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What does "surface finish: 4-8 rms" mean? Answered

I'm looking through parts on McMaster Carr's website, and I came across one of their shafts. What exactly is an "rms" and what does it look like?

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kelseymhBest Answer (author)2011-02-23

I think you left out a bit; that should read "4-8 rms micron". As a physicist, I would rather write that as "4 to 8 (rms) micron" to make it clear that "rms" (i.e., "root mean square") is a modifier. As NM said, it means "highly polished."

Technically, it means that if you were to carefully measure the radius (diameter/2) at a large number of points all over the shaft, you would get a normal distribution ("bell curve" or Gaussian), with a mean centered on the quoted diameter, and a spread (standard deviation, or RMS) of 4-8 microns.

I believe the technical term is "bloody freaking smooth." :-D

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steveastrouk (author)2011-02-24

Its a statement of surface roughness. It SHOULD be qualified by units - microinch or microns. You can buy special test gauges you wipe with a finger nail and compare the feel of the nail on the standard, against the job.

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