Ra is the arithmetic mean roughness and is widely used, but it does not describe individual peaks and valleys well. Rz adds information about vertical roughness depth and can help characterize a surface more completely.
Ra
Ra is the arithmetic mean of the absolute profile deviations from the mean line over the evaluation length. It is popular because it is straightforward to measure and repeat, but two very different profiles can produce similar Ra values.
Rz
Mahr describes Rz as an average roughness depth derived from individual measuring sections. It is more sensitive to the vertical height of the profile than Ra and can reveal surface differences that Ra alone may hide.
Why the cutoff/filter matters
Surface instruments separate roughness from longer-wavelength waviness using filters/cutoff values. A measurement taken with the wrong filter can produce a number that is not comparable to the drawing requirement.
Shop-floor controls
- Confirm whether the drawing specifies Ra, Rz or another parameter.
- Confirm units — µm vs µin can cause large mistakes.
- Measure in the correct direction relative to the lay where specified.
- Use the correct cutoff/evaluation length.
- Keep the stylus and reference surface clean.
This article is general guidance for preliminary engineering and shop-floor understanding. The drawing, applicable standard, machine/tooling OEM instructions, process-specific safety requirements and qualified engineering judgment take priority.