I had taken the 3D roughness value of precision milled Ti6Al4V workpiece surface in Alicona microscope. The parameter I considered is Sa (Average height of selected area) in microns. In Alicona Infinite microscope I got the reading as 11.0355µm, 3.9683µm, 1.4111µm, 1.0857µm, 0.9991524µm, and 0.8205437µm at 2.5X, 5X, 10X, 20X, 50X and 100X magnifications. Which magnification is usually considered and why? Why so much variation in values? kindly help.
PS: Attached other 3D roughness parameter values and image of the sample
You would not expect to get the same value for Sa at a different magnification. As you change the lens (the numerical aperture), the instrument is measuring over a different spatial frequency bandwidth. Your range is different, the resolution is different, the transfer function is different and the slope capability is different. No surface has an "inherent" Sa - the value is dependent on the instrument and filtering conditions. Take a look at Leach and Haitjema 2010 Bandwidth characteristics and comparisons of surface texture measuring instruments Meas. Sci. Technol. 21 032001
Article Bandwidth characteristics and comparisons of surface texture...
Prof. Richard Leach, thanks a lot for enlightening me to the fact that so much factors decide the value of 'Sa' measured. I will go through your paper and I'm positive my doubts will be cleared.
Prof. Giuseppe Cotellessa, thank you for the reply.
The ISO 25178 family describes relevant parameters for your measurement strategy. The required spatial resolution (better speaking: the optical resolution of the objective) depends on the width of your minimal surface feature you want to resolve. Furthermore, from the feature height you might be able to estimate the slope of the surface features, which gives you the required NA of the objective. Read the the ISO 25178 for more details.
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