Roughness means the set of all those small surface irregularities present on the piece, which can be connected to the properties of the piece itself or resulting from mechanical processing, such as milling, turning and grinding. The measuring instruments that allow us measuring roughness are roughness testers.
Thanks to the filters, the operator can get the measurement of surface quality only, deprived of the effects that geometry errors of the piece have on the measured profile.
The measure of the roughness Ra (average roughness) is the arithmetic average value of the absolute values within an evaluation length. However, the roughness Ra defines an average value of surface finish that does not differentiate for the type of irregularity, i.e. profiles with different trends from the same arithmetic mean deviation will have the same value as Ra. RPc is often used to characterize a profile, i.e. peak counting.
Other parameters are also required, such as the mean square roughness Rq, which is the square root of the mean of deviations squares of the real profile within an evaluation length.
Another parameter is Rz (also used in sandblasting) which takes into account the average peak-valley difference of 10 points (5 peaks and 5 valleys) that deviate more within an evaluation length.
The Rmax parameter, on the other hand, consists of the distance between two lines parallel to the average line tangent to the highest peak and the deepest valley. This parameter allows measuring the maximum depth of profile within an evaluation length.
In painting field, the degree of sandblasting roughness is controlled both with instruments for quality control (visual samples) and with instruments for quantitative control (Digital Surface Profile Gauges).