État de surface expliqué : Ra, Rz et leur signification sur une pièce usinée
Comment se mesure la rugosité, quelles valeurs Ra donne chaque procédé, quand la texture agit sur la fonction et pas seulement l'aspect, et comment spécifier un état de surface reproductible.
Rédigé par Engineering Team
La documentation technique est tenue en anglais afin de préserver la précision des termes d'ingénierie.
Surface finish is the specification most often copied from a previous drawing without anyone asking what it is for. That matters, because on some surfaces the texture is purely cosmetic and on others it determines whether a seal holds or a bearing survives.
What Ra actually measures
Ra is the arithmetic mean deviation of the surface profile from its mean line, measured over a defined sampling length. In plain terms: take the profile of the surface, draw a line through its average, and Ra is the average distance of the real surface from that line, ignoring whether it is above or below.
Because it averages, Ra is insensitive to isolated defects. A surface with one deep scratch can have the same Ra as a uniformly rough surface. This is why Rz — the average of the five highest peaks and five deepest valleys — is specified alongside Ra where peak height matters, for example under a seal.
Conversions worth remembering:
- Ra 0.8 µm = 32 µin
- Ra 1.6 µm = 63 µin
- Ra 3.2 µm = 125 µin
- Rz is typically 4 – 7× Ra for machined surfaces
Typical values by process
| Process | Typical Ra | Achievable Ra |
|---|---|---|
| Rough milling | 3.2 – 6.3 µm | — |
| Finish milling | 1.6 µm | 0.8 µm |
| High-speed finish milling | 0.8 µm | 0.4 µm |
| CNC turning | 1.6 µm | 0.8 µm |
| Fine turning / hard turning | 0.8 µm | 0.2 µm |
| Reaming | 0.8 µm | 0.4 µm |
| Surface grinding | 0.4 µm | 0.1 µm |
| Cylindrical grinding | 0.4 µm | 0.1 µm |
| Honing | 0.2 µm | 0.05 µm |
| Lapping | 0.1 µm | 0.025 µm |
| Bead blasting | 1.6 – 3.2 µm | (matte, non-directional) |
| EDM (finish setting) | 0.8 µm | 0.2 µm |
Our standard as-machined finish is Ra 1.6 µm. Anything better is an explicit operation with an explicit cost.
When texture is functional, not cosmetic
Sealing surfaces. An O-ring groove needs a specific finish range: too rough and the ring leaks past the peaks, too smooth and it can stick-slip and roll. ISO 3601 recommends Ra 0.8 – 1.6 µm on the groove sides and better than Ra 0.4 µm on the dynamic sealing surface.
Bearing and sliding surfaces. Oil retention depends on having valleys to hold lubricant. A mirror-polished journal can seize where a honed one with a 0.2 µm Ra and a defined cross-hatch runs indefinitely. This is why cylinder bores are honed, not lapped.
Fatigue-critical parts. Surface roughness concentrates stress. On a component under cyclic load, going from Ra 3.2 µm to Ra 0.8 µm can extend fatigue life substantially, because the peaks that act as crack initiation sites are removed.
Cleanability. Pharmaceutical and food-contact surfaces are specified at Ra 0.8 µm or better, often electropolished, because bacteria colonise the valleys of a rough surface and cleaning cannot reach them.
Coating adhesion. Anodising and painting both want a controlled surface — too smooth and adhesion suffers, too rough and the finish telegraphs the texture underneath.
Direction matters as much as value
A milled surface has lay: the cutter leaves a directional pattern. A turned surface has a helical lay. A blasted surface has none.
For a sliding contact, the lay direction relative to the motion changes friction and wear significantly. Where it matters, specify it with the standard lay symbol (⊥, ∥, X, M, C, R) alongside the Ra value.
How to specify finish so you get what you want
- State Ra and the surface it applies to. A general note of "Ra 0.8 all over" on a part with a deep pocket is an expensive request that probably was not intended.
- Specify Rz too where peaks matter — under seals, and on parts that will be coated.
- State the lay if the surface slides.
- Say whether the finish is before or after coating. Anodising adds roughly half the coating thickness to the surface and slightly increases Ra. A Ra 0.4 µm requirement after Type III hard anodising means the substrate must be considerably better.
- Do not specify better than you need. Going from Ra 1.6 µm to Ra 0.4 µm can add 15 – 30 % to the cost of a milled face, and from Ra 0.4 µm to Ra 0.1 µm usually means adding a grinding operation.
How we measure it
Contact stylus profilometers, calibrated against certified roughness standards, with the cut-off length set per ISO 4288 for the expected roughness range. Where a drawing specifies a surface finish, the measured value appears on the inspection report — not a pass/fail mark.
