ORTHOGRAPHIC PROJECTION
These are two dimensional views since the object is represented on a two
dimensional plane. The orthographic projections of an object should convey
the length, width and thickness, along with other details of the. The
elements to be considered while obtaining orthographic projections are:
i) The object
ii) The plane of projection
iii) The point of sight
iv) The rays of sight
A projection may be obtained by viewing the object from the point of sight
and tracing in correct sequence, the points of intersection between the
rays of sight and the plane on to which the object is projected. A projection
is called orthographic projection when the point of sight is imagined to be
located at infinity so that the rays of sight are parallel to each other and
intersect the plane of projection at right angle to it.
The principles of orthographic projection may be followed in four different
angles or systems, i.e. first, second, third and fourth angle projections. A
projection is said to be first, second, third or fourth angle when the object
is imagined to be in the first, second, third or fourth quadrant respectively .
The most commonly used are the first angle and third angle orthographic
projections.
1
If the HP and the VP intersect as shown above, it produces four quadrants.
The first quadrant, or first angle, is the top right and the third is the
bottom left. If a block is suspended between the VP and the HP in the first
and third angles, you can see how the views are projected onto the two
planes. So far we have obtained only two views of the block, one on the VP
and one on the HP. With a complicated block this may not be enough. This
problem is easily solved by introducing another plane. In this case it is a
VP and it will show a view of the end of the block and so, to distinguish it
from the other VP, it is called the auxiliary vertical plane (AVP), and the
original vertical plane is called the front vertical plane (FVP).
2
The front view is drawn by projecting the object on the V.P. The top view is
drawn by projecting the object on the H.P. The projection on the AVP as
seen from the left of the object and drawn on the right of the front view, is
called left side view.
First Angle Orthographic Projection
3
Third Angle Orthographic Projection
Orthographic Projection symbols
Symbol Meaning
First Angle Projection
Third Angle Projection
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Symbol Dimensions
E.g. diameter 16
Hidden Lines
While obtaining the projection of an object on to any principal plane of
projection, certain features of the object may not be visible. The invisible
or hidden features are represented by short dashes of medium thickness.
Note: In isometric projection, the arrow points in the direction of the Front
Elevation
Dimensioning
Features in orthographic projection are dimensioned in the view in which
they are best seen. Dimensions should not be repeated if it is inferred in
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another view. Sufficient space between the views must be provided to
facilitate placement of dimensions.
Examples
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Construct the isometric projection of a casting whose orthographic projection is shown
below.
Draw the oblique projection of a casting whose views are shown below
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Draw the oblique projection of a casting whose views are shown below
Draw the isometric projection of a casting whose views are shown below
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