Technical drawing, design process and working to specification
What the exam tests
Reading and producing working drawings, interpreting dimensions and tolerances, and following a design process from brief to evaluation.
Core rules
- Orthographic views: first-angle projection places the view from the left on the right; third-angle places it on the same side. Check the projection symbol.
- Dimensions in millimetres unless stated; do not scale from a drawing. Use datum surfaces and consistent dimension lines.
- Tolerances show permitted variation; a fit needs both size and tolerance. Hidden detail uses dashed lines; centrelines use chain lines.
- Working drawings include title block, scale, material, finish, quantities, assembly notes and a cutting list.
- Design process: brief, research, specification, concept, development, prototype, test, evaluate. Record decisions and constraints.
- Work to specification: check material, thickness, joint, finish and tolerance before cutting; mark face side and face edge.
Worked example
A drawing shows a rail 600 mm long with a ±0.5 mm tolerance and a 20 mm tenon at each end. The overall length is dimensioned from a datum at one end: 600 ±0.5 mm. Set out from the datum, not from both ends, so errors do not accumulate. The reasoning: a single datum controls the overall length and tenon positions.
Traps
Scaling a dimension from a print; confusing first- and third-angle; ignoring the title block note all dimensions mm; adding tolerances to both ends and doubling error; changing a specification without recording it; assuming a drawing is to scale.