Smoother Than a Baby’s Bottom – Measuring Surface Finish

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So just how smooth is a baby’s bottom? A verbal description would only be subjective. In machining though, a more definitive rule of measure is a mandate. In the United States, surface finish is usually specified using the ASME Y14.36M standard. The other common standard is International Organization for Standardization (ISO) 1302.

Surface texture is measured by three components:

  • Lay – where the machining marks are parallel typical of grinding and sanding operations.
  • Surface Roughness – where surface irregularities and finely spaced. They can be radial, cross hatched, or random.
  • Waviness – is the measure of irregularities with a spacing greater than surface roughness. This can be caused by tool chatter or warping.

The most common method to measure surface roughness is to use a diamond stylus Profilometer. For those of us old enough to remember the days of vinyl recordings, it uses the same concept as a record player. As the stylus is dragged across the surface it produces a frequency. This is deciphered by the instruments electronics producing a digital readout. The disadvantage of a Profilometer is that it is not accurate when the size of the features of the surface are close to the same size as the stylus.

Non-contact surface measurements are more accurate but more complicated. Here are some links if you want to read up on them:  interferometry, confocal microscopy, focus variation, structured light, electrical capacitance, electron microscopy, and photogrametry. For most manufacturing operations contact measurement are the norm.

A third “quick and dirty”  method is to use a surface finish scale. This is a flat piece of plastic with various micro finishes. Measurement is by a tactual comparison. You run your finger over the piece you are inspecting and then find the finish on the board that feels the same.

 Back in my machining days this was a source of many heated debates. My callused fingers could not detect with the same resolution as a female inspector with soft supple hands.

I hope you found this article informative. If  you need assistance with your application where you think compressed air products would be of benefit, feel welcomed to give me a call.

Joe Panfalone
Application Engineer

Phone (513) 671-3322
Fax   (513) 671-3363
Web: www.exair.com 
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