Orthographic Projection (First and Third Angle)
This domain tests your ability to produce a correctly arranged multi-view orthographic drawing and to read one. The exam usually gives a pictorial view and asks for front, top and side views, or the reverse.
The two systems
In first-angle projection the object sits between the observer and the projection plane, so a view taken from the left is drawn on the right of the front view, and the view from the top is drawn below it. In third-angle projection the plane lies between observer and object, so each view appears on the side from which it was taken: left view on the left, top view above. The identifying symbol is a truncated cone drawn as a trapezium plus two concentric circles, arranged according to the same rule.
Setting out
Choose as the front view the direction that shows the most characteristic shape and the fewest hidden features. Align views by projection lines: heights carry across horizontally to side views, widths carry down to the top view, and a 45° mitre line transfers depth between top and side views. Use 2H for projection lines, HB for visible outlines, dashed thin lines for hidden detail and chain lines for axes of symmetry.
Line precedence
Where lines coincide, draw in this order: visible outlines, hidden detail, centre lines, then construction and dimension lines. A visible edge always overrides a hidden edge.
Worked example
An L-shaped block, 60 mm wide, 40 mm deep and 50 mm high, with the upright 20 mm thick at the left and the foot 15 mm thick. Front view: 60 × 50 outline with an internal step. Top view (below, first angle): 60 × 40 rectangle with a hidden line where the upright meets the foot. Side view (right, first angle): 40 × 50 with the step shown as a visible line because the profile is seen edge-on.
Traps
Putting a left view on the left in first angle; omitting hidden lines and losing marks; failing to align heights between front and side views; drawing a mitre line at the wrong angle; choosing a front view that produces a tangle of hidden detail.