Many objects, however, have no definite top,bottom, front, or back—as many types ofmachine parts, for example. With an object ofthis kind, you can select a surface and call it thefront, and select another and call it the top,according to convenience. However, it is a generalrule that an object should be shown in theposition it customarily occupies.One-view drawings are permissible for objectsfor which one view and such features as thicknessor length, stated as a dimension or note, cancompletely define the object.NORMAL AND NON-NORMAL LINES.—In a multi-view orthographic projection, aNORMAL line is one that is parallel to two ofthe planes of projection and perpendicular to thethird. A line that is parallel to a plane ofprojection will appear on that plane in its truelength (to the scale of the drawing). A line thatis perpendicular to a plane of projection willappear on that plane as a point.A line that is perpendicular to one plane ofprojection must of necessity be parallel to theother two. But a line that is parallel to one planeof projection may be oblique (neither parallel norperpendicular) to one or both of the others. A linethat is oblique to one or more of the planes ofprojection is called a NON-NORMAL LINE.If a non-normal line is parallel to a plane ofprojection, it will appear on that plane in its truelength. However, it will appear foreshortened inFigure 5-20.-Foreshortening of a line inprojection.a multi-viewa view on a plane to which it is oblique. A non-normal line may, of course, be oblique to all threeplanes of projection, in which case it will appearforeshortened in all regular views of the object.A REGULAR VIEW is a view on one of the threeregular planes of projection (horizontal, vertical,or profile). Views on planes other than the regularplanes are called AUXILIARY VIEWS. Auxiliaryviews will be discussed later in this chapter.A single-view projection of a block is shownin the upper left corner of figure 5-20. Thisblock is presumed to be placed for multi-viewprojection with the front parallel to the verticalplane, the bottom parallel to the horizontal plane,and the right side parallel to the profile plane. Theline AB, then, is parallel to the vertical plane, butoblique to both the horizontal and the profileplanes.In the multi-view projections, you can see thatit is only in the views on the vertical plane (thefront and back views) that the line AB appearsin its true length. In the views on the horizontalFigure 5-21.-A circle on a surface oblique to the plane ofprojection projected as an ellipse.5-12
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