
Figure 1. Demonstration of the measurement of Ra. Source: Michigan Metrology
When measuring surface roughness, typically the average roughness, Ra, is specified. Ra is a measure of the average of the absolute value of the variation of the surface texture profile about a best-fit mean line. A typical specification for Ra may be 16 microinches, or 0.4 micrometers.

Figure 1. Demonstration of the measurement of Ra. Source: Michigan Metrology

Figure 1. Demonstration of the measurement of Ra. Source: Michigan Metrology

Figure 2A demonstrates the measurement of surface profile without filtering being comprised of many different spatial wavelengths. Source: Michigan Metrology

Figure 2B demonstrates the measurement of surface profile with only a 0.8-millimeter long wave cutoff applied, resulting in the elimination of the “wavy-like” structures. Source: Michigan Metrology
In addition to specifying the long wave cutoff, a short wave cutoff, ls, must be specified. Typically, the ratio of the long wave cutoff to the short wave cutoff is 100:1. Thus an 8-micrometer short wave cutoff may go along with a 0.8-millimeter long wave cutoff to establish a typical measurement bandwidth.

Figure 2C demonstrates the measurement of surface profile with only an 8-micrometer short wave cutoff applied, which results in the finer spaced structures being attenuated. Source: Michigan Metrology
Per ASME B46.1-1995 (p. 36), a standard on surface texture specification, the long wave cutoff length, lc, must be specified on all drawings. Per ISO 1302-2002 (p. 9), a standard on surface texture, the long wave cutoff length, lc, and the short wave cutoff length, ls, must be specified on all drawings.

Figure 2D demonstrates the measurement of surface profile with both long wave cutoffs and short wave cutoffs applied. Applying both the 0.8-millimeter long wave cutoff and the 8-micrometer short wave cutoff results in this “band limited” profile. Source: Michigan Metrology

Per ISO 1302-2002, the proper specification on a drawing for a surface with a maximum Ra of 4 micrometers with a long wave cutoff of 0.8 millimeter and a short wave cutoff of 8 micrometers is described in Figure 3. Source: Michigan Metrology
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