graphite, and your rpm wasn't relative to the feedrate, as in now it's feeding half as fast but spinning full speed you will get cutter wear. Anyone who doesnt think so can cut a few pockets in graphite using a HSS cobb mill. If you get the rpm and feedrate at the right point that cutter will last years. You spin too fast and feed too slow that cutter won't make it thru more than 1 or two pockets before it wears the edges off.
Doesn't sound like a big deal, but in production milling...could mean a ton of money in cutters.