CHAPTER 3 PROJECTIONS AND VIEWS

When you have read and understood this chapter, you should be able to answer the following learning objectives:

*Describe the types of projections.

*Describe the types of views.

In learning to read blueprints you must develop the ability to visualize the object to be made from the blueprint (fig. 3-1). You cannot read a blueprint all at once any more than you can read an entire page of print all at once. When you look at a multiview drawing, first survey all of the views, then select one view at a time for more careful study. Look at adjacent views to determine what each line represents.

Each line in a view represents a change in the direction of a surface, but you must look at another view to determine what the change is. A circle on one view may mean either a hole or a protruding boss (surface) as shown in the top view in figure 3-2. When you look at the top view you see two circles, and you must study the other view to understand what each represents. A glance at the front view shows that the smaller circle represents a hole (shown in dashed lines), while the larger circle represents a protruding boss. In the same way, you must look at the top view to see the shape of the hole and the protruding boss.

Figure 3-1.-Visualizing a blueprint.

You can see from this example that you cannot read a blueprint by looking at a single view, if more than one view is shown. Sometimes two views may not be enough to describe an object; and when there are three views, you must view all three to be sure you read the shape correctly.

PROJECTIONS

In blueprint reading, a view of an object is known technically as a projection. Projection is done, in theory, by extending lines of sight called projectors from the eye of the observer through lines and points on the object to the plane of projection. This procedure will always result in the type of projection shown in

Figure 3-2.-Reading views.

fig. 3-3. It is called central projection because the lines of sight, or projectors, meet at a central point; the eye of the observer.

You can see that the projected view of the object varies considerably in size, according to the relative positions of the objects and the plane of projection. It will also vary with the distance between the observer and the object, and between the observer and the plane of projection. For these reasons, central projection is seldom used in technical drawings.

If the observer were located a distance away from the object and its plane of projection, the projectors would not meet at a point, but would be parallel to each other. For reasons of convenience, this parallel projection is assumed for most technical drawings and is shown in figure 3-4. You can see that, if the projectors are perpendicular to the plane of projection, a parallel projection of an object has the same dimensions as the object. This is true regardless of the relative positions of the object and the plane of projection, and regardless of the distance from the observer.

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