Projection Methods:
Projection Methods are used to show drawing on a
paper . Projection Methods has mainly two types used in
Engineering Drawing :
1) First Angle Projection
Method
2) Third Angle Projection
Method
The principal projection planes and quadrants
used to create drawings are shown in figure. The object can
be considered to be in any of the four
quadrant.
Principal Projection Planes and Quadrants |
First Angle Projection
In this the object in assumed to be positioned
in the first quadrant and is shown in
figure.
Example of First Angle Method |
The object is assumed to be positioned in between
the projection planes and the observer. The views are obtained
by projecting the images on the respective planes. Note that
the right hand side view is projected on the plane placed
at the left of the object. After projecting on to the
respective planes, the bottom plane and left plane is
unfolded on to the front view plane. i.e. the left plane
is unfolded towards the left side to obtain the Right hand side
view on the left side of the Front view and aligned with the
Front view. The bottom plane is unfolded towards the bottom to
obtain the Top view below the Front view and aligned with the
Front View.
Third Angle Projection
In the third angle projection method, the object is
assumed to be in the third quadrant. i.e. the object behind
vertical plane and below the horizontal plane. In this
projection technique, Placing the object in the third
quadrant puts the projection planes between the viewer and
the object and is shown in figure
Example of Third Angle Method |
Difference between first- and third-angle
projections
Either first angle projection or third
angle projection are used for engineering drawing.
Second angle projection and fourth angle projections are not
used since the drawing becomes complicated.
Symbol of projection
As per BIS standards, Drawing symbols for 1st angle and 3rd
angle are shown in below figure. as previously explained that
in case of first angle method the left hand side view is
shown on right side of front view and vice
verse.The symbol recommended by BIS is to draw
the two sides of a frustum of a cone placed with its axis
horizontal The left view is drawn.
Symbol of first and third angle method |