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CNC & Precision Machining

Surface Roughness (Ra, Rz)

Surface roughness describes the fine, closely spaced irregularities of a machined surface. It is quantified with parameters such as Ra, the arithmetic mean deviation of the profile, and Rz, the maximum height of the profile.

A measured surface profile is separated by filtering into form, waviness and roughness. Ra is the arithmetic mean of the absolute profile deviations from the mean line, Rq is the root mean square deviation and Rz is the sum of the highest peak and deepest valley within a sampling length. Stylus profilometers drag a fine diamond tip across the surface, while optical instruments measure without contact, and areal parameters such as Sa describe whole surfaces.

Roughness affects friction, wear, sealing, fatigue life, coating adhesion and appearance, so drawings specify it for functional surfaces. In machining it depends mainly on feed and tool nose radius, step-over and tool geometry, and is degraded by tool wear, built-up edge, runout and chatter.

Profile parameters and their drawing indication are defined in the ISO 21920 series, which superseded older standards such as ISO 4287 and ISO 1302; areal parameters are in ISO 25178, and ASME B46.1 is widely used in North America. Results depend on filter cut-off, evaluation length and instrument, which must be stated. Ra alone does not fully describe function, because very different surfaces can share the same Ra.

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