Milling: creating a smooth end finish in a slot

[Rob Meades]

Hi again. I've found myself a 2 mm, 2 flute end-mill with radiused corners (0.2 mm radius) and used the FSWizard web calculator, from which I get a 5 mm/second feed rate and 23,000 RPM for cutting in brass. I've just tried using 20,000 RPM and a 4 mm/second feed rate, running 0.3 mm passes to achieve a 3.8 mm pocket in brass. 10 hours in (it's quite a long pocket), towards the end of the last pass (13 passes), the bit started making screeching noises and eventually snapped. You can see from the picture below that it was fairly badly damaged.

Two questions:

1. Am I expecting too much for a bit to work that hard for that long?
2. Should I try, say, a 3 mm/second feed rate and 26,000 RPM (the next step up on my motor)?
3. ...or something else?