Surface Roughness Chart (Ra, Rz, RMS)
Surface roughness indicates the smoothness of a machined part. Specifying a finer finish than necessary exponentially increases manufacturing costs. Use this chart to correlate Ra/Rz values with typical manufacturing processes.
| Ra (µm) | Ra (µin) | RMS (µin) | Rz (µm) Approx. | Typical Machining Process |
|---|---|---|---|---|
| 50.0 | 2000 | 2200 | 200.0 | Flame Cutting, Rough Sand-Casting |
| 25.0 | 1000 | 1100 | 100.0 | Sawing, Rough Turning, Forging |
| 12.5 | 500 | 550 | 50.0 | Drilling, Milling, Shaping |
| 6.3 | 250 | 275 | 25.0 | Turning, Boring, Extruding |
| 3.2 | 125 | 137 | 12.5 | Reaming, Broaching, Cold Rolling |
| 1.6 | 63 | 69 | 6.3 | Grinding, Fine Turning |
| 0.8 | 32 | 35 | 3.2 | Honing, Lapping, Polishing |
| 0.4 | 16 | 18 | 1.6 | Fine Honing, Burnishing |
| 0.2 | 8 | 9 | 0.8 | Superfinishing |
| 0.1 | 4 | 4.4 | 0.4 | Mirror Polish, Buffing |
Terminology
- Ra (Roughness Average): The arithmetic average of absolute values of profile height deviations from the mean line. Most common in North America.
- Rz (Mean Roughness Depth): The average of the 5 highest peaks and 5 lowest valleys over the sampling length. More sensitive to occasional deep scratches than Ra. Commonly used in Europe.
- RMS (Root Mean Square): Similar to Ra but uses the root mean square calculation. Largely obsolete but still found on older blueprints. RMS ≈ 1.1 × Ra.