Roughness Measurement and Control

Roughness refers to the small surface irregularities on a piece, which can either be due to the material’s properties or the result of machining processes like milling, turning, and grinding. The tools used to measure roughness are called roughness testers.

Roughness is measured by analyzing the surface profile over a specific evaluation length. By applying filters, you can isolate the surface quality, removing the effects that geometric errors in the piece might have on the measured profile.

The Ra roughness (average roughness) is the arithmetic average of the absolute values within a given length. However, Ra only gives an average surface finish and doesn’t distinguish between different types of irregularities. For example, profiles with different patterns but the same average deviation will have the same Ra value. To better characterize a profile, RPc is often used, which counts the peaks.

Other important parameters include the root mean square roughness (Rq), which is the square root of the average of the squared deviations of the actual profile over the evaluation length.

Another parameter, Rz, is commonly used in sandblasting and measures the average peak-to-valley difference over 10 points (5 peaks and 5 valleys) with the greatest deviations within the evaluation length.

The Rmax parameter measures the distance between two parallel lines to the mean line, tangent to the highest peak and deepest valley. This gives the maximum depth of the profile within the evaluation length.

In painting applications, the degree of sandblasting roughness is measured and monitored using both qualitative control instruments (visual samples) and quantitative control instruments (sandblasting profile gauges).