Roughness

Roughness refers to the fine irregularities of a surface that cause deviations from an ideal smooth plane. It describes the micro- to nano-scale texture that remains after a manufacturing process and is a key descriptor of surface quality, influencing how a part feels, looks, and performs in contact with other surfaces.

Roughness is typically quantified by profile parameters measured over a defined sampling length. The most common is Ra, the arithmetic average of absolute deviations from a mean line. Rq, the root-mean-square roughness, is similar but gives more weight to larger deviations. Rz represents the average peak-to-valley height over several sampling lengths. Other parameters include Rp (maximum peak height) and Rv (maximum valley depth). Standards such as ISO 4287 and ISO 4286 define these and related metrics.

Measurement methods include contact profilometry, where a stylus drags across the surface, and non-contact techniques such as optical or electronic methods. Optical approaches include laser scanning, confocal microscopy, interferometry, and white-light scanning, which can capture three-dimensional topography without physical contact.

Applications and effects: surface roughness influences friction, wear, lubrication behavior, sealing performance, heat transfer, and the fit between mating parts. It is a major consideration in finishing processes like turning, milling, grinding, polishing, and shot peening, where target roughness values are specified to meet functional or aesthetic requirements.

Roughness is one aspect of surface texture. It is distinct from waviness, which describes broader-scale deviations, and from texture, which encompasses the overall surface characteristics, including deliberate micro-structuring. Roughness is typically reported as a dimensioned value over a defined sampling length.