The word projection means to throw forward. In mechanical drawing, the significance is to throw forward in straight lines. Projection really means, therefore, either the act or the result of projecting parallel rays from the surface of a body and of cutting these rays with a plane, so as to obtain on the plane a shape corresponding point for point with that of the body. The rays are called projecting lines. A plane may be considered transparent, since it is a flat surface having no thickness.
Fig. 95. Body and Its Projection.
In Fig. 95 a body is shown as projecting from its surface projection lines, and these lines are cut by a plane. By connecting the points on the plane made by the projection lines the projection of the body is formed, and it corresponds in shape with the body itself. A projection of this kind is called a view, this name being given it on account of the fact that an observer on the same side of the body as the projection plane would get this view.
It can readily be seen that one view only will not give a complete picture of a solid object. Usually two or more views are necessary, according to the complication of the object or body. When two or three views are shown, they are pictured on two or three planes at right angles to each other. In this way views of two or three sides are shown, and this is usually sufficient to give the idea of the complete form of the object.
The word orthographic means at right angles, and in mechanical drawing, in connection with the word projection, it means that two or more views are projected on planes at right angles with each other. The various views of a body have special names - those showing vertical faces are called elevations, such as front, side, end or rear elevation; a view of the top of a body is called a plan or plan view; and a view of the under side, a bottom view.
In Fig. 96 is shown three faces of a body projected on three planes - the top view on the top plane, the front view on the front plane, and the end view on the end plane. It will be seen that the same body is represented as projecting rays in three directions, and thus the three projections, or views, are obtained. It will also be seen that the three planes with their views have been brought into one plane, that is, the surface of the paper. This brings the top view directly above the front view, and the end view to the right of the front view. The above is a definition of true projection, usually called third-angle projection, and is the method used by practically all draftsmen in this country.
Fig. 96. Projections of Top, Front, and End of Body.
There is, however, a method called first-angle projection, used but little now in this country, although formerly in almost general use. Because draftsmen may have to do at times with old drawings or drawings made in foreign countries, it is well for them to understand first-angle projection. This method bring3 the front view, or elevation, above the top view, or plan, the end view being at the right of the front view. Fig. 97 shows this method of first-angle projection.
Fig. 97. Body and It* First-Angle Projections.
Perhaps a short explanation will make clear the meaning of first- and third-angle projection. In geometry, when two planes intersect at right angles, the angles are designed as first, second, third, and fourth, as numbered in Fig. 97. In first-angle projection, the body is placed in angle 1, and a top view is projected on a plane under the body.
This passes the projection lines back through the body, instead of throwing them out from the surface. In fact, by this method, the body is supposed to turn itself inside out, an absurdity which led to the general abandonment of the method in this country. Third-angle projection places a body in angle 3, and projects a top view on to the plane above it, and a front view on to the plane in front of it. This is true projection.
When a drawing is made by projection, an object is represented just as it would be seen if one eye were closed and the other were directly over each point of the object at the same time. As an illustration of this, place a box on a table and a piece of ground glass a few inches in front of it. Now, stand so that one eye will come directly in line with one corner a, Fig. 98. Make a dot at a1 where the line from the eye to the corner a passes through the glass. Next, move the eye until it is directly in line with the corner b of the box, and put a dot at b1 where the line from the eye to the corner b passes through the glass. Repeat this process, putting dots on the glass at c1 and d1 where the lines from the eye to the corners c and d pass through the glass. Now, connect the points a1, b1, c1 and d1, and the complete projection of the front of the box will be shown by the figure on the glass. This figure is a rectangle, and is the same shape and size as the front of the box.
Fig. 98. Visual Method of Finding Projection.
The first four figures in this textbook, Figs. 95, 96, 97, and 98, are pictorial views given to show the student clearly how the views of objects are projected. The student in drawing orthographic projections does not need to draw the pictorial views, but simply the projections, as illustrated in Figs. 99 to 129, inclusive.