Surface roughness

Surface roughness is the set of fine irregularities (peaks and valleys) that a cutting tool, abrasive grains or another manufacturing process leaves on the surface of a part. Together with waviness and form deviations, it makes up the surface texture. Roughness affects friction and wear, the tightness of joints, fatigue strength, the adhesion of coatings and the appearance of the part.

The most widely used parameter is Ra, the arithmetic mean deviation of the profile, given in micrometers. It expresses the average height of the irregularities, which makes it stable and easy to compare, but insensitive to isolated deep scratches. Rz (maximum height of the profile) is the distance between the highest peak and the deepest valley within the sampling length. It is always larger than Ra, usually several times larger, and there is no exact conversion between the two. The parameters and their drawing indication were defined by ISO 4287 and ISO 1302, which are gradually being replaced by the ISO 21920 series. Areal (3D) surface evaluation is covered by ISO 25178 with the parameters Sa and Sz. US drawings often follow ASME B46.1 and ASME Y14.36 and state Ra in microinches: Ra 0.8 µm corresponds to about 32 µin, Ra 1.6 µm to 63 µin and Ra 3.2 µm to 125 µin.

Typical Ra values: roughing usually 6.3 to 25 µm, finish turning and milling 1.6 to 3.2 µm, fine turning and grinding 0.2 to 0.8 µm, honing, lapping and polishing 0.05 to 0.4 µm. Specified values are chosen from the series 0.1, 0.2, 0.4, 0.8, 1.6, 3.2, 6.3 and 12.5 µm. Sand castings, thermally cut edges and FDM prints are much rougher than machined faces.

On drawings, roughness is indicated with a check-mark-shaped symbol and a value, for example Ra 1.6. A symbol closed with a horizontal bar requires a surface produced by material removal, a symbol with a circle a surface without material removal (an as-cast surface, rolled sheet). The general roughness is stated near the title block, stricter values directly on the functional surfaces. It is usually measured with a diamond-tipped stylus profilometer, and fine or soft surfaces optically.

Specify roughness on functional surfaces: sliding and bearing surfaces, sealing faces for O-rings and shaft seals, fitted and seating faces, surfaces loaded in fatigue and cosmetic surfaces. For ordinary machined surfaces with no special function, Ra 3.2 µm is usually enough, for engineering fits in grade IT7 Ra 0.8 to 1.6 µm is typical, and more precise fits call for even finer values.

Address roughness on prototypes and 3D-printed parts as well if they need to seal, slide or be painted. The required surface is then achieved by subsequent machining, grinding or smoothing.

The most common mistake is to specify Ra 0.8 or finer on the whole part as a precaution. A finer surface often means an extra operation (grinding, polishing) and a significantly higher price. The opposite mistake is a drawing without a general roughness, which leaves the manufacturer to choose the surface by its own habits.

Ra alone does not fully describe a surface: two surfaces with the same Ra can have differently shaped irregularities and behave differently. For sealing and sliding surfaces, consider adding Rz, the material ratio or the lay direction. For radial shaft seals, for example, seal manufacturers usually require Ra of roughly 0.2 to 0.8 µm and a ground surface without spiral marks (lead-free). Blasting, anodizing or painting change the roughness, so state whether the value applies before or after surface finishing. Obsolete symbols from earlier standards on older drawings are worth converting to current notation during a revision.