Machinery's Handbook, 31st Edition
Surface Texture 819 Waviness Height (Wt): The preferred series of maximum waviness height values is listed in Table 4. Waviness height is not currently shown in US or ISO Standards. It is included here to follow present industry practice in the United States. Table 4. Preferred Series Maximum Waviness Height Values mm inch mm inch mm inch 0.0005 0.00002 0.008 0.0003 0.12 0.005 0.0008 0.00003 0.012 0.0005 0.20 0.008 0.0012 0.00005 0.020 0.0008 0.25 0.010 0.0020 0.00008 0.025 0.001 0.38 0.015 0.0025 0.0001 0.05 0.002 0.50 0.020 0.005 0.0002 0.08 0.003 0.80 0.030 Lay: Symbols for designating the direction of lay are shown and interpreted in Table 5. Example Designations.— Table 6 illustrates examples of designations of roughness, waviness, and lay by insertion of values in appropriate positions relative to the symbol. Where surface roughness control of several operations is required within a given area or on a given surface, surface qualities may be designated, as in Fig. 9a. If a surface must be produced by one particular process or a series of processes, they should be specified as shown in Fig. 9b. Where special requirements are needed on a designated surface, a note should be added at the symbol giving the requirements and the area involved. An example is illustrated in Fig. 9c. Surface Texture of Castings.— Surface characteristics should not be controlled on a drawing or specification unless such control is essential to functional performance or ap - pearance of the product. Imposition of such restrictions when unnecessary may increase production costs and in any event will serve to lessen the emphasis on the control specified for important surfaces. Surface characteristics of castings should never be considered on the same basis as machined surfaces. Castings are characterized by random distribution of nondirectional deviations from the nominal surface. Surfaces of castings rarely need control beyond that provided by the production method necessary to meet dimensional requirements. Comparison specimens are frequently used for evaluating surfaces having specific functional requirements. Surface texture control should not be specified unless required for appearance or function of the surface. Specifi cation of such requirements may increase cost to the user. Engineers should recognize that different areas of the same castings may have different surface textures. It is recommended that specifications of the surface be limited to defined areas of the casting. Practicality of and methods of determining that a casting’s surface texture meets the specification shall be coordinated with the producer. The Society of Automotive Engineers standard J435 “Automotive Steel Castings” describes methods of evaluating steel casting surface texture used in the automotive and related industries. Metric Dimensions on Drawings.— The length units of the metric system most gener- ally used in connection with any work relating to mechanical engineering are the meter (39.37 inches) and the millimeter (0.03937 inch). One meter equals 1000 millimeters. On mechanical drawings, all dimensions are generally given in millimeters, no matter how large the dimensions may be. In fact, dimensions of such machines as locomotives and large electrical apparatus are given exclusively in millimeters. This practice is adopted to avoid mistakes due to misplacing decimal points, or misreading dimensions as when other units are used as well. When dimensions are given in millimeters, many of them can be given without resorting to decimal points, as a millimeter is only a little more than 1 ⁄ 32 inch. Only dimensions of precision need be given in decimals of a millimeter; such dimensions are generally given in hundredths of a millimeter—for example, 0.02 millimeter, which is equal to 0.0008 inch. As 0.01 millimeter is equal to 0.0004 inch, dimensions are seldom given with greater accuracy than to hundredths of a millimeter.
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