DRAWING, Mechanical, the art of repre senting objects on paper, or other flat surface, by means of projections of the objects on im aginary planes. A drawing of this sort is exe cuted with the aid of instruments, such as com passes, scales, T-squares, triangles, etc., espe cially designed to facilitate its production. The projections of the objects are areas of the im aginary planes which represent the object and are obtained by drawing imaginary lines from the points of the object to the planes. These imaginary lines may meet in a point or be paral lel. In the former case the representation ob tained on the plane is a conical projection of the object; in the latter, the projectors may be either perpendicular to the plane of projection, giving us orthographic or orthognal projection, or oblique to the plane of projection, giving us oblique projection.
Drawing made on the principle of conical projection is known as perspective drawing and minutely described and a complete record kept for duplication and repairs. This constitutes a graphic or written language. It can be read only by forming a mental picture of the sub ject represented. Skill in this language is indi cated not alone by execution but by the ability to interpret impressions by visualizing clearly in space. The training this interpretation gives in quick, accurate observation and in the power of reading description from lines has a value not understood by those unfamiliar with it.
The principal lines of most buildings, ma chines and structures are generally in three di rections: one vertical, the other two horizontal and at right angles to each other. The object is placed in the first or third angle of space. with these principal lines or dimensional azes parallel to the planes of projection. The position of the projections called “viewe in engineering drawing correspond to the positions assumed by these views when the planes of projection are revolved into the plane ofthe paper.
i Since the object is placed with its principal lines or dimensional axes parallel to the planes of projection, each view shows two dimensions of the object in true size; and any line which runs in the direction of the third dimension is represented by a point. Likewise, any surface
in the direction of the third dimension will be represented by a line. If the plane of projec tion is parallel to the front of the object the resulting projection is called the front view and shows the dimensions of height and length. If parallel to the top or bottom of the object the view shows the dimensions of length and depth. The former is called the top view and the lat ter the bottom view. If the plane is parallel to a side of the object it is called a side view, which shows the dimensions of height and ferent directions and can be shown to better advantage by combining some of the other views with one taken on a plane so placed as to show the dimension of the part not shown in the reg ular views. Usually it is not necessary to pro ject the entire object on this auxiliary plane, but only the part to be shown in true shape:. (See Fig. 3, which is a detailed drawing of a bearing bracket). It is often not possible to show the internal construction of an object clearly by using dotted lines to ex press the invisible parts in an outside view. In such cases the object is drawn as if parts were cut or broken away. Such a view is called a "section?' .(See Fig. '4). In it the exposed portions of the cut surface are indicated by section lining, composed of diagonal lines, usually at 45 degrees and spaced uniformly to depth. Any combination of two or more cent views may be used to express the It is the manner of arranging these views which determines whether the drawing is made in accordance with first or third angle projec tion. The former method was abandoned in this country more than 25 years ago and is now almost obsolete. It is occasionally met in old drawings and in foreign drawings and there fore should be understood.