Surface finish and what it costs

The standard machined finish of about Ra 3.2 µm is fine for most faces. Finer finishes need extra passes or a second process, so specify them only on sealing, sliding and bearing faces.

Sensible defaults

• Ra 3.2: general machined faces.
• Ra 1.6: mating faces, sliding faces, reamed holes.
• Ra 0.8 or finer: bearing seats, seal faces; usually needs grinding or fine turning.

See the surface roughness chart for all grades.

Coatings change dimensions

Type II anodizing adds about 5–25 µm and roughly half of that growth is outward. Hard anodizing adds 25–75 µm. For tight fits, state whether the dimension applies before or after coating, or mask the feature.

Bead blasting

Bead blasting gives an even matte look and hides tool marks, and it is cheap. It slightly roughens the surface, so mask precision faces.