projection in ed

7/26/2019 Projection in ED

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TYPES OF PROJECTION

Engineers are confronted with the task of communicating the design, development and

structures of machines to manufacturers and builders. The shape and size of various parts

of a machine and its structure must be recorded on plane sheets in a systematic way for

communication. The pictorial view of the object does not carry all the details, especially

the inner details and correct shape of complicated parts. Different methods, therefore,

are implied for describing the exact shape based on the projectors! drawn by engineers.

Principle of Projection

"f straight lines are drawn from various points on the contour of an object to meet aplane, the object is said to be projected on that plane. The figure formed by joining, in

correct se#uence, the points at which these lines meet the plane, is called the projection

of the object. The lines from the object to the plane are called projectors.

Projection on a Single Plane

"f straight lines are drawn from the various points on the contour of an object to meet a

plane, the figure obtained on the plane is called the $rojection of the object. The object

is said to be projected on the plane. "n other words, we can say that the projection of an

object on a plane is the shadow of the object on the plane showing each and every edge

line of the object. The imaginary lines drawn from the object to the plane are called

projectors or projection lines. The plane on which the projection of the object is taken is

called plane of projection. %uppose an object is placed in front of a screen and light

thrown on the object &assuming the light rays to be parallel to each other and

perpendicular to the screen' then a true shadow of the object is obtained on the screen.

This shadow is the projection on the object on the plane of screen showing the contourline of the object.


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Types of Projections

The projections are classified according to the method of taking the projection on the

plane. ( classification of projection is shown below)


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Factors on Which Type of Projection Depends

Different views of an object can be drawn by projections. Thus every drawing of an

object will have four things on which projection depends

o *bject,

o $rojectors,

o $lane of projection, and

o *bserver!s eye or station point.

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ethods of Projection
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"n engineering drawing following four methods of projection are commonly used, these

are)

o "sometric projection

o *bli#ue projection

o $erspective projection

o *rthographic projection

The above method represents the object by a pictorial view as an observer sees it. "n

these methods of projection a three dimensional object is represented on a projection

plane by one view only. +hile in the orthographic projection an object is represented by

two or three views on the mutual perpendicular projection planes. Each projection view

represents two dimensions of an object. or the complete description of the three

dimensional object, at least two or three views are re#uired. *rthographic projection

comes under the category of -on$ictorial Drawing!.

!rthographic Projections

The word orthographic means straight description. The straight description here stands

for the parallel projectors from the object to infinity. "f a perpendicular picture plane is

inserted between the projectors, a picture is formed having the same shape and size as

that of the object. "f an observer at position (! moves to infinity, the projectors to his

eyes becomes parallel to the object and he observes the same shape and size as that of

the object. The view so formed is known as the orthographic projection. %imilarly, the

parallel projectors shall form the pictures on the respective picture planes from the

positions / and 0. 1sually two views are sufficient for simple objects, but the help of

three or more views is necessary for complicated objects. These picture planes are

mutually perpendicular to each other and are known as $rincipal $lanes! of projectors,

named 2orizontal $lane &2$', 3ertical $lane &3$' and $rofile $lane &$$'.


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View from Parallel Projection

ethods of !rthographic Projection

The two methods of projections are)

o irst angle projections

o Third angle projections

igure below shows four #uadrants formed by the intersection of horizontal and vertical

planes. The intersecting line of the planes is called the coordinate axis. The revolving

direction of the horizontal plane shows that #uadrants Iand III are 4open4

but II and I" #uadrants become 5closed4 when the horizontal plane coincides with the


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vertical plane. "t is obvious that the closed #uadrant has no use for the purpose of

projectors as the views taken on these will overlap.

Read more about !bjecti#e$ Significance % &tility of 'ngineering

Dra(ing

Four Quadrants

First Angle Projections)This method of projection is popular in Europe, especially in

/ritain. /ureau of "ndian %tandard has also recommended it now. igure 6 shows an object

placed in the first #uadrant. $arallel projectors in the direction (!, from the object,

forms a picture on the vertical plane &3$' which is known as ront 3iew or ront
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Elevation. %imilarly, a parallel projector from the direction of /! forms the picture on the

horizontal plane &2$', known as Top 3iew or $lan. ( mutually perpendicular plane to both

2$ and 3$, known as profile plane &$$' also receives projectors from the object from the

direction 0. The view on the profile plane is known as %ide 3iew or %ide Elevation. The

three planes containing the views are then opened on a plane. The symbol of first angle is

shown in below.

First Angle Projection

(lthough two or three views are enough to reveal an object, the projectors from six

directions of the object in the first angle are shown, if necessary. The views are to be

shown symmetrically. The view from the top &Direction /' placed underneath. The view

from the front &Direction (' is placed in the centre. The view from the left side &Direction

0' is placed on the right side of view (. The view from the right side &Direction D' is

placed on the left side. The view from the bottom &direction E' is placed on the top as

5E4 view. The view from the rear &Direction ' may be placed on the right or left side of 0

or D views.


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Six Views of First

Angle Projection

Third Angle Projections)This system of projection is known as the (merican system. The

object is placed in the 7rd #uadrant. The planes are imagined to be made of transparent

material, say a glass box. The front wall of the box is assumed to be hinged to the other

walls as shown in the figure. The parallel projectors in all the six directions form

respective views on the walls of the box serving as picture planes. The hinged walls of the

box are opened and laid down on a plane. The placement of various views are in a

systematic way. The view from the top is placed above the ront 3iew &3'. The viewfrom the right hand side is placed on the right side of 3. The view from left hand side is

placed on the left side of 3. The view from bottom is placed underneath the 3. The

view from the rear may be placed on the right or the left of the side views. The symbol

for the third angle is given below.


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Comparison of First Angle Projection and Third AngleProjection method

Sl*o) First Angle Projection ethod Third Angle Projection ethod

8

The object is kept in the first

#uadrant.

The object is assumed to be kept in

the third #uadrant.

9

The object lies between the observer

and the plane of projection.

The plane of projection lies between

the observer and the object.

7

The plane of projection is assumed to

be non transparent.

The plane of projection is assumed

to be transparent

6 "n this method, when the views are

drawn in their relative positions, $lan

"n this method, when the views are

drawn in their relative positions,


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&Top view' comes below the elevation

&ront view', the view of the object

as observed from the left side is

drawn to the right of elevation.

$lan comes above the elevation, left

hand side view is drawn to the left

hand side of the elevation.

:

This method of projection is now

recommended by the 5/ureau of

"ndian %tandards4 from 8;;8.

This method of projection is used in

1.%.( and also in other countries.

projection in ed

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