Engineering Drawing Principles and Applications

Introduction

In engineering practice, it is usual to draw two or more than two orthographic projections to show the true shape and size of an object. Such drawings can be correctly interpreted only by those persons who have a sound knowledge of the principles of orthographic projections. To make the drawing more understandable, several forms of one plane projection drawings are used to supplement the orthographic drawings. These are called pictorial drawings, which can be easily understood by everybody without any formal training. Pictorial drawings are mainly used to show complicated shapes such as aircraft, ships, buildings, etc.

Classification of Pictorial Drawings

Pictorial drawings are classified into three categories:

• (a) Axonometric Projection

• (b) Oblique Projection

• (c) Perspective Projection

Axonometric Projection:It is a three-dimensional projection of an object, as shown in Fig. 16.1 (a). It is a form of orthographic projection, since the projectors are perpendicular to the plane of projection and parallel to each other.

Oblique Projection:It is a three-dimensional projection of an object on a projection plane, as shown in Fig. 16.1 (b). In this, the projectors are parallel to each other, but are oblique to the picture plane.

Perspective Projection:It is the most realistic projection. In this, the projectors converge towards the viewer's eye, making different angles to the picture plane, as shown in Fig. 16.1 (c).

Axonometric Projection

It is a form of orthographic projection and is obtained by projecting an object placed in an oblique position to the plane of projection. Axonometric projection is classified as:

• • Isometric Projection

• • Dimetric Projection

• •Trimetric Projection

Out of these, only isometric projection is dealt with here.

Isometric Projection

It is a type of pictorial drawing in which the three dimensions of a solid are not only shown in one view, but their actual sizes can be measured directly from it.

If a cube is resting on one of its corners on the HP, with its solid diagonal perpendicular to the VP, the front view of the cube is its isometric projection as shown in Fig. 16.2.