5 Chapter 1 Introduction to Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. requirement such as perpendicularity could be as numerous as the number of people writing such a specifi cation. Over time, common dimensioning and tol- erancing practices began to develop and result in standards. The United States was a leader in this area of standards development and greatly infl uenced standards throughout the world. However, by the 1990s, many countries had their own standards for dimensioning and tolerancing. Utilization of a recognized standard is essential to clearly communicate dimensions and tolerances defi ning product requirements. It is the responsibil- ity of the person who applies tolerances to master the standardized practices. Properly specifi ed dimen- sional requirements allow for effi cient utilization of today’s industrial capabilities. The methods for completing product defi nition data have advanced along with the capabilities of production and inspection machines. During the 20th century, drawing instruments advanced suffi ciently to enable production of very accurate manually cre- ated drawings. During the last two decades of the 20th century, CAD tools emerged. Now, computer software and hardware have replaced drawing instruments in many companies. Computer-generated product defi nition data can contain dimensional data with accuracies far beyond what could be achieved with manual drafting practices. Although a computer-generated model contains precise defi nition of what was created through the design process, production machines cannot produce perfect parts from the model. In the past few decades, however, machines have improved and can achieve amazing accuracies. Regardless of the precision that is possible today, all machine processes produce a fi nite amount of feature variation in every fabricated part. Many factors in addition to the machine quality affect fea- ture variation. Cutting tools, tool setup, cutting feed and speed, cutting lubricant and coolant, product material, product design, and even temperature can introduce feature variation. Because of these factors, it is essential that the amount of acceptable variation be defi ned so that processes can be correctly selected. Specifi cation of the acceptable variation is done through the application of tolerances. Tolerances are the acceptable dimensional variations that are per- mitted on a feature. There is a responsibility to specify only the degree of dimensional accuracy necessary for the parts to assemble and function properly while meet- ing customer expectations. Specifying a tolerance that is smaller than needed can increase both the cost and time required to produce a part. The practices for showing size, form, orienta- tion, runout, location, and profi le tolerance require- ments are better defi ned today than ever before. These practices are standardized. Standardization provides both a clear and consistent set of practices that may be understood anywhere. Applicable Dimensioning Standards ASME Y14.5 is the main United States standard for dimensioning and tolerancing. Additional stan- dards do exist and address related applications. The most relevant ASME standards for specifi c applications are: ASME Y14.5 Dimensioning and Tolerancing ASME Y14.8 Castings, Forgings, and Molded Parts ASME Y14.41 Digital Product Defi nition Data Practices ASME Y14.46 Product Defi nition for Additive Manufacturing ASME AED-1 Aerospace and Advanced Engineering Drawings ASME Y14.5 provides defi nition for dimen- sions and tolerances. It also shows how dimensions and tolerances are applied on orthographic drawing views and to some extent on models. ASME Y14.8 includes some special dimensioning and tolerancing requirements for castings, forgings, and molded parts. ASME Y14.41 specifi es require- ments for applications in annotated models (models including annotation), saved views in annotated models, and orthographic drawing views generated from model geometry. ASME Y14.41 relies on ASME Y14.5 to explain the meaning of the tolerances. ASME Y14.46 includes special dimensioning and tolerancing requirements for parts that are fab- ricated by additive manufacturing methods. ASME AED-1 includes special dimensioning and toler- ancing requirements for aircraft and to some extent covers assemblies of any product type. Development of methods for defi ning toler- ances through what is commonly referred to as geometric dimensioning and tolerancing (GD&T) or positional tolerancing began in military documents written in the 1940s and evolved into the MIL-STD-8 series of standards starting in the 1950s. The devel- opment of American industry standards began with USASI Y14.5-1966. The following list shows the evo- lution of standards to what is now the ASME Y14.5 dimensioning and tolerancing standard. MIL-STD-8A (1953) MIL-STD-8B (1959) MIL-STD-8C (1963)