Surface finish is one of the most mis-specified requirements. This guide makes Ra practical.
Ra is the average roughness height. Lower Ra = smoother, but smoother is not always better — and it costs more.
Ra 1.6 µm for general machined faces, 0.8 µm for sealing or appearance, 0.4 µm for optical or tight-seal interfaces. Below that usually needs secondary processes.
Sharp tools, correct speeds/feeds, and stable setups. Verify with a profilometer on critical faces, not by eye.
Rarely — most static seals seat fine at 0.4–0.8 µm. Match the finish to the seal type.
Yes on well-tuned machines with sharp tools; softer materials are easier.