Filters

Filters are used to separate the components of a signal on a wavelength basis. In the field of electrical engineering, frequency rather than wavelength is the basis for filtering. In surface metrology, wavelength is the basis of filtering as it can be related to the part being measured. A filter operates a waveform, such as a primary profile, to produce the required output. In order to separate out the roughness and the waviness, a cutoff wavelength ( c) is set. Filters operate depending on the cutoff set for them. ASME  and ISO standards provide a series of cutoffs (0.08, 0.25, 0.8,2.5, 8 mm). All wavelengths below  c in the profile fall into the roughness regime and all others go into the waviness and form regime. Similarly, waviness and form can also be separated by setting a cutoff wavelength ( f). . In addition to c and lf it is also possible to set a short wavelength cutoff  s. This cutoff assures a well-defined bandwidth for roughness ( s -  c). The value of  s is chosen based on a ratio of  c/ s to be either 100, 300, or 1000. The radius of the stylus used to trace the profile is expected to be smaller than  s. The use of  s effectively eliminates one of the variables that could affect the comparison of parameters from different commercial vendors of stylus instruments. This is done by calculating the parameters based on a well-defined bandwidth ( s- c).


Figure 1.  Transmission characteristic of roughness and waviness profiles


References

Surface Texture (Surface Roughness, Waviness, and Lay), an American National Standard, ASME B46.1-2002, New York, NY