Engineering Drawing, Orthographic Projection and CAD
Engineering Drawing, Orthographic Projection and CAD
This domain tests orthographic views, first-angle projection, dimensioning, sections, line types and basic CAD conventions.
Core rules
- In first-angle projection the object sits between the observer and the plane: the top view is placed below the front view, and the left-side view is placed on the right. Malta and much of Europe use first-angle; the symbol shows a truncated cone.
- Each view shows two dimensions; hidden detail uses dashed lines, centrelines use chain lines, and visible outlines use thick continuous lines.
- Dimension to visible edges, not hidden lines. Do not duplicate a dimension. Give diameters with Ø and radii with R. Unless stated otherwise, dimensions are in millimetres.
- Sections show internal detail; hatch adjacent parts at different angles. CAD should use layers, named views and parametric constraints where appropriate.
Worked example
A block 60 mm × 40 mm × 20 mm has a 12 mm diameter through-hole centred in the 60 × 40 face. Front view: 60 × 20 rectangle. Top view, placed below it in first angle: 60 × 40 rectangle with a 12 mm circle. Side view: 40 × 20 rectangle with hidden lines for the hole. Dimension the block once in the view that shows the feature most clearly.
Traps
Mixing first- and third-angle placement; dimensioning a hidden line; double-dimensioning; using the wrong line thickness; forgetting the projection symbol.