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Surface Finish Measurement
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Ground Surface Finish Measurement
Surface Roughness vs. Surface FinishSurface roughness measurement is a quantitative assessment of very closely occurring surface irregularities, and is sometimes used interchangeably with the term surface finish measurement. There is a difference in context, however: while surface roughness is a general term about the amount of surface variation (at a very small scale) on any given surface, whether natural, man-made or found, "surface finish" is a term implying a surface that has been deliberately modified or engineered, with an end in mind.
What is Surface Roughness or Finish?Think of a bump or a dip in a surface as a wave shape (in that it has "height" or amplitude, and "width" or frequency). Surface roughness is broadly understood as a measurement of by how much amplitude the surface departs from a mean and at what frequency those departures are prone to occur.
Sampling of a surface, to measure surface roughness, must take place over an area large enough to average the roughness frequency several times. The sampling size should be at least seven, and likely ten times longer than the frequency of the surface irregularities to be measured.
Surface roughness, and by extension, surface finish, is measured as an average, consisting of several measurements of the minimum sample length, gathered end-to-end if the measurement is linear, or side by side if the measurement is of an area ("areal"). Averages can also be aggregated from several noncontiguous locations.
Depending on the precision of the surface finish specification, surface finish may be qualified with tools of different precision. Optical profilers from ZYGO have the advantage of providing high precision height measurements at any magnification. ZYGO profilers measure surface roughness with better than nanometer and microinch specification - well below the level of detection by the human eye or tactile sensation.
Directionality of Roughness is LayMany manufactured surfaces, and some natural surfaces, have surface features which are directional. Directionality, or "lay," is an important factor when obtaining roughness measurements using a linear or stylus technique, but is not a factor for three dimensional optical profiling.
For example, turned cylindrical parts may have circumferential grooves from the cutting tool, spiraling from one end to the other of the machined cylinder. To measure such a cylinder, one measures roughness perpendicular to the direction of the grooves, so as not to follow the grooves and see no effect from them, and also so that the grooves' frequency can be determined accurately. To measure diagonally across the turning grooves would produce a profile which elongates the frequency.
Lay is not a consideration in measuring roughness of areas, as with a ZYGO optical profiler, because 3D measurements measure in all directions simultaneously. To indicate this, areal roughness is described using a surface parameter. The designation for surface roughness is Sa, according to Birmingham and ISO standards, as opposed to Ra, which is a linear measurement.
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