Possibly your mill could have been chucked out to far and it caused a "micro wobble", or allow it to bend just a titch might have broke it,, not enough cutting oil, a constant bath works best IMO. I've found those small end mills get real hot real quick so I always did a test piece of the same material the end product was made out of to adjust the speed for optimal cutting with out any extra stress to the bit/ end mill. Keeping it cool and clean and chucked close is probably the most important things.
Those might be a couple causes I could think of off the top of my head. I doubt you scrimp on bits, ,,it will cost ya more in the long run by fucking stuff up if you do.
I damn sure miss not having access to all the good stuff we had at work now!! The stuff was old,, but all high end commercial quality stuff from the 40's and some older. But over all they worked pretty good. The lath was starting to get a bad main bearing behind the chuck and it would allow the chuck center to vary about .002-.003,, it was pretty hard to get to precise with that going on!!! But every thing else was OK nice press's, mills, every kinda bit and mill known to man, and a nice stock of all kinds of steel and stainless,, even some exotic stuff. The owners father had one of the main salvage yards in the state, and stuff from GE, IBM, and places like that would end up there. So had a pick of the cream of the crop of materials too,,,, boy do I miss that!!!!