Engineering Drawing
This domain checks whether you can read a mechanical drawing correctly: projections, line conventions, dimensions, sections, tolerances and surface finish. The exam often gives a simple part and asks for the missing view, a dimension interpretation or a symbol meaning.
Projection and line conventions
- In first-angle projection the object lies 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. In third-angle projection the top view is above the front view and the left view is on the left.
- Visible edges are continuous thick lines; hidden edges are dashed; centre lines are long-short chain lines; cutting planes are thick chain lines with arrows; dimension and extension lines are thin continuous lines.
- Dimension from a datum where possible. Avoid double dimensioning and never dimension to a hidden line. Limits and fits show permissible size variation; geometric tolerances control form, orientation and location; surface roughness is given as Ra in micrometres.
Worked example
A drawing specifies a plate thickness as 50 ± 0.2 mm. The lower limit is 50 − 0.2 = 49.8 mm and the upper limit is 50 + 0.2 = 50.2 mm. A measured thickness of 49.75 mm is below 49.8 mm, so the part is rejected even though it is only 0.05 mm below the nominal size. A reading of 50.15 mm is accepted. Reasoning: the tolerance band is the acceptance zone, not a suggestion.
Traps that cost marks
Confusing first-angle and third-angle view placement; treating a hidden line as visible; adding a dimension already given from a datum; and ignoring the tolerance band by comparing only with the nominal size.