Drafting for the Theatre

7. 3D to 2D and Back

79 7.1 Multiview Drawings The very root of technical drafting is the challenge of describing three-dimensional shapes. To be useful, technical drafting must be drawn accurately and provide all of the information essential to successfully construct the needed object. Furthermore the drawing must convey the true size and shape of the object. Once the object is drawn, the description of it should be further enhanced by the addition of dimensions and notes describing materials, finishes, and assembly techniques. Ideally the drawing process works to the benefit of both the drafter and the user. Whenever possible, the drafter should use the drawing as a problem-solving opportunity to resolve many questions that might otherwise wait until the more costly construction phase for answers. All the drawing techniques discussed in this text rely on the conventions of orthographic (right-angle/straight-line) projection . Figure 7.1 illustrates the classification of projections available to the drafter. Two major divisions are (1) central projection and (2) parallel projection. The focus of this and succeeding chapters is the group of drawings known as multiview projections. 7. 3D to 2D and Back 7.1 Types of projection drawings 80 7.2 The Glass Box To illustrate the development of a set of multiview drawings using orthographic projection, an object is placed within an imaginary glass box. Surfaces of the object are then projected to the surface planes of the glass box (fig. 7.2). Once all sides of the object have been projected to their respective planes, the box is unfolded, revealing six views that accurately describe the three-dimensional object in twodimensional terms (fig. 7.3). Because the glass box surfaces were positioned an identical distance from each of the object planes, the distance between views will be identical, a condition very useful to the drafter. Except in very rare instances, there is no need to use all six views to fully describe the object being drawn. In fact most objects can be fully depicted using only three views—the most common views being front, top, and right or left side. Using these views, height, width, and depth dimensions can be shown (fig. 7.4). The term “elevation” is often used in place of “view” when referring to front, rear, or side views, i.e., those views in which height is shown. 7.3 Arrangement of Views The specific arrangement of orthographic views is very important. Proper orientation allows for the straightforward transfer of lines from one view to another, as seen in the ANSI arrangement shown in figures 7.3 and 7.4. This same formal arrangement holds true for computer drafting. When views are placed in the specified “orthographic” orientation, it is considered unnecessary to identify (label) them, which is both a timesaver and reduces visual clutter. If and when views are positioned out of order, the “removed view” must be clearly identified, e.g., “LEFT SIDE ELEVATION—REMOVED.” 7.4 Meaning of Lines Any line within a single orthographic view has three possible meanings. The line can be: • The intersection of two surfaces • The edge view of a surface • The contour end view of a curved surface Contemporary pencil drafting practice discourages the use of shading on working 7.2 The glass box 7.3 Unfolding the glass box 7.4 Standard dimension relationships 81 drawings and is almost never used on shop drawings, since the technique, although attractive , is very time-consuming. Consequently, it is necessary to carefully examine all the views of an object to determine the meaning of the lines, since a surface can be interpreted in several different ways (fig. 7.5). Line AB at the top of the front view might be regarded as the edge view of a flat surface, if we look at only the front and top views and do not observe the curved surface seen in the right-side view. Similarly, the vertical line CD in the front view might be regarded as the edge view of a parallel surface, if we look at only the front and side views. The top view shows that the line is the intersection of an inclined surface. Figure 7.6 further explores the issue of line interpretation. The top view shown is divided into four distinct planes—A, B, C and D. Each represents a surface at a different level. The one top view would accurately represent any of the five of...