engineering drawing

2. PROJECTIONS
INTRODUCTION TO PROJECTIONS
Introduction 4. Perspective projection-when the projectors converge to a
point, the projection thus obtained is known as perspective
projection
I think you all know how to choose scale and how to take
dimensions so let us start with projections you might have
some idea of projection as you might have studied in physics
but here concept is somewhat different so let us start with first
of all is what is it but before this we will discuss some objec-
tives of this chapter.
Now we will discuss the most important and widely used
method by engineers. Yes, orthographic projection is the most
important method out of these so will discuss this in detail.
Before starting orthographic projection we must discuss planes
of references in which we have to draw projections.Chapter Learning Objectives
Planes of ReferencesAfter reading this chapter, you will be able to the following
questions In study of orthographic projection, two principle planes are
used. They are known as coordinate planes. One plane is
horizontal and the other vertical and they intersect each other at
right angles. This can be drawn as
You have seen the planes divide themselves into four angular
spaces or dihedral angles.
The horizontal plane is known as H.P and the vertical plane is
known as V.P.Their line of intersection is known as the ground
line denoted by the line XY.The projection of an object on H.P
is known as the plan and the projection on the V.P is known as
elevation.
• Discuss projection and its methods (in brief).
• Explain multiview projection in detail
• Discuss projection of points in detail.
• Discuss projection of straight lines in detail.
• Discuss various types of projection of planes.
What is Projection?
A projection is defined as a view imagined on to a plane known
as projection plane. In graphic language shape of an object is
described by projection, which is image of the object, formed by
the rays of sight taken in some particular direction from the
object into a picture plane.
Depending upon the orientation of the object, location of the
point of sight, and the direction of lines of sight relative to the
picture plane, different types of projections such as ortho-
graphic, isometric, perspective, oblique etc. can be obtained.
The point, from which the observer is assumed to view the
object, is called station point or the center of projection and
the lines of rays drawn from the object to the plane are called
projectors.
Methods of Projection
There are four methods of projections commonly used
1. Orthographic projection
2. Isometric projection
3. Oblique projection
4. Perspective projection.
Let us discuss small introduction of each one of these so that
we can understand examples
1. Orthographic projection-when the projectors are Orthographic Projection
While projecting any object on a plane if the rays of sight are
taken perpendicular to the plane of projection, the projection
method is then called orthographic projection. In this method
the observer is assumed to be at infinite distance from the plane
of projection, such that the projectors will be parallel to each
other.
perpendicular to the plane of projection the projection is
known as orthographic projection.
2. Isometric projection-when the projectors are parallel but
inclined to one plane of projection, the projection thus
obtained is known as isometric projection
3. Oblique projection-when the projectors are parallel to each
other and oblique to the plane of projection, the projection
drawn is known as oblique projection.

3. To draw side view rotate the AVP in the direction of arrow
shown, so as to make it coincide with the V.P. looking the object
from the left, the left side view is obtained and drawn on the
right of the front view.
There are two systems by which an object can be represented.
a. First angle projection
b. Third angle projection
Both these are named according to the quadrant in which the
object is imagined to be placed for purposes of projection of
projection. As far as the shape and sizes of views are concerned
there is no difference between these two systems. The only
difference lies in the relative positions of the various views.
Orthographic Projection (front view) Principle of First Angle Projection
When the object is situated in first quadrant, that is in front of
V.P and H.P, the object lies in between the observer and the
plane of projection. In this case the object is to be transparent
and the projectors are imagined to be extended from various
points of the object to meet the projection plane. The meeting
points when join in order form an image. This is the principle
of first angle projection.
Initial view Final arrangement of views
Principle of Third Angle Projection
When the object is situated in third quadrant that is in front of
V.P and below H.P, the projection plane lies between the
observer and the object. This means that the plane of projec-
tion is said to be transparent. The intersection of this plane
with the projectors from all the points of object would form an
image on the transparent plane. This is the principle of third
angle projection.
Orthographic Projections (front & top views)
Here the figure shows the two principal planes H.P and V.P and
another auxiliary vertical plane (AVP). AVP is perpendicular to
both V.P and H.P.
Front view is drawn by projecting the object on the V.P and top
view is drawn by projecting on the H.P the projection on the
AVP as seen from the left of the object and drawn on the right
hand of the front view, is called left side view.
Initial view Final arrangement of views

4. Comparison of First Angle and Third Angle
Projection.
If one projects lines from the corners of the object (with each
line at 90 degrees to a surface of the glass), until these lines
intersect the glass, one lay out 6 views each of which represents
the object as it is seen by the various planes of glass.
S.no. First Angle Projection Third Angle Projection
1. The object to be assumed The object to be assumed
in first quadrant. in third quadrant.
2. The object lies between the The plane of projection
observer and the plane of lies between the observer
projection. and the object.
3. The plane of projection is The plane of projection is
assumed to be assumed to be transparent
non- transparent
4. Object is situated on or Object is situated on or
above the H.P above the ground
5. Symbol Symbol
6. Front view comes above Top view comes above
Having projected the respective views of each face of the object
onto the glass surfaces we can unfold the glass.
top view. This method of front view. This method
projection is recommended of projection is used in
by the B.I.S from 1991. U.S.A and also in other
countries.
Making Orthographic Views
Representation drawing and sketching is concerned with what
you see or drawing which you imagine as if you actually saw it.
For making orthographic views we can consider an object
enclosed in an imaginary glass box, positioned such that the
planes of glass are parallel to the major surfaces of the object
and 90 degrees to each other.
Until all the views are laid out in the same plane as the front
view giving the figure.
This process uses principles of orthographic projection the
advantage of this process is that it’s possible to apply a uniform
a system of coordinates to the pictures created in each view. If
we correctly draw the individual views onto the pieces of glass,
fold them up into the middle of box, the resulting intersec-
tions will define the three dimensional object.
From above example we can designate and find out the relative
positions of views as follows:
• View from the front- front view- View from the top-top
view
• View from the left- left side view- View from the right-
right side view
• View from the below-bottom view-View from the rear-
rear view