Introduction
1.
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
2.
If straight lines are drawn from various points on the contour of an object to
meet a plane, the object is said to be projected on that plane. The figure
formed by joining, in correct sequence, 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
3.
If 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 Projection of
the object. The object is said to be projected on the plane. In 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. Suppose 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 contour line of the object.
Fig 1 Projection
Types of
Projections
4.
The projections are classified according to the method of taking the projection
on the plane. A classification of projection is shown below:
Factors
on Which Type of Projection Depends
5.
Different views of an object can be drawn by projections. Thus every drawing of
an object will have four things on which projection depends
(i) Object,
(ii) Projectors,
(iii) Plane of projection, and
(iv)
Observer's eye or station point.
Methods
of Projection
6.
In engineering drawing following four methods of projection are commonly used,
these are:
(a)
Isometric projection
(b)
Oblique projection
(c)
Perspective
projection
(d)
Orthographic
projection
In the above
methods (a), (b) and (c) represent the object by a pictorial view as an
observer sees it. These projections, which come under the category of
‘Pictorial Drawing’ have already been discussed in detail in Chapter7. In
these methods of projection a three dimensional object is represented on a
projection plane by one view only. While 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.
For the complete description of the three dimensional object, at least two or
three views are required. Orthographic projection comes under the category of
‘NonPictorial Drawing’.
Orthographic
Projections
7.
The word orthographic means straight description. The straight description here
stands for the parallel projectors from the object to infinity. If a
perpendicular picture plane is inserted between the projectors, a picture is
formed having the same shape and size as that of the object. If an observer at
position ‘A’ 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. (Refer Fig 2). The view so formed is known as the orthographic
projection. Similarly, the parallel projectors shall form the pictures on the
respective picture planes from the positions B and C. Usually 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 ‘Principal Planes’ of projectors, named Horizontal
Plane (HP), Vertical Plane (VP) and Profile Plane (PP).
Fig. 2 View from Parallel Projection
Methods
of Orthographic Projection
8.
The two methods of projections are:
(a)
First angle projections
(b)
Third angle projections
Figure 3 shows four quadrants 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 quadrants I and III are ”open” but II and
IV quadrants become “closed” when the horizontal plane coincides with
the vertical plane. It is obvious that the closed quadrant has no use for the
purpose of projectors as the views taken on these will overlap.
Fig 3 Four
Quadrants
9. First Angle
Projections. This method of projection is popular in
Europe, especially in Britain. Bureau of Indian Standard has also recommended
it now. Figure 4 shows an object placed in the first quadrant. Parallel
projectors in the direction 'A', from the object, forms a picture on the
vertical plane (VP) which is known as Front View or Front Elevation.
Similarly, parallel projectors from the direction of 'B' forms the picture on
the horizontal plane (HP), known as Top View or Plan. A mutually
perpendicular plane to both HP and VP, known as profile plane (PP) also
receives projectors from the object from the direction C. The view on the
profile plane is known as Side View or Side Elevation. The three
planes containing the views are then opened on a plane as shown in Fig. 5. The
symbol of first angle is shown in Fig 6.
Fig
4 First Angle Projection
Fig 5 Three Views on a Plane
Fig 6 Symbol
for First Angle Projection
10.
Although 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.
(Refer Fig 7). The views are to be shown symmetrically as
shown in Fig 7 (b). The view from the top (Direction B) placed underneath. The
view from the front (Direction A) is placed in the centre. The view from the
left side (Direction C) is placed on the right side of view A. 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 “E” view. The view from the rear
(Direction F) may be placed on the right or left side of C or D views.
Fig
7 First Angle Projection
11.
Third Angle Projections. This system of projection is known
as the American system. The object is placed in the 3rd quadrant. The planes
are imagined to be made of transparent material, say a glass box. (Refer Fig.
8). 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. (Refer Fig. 9). The
placement of various views are in a systematic way. The view from the top is
placed above the Front View (FV). The view from the right hand side is placed
on the right side of FV. The view from left hand side is placed on the left
side of FV. The view from bottom is placed underneath the FV. 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 in Fig 10.
Fig
8 Object in a Transparent Box
Fig 9 Layout
of Various Views
Fig
10 Symbol for Third Angle Projection
12.
Comparison of First Angle Projection and Third Angle Projection method
Sl No.

First Angle
Projection Method

Third Angle
Projection Method

1

The object is
kept in the first quadrant.

The object is
assumed to be kept in the third quadrant.

2

The object
lies between the observer and the plane of projection.

The plane of
projection lies between the observer and the object.

3

The plane of
projection is assumed to be non transparent.

The plane of
projection is assumed to be transparent

4

In this
method, when the views are drawn in their relative positions, Plan (Top view)
comes below the elevation (Front view), the view of the object as observed
from the left side is drawn to the right of elevation.

In this
method, when the views are drawn in their relative positions, Plan comes
above the elevation, left hand side view is drawn to the left hand side of
the elevation.

5

This method of
projection is now recommended by the “Bureau of Indian Standards” from 1991.

This method of
projection is used in U.S.A and also in other countries.
