Engineering quick reference
Surface Finish (Ra) Guide
Ra by machining process, Ra↔Rz↔RMS conversions, and µin↔µm — what finish each process realistically holds.
| Roughness — Ra | Drawing callout | What holds it · how it reads | |||
|---|---|---|---|---|---|
| Ra µin | Ra µm | ISO N | Symbol | Typical processes (what holds it) | Feel / appearance |
| 1 | 0.025 | N1 | ▽▽▽▽ | Polished, Super-finished | Optical quality |
| 4 | 0.1 | N3 | ▽▽▽▽ | Lapped, Honed (precision) | Cosmetic mirror |
| 8 | 0.2 | N4 | ▽▽▽ | Ground, Honed, Fine bore | Near-mirror, no tool marks |
| 16 | 0.4 | N5 | ▽▽▽ | Surface ground, Honed, Fine turn (Swiss) | Very smooth |
| 32 | 0.8 | N6 | ▽▽▽ | Finish mill / turn, Reamed, Surface ground (rough) | Smooth — most CNC default |
| 63 | 1.6 | N7 | ▽▽ | Fine mill, Fine turn, Drilled | Smooth, slight tool marks |
| 125 | 3.2 | N8 | ▽▽ | Standard mill, Standard turn, Hot finish | Visibly tooled |
| 250 | 6.3 | N9 | ▽ | Sand blast, Rough mill, Bandsaw | Grainy, matte |
| 500 | 12.5 | N10 | — | Rough cast, Hot rolled | Coarse to touch |
| 1000 | 25 | N11 | — | As-cast, Coarse sand blast | Visibly textured |
Key: N… = ISO 1302 grade · ▽ = legacy drawing symbol (more ▽ = finer) · CLA / AA ≡ Ra ·tinted row = common CNC as-machined default (Ra 32)
| Ra ↔ Rz ↔ RMS (µin) | |||
|---|---|---|---|
| Ra | Rz ≈×4 | RMS ≈×1.11 | ISO N |
| 1 | 4 | 1.1 | N1 |
| 4 | 16 | 4.4 | N3 |
| 8 | 32 | 8.9 | N4 |
| 16 | 64 | 17.8 | N5 |
| 32 | 128 | 35.5 | N6 |
| 63 | 252 | 69.9 | N7 |
| 125 | 500 | 138.8 | N8 |
| 250 | 1000 | 277.5 | N9 |
| 500 | 2000 | 555 | N10 |
| 1000 | 4000 | 1110 | N11 |
Quick conversions
- 1 µm = 39.37 µin
- 1 µin = 0.0254 µm
- Rz ≈ 4 × Ra
- RMS (Rq) ≈ 1.11 × Ra
- CLA / AA ≡ Ra
- Ra 32 µin = Ra 0.8 µm (N6)
Ra (arithmetic average, AA / CLA per ASME B46.1) is the most common callout. Rz ≈ 4× Ra and RMS (Rq) ≈1.11× Ra are nominal multipliers that vary by process — a legacy “63 RMS” print is ≈ Ra 57 µin, not 63. Process ranges show the finish each operation realistically holds: most CNC mills and lathes default near Ra 32 µin (0.8 µm); finer rungs add grinding, honing, lapping, or polishing as separate operations. For the interactive converter and cost-step detail, see the fullsurface finish reference.
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