Projection methods: 1st angle and 3rd angle projections

Orthographic Projection

Orthographic projection is a way of drawing an 3D object from different directions. Usually a front, side and plan view are drawn so that a person looking at the drawing can see all the important sides. Orthographic drawings are useful especially when a design has been developed to a stage whereby it is almost ready to manufacture.

There are two ways of drawing in orthographic - First Angle and Third Angle.

The principal projection planes and quadrants used to create drawings are shown in Figure 1. The object can be considered to be in any of the four quadrants.

Figure 1. The principal projection planes and quadrants for creation of drawings.

First Angle Projection

• In this the object is assumed to be positioned in the first quadrant and is shown in figure 2. 
• 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 the 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.

Figure 2.  Illustrating the views obtained using  first angle projection technique.

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 plane between the viewer & the object and is shown in figure 3.

Figure 3. Illustrating the views obtained using  first angle projection technique

A summary of  the difference between 1st & 3rd angle projection is shown in Table 1.

Table 1

Note : Second angle projection & fourth angle projection are not used since the drawing becomes complicated.