#ndt Surface finish describes the roughness or irregularity of a surface using a measurement system to quantify the results. This system covers a wide range from extremely rough-textured surfaces all the way to those that are highly polished, mirror-like surfaces. The predominant parameter is Roughness Average, denoted by “Ra,” which is the arithmetic mean deviation of a profile. The higher the number, the rougher the finish. Roughly milled surfaces will be in the 250 to 500 Ra µin (6.3 to 12.5 µm) range. S
Source: NDT