2-D Transformation and Projection 2-D Drawing 26...

Projection Theory

Multi-view Projection

Principle views & View placement

Projection angle

Detail drawing: 1view, 2 view, 3 view

drawing

Creating multi-view drawing

View Selection

2 PRT 137 Engineering Graphicss

Engineering and technical graphics are depend on projection methods.

Two methods commonly used: a) Parallel: object positioned

at infinity & viewed from multiple points on an imaginary line, parallel to the object.

b) Perspective: object positioned at finite distance & viewed from a single point.


Projection theory comprises of principles used to graphically

represent 3D objects on 2D media based on 2 variables:

i. Line of Sight (LOS): an imaginary ray of light between an

observer’s eye and an object.

• Parallel- all LOS are parallel

• Perspective- all LOS start at a point.

ii. Plane of projection: an imaginary flat plane where the

image created by LOS is projected.


A parallel projection

technique

The projection plane is placed

between observer and object.

The projection plane is also

perpendicular to the parallel

line of sight.


Multi-view projection is an

orthographic projection for

which the object is behind the plane of projection.


Multi-view drawings i. Employ multi-view projection technique

ii. Generally 3 views of an object are drawn

iii. Each view is a 2D flat image


The 6 principle views are the

six mutually perpendicular views that are produced by

six perpendicular planes of projection.

Image an object is

suspended in a glass box,

the 6 sides become

projection plane showing

the six views.


The views are laid flat by

“unfolding the glass box”

This forms the basis of two important multi-view drawing

standard:

a) Alignment of views

b) Fold lines

Top and bottom views are all

aligned vertically & share the

same height with dim.

Rear, left, front and right side

are aligned horizontally & share

same height with dim.


Fold lines are imaginary

hinged edges of the

glass box

i. The fold line between the

top & front views is

labeled H/F

ii. The distance from a point

in front view to the H/F is the same as the distance

from the corresponding

point in the top view to

the H/F fold line.


The arrangement of views may vary as shown,

where the top view is considered the central view.


There are two standard arrangement of

all six views of an object

i. First-angle projection

ii. Third-angle projection

Each uses different symbol

The names are derived from the method

used to view the object being drawn


In 1st angle projection, the object is placed

in the first quadrant.

In 3rd angle projection, the object is placed in

third quadrant.


Rules:

Symbols

1st angle

projection

3rd angle

projection

View from

above is placed

underneath

View from

above is placed

above

View from

below is placed

above

View from

below is placed

below

View from left is

placed on right

View from left is

placed on left

View from right

is placed on left

View from right

is placed on

right

(a) 1st angle projection (b) 3rd angle projection


1. Draw border/frame, title block, etc. Draw border, title block

Locating the view

i. should be approx. in

centre

ii. distance between view

= total length available

– view length / 3 Drawing projection lines


2. Light construction of the

views

Draw light/thin horizontal &

vertical lines accordingly for front view

Draw center lines and hidden lines as final

Draw top (or side) view

Project top view (or side) from front view using thin, light construction line

Draw side (or top) view

Project side view (or top) from front view and top (or side) view

All arcs and circles should be lined (final) at this stage


3. Lining in of the views

To darken all visible edges (lines that represent a hard edge)

Done using thick, black pencil (0.5mm, 2B)

Should be done systematically for 3 views

Start with horizontal line at the top of the top/plan view, working down the page using T-square

From left hand side, working across the page, line in all vertical lines, using T-square and set square

Projection lines may be left on the drawing if they are light


4. Write dimensioning, notes,

annotation, etc.

Be aware of redundant dimensioning

5. Complete drawing by writing the rest

of title block, parts list & revision table,

and others


4 basic decisions:

1. Determine the best position. The object must be positioned such a way that the surface of major features are either perpendicular or parallel to glass planes


2. Define the front view. Should show the

object in natural state and show most features


3. Determine the minimum number of

views needed to completely describe

the object.


4. Determine other views that fewest

number of hidden lines

PRT 137 Engineering Graphicss 31

THANK YOU…


2-D Transformation and Projection 2-D Drawing 26...

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