Technical drawing, communication and modelling
Drawing questions test whether you can communicate a three-dimensional object unambiguously: correct view layout, correct line types, and dimensions that nobody has to measure off the sheet.
Projection systems
In first-angle projection (the convention used in Europe and Cyprus), each view is placed on the far side of the object: the view from the left is drawn on the right, and the view from above (plan) is drawn below the front view. In third-angle, views sit on the near side, as if projected onto glass. The projection symbol - a truncated cone shown beside the title block - tells the reader which system applies; the wrong system invalidates the drawing.
Line conventions: continuous thick for visible edges; short dashes for hidden detail; long-short chain for centre lines and pitch circles; thin continuous for dimension, projection and hatching lines. Hatching at 45° shows cut material in a section, and the cutting plane is marked with a chain line and arrows.
Dimensioning rules
Give each dimension once, on the view where the feature is clearest - diameters where the hole appears as a circle (Ø), positions in a view where its centre appears. Dimension to visible edges, avoid dimensioning to hidden lines, and always give the overall length, width and height. Tolerances (±0.2, or limits such as H7/g6) state permitted variation; fits describe clearance, transition or interference.
Other communication
Isometric (30° axes) for 3D explanation, oblique for quick sketches, exploded views with balloon numbers and a parts list for assemblies, scale models for form, mock-ups for ergonomics, working prototypes for function. CAD adds parametric constraints, drawing extraction, rendering, simulation (FEA) and STL export for 3D printing.
Worked example
An L-bracket 80 mm long, 50 mm high, 20 mm deep, with a Ø10 mm hole whose centre is 20 mm from the left edge and 15 mm above the base. Draw the front elevation showing the L-shape, place the plan below it and the view from the left on the right (first angle). Give 80, 50 and 20 once, on the elevation or plan, not both. Give Ø10 once where the hole appears circular, and locate the centre with 20 and 15 in that same view. Add centre lines through the hole in every view. Nothing is repeated and nothing needs scaling.
Traps
- Placing views as third angle while labelling first angle.
- Dimensioning a hole on a view where it is hidden.
- Double dimensioning, which examiners treat as an error rather than a nicety.
- Omitting overall sizes.
- Measuring the printed drawing instead of reading the stated dimension.