Mechanical drawing is a drawing that provides the exact information needed to produce an architectural structure or a manufactured object. Every building, bridge, and dam requires mechanical drawings, as does every automobile, refrigerator, and lathe.
Information in the drawing.
Like a sketch, a mechanical drawing shows how an object should look. However, the drawing is made precisely to scale, and specifications of dimensions, angles, and other measurements are written on it.
The drawing also provides instructions for producing the object. For example, a mechanical drawing of a gear may specify how precise the dimensions must be, the material and heat treatment to be used, the surface finish, and settings for the machines that are to fashion the gear from metal.
Mechanical drawings represent the appearance of an object in different views. A view, also known as an orthographic projection, is made by drawing the various sides of a three-dimensional object on a flat plane such as the surface of a sheet of paper or a computer screen. In the United States, designers and drafters (specialists who prepare mechanical drawings) commonly use third-angle projections, which consist of the top, front, and side views of an object.
Creating a drawing.
A designer often begins the design process by making a freehand sketch of ideas on paper. The designer then develops these ideas on paper or on a computer. Today, most designers or drafters create the final mechanical drawing on a computer. But some still use traditional tools—such as compasses, curves, dividers, protractors, and T squares—to produce the drawing manually on paper, Mylar (a plastic film), or vellum (a high-quality paper). Some drafters use a drafting machine, a device that helps them draw precise vertical, horizontal, and angled lines.
The production of mechanical drawings on computers is called computer-aided design and drafting (CADD). A drafter using a CADD system selects commands from graphics software and other programs. The drawing appears on the monitor and is stored in the computer’s memory. At some design firms, a networked computer system enables several different people to work on different parts of a single drawing. Drafters use a laser printer or a small inkjet printer (a device that squirts tiny drops of ink onto paper) to print most drawings. For large drawings, they may use a pen plotter, which inks a roll of paper with a number of precisely controlled pens; an electrostatic plotter, which works somewhat like an electrostatic copying machine; or an inkjet plotter, which is basically a large inkjet printer. The drawing can also be printed on photographic film. 
In many cases, CADD makes it unnecessary to print a drawing. Workers who are going to produce a part can call up the drawings from computer memory or a disk or tape, and display them on their own monitors. In some instances, workers transmit the drawing electronically to a computer-aided manufacturing (CAM) system. This computerized device controls machines that produce the part.
Designers often use CADD software in the design process. They do this by producing a three-dimensional model—a photographlike drawing of the object on the monitor—then manipulating it in various ways. For example, they can change the object’s dimensions. Or they can move the model about as if the object were rotating in space, thereby obtaining various views. The process of designing and creating mechanical devices on a computer is known as computer-aided design (CAD).
CAD provides additional advantages. For example, an architect can use CAD to “walk through” a model of a building before construction begins.
