As stated by Vmax & Titchener above, I agree that you are probably over stretching the Feed rate due to low RPM. Also, with such small sizes I wonder why 1/2" EM ? & O flute ? There has been no mention as to the work holding or the remains of the material during the So-Called Chatter. I would suspect that what you are calling Chatter is not really what or where your describing. Deflection might be in play as well. Or even dull cutter induced surface character. I didn't see what material your cutting, could be surface work hardening, or grain tearing, or chip re cutting ! It could be any number of things involved here and without a visual or better description, then you could be easily leading all who try to help, astray ! And you end up without a viable solution.