Rq Root mean square roughness Rq
Root mean square roughness Rq · Estado superficial · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “Rq 0.8 on optical surface” states a root mean square roughness rq requirement.
What controls
Root mean square deviation of the profile from the mean line within the evaluation length. It describes the same thing as Ra (the average height of the irregularities), but because the values are squared before averaging, large peaks and valleys carry more weight in the result. That is why Rq is always somewhat greater than Ra for the same surface, and the difference between them grows when the profile is irregular. It is not a convertible parameter: the drawing calls for one or the other, and you measure the one it calls for.
How to verify
Roughness tester set to the Rq parameter, with the cut-off length suited to the expected finish and the stylus traversing perpendicular to the machining marks. Several measurements are taken in representative areas and compared with the drawing limit. On very fine or delicate surfaces, where the stylus could leave marks, non-contact optical instruments are used, checking that the parameter calculated is the same one the drawing calls for.
Workshop guideline values
- General
- Rq on the order of 4 to 8 µm on ordinary machined surfaces.
- Accuracy
- Rq on the order of 1 to 2 µm on ground or close-fitting bearing surfaces.
- High precision
- Rq below 0.5 µm on optical, honed or polished surfaces.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Converting Ra to Rq with a fixed factor: the relationship depends on the profile shape of each process.
- Measuring Ra because that is what the instrument is set to when the drawing calls for Rq.
- Forgetting that Rq penalizes local defects and looking for the cause in the general cutting parameters.
How to get it in the workshop / if it comes out of tolerance
It appears mainly in optical specifications, coatings and documentation of English-speaking origin, where it is called RMS; on ordinary mechanical drawings Ra is still the norm. If the drawing calls for Rq, set the roughness tester to Rq and do not measure Ra and then estimate: the relationship between the two depends on the shape of the profile and is not a constant.
If Rq fails while Ra looks correct, look for isolated peaks or valleys: a dragged chip, a restart mark from a pass or a burr from an insert change drive up the parameter without worsening the overall finish. The process corrections are the same as for Ra (lower feed, larger nose radius, sharp edge, avoiding stops mid-pass), but here eliminating local defects is especially important.
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