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11 September, 07:11

Surface roughness can be measured as either an arithmetic average (aa) or a root-mean-square (rms) average. which value is almost always greater for most surfaces:

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  1. 11 September, 07:16
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    Rms

    The rms average amplifies the effect of isolated large deviations from the mean, while the arithmetic average evens out the larger deviations. For instance, let's assume you have a nice surface finish that is about + / - 3 microns from the desired surface and have one large defect of + 10 microns. Let's see what happens with the math:

    Surface roughness measurements:

    -2.49, 2.5, - 1.67, - 1.62, - 1.72, 0.4, - 1.75, - 1.63, 2.07, 10

    The AA value for the 1st 9 values in the above list is 1.761111

    The RMS value for the 1st 9 values is 1.855505

    Notice that they're both fairly close to each other.

    Now let's recalculate using all 10 values.

    AA value is 2.585. A jump of 0.8238.

    RMS value is 3.6192. A jump of 1.7637. The increase in the RMS value is over twice that of the AA value.
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