16 GD&T: Application and Interpretation Copyright Goodheart-Willcox Co., Inc. manufacturer incorrectly assumed an angle of pro- jection. As a result, parts were fabricated incorrectly. In one case, a structural part for a parking garage was fabricated using the wrong projection system, and the incorrectly made parts were put into the concrete columns before the error was noticed. In this exam- ple, either the engineer made a mistake by not show- ing the applicable angle of projection symbol, or the fabricator read the drawing incorrectly. Regardless of who made the mistake, there was potential for loss of lives because of not understanding the angle of projection. With the vast amount of global man- ufacturing that takes place, it is important to avoid drawing interpretation errors that could potentially affect product quality and safety. Figure 1-11 shows the symbols for third angle projection and fi rst angle projection. Only one of them should be shown on a drawing. The projection symbol is typically shown in or near the title block on the drawing sheet. Orthographic views in this textbook are drawn using the third angle projection system unless noted otherwise. Where orthographic views are used, each adja- cent view appears as though the part is rotated 90°. Features in adjacent views can be related to one another by drawing or visualizing projection lines between the views. When drawings are manually cre- ated, the projection lines are typically drawn lightly so they do not appear in copies of the drawing. If all of the projection lines were visible on a complex part, it would be very hard to tell the difference between the part geometry and the projection lines. If the part is complex, it may be helpful to draw some of the projection lines. However, even when no projection lines are visible, it is possible to mentally connect the features in adjacent views. Figure 1-12 shows a draw- ing with projection lines. The projection lines shown are not intended to be part of the fi nal drawing but are included to show the relationship between fea- tures in the views. Future Trends A signifi cant change has taken place in industry over the past few decades. In some companies, the advancement and integration of software tools has resulted in replacement of manual drafting methods. Even small companies can now afford to use CAD systems for product design. See Figure 1-13. Ø2X .5X 2X Third angle projection symbol Ø1X Ø2X .5X 2X First angle projection symbol Ø1X X = Character height on drawing X = Character height on drawing Goodheart-Willcox Publisher Figure 1-11. First and third angle projection symbols. Projection lines Goodheart-Willcox Publisher Figure 1-12. Projection lines relate features in adjacent views. Goodheart-Willcox Publisher Figure 1-13. Manual drafting methods have generally been replaced by the use of CAD software for product design.