Need Help! ARC ON WITH END MILL

I'M GOING TO INTERPOLATE A DRILLED HOLE WITH A END MILL. WHEN I FEED OVER IN X IT CHATTERS REAL BAD, I WAS TOLD I NEED TO ARC IN TO THE CUT.
DO I PUT A "R" ON LINE WITH "X" FEED OVER? IS THERE A RULE OF THUMB ON THE SIZE END MILL TO THE AMOUNT OF ARC IN? NEED HELP THANKS