Ordliste
Ra value — definition
A measure of surface roughness, given as the average deviation from the mean line in micrometres.
Ra is the most common roughness parameter and is stated in micrometres. A low Ra means a smooth surface.
For castings, Ra is determined by three things: how smooth the die surface is, which process is used, and how worn the tool is. High pressure die casting generally gives a finer surface than gravity casting, and zinc gives a finer surface than aluminium.
An important caveat: Ra does not describe appearance. A surface can have a low Ra and still show visible flow marks, parting lines or ejector marks. If a face has to look good, stating Ra is not enough — you need a visual reference, typically an approved sample.
The Ra we hold as cast is stated per part.