A true mirror finish on turned aluminum comes down to five variables working together — miss one and you'll get a dull or chattery surface no matter how good the rest of your setup is.
1. Tool sharpness and geometry
A sharp, polished-rake insert or HSS tool with a generous nose radius (0.015"–0.031") is non-negotiable. A dull edge tears the material instead of shearing it.
2. Speed
Run aluminum fast — typically 600–1000+ SFM with carbide. Too slow invites built-up edge (aluminum welding itself to the tool), the #1 cause of a dull, scratchy finish.
3. Feed and depth on the finish pass
Light finishing passes — 0.001" to 0.003" depth of cut — with feed matched to your nose radius. Aim for a feed rate roughly 1/3 to 1/2 of the nose radius; too slow a feed at a small radius can rub rather than shear.
4. Cutting fluid
Aluminum benefits enormously from flood coolant or even a light mist/lubricant on the finish pass — it prevents the built-up edge that ruins finish.
5. Rigidity
Minimize tool overhang and stickout. Chatter marks — even ones too fine to feel — scatter light and kill the mirror effect. If you see any chatter, fix rigidity before touching anything else.