I'm beginning to get the hang of cnc machining in ally - haven't welded an end mill to the workpiece for days! - but I'm still getting some chatter.
I'm cutting 1mm into duraluminium with a 2mm carbide high helix single flute bit. Spindle is running at 18,000 and feed rate is .5 m/min. I'm taking .2mm depth cuts each pass (ok, I'm a coward after breaking too many expensive bits) and using air and a coolant recommended by the tool suppliers. I'm rubbish at doing the maths to determine the cutting parameters, and everyone seems to have a different view. Machine is substantial with a 4' x 8' bed weighing close on a ton.
Somewhere there is a bow in your material. How big of piece of material are you cutting? You need to find a way to clamp your material down better.