Lecture - 10
Projection
Consider an arrangement
for a simple phenomenon of shadow formation. When an object is
placed between a light source and a screen, the light rays coming
from the source are obstructed by the object. As a result, a shadow is
formed on the screen behind the object.
This occurs because light
travels in straight lines, and any object that blocks the light prevents it
from reaching the screen in the region directly behind the object.
In this setup, the shadow
formed is larger than the actual size of the object.
If the light source is
moved farther away from the object, the size of the shadow decreases.
This happens because the rays from the source become less divergent,
resulting in a smaller projection of the object on the screen.
When the light source is placed at an infinite distance, the light rays that reach the object are almost parallel to each other. In such a case, the shadow formed on the screen is nearly equal in size to the actual object.
Projection
is a technique used in engineering drawing to show the size, shape, and
features of an object by projecting its edges and surfaces onto a reference
plane using straight lines.
·
The lines of sight are popularly called
projectors.
·
The planes on which the drawings are made
are called planes of projection.
Types of Planes of Projection
1.
Horizontal Plane (HP)
2.
Vertical Plane (VP)
When these planes
intersecting each other at right angle, divide the space into four dihedral
angles or quadrants. The line of intersection between these planes is called a
reference line. Any position in space with reference to the principal planes
can be defined as in figure below:
Classification Of Projections
Orthographic Projection
The term orthographic
comes from the Greek word orthos, meaning perpendicular. In
orthographic projection, the observer is assumed to view the object from an infinite
distance, ensuring that the rays of sight (projectors) are:
- Parallel to each other
- Perpendicular to the plane of
projection
Orthographic projection can produce:
1. Single
pictorial views – showing all three dimensions in one view.
2. Multi-view
drawings – each view shows only two dimensions, such as:
o Front
view (height and width)
o Top
view (width and depth)
o Side
view (height and depth)
Multi-View Drawing
Multi-view drawing
requires two or more orthographic projections to accurately define the
shape of a three-dimensional object. Each orthographic view is a two-dimensional
representation, showing only two of the three dimensions—typically
height, width, or depth.
Since no single view
can provide complete information about the object, multiple views are
required. These views must be correlated and interpreted together to
understand the full shape and structure of the object.
Because of this, the arrangement
and relationship among the views are interdependent. Over time,
standardized conventions and rules have been established to ensure
consistency and clarity in technical drawings.
Orthographic Projection Planes
Orthographic projections
are primarily drawn on two principal planes, also referred to as reference
planes:
- Vertical Plane (VP)
– usually used for the front view
- Horizontal Plane (HP)
– typically used for the top view
These two planes are perpendicular
to each other, and they divide the 3D space into four quadrants,
commonly referred to as angles:
1. First
Angle
2. Second
Angle
3. Third
Angle
4. Fourth
Angle
Depending on the position of the object, the orthographic projection can be
classified as follows:
1. First
angle projection:
The object lies in the first angle, i.e.,
above H.P. and in front of V.P.
2. Second
angle projection:
The object lies in the second angle, i.e.,
above H.P. and behind V.P.
3. Third
angle projection:
The object lies in the third angle, i.e.,
below H.P. and behind V.P.
4. Fourth
angle projection:
The object lies in the fourth angle, i.e., below H.P.
and in front of V.P.
The following terms are frequently used in multi-view drawings:
Vertical
plane: Vertical plane, also known as front reference plane,
is assumed to be placed vertically and is denoted by V.P.
Horizontal
plane: Horizontal plane, also known as horizontal reference
plane, is assumed to be placed horizontally and is denoted by H.P. It is
perpendicular to V.P.
Profile
plane: A plane perpendicular to both the above planes is
known as a profile plane. The plane on the right end of the planes is known is
right profile plane while the plane on the left end is known as left profile
plane.
Reference
plane: All the above mentioned mutually perpendicular planes
are called reference planes.
Principal plane:
It is an alternative name of the reference plane.
Reference
line: The line of intersection between the principal planes
is known as a reference line. It is also popularly called xy line.
Front
view: The view of an object by observing it from the front
and drawn on the V.P. is called front view (F.V.) or elevation.
Top
view: The view of an object by observing it from the top
and drawn on the H.P. is called top view (T.V.) or plan.
Side
view: The view of an object by observing it from the
left-hand side or right-hand side and drawn on a profile plane is called side
view or end view.
Features of First Angle Projection
1. Object
Position:
The
object is located in the first quadrant, i.e., in front of the
Vertical Plane (V.P.) and above the Horizontal Plane (H.P.).
2. Observer's
Position:
The
object is placed between the observer and the plane of projection.
3. Top
View Placement:
The
top view (plan) is drawn below the front view.
4. Left-Hand
Side View Placement:
The
left-side view is drawn on the right side of the front view.
5. Right-Hand
Side View Placement:
The
right-side view is drawn on the left side of the front view.
Fig: (a) Front view on
V.P. (b) Top view on H.P. (c) Left-hand side view on P.P.
Features of Third Angle Projection
1. Object
Position:
The
object is located in the third quadrant, i.e., behind the Vertical
Plane (V.P.) and below the Horizontal Plane (H.P.).
2. Observer's
Position:
The
plane of projection lies between the observer and the object.
3. Top
View Placement:
The
top view (plan) is drawn above the front view.
4. Left-Hand
Side View Placement:
The
left-side view is drawn on the left side of the front view.
5. Right-Hand
Side View Placement:
The
right-side view is drawn on the right side of the front view.
Fig. (a) Front view on
V.P. (b) Top view on H.P. (c) Left-hand side view on P.P
Conversion of Pictorial View into Orthographic Views
Problem 1: Pictorial
view of an object is shown in Fig. Using first angle projection, draw its (a)
front view from the X-direction, (b) top view and (c) left-hand side view.
Fig. (a) Pictorial view (b) Orthographic views
Problem 2: Pictorial
view of an object is shown in Fig. below. Using first angle projection, draw its
(a) front view, (b) top view and (c) right-hand side view.
Fig. (a) Pictorial view (b) Orthographic views
Problem 3:
Pictorial view of an object is shown in Fig. below. Using first angle
projection, draw its (a) front view, (b) top view and (c) right-hand side view.
Fig. (a) Pictorial view (b) Orthographic views
Problem 4: Pictorial
view of an object is shown in Fig. below. Using first angle projection, draw
its (a) front view, (b) top view and (c) side view.
Exercise
Draw three views of the
objects shown in Figs. below using first angle projection.
