CBSE Book Class 11 Engineering Graphics Orthographic Projection

ORTHOGRAPHIC PROJECTION

4.1 INTRODUCTION

We, the human beings are gifted with power to think. The thoughts are to be shared. You will appreciate that different ways and means are available to us. Can you name some? We communicate by signs/gestures/body language/talking/singing and dancing etc. In order to explain our ideas/thoughts both the listener and speaker must understand the language spoken. All our ideas originate in our mind, some are creative/innovative.

What is being imagined in our mind is to be given a reality. In Engineering, we notice that all the objects around us are three dimensional solids. This is to be explained in two dimensions i.e. on the paper/a drawing sheet. We employ a unique method to achieve this. This is called as orthographic method of projection. This method has certain rules which are universal. This method employs the drawing consisting of straight lines, rectilinear figures, arcs and circles. Such a skill can be easily acquired. This is essential, as the Engineering Graphics is for masses.

An object is represented by drawing the boundaries of all the surfaces of the object. The boundary of a surface may be made up of straight lines or curved lines or both. As each curved or straight line is made up of a number of points, the theory of orthographicprojection is logically started with the projection of points. Then we study about projection of lines, planes and solids. In this unit, we aim to introduce some basic concepts of solid geometry. To begin the solid geometry, let us first understand the solids in detail.

4.1.1 INTRODUCTION TO SOLIDS

Whenever we look around, we see many objects which are three dimensional like cube (dice), cuboid (match box, lunch box, notebook etc.), prism (packaging box), cylinders (water bottle, gas cylinder etc.), cone (softy, tent etc.) sphere (ball). Let us now study about these solids and its projections.

4.1.1.1 SOLIDS

A solid is a three dimensional object, i.e. it has three dimensions, viz; length, breadth and height or thickness. It is bounded by plane or curved surfaces. Atleast 2 orthographic views are required to represent a solid on a flat surface. Sometimes, additional views become necessary to describe a solid completely.

The solids under study can be divided into 2 main groups viz;

1. POLYHEDRA – solids bounded by plane surfaces such as prisms, pyramids Fig. 4.1, 4.2

2. SOLIDS OF REVOLUTION – solids formed by revolution of linear figures such as cylinder, cone and sphere Fig. 4.5. NOTE : In our study, only right regular solids are discussed. Such solids have their axis perpendicular totheir base.

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