Surface Roughness Standards
Understanding roughness parameters is critical for specifying functional surfaces. This guide explains Ra, Rz, Rmax and how to specify them correctly.
Key Parameters
| Parameter | Description | Typical Range |
|---|---|---|
| Ra | Arithmetic average roughness. Most common. | 0.025-50µm |
| Rz | Average of 5 highest peaks minus 5 lowest valleys | 0.1-200µm |
| Rmax/Rt | Maximum peak-to-valley height | 0.2-500µm |
| Rq (RMS) | Root-mean-square, roughly 1.1x Ra | 0.03-55µm |
Ra vs Rz
Ra averages out peaks and valleys. Rz is more sensitive to extreme features. Use Rz for: sealing surfaces (deepest scratch determines leakage), bearing surfaces (peaks wear first), and safety-critical applications.
Roughness by Process
| Process | Ra (µm) | Rz (µm) |
|---|---|---|
| EDM rough | 3.0-12.0 | 20-80 |
| Grinding | 0.2-3.2 | 1.5-20 |
| Turning | 0.8-6.3 | 5-40 |
| Lapping | 0.05-0.4 | 0.4-3 |
| Polishing | 0.025-0.2 | 0.2-1.5 |
Specification Best Practices
- Don't over-specify: Ra 0.4 vs 1.6 can double cost. Specify max, not min.
- By function: Ra 3.2 for clearance, 1.6 for press fits, 0.8 for sliding, 0.4 for seals
- Indicate surface lay direction with ISO 1302 symbols
- Specify measurement method (contact ISO 4287 vs non-contact ISO 25178)