Projection Methods: 1st Angle and 3rd Angle Projections

Projection Methods

Universally either the 1^st angle projection or the third angle projection methods is followed for obtaining engineering drawings. The principal projection planes and quadrants used to create drawings are shown in figure 16. The object can be considered to be in any of the four quadrant.

First Angle Projection

In this the object in assumed to be positioned in the first quadrant and is shown in figure 17 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

Figure  illustrates the difference between the  1^st angle and 3^rd angle projection techniques. A summary of the difference between 1st and 3rd angle projections is shown

 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. This is being  explained with illustrations in the lecture on Projections of points (lecture 18).

Symbol of projection

The type of projection obtained should be indicated symbolically in the space provided for the purpose in the title box of the drawing sheet. 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.