Machinery's Handbook, 31st Edition
812 Surface Texture Roughness sampling length (l) is the sampling length within which the roughness average is determined. This length is chosen to separate the profile irregularities des ignated as roughness from those irregularities designated as waviness. It is different from evaluation length (L) and the traversing length. See Fig. 3. Evaluation length (L) is the length the surface characteristics are evaluated. The evalua tion length is a key specification of a measuring instrument. Traversing length is the profile length traversed to establish a representative evaluation length. It is always longer than the evaluation length. See Section 4.4.4 of ANSI/ASME B46.1-2009 for values that should be used for different types of measurements. Cutoff is the electrical response characteristic of the measuring instrument which is selected to limit the spacing of the surface irregularities to be included in the assessment of surface texture. Cutoff is rated in millimeters. In most electrical averaging instruments, the cutoff can be user-selected and is a characteristic of the instrument rather than of the surface being measured. In specifying the cutoff, care must be taken to choose a value that will include all the surface irregularities to be assessed. Waviness sampling length (l) is a concept no longer used. See waviness long-wavelength cutoff and waviness evaluation length. Roughness Parameters.— Roughness refers to the fine irregularities of the surface tex - ture resulting from the production process or material condition. Roughness average (Ra or R a ), also known as arithmetic average (AA), is the arithmetic average of the absolute values of the measured profile height deviations divided by the evaluation length, L. This is shown as the shaded area of Fig. 4 and generally includes sampling lengths or cutoffs. For graphical determinations of roughness average, the height deviations are measured normal, or perpendicular, to the chart center line. Y'
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Fig. 4. Roughness average is expressed in micrometers (μm). A micrometer is one millionth of a meter (0.000001 meter). A microinch ( μ in) is one millionth of an inch (0.000001 inch). One microinch equals 0.0254 micrometer (1 μ in. = 0.0254 μ m). Roughness average (Ra) value from continuously averaging meter reading may be made of readings from stylus-type instruments of the continuously averaging type. To ensure uniform interpretation, it should be understood that the reading that is considered significant is the mean reading around which the needle tends to dwell or fluctuate with a small amplitude. Roughness is also indicated by the root-mean-square (rms) average, which is the square root of the average value squared, within the evaluation length and measured from the mean line shown in Fig. 4, expressed in micrometers. A roughness-measuring instrument calibrated for rms average usually reads about 11 percent higher than an instrument cali brated for arithmetical average. Such instruments usually can be recalibrated to read arithmetical average. Some manufacturers consider the difference between rms and AA to be small enough that rms on a drawing may be read as AA for many purposes.
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