ENGINEERING

DRAWING

LECTURE 3

Theory of Projection and Orthographic

Projection

• Projection:

Any kind of representation of an object on a paper, screen or similar

surface by drawing or by photography is called the projection of that

object.

Projection Methods

Parallel

Pictorial Orthographic

Perspective

Projection Theory

• The projection theory is used to graphically represent 3-D objects on 2-D

media (paper, computer screen).

• The projection theory is based on two variables:

i. Line of sight

ii. Plane of projection (image plane or picture plane)

Line of sight is an imaginary ray of light between an

observer’s eye and an object.

• There are 2 types of LOS : parallel and converge

Plane of projection is an imaginary flat plane which the

image is created.

Disadvantage of Perspective Projection

1. It is difficult to create.

2. It does not reveal exact shape and size.

Pictorial Projection

• The Projection in which the length, breadth (width) and height of an object

is shown in one view is known as pictorial projection.

Orthographic Projection

• Orthographic projection is a parallel projection technique in which the

parallel lines of sight are perpendicular to the projection plane.

Orthographic Projection

The Glass Box Approach

Opening the Box

Final Views

Six Orthographic Views:

Laying out all six views

Three Primary Views

Construction of Views

First and Third Angle Projections

• First Angle – International

• Three Angle – USA

Comparison of First and Third Angle Projections

FIRST ANGLE PROJECTION THIRD ANGLE PROJECTION

The object is assumed to be situated in the first

quadrant.

The object is assumed to be situated in the third

quadrant.

The object lies in between the observer and the

plane of projection.

The plane of projection lies in between the

observer and the object.

View from above (top view) is drawn below of

front view.

View from above (top view) is drawn above of

front view.

View from below (bottom view) is drawn above of

front view.

View from below (bottom view) is drawn below of

front view.

View from left (left side view) is drawn on the right

of front view.

View from left (left side view) is drawn on the left

of front view.

View from right (right side view) is drawn on left of

front view.

View from right (right side view) is drawn on right

of front view.

View from the rear (rear view) is drawn on left or

right of front view.

View from the rear (rear view) is drawn on left or

right of front view.

First Angle Projection

Third Angle Projection

Symbol of first and third angle projection

Dimensioning

Placing of Dimensions

Basic Line Types

Meaning of Lines

• Visible lines represent features that can be seen in the current view.

• Hidden lines represent features that can not be seen in the current view.

• Center line represents symmetry, path of motion, centers of circles, axis of

axisymmetric parts.

• Dimension and Extension lines indicate the sizes and location of features

on a drawing.

Good Practice

Exercise

• Complete three orthographic views of the object shown on the next slide

(Third Angle Projection).

• Include visible, hidden, and center lines where appropriate.

Object for exercise