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Surface Roughness

JacobCheverie
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)?
  • 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)?

    I assume this is due to Z-range of the profilometer you are using, right? But honestly, you shouldn't change it. Are you able to reduce the sample interval to 3 to make sure the correct filter is applied?

    Are you able to check the surface radially versus axially?
  • in reply to bfire85
    That is correct. We have had many instances where even cutting the interval to 1 is not sufficient.

    For example, at a required roughness the standard dictates we should use a .030 cutoff, so even a .030 over 1 interval will require .090 of surface to account for end effects. Sometimes we will not even have the .090 required (or are limited by Z-range). My idea is to measure all of the possible surface (maybe .050) and then apply the .030 filter to the measured data.

    Checking radially versus axially is not an option, there is part material in the way.
  • in reply to bfire85
    We have a GD25 as well. Ancient software, running XP. At that point, have you tried shutting off the automatic contact, finding the peak of the sphere as to not max out the high end, then back tracking the part so the level starts at the low end then tracing it for 0.069"?
  • 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)?

    Yes. It would be wrong.
  • in reply to bfire85
    I will have to give that a shot. Thanks for the advice.
  • It would give an artificially lower surface roughness evaluation.
  • in reply to dph51
    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?
  • JacobCheverie, if you measure a length of 0.020 and try to evaluate a length of 0.030, you will probably get an error message, somewhere along the lines of “insufficient data”.

    In theory, you would use 5 sample lengths and evaluate that. Can you evaluate 1 sample length? Well, we did. Was it correct? No. But engineering and manufacturing wanted a number and we gave them a number. The feature had a length of 1.75 mm and specified a cutoff length of 0.8.

    Hope this helps.
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