I've been browsing around the web for a while now trying to get some kind of solid answer about SFM but I can't seem to find it. From what I've read so far:

Lets say I'm machining Aluminum that has a diameter of .500. I've read so many different things but one of the charts I've seen for SFM says it should be 150-400. So lets say...400 that would mean my max rpm would be 3200? But what if I'm cutting something that .202 in dia. it says my max rpm shoud be ....7920?

I just need someone to explain to me how deciding feed rates and sfm actually works...I've really been learning on the fly and have been getting by making parts that are ok...but I know they can be better.

Thanks for any help.