Surface Roughness

Theory

Ra is the most common roughness parameter (ISO 4287). Typical values: ground surfaces 0.1–0.4 µm, milled 0.8–3.2 µm, FDM printed 5–25 µm (layer lines visible), SLA 1–5 µm. Rz (ten-point height) captures peak extremes better than Ra for functional surfaces.

Application

FDM surfaces normal to the Z-axis (top/bottom) are smoother than vertical side walls. Layer height is the dominant Ra driver: 0.1 mm layers yield ~5–8 µm Ra; 0.3 mm layers yield ~15–25 µm. For sealing surfaces, sand or vapor-smooth after printing.

Common mistakes

Specifying Ra on an AM drawing is misleading without specifying orientation. Top surfaces and side surfaces of the same print can differ by 10× in Ra. Always specify the measurement direction and layer orientation on the drawing.

Related terms: GD&T (Geometric Dimensioning & Tolerancing), Dimensional Accuracy, Build Orientation