Machinery's Handbook, 31st Edition
Surface Texture 815 Type IV, Profiling Contact Skidded Instruments: Uses a skid as a datum to eliminate lon ger wavelengths; thus cannot be used for waviness or errors of form. May have a selection of filters and parameters and generates an output recording of filtered and skid-modified profiles. Examples include: 1) skidded, stylus-type with LVDT vertical measuring trans ducer and 2) fringe-field capacitance (FFC) transducer. Type V, Skidded Instruments with Parameters Only: Uses a skid as a datum to eliminate longer wavelengths; thus cannot be used for waviness or errors of form. Does not generate a profile. Filters are typically 2RC type and generate Ra, but other parameters may be available. Examples include: 1) skidded, stylus-type with piezoelectric measuring transducer and 2) skidded, stylus-type with moving coil measuring transducer. Type VI, Area Averaging Methods: Used to measure averaged parameters over defined areas, but do not generate profiles. Examples include: 1) parallel plate capacitance (PPC) method; 2) total integrated scatter (TIS); 3) angle resolved scatter (ARS)/bi-directional reflectance distribution function (BRDF). Selecting Cutoff for Roughness Measurements.— In general, surfaces will contain irregularities with a large range of widths. Surface texture instruments are designed to respond only to irregularity spacings less than a given value, called cutoff. In some cases, such as surfaces in which actual contact area with a mating surface is important, the larg- est convenient cutoff will be used. In other cases, such as surfaces subject to fatigue fail- ure, only the irregularities of small width will be important, and more significant values will be obtained when a short cutoff is used. In still other cases, such as identifying chatter marks on machined surfaces, information is needed on only the widely space irregulari- ties. For such measurements, a large cutoff value and a larger radius stylus should be used. The effect of variation in cutoff can be understood better by reference to Fig. 6. The pro file at the top is the true movement of a stylus on a surface having a roughness spacing of about 1 mm and the profiles below are interpretations of the same surface with cutoff value settings of 0.8 mm, 0.25 mm and 0.08 mm, respectively. It can be seen that the trace based on 0.8 mm cutoff includes most of the coarse irregularities and all of the fine irregularities of the surface. The trace based on 0.25 mm excludes the coarser irregularities but includes the fine and medium fine. The trace based on 0.08 mm cutoff includes only the very fine irregularities. In this example, the effect of reducing the cutoff has been to reduce the roughness average indication. However, had the surface been made up only of irregulari ties as fine as those of the bottom trace, the roughness average values would have been the same for all three cutoff settings. In other words, all irregularities having a spacing less than the value of the cutoff used are included in a measurement. Obviously, if the cutoff value is too small to include coarser irregularities of a surface, the measurements will not agree with those taken with a larger cutoff. For this reason, care must be taken to choose a cutoff value which will include all of the surface irregularities it is desired to assess. To become proficient in the use of continuously averaging stylus-type instruments, the inspector or machine operator must realize that for uniform interpretation the reading considered significant is the mean reading around which the needle tends to dwell or fluctuate under small amplitude. Drawing Practices for Surface Texture Symbols.— American National Standard ANSI/ASME Y14.36-2018 establishes the method to designate symbolic controls for surface texture of solid materials. It includes methods for controlling roughness, wavi- ness, and lay, and provides a set of symbols for use on drawings, specifications, or other documents. The standard is expressed in SI metric units, but US customary units may be used without prejudice. Units used (metric or non-metric) should be consistent with the other units used on the drawing or documents. Approximate non-metric equivalents are shown for reference.
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