28 Geometric Dimensioning and Tolerancing Copyright Goodheart-Willcox Co., Inc. GD&T symbols to represent specific dimensioning and GD&T requirements. CADD automates the application of GD&T symbols, but the user must apply dimensions and symbols correctly using proper drafting practices and drawing techniques. The advantage of using CADD for GD&T exists because the designer can place GD&T symbols on a drawing in much less time than it takes to manually draw the same symbols. The added advantage occurs when it becomes necessary to revise or change a drawing. Making engineering changes that often take many hours manually can be done in a few minutes using CADD. It becomes an easier task to remove or change symbols using the software and then print or plot a new representation. As previously discussed, the software tools available for applying GD&T on 2D drawings vary depending on the software used. Autodesk AutoCAD provides the TOLERANCE command for adding GD&T information to a drawing. This command accesses the Geometric Tolerance dialog box. See Example 1-23. This is the primary method for drawing and editing 2D feature control frames, geometric tolerancing symbols, and datum feature symbols in AutoCAD. Other commands are available in AutoCAD for creating dimensions, symbols, and text to completely specify GD&T requirements. Model-Based Definition (MBD) GD&T is most often applied to features shown on 2D multiview engineering drawings. Designers prepare 2D drawing views and annotations without creating 3D models, or by referencing 3D model data when a model is created. As new technology emerges and industry and company practices evolve, annotated models are becoming a popular approach to design and drafting. An annotated model is a 3D model in which dimensions, GD&T information, and other annotations defining the product are applied directly to the model geometry. In some companies where annotated models have become the primary engineering documentation, models have eliminated engineering drawings. Model-based definition (MBD) is the dimensioning, tolerancing, and annotating of 3D models, most often solid models, in a model-based design environment. Digital product definition (DPD) is another name for MBD. The software tools used to apply MBD are often referred to as 3D annotation tools or product and manufacturing infor- mation (PMI) tools. Product and manufacturing information (PMI) refers to model annotations such as dimensions, GD&T specifications, notes, and surface finish speci- fications. Refer to the solid model of the hub in Example 1-22B. In this example, PMI tools were used to define the GD&T information within the model. Goodheart-Willcox Publisher Example 1-23. Using the AutoCAD Geometric Tolerance dialog box to draw a feature control frame on a 2D engineering drawing.