What is the difference between Ra and Rz in roughness measurement?

Ra and Rz are the two most commonly used parameters in surface roughness evaluation, but they are different in definition and applicable scenarios.

Ra (arithmetic mean roughness)

Ra is the arithmetic mean deviation of the contour, that is, the arithmetic mean of the distance from each point on the contour to the midline within the sampling length. The Ra value is stable and reliable, and responds evenly to surface microscopic features. It is currently the most widely used roughness parameter.

Rz (ten-point height of microscopic unevenness)

Rz is the sum of the average of the five maximum peak heights and the five maximum valley depths. Rz is sensitive to individual peaks and valleys on the surface and can reflect extremes in surface quality.

main difference

1. Stability: Ra is more stable than Rz and is less affected by individual abnormal points.
2. sensitivity: Rz is more sensitive to local defects such as scratches and dents on the surface.
3. Applicability: Ra is suitable for surface evaluation with most general accuracy requirements; Rz is suitable for occasions with special requirements for surface peak height.

Practical application suggestions

In quality control and routine testing, it is recommended to use the Ra parameter first. For occasions such as sealing surfaces and mating surfaces that are highly sensitive to surface peaks and valleys, it is recommended to use both Ra and Rz for evaluation.