Engineering Drawing and Trade Drawing
Engineering Drawing and Trade Drawing
This domain assesses reading and producing trade drawings: orthographic views, isometric sketches, dimensioning, sectioning, symbols, and limits and fits. The exam favours interpretation of simple mechanical parts.
Core rules
- First-angle projection: the object lies between observer and plane; the top view is placed below the front view, and the view from the left is placed on the right.
- Visible edges are continuous thick lines; hidden edges are dashed; centre lines are chain lines.
- Dimensions show true size, not scaled size; do not repeat a dimension in two views.
- A fit such as Ø20 H7/g6 means nominal 20 mm, hole tolerance H7, shaft tolerance g6; lower-case letters are used for shafts.
- Sections use hatching at 45° and omit hidden lines unless needed.
Worked example
A drawing notes Ø30 H7/p6. The nominal size is 30 mm. H7 is a hole tolerance and p6 is a shaft tolerance; because the shaft letter is lower-case, it is an external feature. This is an interference fit, so the shaft is intended to be slightly larger than the hole and assembly needs force or heat/cooling. If the note were Ø30 H7/g6, it would be a clearance fit. Reasoning from the letter case and tolerance position prevents reversing hole and shaft.
Traps
Mixing first- and third-angle placement; dimensioning a scaled drawing instead of using the stated value; treating H7 as a shaft; and missing the Ø or R symbols.