Ra, Rz, and Rq are the three surface roughness parameters you will encounter most often.
Surface roughness is measured using a contact profilometer.
Sampling Length and Evaluation Length
The sampling length (lr) is the reference length over which a single roughness measurement is taken.
| Ra range | Sampling length (lr) | Evaluation length (ln) |
|---|---|---|
| Ra ≤ 0.02 µm | 0.08 mm | 0.4 mm |
What the Parameters Measure
RaRoughness Average
The arithmetic mean of all profile deviations from the centreline.
Rz to Ra Relationship
For most machined surfaces, Rz is approximately 4–7 times Ra.
| Process | Rz / Ra ratio | Example |
|---|---|---|
| CNC turning | 4 – 5× | Ra 0.8 µm ≈ Rz 3.2 – 4.0 µm |
Expected Ra by Machining Operation
| Process | Ra range | Notes |
|---|---|---|
| Rough milling | 3.2 – 6.3 µm | Heavy stock removal |
Ra to N-Grade Equivalents
N-grades (ISO 1302) are an older notation still found on legacy drawings.
| N-grade | Ra equivalent |
|---|---|
| N12 | 50 µm |
Inspection Notes
- Always measure roughness perpendicular to the lay.