Engineering Drawing Principles and Applications

Introduction

In this chapter, an attempt is being made to introduce the readers to the projection of points. Following the treatment here it will be easy for them to understand the projection of lines, planes and solids in the subsequent chapters. For the projection of points, the quadrant system is considered and a point lying in space is assumed in any one of the four quadrants that are obtained by the intersection of two principal planes. A point can lie with reference to both the reference planes, i.e., HP and VP. Its projections are obtained by extending projectors perpendicular to the planes.

In order to obtain the projection of a point lying in three-dimensional space on a two-dimensional plane (drawing sheet), the principal plane HP is rotated clockwise through 90° and made co-planner with the VP. This process coverts the three-dimensional quadrant system into two-dimensional front and top views, i.e., the front view VP is obtained above x-y line where this x-y line represents the elevation or front view of HP; similarly the top view of HP is obtained below the x-y line, here the x-y line represents the top view of VP.

Projection of a Point Lying in the First Quadrant

The pictorial view Fig. 8.1(a) shows a point A lying in the first quadrant, i.e., above the HP and in front of the VP. When the point is viewed in the direction of l, the view from front a’ is obtained as the intersection point between the ray of sight through A and the VP. When the point is viewed in the direction m, the top view (a) of point ‘A’ is obtained as the ray of sight intersect with HP at a. Similarly front view (a’) of ‘A’ is obtained when the ray of sight coming from l direction intersects VP at a'. Hold VP and rotate HP 90° in the clockwise direction; these projections are seen in the Fig. 8.1(b). The front view a’ is above x-y and top view below it.

The line joining a’ and a (which is called as projector) intersects x-y at right angle (90°) at a point a.