JSS 3, TOPIC 4: ORTHOGRAPHIC PROJECTION

Definition:
The word orthographic is coined from the word orthogonal (or perpendicular) and  graphic (or view). Orthographic projection can be defined as a two-dimensional drawing that shows different views of a structure. It contains the vertical and the horizontal planes as shown below. 

                                                           Angles of Projection.

i.First angle projection: Where the plan of a structure is positioned below the quadrant. E.g.
                                    1^st angle:   F.E.
                                                   PLAN

ii.Third angle projection: This is a condition where the plan of a structure is positioned above the quadrant. E.g. 
                                  3^RD angle: PLAN  
                                                     F.E. 

                       
                                                                   Elevations(views) .

In orthographic projection, there three basic views reffered to as elevations. These elevations are as follows:
(i).Plan: This is the view of an object as seen or projected from the top.
(ii).Front elevation: This is the view of an object as seen or projected from the front side of the object or block.
(iii).End(side) Elevation: This view is sometimes reffered to as the auxiliary vertical plane in some text books. It is the view showing the right or left side of any block when viewed from the front.

                          Steps in Orthographic Projection and Worked Illustrations.   

(i).Make a sketch of the required views.
(ii).Draw both the vertical and the horizontal axis to intersect midway.
(iii).Determine the placing of the views.
(iv).Begin drawing with either the plan or front views.
(v).From either of the views drawn above, project lines for the other view.
(vi).Draw the reflector line at 45⁰ .
(vii).Project the horizontal lines from the front elevation.
(viii).Project the plan to meet the reflector line of the 45⁰.
(ix).Take note of hidden details and outline the elevations to complete the job.

                                                 Worked Examples.

Draw the block shown on page 168 of the J.N. Green in orthographic projection in both 1^st and 3^rd angle projections, using arrow x as the front elevation.
( the teacher sketches the block on the board  and demonstrates different examples on the board for students to follow practical procedure involved in orthographic projection).

    
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