Machinery's Handbook, 31st Edition
STANDARD Drafting Practices 625 Surface Texture Symbols.— A detailed explanation of the use of surface-texture sym bols from American National Standard ANSI/ASME Y14.36 begins on page 815, while ISO surface-texture techniques, standardized in ISO 1302, are explained beginning on page 822. Geometric Dimensioning and Tolerancing (GD&T).— ASME Y14.5-2018, “Dimen- sioning and Tolerancing,” covers dimensioning, tolerancing, and similar practices for en- gineering drawings and related documentation. The mathematical definitions of dimen - sioning and tolerancing principles are given in the standard ANSI/ASME Y14.5.1M-1994 (R2012). ISO standards ISO 8015, ISO 1101 and ISO 26921 contain a detailed explanation of ISO geometric dimensioning and tolerancing practices. Those ISO practices involving GD&T and other selected ISO standard practices are shown in contrast to ANSI/ASME practices where applicable. Geometric dimensioning and tolerancing provides a comprehensive system for symbolically defining the geometrical tolerance zone within which features must be contained. It provides an accurate transmission of design specifications among the three primary users of engineering drawings: design, manufacturing, and quality assurance groups. Some techniques introduced in ASME Y14.5-2018 have been accepted by ISO. These techniques include projected tolerance zone, the three-plane datum concept, total runout tolerance, multiple datums, and datum targets. Although Y14.5 follows ISO practice closely, there are still differences between ISO and US practice. Summaries of commonly used symbols in ASME Y14.5 and ISO standards are given in Table 4 and Table 5, respectively. One major area of disagreement is the ISO “principle of independency” versus the “Tay lor principle.” Y14.5 and standard US practice both follow the Taylor principle, in which a geometric tolerancing zone may not extend beyond the boundary (or envelope) of perfect form at MMC (maximum material condition). This boundary is prescribed to control variations as well as the size of individual features. The US definition of independency further defines features of size as being independent and not required to maintain a perfect relationship with other features. The “envelope principle” is optional in treatment of these principles. A summary of the application of ASME geometric control symbols and their use with basic dimensions and modifiers is given in Table 6. ASME Y14.5 features metric SI units (the International System of Units), but customary units may be used without violating any principles. On drawings where all dimensions are either in millimeters or in inches, individual identification of linear units is not required. However, the drawing should contain a note stating “Unless otherwise specified, all dimensions are in millimeters” (or in inches , as applicable). According to Y14.5, all dimensions are applicable at a temperature of 20 ° C (68 ° F) unless otherwise specified. Compensation may be made for measurements taken at other temperatures. Angular units are expressed in degrees and decimals of a degree (35.4) or in degrees ( ° ), minutes ( ′ ), and seconds ( ′′ ), as in 35 ° 25 ′ 10 ′′ . Where decimal degrees less than one are specified, a zero shall precede the decimal value. A 90-degree angle is implied where center lines and depicting features are shown on a drawing at right angles and no angle is specified. A 90-degree BASIC angle applies where center lines of features in a pattern or surface shown at right angles on a drawing are located or defined by basic dimensions and no angle is specified. Basic to all US practice is that orthographic drawing views are arranged in 3rd angle projection while Europe and Asia generally follow ISO practices that default to 1st angle projection. In 1st angle projection, the orientation of the views is reversed from US prac tice. See Fig. 1a for 3rd angle projection and Fig. 1b for 1st angle projection. Note the graphical symbol at the bottom of each, which indicates the projection system in use for the drawing. ANSI, ASME and ISO practice allow the other projection system, but the use of non-default practice must be declared by use of the graphical symbol.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online