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How to Read Surface Finish Data

These days, we expect consumer products to have a smooth and sleek design. You might call it the ipod syndrome. This increased consumer attention on product surfaces has forced engineers, mechanics and designers to pay extra attention to surface finish. The data on surface finish measure the height, depth, and average roughness using sophisticated instruments. There are, however, other measures of surface finish such as waviness and lay, but for producers roughness is typically the single biggest concern, since it is directly related to the quality of the machines used.

Instructions

- 1
  Find out the Ra value. The Ra, or the Arithmetic Average Roughness, is the most commonly used data to measure roughness in surface finish. It is interpreted as the average irregularities of the surface from a mean line drawn on the surface. The unit for measurement is usually a micrometer (µm). For a list of Ra values for common processes, see Reference 1.
- 2
  Calculate the Root Mean Square value, Rq. Unlike the Ra, the Rq is determined by squaring the irregularities from the mean line before taking its average. It is a more robust measure because it gives a bigger weight to larger irregularities.
- 3
  
  Surface roughness, like the machine that produces it, has peaks and valleys.
  
  Check the Rp, Rv, and Ry values. The Rp value gives the maximum peak height of the irregularity. The Rv denotes the maximum valley depth, and the Ry number denotes the maximum peak-to-valley roughness. These data, together with the average roughness and root square roughness, give a fuller picture of any surface finish.

Surface Finish Specifications

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