Surface Finish - 3: The Theory Behind Surface Finish Values
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Surface finishes are determined by an averaging process. The graphic to the left could be highs and lows of a surface measured in a specific direction (lay). The surface value for this sample would be found like this:

First get the profile of the surface finish and draw the centerline of the profile (shown in red).

Next divide the profile in equal sections and find the area of each section.

Finally divide the total of all areas by the number of sections that you used to divide the profile..

You will end up with Ra, the value of the surface finish using the arithmetic average value.

Other values such as Rmax can also be calculated according to the standard used.

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label: surface finish, finish, symbols, ANSI/ASME B46.1, forumlas, measuring, costs