What is the surface roughness Ra?

The surface of the workpiece has complex undulations, with continuous peaks and valleys of different heights and intervals. Larger undulations are called "flatness", while smaller undulations are called "surface roughness", that is, Surface Roughness. Roughness Ra, this seemingly simple number, is actually an exquisite display of the microscopic world on the surface.

We can imagine the surface as a small landscape in which tiny particles and uneven topography unfold before our eyes. The roughness Ra is like the footprints left in this landscape, measuring the average height change of the surface in microns. So, specifically, what does Ra mean? Use the picture below to explain.

Ra is the arithmetic mean roughness, which represents the absolute value of roughness on the reference length (the blue part in the figure is originally below the average line, but in order to intuitively express Ra, it is calculated using the absolute value, so it is inverted to the top) The average value, this absolute value is relative to the average height. So more precisely, Ra is the average deviation of the height of each point on the surface from the mean height.
Of course, in the roughness fluctuation curve, in addition to Ra, there are also peaks and valleys and root mean square. As shown in the figure below, Rp represents the peak, Rv represents the valleys, and Rq represents the root mean square.

Rv: The calculated distance between the lowest valley of the profile and the average line within the evaluation length. Rp: The calculated distance between the highest peak of the profile and the average line within the evaluation length. Rz: The calculated deviation between the highest peak and lowest trough within the evaluation length. Rq: Also known as RMS, calculated over the evaluation length, this is the root mean square value of the change in profile height relative to the mean line.

Behind the number Ra lies the stringent requirements of manufacturing engineers for surface quality. Imagine running your finger across the surface of a metal and feeling tiny particles. This is what the roughness Ra describes.

For manufacturing high-precision parts, these tiny particles can be a source of quality defects. Therefore, the microscopic roughness Ra value has become a key technical indicator in the manufacturing industry.