Engineering Drawing Principles and Applications

Introduction

A solid is a three-dimensional object having length, breadth and thickness. In engineering practice, one often comes across solids bounded by simple or complex geometric surfaces. To represent a solid in orthographic projections, the number and types of views necessary will depend upon the type of solid and its orientation with respect to the principal planes of projections. Sometimes, additional views projected on auxiliary planes become necessary to describe a solid completely.

Types of Solids

Solids may be divided into two main groups:

(i) Polyhedra

• (a) Regular Polyhedra

• (b) Prisms

• (c) Pyramids

(ii) Solids of Revolution

• (a) Cylinders

• (b) Cones

• (c) Sphere

Polyhedra: A polyhedron is defined as a solid bounded by planes called faces.

(a) Regular Polyhedra: A polyhedra is said to be regular if all its faces are similar, equal and regular. There are five regular polyhedra which may be defined as stated below:

• • Tetrahedron: It has four equal faces, each are equilateral triangle as shown in Fig. 12.1

• • Cube or hexahedron: It has six equal faces, each of which is a square as shown in Fig. 12.2.

• • Octahedron: It has eight equal faces, each of which is an equilateral triangle as shown in Fig. 12.3.

• • Dodecahedron: It has twelve equal faces, each of which is a regular pentagon as shown in Fig. 12.4.

• • Icosahedron: It has twenty equal faces, each of which is an equilateral triangle as shown in Fig. 12.5.

(b) Prisms: A prism is a polyhedron having two equal ends or bases, parallel to each other. The two bases are joined by faces which are rectangles or parallelograms. The imaginary line joining the centres of the bases is called the axis.

A right regular prism has its axis perpendicular to the base. All its faces are equal rectangles as shown in Fig. 12.6.

(c) Pyramids: A pyramid is a polyhedron having one base and a number of triangles as faces meeting at a point called as vertex or apex. The imaginary line joining the centre of the base with its apex or vertex is called its axis.