Blog

Are there any differences between the roughness standards Ra and Rz?

It is estimated that when observing the roughness markings on mechanical component drawings, both Ra and Rz marking methods have been seen, with Ra marking being more common. Some students have noticed that there are also many marking methods for Rz in foreign countries.

I don't know if everyone is clear about the meanings represented by the two, but due to changes in standards, it is estimated that many people are confused about the definition of roughness. Ra refers to the arithmetic mean deviation of the contour, which has not changed, while Rz was defined as the "micro roughness ten point height" in the old standard. In the new standard, it is the height of the sum of the maximum contour peak height Zp and the maximum contour valley depth Zv within a sampling length, which is the Ry of the old standard. However, currently, many surface roughness measurement programs still use Rz according to the old standard, as long as everyone knows this difference.

Definition of Rz roughness in the old standard

Definition of Rz roughness in the new standard

So here's a question for everyone, when will you mark Ra on the drawing? When will you mark Rz? Are you a bit confused? Many people may have never considered it, and the usual practice is to label whatever is on the previous drawings, or label whichever is most widely used. Few people think about what my application scenario is? What kind of quality control goal do I want to achieve? What is the purpose of marking this roughness? Today we will start our discussion from the most crucial point, hoping it will be helpful to you. From the previous definition, we know that Ra cares about the "average" of the entire evaluation length, and it does not care about local defects or extreme peak valley differences at individual locations; And Rz cares about the extreme height differences of local mountain peaks and valleys, which cannot reflect the overall contour uniformity.

The following two images are provided for everyone to carefully experience the difference between Ra and Rz. 

Schematic diagram of surface profile with the same Rz but different Ra

Schematic diagram of surface profile with the same Ra but different Rz

The surface contour diagram of the same Rz but different Ra can be considered. When do you care more about whether there is a deep groove in a local area? Should it be occasions that require sealing and stress concentration control? Besides, most of the time, such as in the case of mating surfaces, we only focus on the overall distribution. Therefore, if it is a sealing surface or a surface with high fatigue strength requirements, it is recommended to use Rz to label the roughness (if necessary, other roughness parameters need to be constrained together), and in most other cases, Ra can be labeled.