Chapter-3

Theory of ProjectionTheory of Projection

Engineering Graphics

AutoCAD Theory of Projections

3

Projections

Multiview projections

Single view projections

Axonometric and Perspective View

To draw in perspective, draw a horizon line and draw a vanishing point anywhere on the horizon. Lines which are parallel in real life are drawn to intersect

Axonometric and Perspective View

Systems of Projection

Projection

Multi view projection Single view projection

(Orthographic projection) (isometric projection)

Natural / Direct View Method

Glass Box Approach

• Place the object in a glass box

• Freeze the view from each

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

� Project all key pointsto get front view

Glass Box Approach

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to get front view

� Project all key pointsto get top view

Glass Box Approach

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to get top view

� Project all key pointsto get right side view

Glass Box Approach

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to get right side view

� Now unfold the glassbox

Glass Box Approach

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box

Glass Box Approach

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� Object in a glass box � Unfolded glass box

Elements of projection

Planes of projections

• Basically 2 planes

• Intersecting at right angles

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

• Reference line xy• Reference line xy

• Front View (FV) or Elevation

• Top View (TV) or Plan

• Third is the PP (Profile Plane)

DIRECTION OF

VISION FOR TV

VPIST

IIND

III RD

IVTHX

Four quadrants

• Position of the object is described by above,

below, behind, in front of

• Planes are assumed to be transparent

• Can the object be situated in any of the • Can the object be situated in any of the

quadrants??

• If object in II or IV quadrant, TV and FV will

overlap.

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

1st Angle Method of Method of Projection

3rd Angle Method of Method of Projection

I.S.I. Code of Practice

• The method of first-angle projection is the Britishstandard practice.

• The third-angle projection is the standardpractice followed in America and in the continent ofEurope.

• In our country, the first-angle projection methodwas formerly in use. The Indian StandardInstitution (I.S.I.), in its earlier versions of ‘IndianStandard Code of Practice for General Drawing’published in 1955 and revised in 1960 hadrecommended the use of third-angle projectionmethod.

In the second revised version of thisstandard published in December 1973, thecommittee responsible for its preparation has leftthe option of selecting first or third-angleprojection method to the users.

Contd….

Persons engaged in engineering professionmay come across drawings from industries andorganizations following any one method.

It is therefore necessary for them to beperfectly conversant with both the methods.

Example

Symbolic Representation