Orthographic Projections _ Engineering Graphics/Drawing

Orthographic Projection

TOPICS

Object representation

Glass box concept

Line convention

Multiview projection

OBJECT REPRESENTATION

Axonometric projection(Pictorial View)Multiview projection

MULTIVIEW PROJECTIONThree principle dimensionsof an object …

Width Depth

Height

Width

Hei

ght

Depth

Dep

th

… can be presented onlytwo in each view.

Adjacent view(s)is needed tofulfill the sizedescription.

1. Revolve the object with respect to observer.

TO OBTAIN MULTIVIEW REPRESENTATION OF AN OBJECT

2. The observer move around the object.

REVOLVE THE OBJECT

Front view Right side view

Top view

OBSERVER MOVE AROUND

Front view Right side view

Top view

THE GLASS BOX CONCEPT

Bottom view

Left side view

Rear view

HeightWidth

Dep

th

History

Standard Views of Primitive Solids

PROJECTION METHOD

Perspective

Oblique Orthographic

Axonometric Multiview

Parallel

PROJECTION THEORY

The projection theory is based on two variables:

1) Line of sight

2) Plane of projection (image plane or picture plane)

The projection theory is used to graphically represent

3-D objects on 2-D media (paper, computer screen).

Line of sightLine of sight is an imaginary ray of light between an

observer’s eye and an object.

Line of sight

Parallel projectionParallel projection

Line of sight

Perspective projectionPerspective projection

There are 2 types of LOS : parallel convergeand

Plane of projectionPlane of projection is an imaginary flat plane which

the image is created.

The image is produced by connecting the points where

the LOS pierce the projection plane.

Parallel projectionParallel projection Perspective projectionPerspective projection

Plane of projection Plane of projection

Disadvantage ofPerspective Projection

Perspective projection is notnot used by engineer for

manu-facturing of parts,

because

1) It is difficult to create.

2) It does not reveal exact

shape and size.Width is distorted

Comparison of Multi-view, Axonometric, oblique, and perspective projections

1. Multiview Projection 2. Axonometric Projection

3. Oblique Projection 4. Perspective Projection

5

Orthographic projectionOrthographic projection is a parallel projection technique

in which the parallel lines of sight are perpendicular to the

projection plane

MEANING

Object views from top

Projection plane

1

2

3

4

51 2 3 4

Orthographic Projection

of Object Features

OBJECT FEATURESEdges are lines that represent the boundary

between two faces.

Corners Represent the intersection of two ormore edges.

Edge

Corner

Edge No edge

No corner No corner

Surfaces are areas that are bounded by edgesor limiting element.

Limitingelement

is a line that represents the last visiblepart of the curve surface.

Surface Surface Surface

LimitLimit

OBJECT FEATURES

PROJECTION OF OBJECTThe views are obtained by projecting all object features to the picture plane.

You have to project the remaining surfaces which areinvisible too !

s

s

s

PROJECTION OF OBJECT

PROJECTION OF OBJECT

Line Convention

LINE CONVENTION

Precedence of coincide lines.

Hidden line drawing.

Center line drawing.

PRECEDENCE OF LINEVisible

lineOrder ofimportance

Hiddenline

Centerline

HIDDEN LINE PRACTICEHidden line should join a visible line, except itextended from the visible line.

Correct

No !

Join

Leavespace

Correct No !

Hidden line should join a visible line, except itextended from the visible line.

Leavespace

Leavespace

HIDDEN LINE PRACTICE

Hidden line should intersect to form L and Tcorners.

Correct

No !

L T

HIDDEN LINE PRACTICE

Hidden arcs should start on a center line.

HIDDEN LINE PRACTICE

CENTER LINE PRACTICEIn 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

Leave the gap when centerline forms a continuation with a visible or hidden line

Leavespace

Leavespace

Leavespace

Leavespace

Center line should always start and end withlong dash.

CENTER LINE PRACTICE