I am reaming 6061-T6 tube about 2.25" long and using a 0.376" straight flute chucking reamer. On the down feed it has a nice cutting sound but when pulling it back up it chatters quite a bit. Are there any must-do's when reaming 6061?

First thing to do is stop the spindle, then retract the quill. No more chatter.

Tom

Are you using a lathe or mill , if a lathe I just hold the shank with a pair of pliers while withdrawing , with a mill a use 2 pieces of wood usually 1/2"x1/2" 6" or 8"s long wrap a rubber band around 1 end put it over the shank and squeeze the ends together you need to stop the vibes from occuring JMHO

Stop the spindle after reaming, withdraw ream.