Surface roughness: Surface roughness is the fine texture of a part's surface, usually given as Ra in micrometres, the average height of peaks and valleys measured along the surface.
Every manufacturing process leaves a texture. A milled face shows tool marks, a printed part shows layer lines, a powder part feels slightly grainy. Surface roughness puts a number on that texture so a buyer can say what they need and a shop can say what it can deliver.
The most common measure is Ra, the arithmetic average of the surface’s deviation from its mean line, in micrometres (µm). Lower is smoother. Rz, the average height from peak to valley, is also widely used and gives larger numbers for the same surface, so the two cannot be compared directly.
As a general guide, a standard CNC-machined finish is often around Ra 3.2 µm, with Ra 1.6 µm or 0.8 µm available at extra cost. 3D-printed surfaces are generally rougher as printed and depend on layer height, orientation and process, and they are often smoothed by post-processing such as sanding, tumbling or vapour smoothing.
Example: a buyer asks for Ra 0.8 µm on a sealing face of a machined part and Ra 3.2 µm everywhere else. The shop adds a finishing pass for that one face only. Asking for Ra 0.8 µm on every face would add cost with no benefit.
Surface roughness is often confused with tolerance. A surface can be smooth and the wrong size, or the right size and rough. Read surface finish options for how finishes compare.
Go deeper: Surface finishes and what they add to cost