GD&T: Application and Interpretation, 7th Edition Page 13 (29 of 496)

13 Chapter 1 Introduction to Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. resolved value and the applied dimension value have the same number of decimal places, they must be exactly the same. Dimension A dimension shows a dimensional requirement for one or more features in an annotated model or on a drawing view. A length dimension includes the following elements: a dimension value, a dimension line, extension lines, and arrowheads. See Figure 1-8. Other elements may be shown as part of a dimen- sion to represent specifi c applications. Examples of other elements used with dimensions include the following: ■ A directly applied tolerance ■ An indication of a basic dimension ■ An origin symbol used in place of one arrowhead to indicate the origin of a dimension ■ An arc length indication ■ An out-of-scale indication These elements and other applications are explained in the chapters on dimensioning. The dimension value is typically a single value that is shown as a basic dimension or has a tolerance directly applied. In some instances, the dimension value may be shown with two values that represent the maximum and minimum limit. The dimension value term that is defi ned in this book is used in addition to the terminology defi ned in ASME Y14.5. ASME Y14.5 uses the term dimension to refer to the set of elements described above and also uses the same term for the shown value. Using the term dimension to describe the shown value can cause confusion. It makes little sense to refer to the shown value as “the dimension shown in the dimen- sion.” It is much easier to understand a reference to “the dimension value shown in the dimension.” Nominal Values The ASME Y14.5 and B4.1 standards defi ne nominal size as the size value or condition of a fea- ture by which a feature is known. Stating it another way, a nominal size designation is a name for a cer- tain size feature. The nominal value for a feature of Goodheart-Willcox Publisher Figure 1-7. An annotated model includes the model and annotations such as dimensions, tolerances, and notes. Features in the model may be queried to display dimensional data. The displayed value may be expressed to a different precision than the queried value depending on the software settings. Dimension Dimension value Extension line Dimension line Goodheart-Willcox Publisher Figure 1-8. Elements of a dimension are shown.

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13 Chapter 1 Introduction to Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. resolved value and the applied dimension value have the same number of decimal places, they must be exactly the same. Dimension A dimension shows a dimensional requirement for one or more features in an annotated model or on a drawing view. A length dimension includes the following elements: a dimension value, a dimension line, extension lines, and arrowheads. See Figure 1-8. Other elements may be shown as part of a dimen- sion to represent specifi c applications. Examples of other elements used with dimensions include the following: ■ A directly applied tolerance ■ An indication of a basic dimension ■ An origin symbol used in place of one arrowhead to indicate the origin of a dimension ■ An arc length indication ■ An out-of-scale indication These elements and other applications are explained in the chapters on dimensioning. The dimension value is typically a single value that is shown as a basic dimension or has a tolerance directly applied. In some instances, the dimension value may be shown with two values that represent the maximum and minimum limit. The dimension value term that is defi ned in this book is used in addition to the terminology defi ned in ASME Y14.5. ASME Y14.5 uses the term dimension to refer to the set of elements described above and also uses the same term for the shown value. Using the term dimension to describe the shown value can cause confusion. It makes little sense to refer to the shown value as “the dimension shown in the dimen- sion.” It is much easier to understand a reference to “the dimension value shown in the dimension.” Nominal Values The ASME Y14.5 and B4.1 standards defi ne nominal size as the size value or condition of a fea- ture by which a feature is known. Stating it another way, a nominal size designation is a name for a cer- tain size feature. The nominal value for a feature of Goodheart-Willcox Publisher Figure 1-7. An annotated model includes the model and annotations such as dimensions, tolerances, and notes. Features in the model may be queried to display dimensional data. The displayed value may be expressed to a different precision than the queried value depending on the software settings. Dimension Dimension value Extension line Dimension line Goodheart-Willcox Publisher Figure 1-8. Elements of a dimension are shown.

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12 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. To avoid violating copyright of the ASME standards, and to simplify understanding, the defi ni- tions given throughout this book and in the Glossary are not quotations of the ASME standards but are consistent with the intent of those standards. Product Defi nition Data Set The requirements of a product are defi ned in one or more data fi les or documents called a product defi - nition data set. A product defi nition data set refers to the entire collection of documentation used to provide a complete description of the requirements for a prod- uct. The product defi nition data set includes items such as an annotated model, a multiview drawing, notes lists, and a list of required materials. Model In a product defi nition data set, a model refers to the model geometry that defi nes the part and any required supplemental geometry used to communicate requirements. See Figure 1-6. The model geometry is all of the defi ning geometry of the part itself. Supplemental geometry refers to geometric elements that are not part of the physical model geometry but are used to help communicate requirements pertaining to the model. Supplemen- tal geometry includes elements such as points, lines, curves, planes, surfaces, and solids. The model includes attributes defi ning the feature dimensions that are used to create the geom- etry. Attributes are dimensions and noted require- ments that may or may not be visible but are available by interrogation of the model. The attributes in the model are stored with accuracy to a number of dec- imal places defi ned by the system capabilities and software settings. Annotated Model Annotation is visible textual content shown in a model to establish required information such as dimensions, tolerances, and notes. An annotated model is a model that includes annotation to describe the product. See Figure 1-7. Annotation may be directly applied to the model geometry and may be associated with model features so that the features highlight when the annotation is selected or queried. Examples of annotation applied to model geometry are the 2.25″ dimension and the noted hole size in Figure 1-7. The straightness tolerance in the fi gure is an example of annotation applied to supplemen- tal geometry. The straightness tolerance is applied to a line that indicates the direction of application. The line is supplemental geometry and is not a feature on the part, but it is essential because it establishes the direction of the required straightness tolerance. A dimension value displayed on a model must be in agreement with the corresponding value obtained when the model geometry is queried. When a model is created, some features in the model geometry may be directly created and others may be a result of intersections or unions of two or more features. Regardless of how features are created, each feature has model values associated with the feature geometry. Model values are expressed to the precision defi ned by the software and are as accurate as the number of decimal places calculated by the software. A model feature may be queried to obtain a model value, but the queried value, when displayed, may be limited to fewer decimal places based on sys- tem settings. The queried value rounded off is called a resolved value. Refer to Figure 1-7. If a dimension is applied to the same feature, that dimension may be limited to fewer decimal places than the resolved value. Thus, the displayed value may not be the exact same value as the resolved value. However, if the Goodheart-Willcox Publisher Figure 1-6. A model includes the model geometry defi ning the part and any needed supplemental geometry.

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14 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. size is not required to be the same as the dimension value, it may not be either dimension limit, and it may not even be a value that lies within the mini- mum and maximum acceptable dimension limits. Examples include nominal sizes for thread and pipe. The common designation of a #10 screw thread is .190-24UNC-2A thread. The decimal value .190 is a name by which the thread size is known. It is similar to the #10 designation except that it gives a value relatively close to the major diameter of the thread. The limits of size for this screw thread are .1818″–.1890″. From this example, it can be seen that the nominal size designation is not always within the limits of size. Nominal sizes for common steel pipe are also general designations. For example, anyone familiar with plumbing or pipe fi ttings knows that a 1/2″ diameter pipe is not .500″ diameter. Yet it is the size designation by which a particular size pipe is known. In this book, the term nominal is used to refer to a general size designation. This is in agreement with the defi nition provided by ASME standards. There is a common use for the term nominal that is not supported by an ASME defi nition, but it occurs often enough that it can be considered a collo- quialism. Nominal is often used as a term to identify the design condition of model geometry or the dis- played dimension values on an annotated model or a drawing view. Although these uses are not techni- cally in agreement with the ASME defi nition of nom- inal, they are common enough that the intent is often understood. To avoid the risk of misunderstanding, it is better to use the term nominal only as defi ned by ASME standards. Orthographic Projection Because of the widespread use of models in product design, not everyone entering industry may understand the 2D orthographic views that appear on a drawing sheet or in the saved views of an anno- tated model. To ensure a basic understanding of orthographic views, this section provides a brief intro- duction to the common methods used to create them. When orthographic views are manually created, the views are typically arranged to make it easy to relate one view to another. The process used to man- ually create multiple orthographic views is called orthographic projection. When orthographic views are created from a model, the view creation process is mostly completed by the CAD software. Even though a manual orthographic projection process is not used when creating a multiview drawing from a model, it is helpful to arrange the views as though they were projected manually. A basic introduction to orthographic views is provided here. Understanding how to “read” the views is important to correctly understand GD&T as it is presented in this book, and also to under- stand the annotated models and orthographic views encountered in industry. It is beyond the scope of this book to completely describe the orthographic view creation process. Drafting textbooks provide a thorough explanation of view creation and rules that must be followed in creating the orthographic views. The following information should be suffi cient to understand the tolerancing practices illustrated in the fi gures of this book. Angles of Projection There are two projection systems that may be used to establish a known relationship between orthographic views. These projection systems are known as fi rst angle projection and third angle projection. The angle of projection must be noted on the drawing to prevent misunderstanding of the views. The United States typically uses third angle projection, but it should not be assumed to be appli- cable. Some countries use fi rst angle projection, but the projection angle used throughout a country is not always consistent. In both systems, the views appear as though the features on the part are projected onto a fl at surface, such as a drawing sheet. Adjacent views are drawn as though the part is rotated 90° between the views, or the viewing line of sight is rotated 90°. Relationship between Views The view that shows the most detail about the part is usually selected as the front view. The front view is usually shown in its natural orientation that is, the bottom of the part is at the bottom of the view. Once the front view is selected, the number of additional views needed to completely describe the part is deter- mined. For some simple parts, it is possible to use only one view, but usually two or more views are needed. Figure 1-9 shows the standard locations for six orthographic views that are drawn using third angle projection. For third angle projection, the example of a house makes the process fairly simple to under- stand. If the front of a house is shown in a front view, then it makes sense for the top of the house to be positioned above the front view. Similarly, the right side of the house would be drawn to the right of the front view. Third angle projection works the same way regardless of the part depicted. When looking at what was selected as the best front view of a part, rotating that part 90° to look at the top would create a top view that goes above the front view.

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