Orthographic Projection (First and Third Angle)
Orthographic Projection (First and Third Angle)
This domain tests the arrangement of related views, projection symbols, hidden detail and the difference between first- and third-angle layouts. The object does not move; the viewing direction and projection plane position change.
Core rules
- First angle: the object lies between observer and plane. The top view is placed below the front view and the left side view to the right of the front view.
- Third angle: the plane lies between observer and object. The top view is above the front view and the right side view to the right.
- The symbol identifies the method: first angle shows the truncated cone's circles on the right of its trapezoid; third angle shows circles on the left.
- Align views: widths match between front and top; heights match between front and side; depths match between top and side.
- Hidden edges are dashed and must align with the visible feature they represent. Centre lines locate axes but are not edges.
Worked example
A 60 x 40 x 30 mm block has a vertical 20 mm diameter through-hole centred. In first angle, draw the front view as a 60 x 30 rectangle with two dashed vertical lines 20 mm apart, centred. The top view goes below: a 60 x 40 rectangle with a 20 mm circle. Reasoning: the hole is invisible from the front, so it is hidden; from above it is visible as a circle. Project the hole's left and right extremes down to the top view.
Traps
- Placing the top view above the front view in first angle.
- Drawing hidden lines where the feature is visible.
- Forgetting the projection symbol.
- Mixing side-view sides.
- Using a thick line for a centre line.