11.1 MEASURES OF ROUGHNESS

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A simple measure of roughness is the average area per unit length that is off the centre line (mean). We will call this the Centre Line Average (CLA), or Arithmetic Average (Ra), the units are minches.

To calculate the roughness using samples at evenly spaced positions,

The roughness can also be calculated by area,

In both cases the mean line is located so the sum of areas above the line is equal to the sum of areas bellow the line.

As an example we can examine a surface that has a triangular profile,

One of the instruments that we will use is the Surfcom. If we were to have obtained the graph above from this device, we would have to use the following formula to determine the true values,