Roughness refers to the fine irregularities of a surface, peaks and valleys of the order of a micrometre left by the tool or the process. It is measured with a stylus profilometer, or by optical methods, and expressed through parameters defined in ISO 4287. The most common is Ra, the arithmetic mean deviation of the profile, expressed in micrometres; Rz, which covers the extreme heights of the profile, is also widely used. The indication on a drawing falls under ISO 1302, updated by the ISO 21920 series.
Some orders of magnitude: ordinary machining (milling, turning) often gives an Ra of around 1.6 to 3.2 µm; a careful finish comes down towards 0.8 µm; grinding and polishing reach lower values. These figures vary with the material, the tool and the cutting parameters. The same Ra can also correspond to different textures: the direction of the lay matters too.
Roughness plays a part in friction and wear, sealing (gasket face, piston rod), fatigue resistance, the keying of a paint or bond, and of course appearance. Each finer step adds finishing passes, specific tools or an extra operation, hence cost: a low Ra is required only on functional surfaces. Treatments also change the surface finish: blasting increases it and gives a matt appearance, whereas polishing reduces it.
In a quote request
State the roughness only on the surfaces that need it, with the parameter wanted (Ra most often), and a general value for the rest of the drawing, for example Ra 3.2. Specify whether the requirement applies before or after treatment, and the inspection method expected. To choose or convert a value, the roughness tool and the guide on machining tolerances will help; metrology explains how it is verified.