Geometry is the branch of mathematics that deals with lines, angles, figures and certain assumed properties in space. Applied geometry, as used in drawing, makes use of these properties to accurately and correctly represent objects graphically. In the past, draftsmen utilized a variety of instruments with various scales, shapes and curves to make their drawings. Today, computer software graphics programs showing drawings provide nearly any scale, shape and curve imaginable, outdating the need for additional instruments.
A number of methods are used to illustrate objects graphically. The most common are orthographic projections, pictorial drawings, diagrams, and flowcharts.
Orthographic Projection Drawings
In order to show the exact size and shape of all the parts of complex objects, a number of views are necessary. This is the system used in orthographic projection.
In orthographic projection, there are six possible views of an object, because all objects have six sides—front, top, bottom, rear, right side, and left side. Figure 2‑12(a) shows an object placed in a transparent box, hinged at the edges. The projections on the sides of the box are the views as seen looking straight at the object through each side. If the outlines of the object are drawn on each surface and the box opened as shown in (b), then laid flat as shown in (c), the result is a six view orthographic projection.
It is seldom necessary to show all six views to portray an object clearly; therefore, only those views necessary to illustrate the required characteristics of the object are drawn. One-, two-, and three-view drawings are the most common. Regardless of the number of views used, the arrangement is generally as shown in Figure 2-12, with the front view as principal view. If the right side view is shown, it will be to the right of the front view. If the left side view is shown, it will be to the left of the front view. The top and bottom views, if included, will be shown in their respective positions relative to the front view.
One-view drawings are commonly used for objects of uniform thickness such as gaskets, shims, and plates. A dimensional note gives the thickness as shown in Figure 2-13. One-view drawings are also commonly used for cylindrical, spherical, or square parts if all the necessary dimensions can be properly shown in one view.
When space is limited and two views must be shown, symmetrical objects are often represented by half views, as illustrated in Figure 2-14.
Aircraft drawings seldom show more than two principal or complete views of an object. Instead, there will be usually one complete view and one or more detail views or sectional views.