Drafting

Drafting, also called mechanical drawing, is the process of representing topography, engineering works, buildings, and pieces of machinery by means of conventionalized drawings called mechanical or engineering drawings (see Drawing). The techniques of drafting are an important part of all types of engineering and manufacturing and are also used in such diverse fields as architecture and geology.

The fundamental purpose of a mechanical drawing is to convey the exact shape and dimensions of an object. An ordinary perspective drawing gives no information about hidden details of the object and is not drawn to scale. The technique of conventional drafting is to use two or more projections to represent solid objects (see Geometry). These projections are views or plans of the object and are not complete in themselves, but when taken together they represent every dimension and detail of the object.

The primary view or projection of a mechanical drawing is the front view, or elevation, which usually shows the side of the object having the largest dimensions. Above the elevation the drafter draws the top view, or plan, which shows the object as viewed from directly above. When these two views are not sufficient, other views are added: a side view, or side elevation;auxiliary views, based on special points of view outside the object and designed to show external details that are not otherwise clear; and sectional views, or sections, which represent the object as partly cut away to show internal detail. Each line in a mechanical drawing represents a contour or a meeting of two surfaces. Unbroken lines represent visible contours and surface junctions, and dotted lines represent hidden ones. The drawing indicates the exact dimensions of all lines necessary to define the object exactly.

A number of special conventions are used in topographic, architectural, and other forms of drafting to give information that will be needed by the user of the drawing. In topographic drafting, special signs are used to show types of land and elevations and depressions of the land (see Map). In architectural and mechanical drafting, specific linings, shadings, and cross-hatchings indicate different kinds of structural materials. When practical, mechanical drawings are drafted to be precisely the same size as the object represented. Drawings of small details of machinery, however, are frequently drawn with all dimensions enlarged according to a given ratio, while maps and drawings of large engineering works are drawn with dimensions reduced. The ratio of enlargement or reduction of a drawing is known as its scale and is always marked on the drawing.

Today, most drafting work is done on computers. Mechanical drawings done on a computer are easier to manipulate than those done on paper. Also, computers make the design and production process more efficient. For example, if the specifications of a small part of a larger machine are changed on a computer, the computer can calculate how the change affects the rest of the machine, before the machine is actually built. See Computer-Aided Design/Computer-Aided Manufacture.