174 Print Reading for Industry Copyright Goodheart-Willcox Co., Inc. P rint reading requires the reader to understand the shape description of the object. This is called visualiza- tion. Previous units primarily focused on describing the shape of an object or part with different view sys- tems, sectional techniques, and special symbols for screw threads. The second major component of print reading is the size description. The reader must under- stand the size of a part as specified on the print. The size description can simply be called dimensioning. Dimensions are annotations to the views describing the size or location of each feature. Dimen- sioning can be a very difficult area of drafting to learn. The drafter must not only know the mechanics of how to create all the dimension and extension lines, including sizes and spacing, but also how to choose which dimension to place and where to best place the dimension. Sometimes, this can be a real challenge. Unfortunately, many drafters and engineers have not had sufficient training in how to properly dimension a drawing. Therefore, some prints may not follow standard recommendations for dimensioning as well as they should, making them harder to read. This unit explains some of the rules a drafter should use to choose and place dimensions. This will assist the print reader in knowing which dimen- sions to expect and where to look for a dimension. As a print reader, you can be a valuable team member by identifying ways in which to improve a drawing with more concise or appropriate dimensioning. The primary standard that covers dimensioning practices is ASME Y14.5, entitled Dimensioning and Tolerancing. Units 10 and 13 of this text also rely heavily on this standard. The ASME Y14.41 Digital Product Definition Data Practices standard addresses the use of computer models and data sets to organize, control, and manage the data associated with parts. Machines can now manufacture, check, and analyze parts without many of the dimensions and notes previously defined by two-dimensional drawings and prints. The ASME Y14.31 Undimensioned Drawings standard helps set forth guidelines for undimensioned draw- ings, such as undimensioned printed circuit drawings and flat pattern drawings. The Y14.41 and Y14.31 stan- dards are assisting industry with new ways to perform some of the tasks defined in this unit. These standards are discussed in more detail later in this unit. Dimensioning Mechanics Dimensioning mechanics can be defined as the instructions or guidelines for what size and spacing to use for all of the components of the dimension. For example, arrowhead appearance, lettering size, and spacing for extension lines are all part of dimensioning mechanics. Even CAD systems that automatically create dimensions need to be set up and managed so that the automatic dimensioning conforms to standards. Simply put, mechanics is the part of dimensioning concerned with what dimen- sions look like, rather than where dimensions are placed or whether or not they are even needed. Lines Used in Dimensioning The standard lines used in dimensioning are presented in Unit 2, which include the extension line, dimension line, and leader line. Figure 9-1 not only Goodheart-Willcox Publisher Figure 9-1. This illustration describes some of the dimensioning mechanics the drafter uses to place dimension lines and extension lines in a clear and organized manner. Extension line Dimension line 1/4″ or more 1/8″ 1/8″ 1/8″ 1/16″ 3/8″ or more