What is surface roughness Ra?

author: YITECH

2025-01-02

Our precision engineering ensures a surface roughness of less than Ra 0.8 for our metal components, providing a high level of accuracy and quality for our customers...

The surface of the workpiece has complex fluctuations, with continuous peaks and valleys of different heights and intervals. The larger fluctuation is called "flatness", and the smaller fluctuation is called "Surface Roughness", that is, surface roughness.

And the roughness Ra, this seemingly simple number, is actually a subtle manifestation of the microscopic world of the surface.

We can think of the surface as a small landscape in which tiny particles and bumpy terrain unfold before our eyes.
The roughness Ra, like the footprints left in this landscape, measures the average elevation of the surface in microns. So, specifically, what does Ra mean? Use the following picture to explain.

Ra is the arithmetic average roughness, representing the absolute value of the roughness on the reference length (the blue part in the figure was originally below the average line, but in order to intuitively express Ra is calculated using the absolute value, so it is reversed to the top). This absolute value is relative to the average height. So more accurately, Ra is the average of the height of each point on the surface away from the average height.

Of course, in the roughness fluctuation curve, in addition to Ra, there are peaks and valleys and root mean square said. In the figure below, Rp means Peak, Rv means Valleys, and Rq means root mean square.

Rv: The calculated distance between the contour's lowest trough and the mean line within the evaluated length.

Rp: The calculated distance between the highest peak of the profile and the mean line within the evaluated length.

Rz: The deviation between the calculated highest peak and lowest trough within the evaluation length.

Rq: Also known as RMS, calculated within the evaluation length, which is the root-mean-square value of the change in contour height relative to the mean line.

Behind this number lies the demanding requirements of manufacturing engineers for surface quality. Imagine sliding your finger across the surface of a metal and being able to feel tiny particles, which is exactly what roughness Ra describes.

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

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DongGuan YiTai Electronic Technologies Co.,Ltd

DongGuan YiTai Electronic Technologies Co.,Ltd is a manufacturing service provider located in Dongguan, China.

YiTai specialized in CNC turning machining and sheet metal fabrication mainly. As a member of Hung Mou Group, we focus on the overseas marketing development. And based on our parent company’s manufacturing capability and resources, YiTai also expended machining services such as die casting, injection molding, aluminum profile extrusion, 3D printing, which are committed to providing customers with one-stop purchasing services and experience.

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