Chapter-3

Theory of Projection

Object Description on a plane paper

3-D description with true dimensions

3-D description with no true dimensions

(Perspective View)

2-D description with true dimensions

object

Elements of projection

Plane of projection

Observer

Projectors

Elements of projection

Systems of Projection

Projection

Multi view projection Single view projection

(Orthographic projection) (Pictorial projection)

Two Methods• Natural Method

(Direct View Method)

• Glass Box Method

Natural / Direct View Method

Glass Box Method

Why do we need 2-Dimensional Views?

• Reading of the drawing for design

and analysis

• Selection of best manufacturing

process from the available

• Assembly

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Glass Box Approach

• Place the object in a glass box

• Freeze the view from eachdirection (each of the sixsides of the box) andunfold the box

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� Project all key pointsto get front view

Glass Box Approach

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� Project all key pointsto get top view

Glass Box Approach

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� Project all key pointsto get right side view

Glass Box Approach

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� Now unfold the glassbox

Glass Box Approach

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� Object in a glass box

Glass Box Approach

� Unfolded glass box

Planes of projections

• Basically 2 planes

• Intersecting at right angles

• V.P (Vertical Plane) & H.P. (Horizontal plane)

• Reference line xy

• Front View (FV) or Elevation

• Top View (TV) or Plan

• Third is the PP (Profile Plane)

Planes of Projection (Principal Planes and Principal

axes)

HP, VP and PP X, Y and Z

DIRECTION OF

VISION FOR TV

VP

IST

IIND

IIIRD

IVTHX

Four quadrants

• Object can be situated in any of the quadrants

• Position of the object is described by above, below, behind, in front of

• Planes are assumed to be transparent

L (X) D (Z) H (Y)

1st Angle

Method of

Projection

3rd Angle

Method of

Projection

I.S.I. Code of Practice

• The method of first-angle projection is the British

standard practice.

• The third-angle projection is the standard

practice followed in America and in the continent of

Europe.

• In our country, the first-angle projection method

was formerly in use. The Indian Standard

Institution (I.S.I.), in its earlier versions of ‘Indian

Standard Code of Practice for General Drawing’

published in 1955 and revised in 1960 had

recommended the use of third-angle projection

method.

In the second revised version of this

standard published in December 1973, the

committee responsible for its preparation has left

the option of selecting first or third-angle

projection method to the users.

Persons engaged in engineering profession

may come across drawings from industries and

organizations following any one method.

It is therefore necessary for them to be

perfectly conversant with both the methods.

Contd….

Example

Symbolic Representation

Orthographic Projections

• Right-angled drawing

• Multi view is also popular as Orthographic projection.

Types of Surfaces

1. Principal surface

2. Inclined surface

3. Skew surface

4. Curved surface

Principal Surface

The surface, which is parallel to one reference plane and perpendicular to other two, is referred as principal surface.

VP

HP

PP

Principle surface

It shows two line views and one area view as true area.

Horizontal surfaces are always principal surfaces but

not the vertical.

Inclined Surface

The surface, which is inclined to two reference planes and

perpendicular to other, is referred as an inclined (or slant)

surface.

It shows one line view and two area-views with apparent

size as shown in Figure. In this type of surface, in general,

at least one edge of the surface is along the principal axis.

Inclined Surface

Skew Surface

The surface, which is inclined to all three principal planes

(HP, VP and PP), is referred as a skew (or oblique) surface.

It shows 3 area-views with apparent size as shown in Figure.

In this type of surface, none of the edges of the surface is

along the principal axis.

Concept of invisible (hidden)

line

Object is a single piece

Assembly Single Piece

Precedence of lines

• Continuous line over hidden line

• Hidden line over centerline

Curved SurfaceThe surface, which has constant radius such as arc or circle, is

referred as curved (or rounded surface).

In this type of surface, the last visible part of the curved surface

falls in line view.

Last visible part of curved

surface falls into line view

Representation of curved

surface

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Object as an Assembly

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Object as a Single Piece

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Object as a Single Piece

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Assembly versus Single Piece Object

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Precedence of Lines

Types of Drawing

• Single View Drawing

• Two View Drawing

• Three View Drawing

Single View Drawing

Two View Drawing

Three View Drawing