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.