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Surface Roughness

Surface Roughness per ASME B46.1

Everywhere I've seen states that the wavelength of the filter is equal to the sampling length. Is this a firm requirement? We are measuring spherical surfaces and we can't quite get the amount of surface measured that we would need to use the filter that matches the roughness requirement. My idea was to measure as much of the surface that we can, and then apply the filter at whatever wavelength we want to for roughness evaluation.

For example, 5 sampling intervals of width .010 are measured with two .010 intervals at the ends to account for end-effects. The evaluation length is .050. Would it be wrong to apply a filter of wavelength .003 (or anything other than .010)?
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  • It would give an artificially lower surface roughness evaluation.
  • Only in the event that you made the filter cutoff wavelength shorter. I am proposing that I measure an arbitrary length of the surface and then apply the correct filter cutoff to the measured data.

    If a 63 finish is called out, I must use a Lc = .030 cutoff wavelength. Unfortunately, my measuring software will only allow me to measure in some increment of .030 (really .060 + .030x for x in [1, 5]). If I don't have enough surface to measure, or if the curvature of the surface is too high, can I measure .020 and then apply a .030 filter to the measured data?

    Must the sampling length be a multiple of the filter cutoff wavelength?
Reply
  • Only in the event that you made the filter cutoff wavelength shorter. I am proposing that I measure an arbitrary length of the surface and then apply the correct filter cutoff to the measured data.

    If a 63 finish is called out, I must use a Lc = .030 cutoff wavelength. Unfortunately, my measuring software will only allow me to measure in some increment of .030 (really .060 + .030x for x in [1, 5]). If I don't have enough surface to measure, or if the curvature of the surface is too high, can I measure .020 and then apply a .030 filter to the measured data?

    Must the sampling length be a multiple of the filter cutoff wavelength?
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