Orthographic Projection

Why Orthographic Projection is used in technical drawing

Orthographic projection is a method of producing a number of
separate two-dimensional inter-related views, which are mutually at right angles to
each other. Using this projection, even the most complex shape can be fully described.

Orthographic projection is based on

two principal planes — one horizontal
(HP) and one vertical (VP) —
intersecting each other

3-D projections are useful in that they provide an image that is similar to the image in
the designer’s mind’s eye. But 3-D projections are often weak in providing adequate
details of the object, and there is often some distortion of the object.
For instance, a circular hole becomes an ellipse in an isometric 3-D projection.
Orthographic projection are used to overcome the weaknesses of 3-D projections.
Orthographic projections are a collection of flat 2-D drawings of the different sides of
an object.
Orthographic view
Orthographic projection is a parallel projection technique
in which the parallel lines of sight are perpendicular to the
projection plane
3-D projections are useful in that they provide an
image that is similar to the image in the designer’s
mind’s eye. But 3-D projections are often weak in
providing adequate details of the object, and there is
often some distortion of the object.
For instance, a circular hole becomes an ellipse in
an isometric 3-D projection. Orthographic
projection are used to overcome the weaknesses
of 3-D projections. Orthographic projections are a
collection of flat 2-D drawings of the different sides
of an object.
Orthographic view
Orthographic view depends on relative position of the object to
the line of sight. It uses multiple views of the object, from points of
view rotated about the object's center through increments of 90°.

The views may be

considered to be obtained
by rotating the object
about its center through
increments of 90°.
Orthographic View

Advantage It represents accurate shape and size.

Disadvantage Require practice in writing and reading.

Example Multiviews drawing (2-view drawing)
Orthographic View
Only two forms of orthographic On engineering drawings, the
projections are used: first-angle projection angle is
projection (‘European ISO-E’) denoted by an international
and third-angle projection symbol consisting of
(‘American ISO-A’). a truncated cone, respectively
for first-angle
(FR) and third-angle (US):

(‘European ISO-E’) (‘American ISO-A’)

View Selection Steps
STEP 1 : Orient the Object
Orient the object to the best position relative to a glass box. The
object should presents its features in actualsize and shape
in orthographic views.
GOOD NO !
STEP 2 : Select a Front View

The object’s longest dimension should be

presented as a width.

First choice Second choice

Waste more space

Inappropriate
STEP 2 : Select a Front View
Choose the view that have the fewest number of
hidden lines.

GOOD Inappropriate
STEP 3 : Select an Adjacent View
Choose the view that have the fewest number of
hidden lines and minimum number of views that
can represent the major features of the object

GOOD

Inappropriate

GOOD
Inappropriate
Object Features

Edges are lines that represent the boundary

between two faces.

Corners Represent the intersection of two or

more edges.
Edge Edge No edge

Corner No corner No corner

Object Features

Surfaces are areas that are bounded by edges

or limiting element.
Limiting is a line that represents the last visible
element part of the curve surface.
Surface Surface Surface

Limit
Limit
Standard Views of Primitive Solids
Fillets, Rounds & Chamfers
Edge Lines – Principal & Inclined
Principal lines appear vertical,
horizontal or as point views.
Inclined lines appear inclined in
one view.

Edge lines are lines that

represent the boundary
between two faces
Edge Lines – Oblique

Oblique line appears inclined in all views

Type of Planes

Oblique

Oblique
Principal surface Inclined
Principal Planes
Principle planes are parallel to principal orthographic
planes

Principal planes appear in true

size in one plane and as an
edge view in the other two
planes.
Inclined Planes
Inclined planes are perpendicular to two opposite
orthographic planes.
Oblique Planes
Oblique planes are neither parallel nor
perpendicular to any principal orthographic planes.

Oblique
surface
Top view
Basic Line Types

Standards are set of rules that govern how Technical

drawings are represented.

Drawing standards are used so that drawings convey

the same meaning to everyone who reads them.
Basic Line Types

Name according
Types of Lines Appearance
to application

Continuous thick line Visible line

Continuous thin line Dimension line

Extension line
Leader line

Dash thick line Hidden line

Chain thin line Center line

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 axisymmetrical parts

Dimension and Extension lines indicate the sizes and

location of features on a drawing
Example : Line conventions in engineering drawing
LINE CONVENTION

Precedence of coincide lines.

Hidden line drawing.

Center line drawing.

 PRECEDENCE OF LINE
Order of Visible Hidden Center
importance line line line

Coincide
with
center
line
EXAMPLE : Already met convention practice

Front view

Using a dash line for CONVENTION

representing the hidden edge. PRACTICE
Hiden Line Practice
Hidden line should join a visible line, except it
extended from the visible line.

Leave
space

Correct

Join

No !
Hiden Line Practice
Hidden line should join a visible line, except it
extended from the visible line.

Leave Leave
space space

Correct No !
 HIDDEN LINE PRACTICE
Hidden line should intersect to form L and T
corners.

L T Correct

No !
Hiden Line Practice

Hidden arcs should start on a center line.

Center Line Practice
In circular view, short dash should cross at the intersections of center
line.
For small hole, center line is presented as thin continuous line.
Center line should not extend between views.

Leave space
Leave space
Center Line Practice
Leave the gap when centerline forms a continuation
with a visible or hidden line
Center line should always start and end with
long dash.
Leave Leave Leave
space space space

Leave
space
 D
G
B C
E D
F
A
F S
C
B
T

A
F E F G
 D

C
G
C S
E F G
F A B I H
I B
H A

D
F
F

E
T
 C
A B D E
E B
D A S
F

F
T C
 A
C
H
B F S
D

A
B C D

F
T