engg drawing

Introduction

1

What is Engineering Graphics

Language used for accurate description of physical objects

Definition of Engineering Graphics: A graphic representation of physical objects

and relationships.

The graphic representation is accomplished by describing both shape and size

Objective of Engineering Graphics

To be able to express ourselves clearly and be able to understand what is written by

others

Two fundamental methods used in Engineering Graphics

• Freehand

• With instruments

2

Main Constituents of Engineering Graphics

• Shape is described by means of projection – a process of causing an image to be

formed by lines (projectors) taken in a particular direction from an object to a

particular plane

• Two main types of projections are used

• Orthographic

Separate views are made and properly arranged to show all the details

of the 3D object on mutually perpendicular planes. Each projection

represents two dimensions of the object

• Pictorial

The 3D object is represented on a plane by one view only

Shape Description

Size Description

• Size is shown by ‘dimensions’ which show linear distances, diameters, radii, angles

• Need to show only those dimensions which convey the information in the best

possible way

3

Orthographic Projection

Projectors are parallel to each other and perpendicular to the plane of projection

Two reference or principal

planes of projection

V.P. – vertical (frontal) plane

H.P. – horizontal plane

Projection on V.P. – front view

or elevation

Projection on H.P. – top view

or plan

Intersection of the V.P. and the

H.P. is called the reference line

and is denoted by xy

Ref: Engineering Drawing by N. D. Bhatt et. al

4

Methods of Orthographic Projects

Only the 1st and the 3rd quadrants are opened out

Basic Four Quadrants

• First Angle Projection

• Third Angle Projection


5

First Angle Projection

• Object is assumed to be situated

in the first quadrant

• Object lies between the observer

and the plane of projection

• The top view comes below the

front view

• Each projection shows the view

of the surface which is remote

from the plane on which it is

projected

• The view of the of the object as

observed from the left-side is

drawn to the right side of the

front view and vice versa

Recommended by the Bureau of Indian Standards (B.I.S)Ref: Engineering Drawing by N. D. Bhatt et. al

6

Third Angle Projection

• Object is assumed to be situated

in the third quadrant

• Plane of projection lies between

the observer and the object

• The top view comes above the

front view

• Each projection shows the view

of the surface which is nearest

to the plane on which it is

projected

• The view of the of the object as

observed from a particular side is

drawn to the same side of the

front view


7

Symbols for Method of Projections

On any drawing it essential to indicate the method of projection adopted


8

Six Views of an Object

• In some cases two views (front and top) may not be sufficient to describe the object

completely

• Need to use additional views (right hand side view, left hand side view, bottom view

and back view)

• Method: Assume that the object is enclosed in a square box and carry out the

projections on the faces of the box using either first angle or third angle method


9

Dimensioning

• Size is shown by ‘dimensions’ which show linear distances, diameters, radii, angles

• Need to show only those dimensions which convey the information in the best

possible way

Placing of Dimensions

Aligned System Unidirectional System


10

Rules for Dimensioning

• Dimensioning should be done completely

• Each dimension should be given only once

• Dimensions should be placed outside the views

• Mutual crossing of the dimension lines should be avoided

• Dimensioning of hidden lines should be avoided

• Dimension lines should not cross any other lines of the drawing

• An outline or a center line should not be used as a dimension line

• A zero is should always precede the decimal when the dimension is less than

one, i.e. write 0.5 instead of .5.

11

Dimensioning of Some Common Features

Dimensioning of Circles Dimensioning of Radii

• Dimension figure should be preceded

by ø• Holes should be dimensioned in the view

in which they appear as circles

• Hole center should be located by center

lines

• Dimension figure should be preceded

by R• Arcs of circles should be dimensioned

by respective radii

• Dimension line of the radius should

pass through the center of the arc


12

Types of Lines

• Outline/Object lines (thick)

• Hidden lines (thin)(dash space dash space ….)

• Dimension lines (thin)

• Leader lines (thin)

• Center lines (thin)(long dash space short dash space long dash ....)

• Extension lines (thin)

• Section lines (thin)

(inclined at 450)

• Break lines (thick)

• Cutting plane lines (thick)

Each line has a definite meaningRef: Engineering Drawing by N. D. Bhatt et. al

13

Precedence of Lines

Coincident lines may exist in the same view. In such a case, use the following

order to establish the precedence

• Visible object lines

• Hidden lines

• Center line or cutting plane lines

• Break lines

• Dimension and extension lines

• Crosshatch/section line

14

Problem 1

Draw the front view, top view and the left hand side view of the object


Problem 1 - Solution

Ref: Engineering Drawing by N. D. Bhatt et. al 15

Important to indicate the

projection method used. Has

to be there in all your drawings

16

Problem 2

Draw the front view, top view and the left hand side view of the object


17

Problem 2 - Solution


Important to indicate the

projection method used. Has

to be there in all your drawings

engg drawing

Documents

show linear

angle projection

engineering

dimension

hidden lines

angle projection

graphic representation

engineering