Spacing Parameter

The parameters are differentiated by their prefix where P is for the primary or the raw profile, R is for the roughness profile, and W is for the waviness profile.  The ISO 4287-1997 defines some parameters within a sampling length and others within an evaluation length.  ISO 4288 indicates that parameters defined over the sampling length should be calculated based on only one sampling length if the user wants to calculate parameter estimate. On the other hand, if the user wants to calculate an average parameter estimate, the average parameter estimated is calculated by taking an average of the parameter estimates from all available sampling lengths.
 

Psm, Rsm, Wsm: mean width of the profile elements

 Psm, Wsm, Rsm = 1/N* sum from j=1 to N of Smj

Spacing and height discrimination are required. If they are not specified, then height discrimination will be 10% of Rz and spacing discrimination is 1 % of the sampling length.  For a roughness and waviness profile, the sampling length is equal to a cutoff.

This plot shows how Sm defined in ISO and ASME is determined

Figure 1 Sm parameter is calculated according to the ASME B46 Standard. For the ISO 4287 standard, the evaluation length would be replaced by the sampling length.


References

ISO 4287, Geometrical Product Specifications (GPS) -- Surface texture: Profile method -- Terms, definitions and surface texture parameters, 1997

ISO 4288, Geometrical Product Specifications (GPS) -- Surface texture:  Profile method-- Rules and procedures for the assessment of surface texture

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