university of palestine college of engineering & urban...

Engineering Drawing

Lecture 5PROJECTION THEORY

Lecturer:Eng. Eman Al.SwaityEng.Heba hamad

University of PalestineCollege of Engineering & Urban PlanningFirst Level

PROJECTION METHOD

PART 1

TOPICS

Projection methods

Orthographic projection

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

Orthographic 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

5

1

2

3

4

ORTHOGRAPHIC VIEW

Orthographic view Orthographic view depends on relative position of the object

to the line of sight.

Two dimensions of anobject is shown.

Three dimensions of an object is shown.

Rotate

Tilt

More than one view is neededto represent the object.

Multiview drawingMultiview drawing

Axonometric drawingAxonometric drawing

Orthographic projection technique can produce either

1.

1. Multiview drawingMultiview drawingthat each view show an object in two dimensions.

2.

2. Axonometric drawingAxonometric drawingthat show all three dimensions of an object in one view.

Both drawing types are used in technical drawing for

communication.

NOTES

ORTHOGRAPHIC VIEW

Axonometric (Isometric) Drawing

Easy to understand

Right angle becomes obtuse angle.

Circular hole becomes ellipse.

Distortions of shape and size in isometric drawing

AdvantageAdvantage

DisadvantageDisadvantage Shape and angle distortion

Example

Multiview Drawing

It represents accurate shape and size.AdvantageAdvantage

DisadvantageDisadvantage Require practice in writing and reading.

Multiviews

drawing (2-view drawing)Example


PART 2

TOPICS

Object representation

Glass box concept

Line convention

Orthographic projection of point,line, plane, surface and object.

Multiview projection

Isometric Sketching

OBJECT REPRESENTATION

Axonometric projection

Multiview projection

MULTIVIEW PROJECTION

Three principle dimensionsof an object …

Width Depth

Height

Width

Heig

ht

Depth

Depth

… 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(Elevation)

Right side view

Top view (Elevation)

OBSERVER MOVE AROUND

Front view Right side view

Top view

THE GLASS BOX CONCEPT

Bottom view

Left side view

Rear view

Height

Width

Dep

th

History

MULTIVIEW PROJECTION

Click on the picture to play video


of Object Features

OBJECT FEATURES

Edges are lines that represent the boundary

between two faces.

Corners Represent the intersection of two or

more edges.

Edge

Corner

Edge No edge

No corner No corner

Surfaces are areas that are bounded by edges

or limiting element.

Limitingelement

is a line that represents the last visible

part of the curve surface.

Surface Surface Surface

LimitLimit

OBJECT FEATURES

A

B

PROJECTION OF POINT(S)

AF

BR

AT

BFAR

BT

AFAR

AT

BFBR

BT

Equaldistance

A

B

AF BF BRAR

AT

BT

BR

AR

AF BF

AT

BT

True length

NORMAL LINETrue lengthPoint

Equallength

PROJECTION OF LINE

AB

AF BF BRAR

AT

BT

INCLINED LINEForeshortened

BR

AR

AF

BF

Foreshortened

AT

BT

True length

A

Equallength

PROJECTION OF LINE

AB

AF

BF BR

AR

AT

BT

OBLIQUED LINE

A

Equallength

B

ForeshortenedForeshortened

Foreshortened

BR

AR

AF

BF

AT

BT

PROJECTION OF LINE

BC

A

PROJECTION OF PLANE

BF AF,CF CRAR,BR

AT

CT

NORMAL PLANE

Equallength

EdgeEdge

True size

CR

AR,BR

AF,CF

BF

AT

BT

CT

BT

BC

BF AF

CR

AR,BR

AT

CT

INCLINED PLANE

A

Equallength

BT

C

CF

Edge

CR

AR,BR

Foreshortened

BT

CT

AT

AF

CF

Foreshortened

BF

PROJECTION OF PLANE

BC

BF

AF

CR

AR

AT

CT

OBLIQUED PLANE

A

Equallength

BT

C

CF

B

BR

Foreshortened

CR

AR

BR

AF

BF CF

Foreshortened

AT

BT

CT

Foreshortened

PROJECTION OF PLANE

You have to project the remaining surfaces which are invisible too !

PROJECTION OF OBJECTThe views are obtained by projecting all object

features to the picture plane.

(Elevation)

s

s

s

PROJECTION OF OBJECT

(Elevation)

PROJECTION OF OBJECT

(Elevation)

Line Convention

LINE CONVENTION

Precedence of coincide lines.

Hidden line drawing.

Center line drawing.

PRECEDENCE OF LINE

Visibleline

Order ofimportance

Hiddenline

Centerline

HIDDEN LINE PRACTICE

Hidden 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 should intersect to form L and Tcorners.

Correct

No !

L T


Hidden arcs should start on a center line.


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 with

long dash.

CENTER LINE PRACTICE

IsometricSketching

2. Define an isometric axis.

3. Sketching the enclosing box.

4. Estimate the size an and relationship of each

details.

5. Darken all visible lines.

1. Place the object in the position which its shape

and features are clearly seen.

Sketch from an actual object

STEPS

1. Positioning object.

2. Select isometric axis.

3. Sketch enclosing box.

4. Add details.

5. Darken visible lines.

Sketch from an actual object

1. Positioning object.

2. Select isometric axis.

3. Sketch enclosingbox.

4. Add details.

Sketch from an actual objectSTEPS

5. Darken visible lines.

Note In isometric sketch/drawing), hidden lines are omitted

unless they are absolutely necessary to completely

describe the object.

End of The Lecture

PROJECTION THEORY

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