### Lecture_4.2

```ENS 207 ENGINEERING GRAPHICS
Lecture 4: Orthographic projections, Multiview
drawing
1
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:
1) Line of sight
2) Plane of projection (image plane or picture plane)
MEANING
Orthographic projection is a parallel projection technique
in which the parallel lines of sight are perpendicular to the
projection 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
Parallel projection
Perspective projection
Line of sight
Line of sight
Which Views to Present?
 General Guidelines
 Pick a Front View that is most descriptive of object
 Normally the longest dimension is chosen as the width (or
depth)
 Most common combination of views is to use: Front, Top, and
Side View
 Views other than the Principal Views are called Auxiliary Views
5
AU 2005
8
Methods of Drawing Orthographic Projections
First Angle Projections Method
Here views are drawn
by placing object
Third Angle Projections Method
Here views are drawn
by placing object
( Fv above X-y, Tv below X-y )
TV
FV
X
TV
( Tv above X-y, Fv below X-y )
Y
X
Y
FV
G
L
First Versus Third Angle Projection

Third Angle Projection Associated with English
units
ANSI Symbol

First Angle Projection Associated with SI units
Conventional Orthographic Views
Width
Top View
Front View
AU 2005
Depth
Right
Side
View
Height
16
Step 1 – Lightly Block Three Views
Use very light
lines for drawing
in the
construction
lines
AU 2005
17
FOR T.V.
ORTHOGRAPHIC PROJECTIONS
FRONT VIEW
x
PICTORIAL PRESENTATION IS GIVEN
DRAW THREE VIEWS OF THIS OBJECT
BY FIRST ANGLE PROJECTION METHOD
L.H.SIDE VIEW
y
TOP VIEW
FOR T.V.
ORTHOGRAPHIC PROJECTIONS
FRONT VIEW
X
Y
TOP VIEW
PICTORIAL PRESENTATION IS GIVEN
DRAW THREE VIEWS OF THIS OBJECT
BY FIRST ANGLE PROJECTION METHOD
L.H.SIDE VIEW
FOR T.V.
ORTHOGRAPHIC PROJECTIONS
FRONT VIEW
X
Y
TOP VIEW
PICTORIAL PRESENTATION IS GIVEN
DRAW THREE VIEWS OF THIS OBJECT
BY FIRST ANGLE PROJECTION METHOD
L.H.SIDE VIEW
FOR T.V.
ORTHOGRAPHIC PROJECTIONS
FRONT VIEW
X
PICTORIAL PRESENTATION IS GIVEN
DRAW THREE VIEWS OF THIS OBJECT
BY FIRST ANGLE PROJECTION METHOD
L.H.SIDE VIEW
Y
TOP VIEW
FOR T.V.
ORTHOGRAPHIC PROJECTIONS
FRONT VIEW
X
Y
TOP VIEW
PICTORIAL PRESENTATION IS GIVEN
DRAW THREE VIEWS OF THIS OBJECT
BY FIRST ANGLE PROJECTION METHOD
L.H.SIDE VIEW
FOR T.V.
ORTHOGRAPHIC PROJECTIONS
FRONT VIEW
X
Y
TOP VIEW
PICTORIAL PRESENTATION IS GIVEN
DRAW THREE VIEWS OF THIS OBJECT
BY FIRST ANGLE PROJECTION METHOD
L.H.SIDE VIEW
FOR T.V.
ORTHOGRAPHIC PROJECTIONS
450
30
FV
40
X
Y
30 D
TV
40
O
PICTORIAL PRESENTATION IS GIVEN
DRAW FV AND TV OF THIS OBJECT
BY FIRST ANGLE PROJECTION METHOD
15
40
PICTORIAL PRESENTATION IS GIVEN
DRAW FV AND TV OF THIS OBJECT
BY FIRST ANGLE PROJECTION METHOD
ORTHOGRAPHIC PROJECTIONS
30
FOR T.V.
FV
RECT.
SLOT
10
50
35
10
X
20 D
TV
60 D
30 D
TOP VIEW
Y
HIDDEN LINES
Hidden lines are used to represent surfaces that are not directly visible in
an orthographic view.
SLANTED SURFACES
Slanted surfaces are surfaces that are not parallel to either the
horizontal or vertical axis.
PROJECTION BETWEEN
VIEWS
COMPOUND LINES
A compound line is formed when two slanted surfaces
intersect. The true length of a compound line is not shown in
the front, top, or side views.
OBLIQUE SURFACES
Oblique surfaces are surfaces that do not appear
correctly shaped in the front, top, or side views
ROUNDED SURFACES
Rounded surfaces are surfaces that have constant radii,
such as arcs or circles. Surfaces that do not have constant
radii are classified as irregular surfaces
CASTINGS
Casting is one of the oldest manufacturing processes. Metal is heated to liquid
form, then poured into molds and allowed to cool. The resulting shapes usually
include many rounded edges and surface tangencies
```