What is surface roughness?

Surface roughness is the short-wavelength part of surface texture: the fine peaks and valleys left once form and waviness have been filtered out of the measured profile. Ra is the arithmetic mean of the absolute deviations of the profile from its mean line. Rz is the height from the highest peak to the deepest valley within a sampling length, in the usual definition averaged over the sampling lengths of the evaluation length. Both are given in µm. On the same profile Rz is larger than Ra, but there is no fixed conversion.

What does surface roughness depend on?

Roughness comes from whatever process forms the surface last, and that need not be machining. In milling and turning, feed, corner radius or edge geometry, tool wear, vibration, coolant and the material all play a part. Grinding, honing, lapping, polishing, blasting and roller burnishing each leave their own texture, and a finish such as anodising, pickling or coating can change the roughness too. For turning, the textbook estimate of the theoretical peak-to-valley height is about f² / (8 × r), with f the feed per revolution and r the corner radius, both in mm. That is an estimate from the geometry of the cutting edge, not a measurement: the measured value departs from it through wear, built-up edge, vibration and material behaviour.

What does the standard say about surface roughness?

The parameters and the drawing indication are set in ISO standards. The ISO 21920 series of 2021 replaces ISO 1302 for the indication and ISO 4287 and ISO 4288 for the parameters and the measurement rules; older drawings and inspection reports still refer to the earlier standards. A roughness value is only unambiguous with its conditions: the parameter, the filter, the evaluation length and the acceptance rule. Where these are not stated, the defaults of the standard the drawing refers to apply. The symbol also says something about the process: with a bar, material must be removed; with a circle, no material may be removed; left open, any process will do. An Rz on an older Japanese or German drawing may follow a different definition from today's ISO Rz.

How does surface roughness work in practice?

A general roughness requirement near the title block applies to every surface without its own indication; tighter values sit on functional surfaces such as a seal or a sliding face. Whether a value is reachable in the last pass depends on machine, tooling, material and setup. A low value can call for a fine finishing pass, grinding or polishing, sometimes subcontracted. It is measured with a stylus or optical roughness tester; a comparison specimen gives an impression, not a measured value. What a requirement costs in time and money differs per surface, process and shop.

What is surface roughness often confused with?

Roughness is a requirement on the surface, not an operation and not a finish: the same requirement can come from milling, grinding or another process. Nor is roughness flatness: flatness is a form tolerance over the whole surface, roughness concerns the fine texture within a short evaluation length. And Ra is not Rz: a number without its parameter is ambiguous.

How do you see surface roughness in Blake?

In Blake the roughness sits with the part as a proposal from the drawing, together with material, tolerances, finishing and threads. You confirm or correct it; who did so and when stays visible, with a note if you add one. More on requests.