Umm....if your starting cap failed while you were milling anything, you either have a problem with the centrifugal switch in the motor sticking, or are loading something so heavily that the motor slows down below the point where that switch would normally kick the capacitor out.
the funny thing is that both time the cap went bad i was milling delrin.
In a single phase motor, a second start winding with a capacitor to shift the phase slightly is used to start the motor. In order to keep this winding (and capacitor) from remaining engaged after the motor comes up to speed, a centrifugal switch is typically used. Flyweights rotating at speed separate a pair of contacts. If the motor slows enough, theoretically these contacts could close again, but not that likely. ON the other hand, the adjustment of the weight assembly and the contact assembly can go out of whack or for that matter the contacts could have become welded.
This could explain why you have blown two of them, too (if the root problem with the cap. staying in circuit has not been fixed.)
Another potential issue is that something is bound up, causing excessive motor load and the motor is trying to start heavily loaded. In a mill, this should not otherwise be the case as you don't start it up with the cutter in the works. Other possible causes could be siezed bearings or as someone else mentioned, cold, overly viscous lubricant.
But yes...as has already been stated, cheap capacitors are famous for puking their guts.