Engineering Drawing Principles and Applications

Introduction

There is a huge number of objects of different shapes which have dimensions in the three directions, i.e., length, breadth and thickness. However, these three-dimensional objects are difficult to describe on a two-dimensional sheet. Therefore, in the language of graphics, the shape of an object is described by its projection. A projection is the image of an object formed by rays of sight coming from the same direction as the object. The image is taken on a picture plane, and it appears to an observer stationed at a point from or towards which the projection is made. Therefore, in order to draw the projections of an object the following elements are considered:

• (i) An object or body (point, line, plan, solid)

• (ii) The projection plane (HP, VE, PP)

• (iii) The point of sight (Observer)

• (iv) The lines of sight or rays of sight

Methods of Projections

In an engineering drawing, four methods of projection are generally used, which are as follows:

• (i) Orthographic Projections

• (ii) Axonometric Projections

• (iii) Oblique Projections

• (iv) Perspective Projections

The first type, i.e., orthographic projection, is one of the most prevalent methods, in which an object can be represented by two or more than two views, on mutually perpendicular planes of projections or picture planes. However, a maximum of six views (front, top, rear, bottom and both left and right side) are used to represent an object. Each view represents two dimensions of an object. For the complete description of a three-dimensional object, at least two views are required. Whereas, in the other three methods, the objects are represented by a pictorial view only, a three-dimensional object is represented on a projection plane by single view only.

Perspective projection

Perspective projection represents objects as perceived by the human eye(s) (refer to Fig. 7.1). It is a pictorial drawing by the intersection of observer's visual rays (lines of sight) converging on a plane (picture plane). The observer's eye-station point or point of sight-is located at a finite distance from the picture plane (refer to Fig. 7.1). Depending on the position of the picture plane, the size of the projection may vary.