Surface roughness on drawings: how to specify Ra without overpaying
Specify roughness only on surfaces where it affects function: sealing, sliding, bearing seats, fatigue or appearance. Use a general roughness for everything else, typically what the process gives naturally. Ra is the arithmetic mean roughness; common values are around 3.2 µm for normal machining, 1.6 µm for good fine machining and 0.8 µm or less for ground or precision surfaces.
Guilherme Rodrigues ItinoseUpdated 2 min read
Key takeaways
- Roughness costs time: every step finer can mean another pass or grinding.
- Put a general roughness in the title block and call out only exceptions.
- Match the value to function, not habit.
- Ra alone does not describe everything; some functions need other parameters.
- Make sure the process and inspection can deliver the value.
A blanket “Ra 0.8 all over” note looks careful but makes parts slower and more expensive without improving anything that matters.
What Ra measures
Ra is the arithmetic mean of how far the surface profile deviates from its mean line. Lower values mean a smoother surface. It is simple and widely measured, which is why it dominates drawings.
Typical values by process
| Process | Typical Ra (µm) |
|---|---|
| Rough turning or milling | 6.3 – 12.5 |
| Normal machining | 3.2 |
| Fine turning or milling | 1.6 |
| Reaming, fine boring | 0.8 – 1.6 |
| Grinding | 0.4 – 0.8 |
| Lapping, honing, polishing | 0.1 – 0.4 |
Specify by function
- Structural or non-contact faces: general roughness from the title block.
- Sliding surfaces: finer finish, often with lubrication requirements.
- Static seals and O-ring grooves: follow the seal supplier.
- Bearing seats: follow the bearing manufacturer.
- Visible surfaces: define appearance separately when needed.
How to write it
Put a general requirement near the title block and call out only exceptions with the surface texture symbol defined in ISO 21920-1. State the measuring direction or lay only when it matters.
Our manufacturing documentation reviews roughness and tolerances so drawings ask for what the part needs, not more.
When to outsource this
- Your drawings carry Ra 0.8 on every face
- You need to define sealing or sliding surfaces correctly
When not to
- Parts where the natural process finish is clearly enough
FAQ
What is Ra?
The arithmetic mean of the absolute deviations of the profile from its mean line over the evaluation length. It is the most common roughness parameter.
Which standard defines the symbol?
ISO 21920-1 defines the indication of surface texture on drawings; older drawings reference ISO 1302, which it replaced.
What Ra for an O-ring groove?
Follow the seal manufacturer's recommendation; sealing surfaces usually need finer finishes than structural faces.
Is Ra 0.4 always better?
No. Very smooth surfaces can be worse for lubrication or adhesion, and they cost more. Specify what the function needs.
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