Surface Roughness Ra Explained: Machining Finish Chart

What Is Ra

Ra (arithmetic average roughness) is the most common surface finish parameter, in μm - smaller is smoother.

Common Values

• Ra50-12.5: Rough turning, sawing
• Ra6.3: Rough turning/milling
• Ra3.2: Finish turning/milling (general fits)
• Ra1.6: Finish turning/grinding (critical fits)
• Ra0.8-0.4: Grinding/lapping (precision fits)
• Ra0.2 below: Lapping/polishing (seals, races)

Why It Matters

• Too rough: fast wear, loose fit, stress risers
• Too smooth: costly, poor oil retention
• Match grade to function

Inspection

Profilometer or comparator samples. Huami controls finish to Ra1.6-3.2 with full inspection.