9 Chapter 1 Introduction to Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. Government and military design personnel need a thorough knowledge of dimensioning and tolerancing principles. Many government design and production contracts require that product defi ni- tion be completed in compliance with dimensioning and tolerancing standards. Product defi nition completed in compliance with the applicable standards will be accepted by customers and suppliers with less need for revision than if there is incomplete compliance. Revisions that are necessary because of noncompliance with stan- dards are usually made at the contractor’s expense. Proper application of the dimensioning and tolerancing principles will make part requirements clearer and result in tolerance zones that are deter- mined by design function. Both of these factors can reduce part cost. Required Dimensioning Skills Dimensions and tolerances must be correctly applied in the product defi nition whether done on a model or a drawing. This requires that standard application methods and symbology be under- stood. The ability to interpret the symbols applied to a design, as well as their meanings, is necessary because the parts must be produced according to what is specifi ed. The ability to explain dimensioning and tolerancing requirements is needed when others with less knowledge on this subject need assistance. The correct application of tolerancing symbol- ogy is only part of the needed tolerancing capability. It is also necessary to determine the correct tolerance values, because showing the tolerances on the model or on the drawing views is benefi cial only if the toler- ance values are achievable and functional. For inter- facing features in an assembly, the best way to be sure tolerance values are correct is to calculate them. Other means of assigning tolerance values, such as similarity to previous designs or determination by judgment, introduces the risk of tolerances that may not result in the desired fi t and function of the parts. Application Decades ago, the methods of showing tolerances were limited to tolerances applied directly to numer- ical values (such as size dimensions) and geometric controls that were explained in notes. The practice of using notes to specify geometric requirements often resulted in uncertainty about the meaning of the note. It was not until the establishment of stan- dard symbols that tolerancing methods progressed beyond the ambiguities of notations. Symbols shown in the ASME standards provide the means for defi ning requirements and remov- ing the ambiguity that can result from using notes. Figure 1-4 shows a perpendicularity requirement for a dowel pin. This tolerance specifi cation has a well-defi ned meaning (as will be shown in the chapter on orientation tolerances). The same control expressed through a note takes more room and is subject to the language skills of the writer and the reader. Proper application of dimensions and tolerances is usually the responsibility of the design engineer, designer, or drafter. Therefore, he or she must have a good knowledge of dimensioning and tolerancing application principles. Various others may review the design before it is approved for release and those reviewers are expected to help ensure the product defi nition is correct. The reviewers must therefore have an understanding of dimensioning and tol- erancing principles for their review to be valid and reliable. Individuals applying dimensions and tolerances must use standard symbols in compliance with the methods defi ned in the applicable release of ASME Y14.5 and other relevant standards. The standard symbols are important where it is desired that others understand the shown information. Nonstandard symbols should be avoided, because nonstandard symbols have no defi ned meaning. This can be somewhat alleviated by doc- umenting a defi nition for any nonstandard symbol, and some large corporations do develop their own company standards that expand on the contents of the ASME standards. When this is done, the product defi nition or contractual agreements with suppliers should include reference to any company-specifi c standards that are applicable. Goodheart-Willcox Publisher Figure 1-4. Symbology is used to defi ne geometric tolerances.