Page 44 - 4234

Figure 5.2

5.3 PRINCIPLES OF SECONDARY PROJECTIONS

In general for any secondary frontal plane of projection we have the following propositions:
- The plane may be viewed from either side, irrespective of the position of the object, but
the direction of sight must always be at right angles to the plane, and the projectors must
be drawn at right angles to x 14.
- Distances above Π 1 must be laid off on the farther side of x 14, and distances below Π 1 on
the near side.
- All vertical distances, either above or below Π 1, remain unchanged.
For any secondary horizontal plane of projection we have the similar propositions.

Simplification of problems by means of secondary projections
In the solution of problem, there is no advantage in introducing a secondary plane of
projection unless the new projection is in some way simpler than the original projections.
A point always projects as a point, and cannot be made any simpler.
The simplest projection of a straight line is a point. This projection can be obtained by a single
secondary plane of projection only when the given line is parallel to one of the original coordinate
planes. Let the line AB be parallel to Π 1 (Fig. 5.3); then if x 14 is taken perpendicular to A 1B 1, the
line AB will be seen endwise, and the projection A 4B 4 will become a point.

Figure 5.3

44