Mechanical Drawing is used to convey precise information from one person to another so that a pattern maker will have a true drawing of an object, giving correct dimensions and instructions before he can make a pattern, from this, the foundry man can make a rough casting.
The machinist must have a drawing from which he can obtain accurate information to enable him to take the rough casting and by slotting, planing, drilling, grinding, chipping or turning, they can produce the finished fixture as designed by the drafter.
Contractors, builders, architects, and engineers of all kinds, must have accurate drawings to enable them to produce satisfactory results in their work. To do this, it is necessary that working drawings should be made according to certain principles and methods thoroughly understood by the person who makes the drawing and the person who uses it.
The mechanical drawing is the common language of all mechanics and machinists. By implementing it, the ideas of the designer, architect or engineer are transmitted or explained to the worker. There is hardly a area of work in the great field of industry where the knowledge of drawing is not used universally. It is a true statement that there is no industry that does not require a knowledge of drawing and the employment of drafter. Illustration of machine and fixtures, by the aid of the mechanical and working drawing, is the necessary first step in the building of such machines and fixtures.
If we then consider mechanical drawing as a language, we understand then that it is to be used to convey thoughts and ideas. Orthographic projection, which is a division of descriptive geometry, is its grammar and the foundation upon which is built all kinds of correct mechanical drawings. It is in fact the art of representing any object so accurately that a skilled workman, upon inspecting the drawing, should be able to make the object of exactly the materials and dimensions shown, without any further verbal or written instructions from the designer.
The objects illustrated may be machines, implements, buildings, utensils, or ornaments. They may be constructed of many various materials. The drawings may be linear, shaded and colored, or plain. Because of the nature of the information to be conveyed, they must be drawn to scale, but various geometrical methods may be employed.
The purpose of a mechanical drawing is not to present a picture of an object as drawn by an artist or as seen by the eye, but to furnish a graphical representation of the actual proportions and shape of an item. This is done by making projections of the edges of the object using imaginary horizontal and vertical planes. These projections show the correct geometrical relations of the various dimensions and parts of the object or structure. Since two dimensions are shown in each projection, the use of two or more projections gives all of the dimensional relations.