Surface Roughness Guide
What Surface Roughness Means
Surface roughness describes how smooth or irregular a manufactured surface is. On drawings it is used to control sealing, friction, wear, fit, coating behavior, and visual finish. In most cases the practical default is Ra.
Where To Use Which
| Surface | Best starting callout | Why |
|---|---|---|
| General machined faces | Ra | Simple and widely understood. |
| Sealing faces | Ra, sometimes with lay note | Direction and finish stability affect sealing. |
| Sliding or wear surfaces | Ra + Rz | Peaks influence wear and lubrication film. |
| Bearing seats and fit diameters | Ra with controlled range | Fit repeatability is the main concern. |
| Cast, forged, or additive raw surfaces | General note only | Do not over-specify non-critical raw faces. |
Common Values
| Ra (µm) | Ra (µin) | Typical meaning |
|---|---|---|
| 0.2 | 8 | Fine finishing, precision contact |
| 0.4 | 16 | Grinding, fine fit surfaces |
| 0.8 | 32 | Seal and sliding starting point |
| 1.6 | 63 | Common controlled machined finish |
| 3.2 | 125 | General machining |
| 6.3 | 250 | Rough machining or non-critical faces |
Frequently Asked Questions
What does Ra mean in surface roughness?
Ra is the arithmetic average of the absolute profile deviations from the mean line. It is the most common parameter used to specify machined surface finish on drawings.
What is the difference between Ra, Rz and Rt?
Ra averages all profile deviations, Rz is the average of the largest peak-to-valley heights inside the sampling lengths, and Rt is the single largest peak-to-valley distance over the full evaluation length. Rz and Rt are more sensitive to local peaks and outliers than Ra.
Can I convert Ra directly to Rz?
Not reliably. Ra and Rz describe different profile characteristics. A rough rule of thumb may exist for a given process, but an exact conversion requires measured data or a process-specific correlation.
Which Ra value should I specify for a sealing surface?
For static O-ring or gasket sealing, Ra around 0.4–0.8 µm is a typical starting point, often combined with a lay direction note. Dynamic seals normally need Ra 0.2–0.4 µm.
What standards are commonly used for surface finish?
ISO 1302 (and the newer ISO 21920 series) is widely used for surface texture indication on drawings, while ASME B46.1 is common in the United States for terminology and measurement practice.
How do I read a surface finish symbol on a drawing?
The surface texture symbol carries the parameter (e.g. Ra), the upper and/or lower limit, the sampling length and the lay direction. Always check whether the limit is a maximum (default) or a 16 % rule limit per the standard.