Surface Roughness Conversion - The Difference Between Ra and Rz

Currently one of our customer inquiry us one technical question: when he is regularly machining Cylinder Heads & Blocks, for head contact face in case of block and block contact face in case of head specifies surface finish of 15 Rz. After finish milling when we check for Rz it is between 15 to 20 Rz, but if we check Ra it is within 1 µRa. Here comes a question: what is the difference between Ra & Rz measurements?

Average Roughness (Ra) and Mean Roughness depth (Rz) are two different but associated methods to measure roughness. 

Roughness on a machined surface results from the machining process itself. Type of tool, speed of the tool, machine geometer, and the conditions of the machining environment can all cause roughness. Ra is calculated using an averaging process. Taking all the peaks and valleys across a surface, and neutralizing extreme outliers, the Ra algorithm is perfect for watching the texture of a surface and guaranteeing consistency in measurement. Rz, on the other hand, averages the 5 highest and 5 lowest peaks on a surface. Rz calculations are done through 3 different equations, and they generally are more impacted by extreme outliers. 

There are ways in which to convert Ra surface roughness measurements to Rz surface roughness measurement, dependent upon the acceptance criteria used by the manufacturer.

For most new technical engineers, to have a full understanding of surface roughness is the key factor to design a perfect print for any project. Here we’d love to share our knowledge of how is the difference between Rz, Ra, Rt, ISO Roughness grade numbers. And you will benefit it definitely.

While it is best to measure using the parameter specified in the print, there are rules of thumb available that can help clear up the confusion and convert Ra to Rz or Rz to Ra.

The methodology of measurement and what is measured are quite different. This is critical to understand if you will not be paid for your parts because the Ra you measured is not in fact the Rz surface profile that customer specified.

·Ra and Rz are different parameters of roughness.

·Ra is the average roughness of a surface.

·Rz is the difference between the tallest "peak" and the deepest "valley" in the surface.

Notice: You must to verify is your instrument (rugosimeter) able to show you the Rz.

There is not one parameter to convert from one unit to the other, because the Rz depends of the way you are obtaining the roughness (machining, grit blasting, polishing, etc.) 

While it is best to measure using the parameter specified in the print, there are rules of thumb available that can help clear up the confusion and convert Ra to Rz or Rz to Ra.

Here is a common conversion table with also roughness grade numbers:

 If you are still not clear how to distinguish the surface roughness difference, just click here.