You could run 1/2" diameter up to 10,000+ rpm in your lathe if you could . What you are lacking on your manual is CSS ( constant surface speed) . When you cut at the same feed at a smaller diameter you are essentialy pushing the aluminum rather than shearing it . When programming in on a CNC lathe in CSS on a 6" diameter for instance at 1200 css , the rpms may be 600 but as the diameter gets closer to X zero , you will reach your machines max rpm or whatever speed limit you programmed . Use a lubricant , WD 40 or equivalent so the tool doesn't load . Increase speeds as part gets smaller . Grind a chipbreaker in your tool if possible.