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The arithmetic average of the absolute values of the profile height deviations recorded within the evaluation length and measured from the mean line. Ra is equal to the sum of the areas of the profile (above and below the mean line) divided by the evaluation length L, which generally includes several sampling lengths or cutoffs. For graphical determination of roughness, the height deviations are measured normal to the chart centerline.
This measurement conforms to ANSI/ASME B46.1 “Surface Texture – Surface Roughness, Waviness and Lay”. Ra [also known as centerline arithmetic average (AA) and centerline average (CLA)] is usually expressed in microinches (µin), and performed by moving a stylus or profilometer in a straight line along the surface. A consistent and measurable surface finish can be specified for a desired roughness i.e., 9-11 microinch.
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