Meaning
Arithmetic average of the absolute values of the profile height deviations from the mean line represents the standard metric for evaluating surface texture. Measured across a sampling length, surface roughness ra indicates the microscopic texture of a machined or cast metal component. This value is used to specify the finish required for mating parts and sealing surfaces.
Profile Deviation
Diamond-tipped stylus instruments are dragged across the surface to record the microscopic peaks and valleys. The resulting measurements of surface roughness ra are used to verify that the machining process remains within the design limits. These values are recorded in the quality control documentation.
Frictional Performance
Low roughness values reduce friction and wear on moving components, thereby extending the service life of the machinery. However, achieving an extremely low surface roughness ra requires expensive finishing steps like lapping or polishing. This requirement must be balanced against the performance needs of the system.
Contractual Tolerance
Supply agreements for precision parts in the aerospace or medical sector specify the maximum and minimum roughness values for every surface. If a delivered batch exhibits a surface roughness ra that exceeds the specified limit, the buyer has the right to reject the shipment. The supplier must then rework the parts at their own expense or face late-delivery penalties.
This commercial pressure ensures that the machining workshop maintains high standards of quality.
