l7 projection theory

8/10/2019 L7 Projection Theory

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Engineering Drawing

Lecture 5

PROJECTION THEORYLecturer:Eng. Eman Al.SwaityEng.Heba hamad

University of PalestineCollege of Engineering & Urban PlanningFirst Level


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PROJECTIONMETHOD

PART 1


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TOPICS

Projection methods

Orthographic projection


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PROJECTION METHOD

Perspective

Oblique Orthographic

Axonometric Multiview

Parallel


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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).


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Line of sightLine of sight is an imaginary ray of light between anobservers eye and an object.

Line of sight

Parallel projectionParallel projection

Line of sight

Perspective projectionPerspective projection

There are 2 types of LOS : parallel convergeand


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Plane of projectionPlane of projection is an imaginary flat plane whichthe 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


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Disadvantage of Perspective 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


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OrthographicProjection



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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


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ORTHOGRAPHIC VIEWOrthographic viewOrthographic 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


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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


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Axonometric (Isometric) Drawing

Easy to understand

Right angle becomes obtuse angle.

Circular holebecomes ellipse.

Distortions of shape and size in isometric drawing

AdvantageAdvantageDisadvantageDisadvantage Shape and angle distortion

Example


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Multiview Drawing

It represents accurate shape and size.AdvantageAdvantageDisadvantageDisadvantage Require practice in writing and reading.

Multiviews

drawing (2-view drawing)Example


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PART 2


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TOPICS

Object representation

Glass box concept

Line convention

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

Multiview projection

Isometric Sketching


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OBJECT REPRESENTATION

Axonometric projection

Multiview projection


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MULTIVIEW PROJECTION

Three principle dimensionsof an object

Width Depth

Height

Width

H e i g h t

Depth

D e p t h

can be presented onlytwo in each view.

Adjacent view(s)

is needed tofulfill the sizedescription.


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1. Revolve the object with respect

to observer .

TO OBTAIN MULTIVIEWREPRESENTATION OF AN OBJECT

2. The observer move around the

object.


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REVOLVE THE OBJECT

Front view(Elevation) Right side view

Top view (Elevation)


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OBSERVER MOVE AROUND

Front view Right side view

Top view


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THE GLASS BOX CONCEPT

Bottom view

Left side view

Rear view


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Height

Width

D

e p t h

History


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Click on the picture to play video


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of Object Features


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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


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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


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A

B

PROJECTION OF POINT(S)

AF

BR

AT

BF AR

BT

AF AR

AT

BFB

R

BT

Equaldistance


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A

B

AF BF BR

AR

AT

BT

BR

AR AF BF

AT

BTTrue length

NORMAL LINETrue lengthPoint

Equallength

PROJECTION OF LINE


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A B

AF BF BR

AR

AT

BT

INCLINED LINEForeshortened

BR

AR

AFBF

Foreshortened

ATBT

True length

A

Equallength

PROJECTION OF LINE


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A B

AF

BF BR

AR

AT

BT

OBLIQUED LINE

A

Equallength

B

ForeshortenedForeshortened

Foreshortened

BR

AR

AF

BF

ATBT

PROJECTION OF LINE


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B C

A

PROJECTION OF PLANE

BF AF,C F C R AR,B R

AT

C T

NORMAL PLANE

Equallength

EdgeEdge

True size

C R

AR,B R AF,C F

BF

AT

BTC T

BT


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B C

BF AF

C R

AR,B R

AT

C T

INCLINED PLANE

A

Equallength

BT

C

C F

Edge

C R

AR,B R

Foreshortened

BTC T

AT

AF

C F

Foreshortened

BF

PROJECTION OF PLANE


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BC

BF

AF

C R

AR

AT

C T

OBLIQUED PLANE

A

Equallength

BT

C

C F

B

BR

Foreshortened

C R

AR

BR

AF

BF C F

Foreshortened

AT

BTC T

Foreshortened

PROJECTION OF PLANE


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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)


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s

s

s

PROJECTION OF OBJECT

(Elevation)


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PROJECTION OF OBJECT

(Elevation)


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Line Convention


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LINE CONVENTION

Precedence of coincide lines.

Hidden line drawing.

Center line drawing.


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PRECEDENCE OF LINE

VisiblelineOrder of importance Hiddenline Center line


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HIDDEN LINE PRACTICE

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

Correct

No !

Join

Leavespace


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Correct No !

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

Leavespace

Leavespace



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Hidden line should intersect to form L and Tcorners.

Correct

No !

L T



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Hidden arcs should start on a center line.



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CENTER LINE PRACTICEIn circular view, short dash should cross at theintersections of center line.

For small hole, center line is presented as thincontinuous line.

Center line should not extend between views.

Leave space Leave space


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Leave the gap when centerline forms a

continuation with a visible or hidden line

LeavespaceLeavespaceLeavespace

Leavespace

Center line should always start and end with

long dash.

CENTER LINE PRACTICE


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Isometric

Sketching


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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 shapeand features are clearly seen.

Sketch from an actual object


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STEPS

1. Positioning object.

2. Select isometric axis.

3. Sketch enclosing box.

4. Add details.

5. Darken visible lines.

Sketch from an actual object


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1. Positioning object.

2. Select isometric axis.

3. Sketch enclosing

box.4. Add details.

Sketch from an actual objectSTEPS

5. Darken visible lines.

Note In isometric sketch/drawing), hidden lines are omittedunless they are absolutely necessary to completelydescribe the object.


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End of The LecturePROJECTION

THEORY

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