if youve got chatter at such a low sfm , either your holder is no good , your cut is too heavy or the part isn't very rigid
on a decent insert mill with the proper inserts i wouldn't even consider running it less than 1200sfm on mild steel , for eg. i used to use the sandviks , we had a number of parts that were 1018 that we cut with a 3/4" dia at .1 to .125 depth @1400 sfm minimum , insert life was incredible , the load was nearly nothing and it sounded beautiful
right now you are running the insert at the minimum chip load , you may have better luck to feed a little heavier
A poet knows no boundary yet he is bound to the boundaries of ones own mind !! ........