The arithmetic mean of the absolute deviation of the surface profile from its mean line.
Ra is the most widely specified roughness parameter because it is stable, reproducible and easy to measure. It is quoted in micrometres or microinches, and ISO 4287 assigns it grade numbers N1 to N12 in steps of √10.
Its weakness is that it says nothing about profile shape. A surface with many small scratches and one with a few deep valleys can share an Ra while behaving completely differently in sealing, fatigue and wear.
Assuming Ra describes the surface. It describes one statistic of the profile. Where peaks matter — sealing, fatigue — specify Rz as well.
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