Technical drawing
Technical drawing is examined through a drawing you must read or complete: views, sections, dimensioning, surface-finish and weld symbols. Questions typically ask what a symbol means, which dimension is wrong, or which view or section completes the part.
Projection
European workshops use first-angle projection (ISO-E): the object lies between the observer and the projection plane, so the view taken from the left is drawn to the right of the front view. Centre lines are thin chain lines, hidden edges are dashed, and a cutting plane is a thick chain line ending in arrows labelled A–A; the resulting section is titled A–A.
Sections
Hatching is thin, at 45°, and adjacent parts in the same section are hatched in opposite directions or at different spacing. Shafts, bolts, pins, keys and ribs are not hatched when the cutting plane passes along their axis, because a lengthwise cut would mislead the reader. A half section removes one quarter and is used on symmetric parts to show the bore without a second view.
Dimensioning
Dimension to visible outlines, never to hidden edges. Use Ø for diameters, R for radii, □ for square, M20×1.5 for a metric thread and 4×Ø9 for a hole pattern. Give each feature once: over-dimensioning, such as closing a chain and also repeating the overall length, is an error that costs marks. Untoleranced dimensions are covered by a general note such as ISO 2768-mK; reference dimensions are written in parentheses.
Surface finish and welds
Roughness is stated as Ra in micrometres (3.2, 1.6, 0.8) on the tick symbol; a bar across the tick means material removal is required. A weld is shown by an arrow to the joint, with the symbol on the arrow side or on the other side of the reference line and the size in front, e.g. a fillet a=5.
Worked example
A stepped shaft is drawn with overall length 120 and chain dimensions 35 and 48 from the left face. The remaining step is 120 − 35 − 48 = 37 mm. Writing 35, 48, 37 and also 120 over-constrains the drawing; the correct solution dimensions the three steps and puts (120) in parentheses as a reference dimension.
Traps
Hatching a shaft in longitudinal section, mixing first- and third-angle placement, omitting centre lines on hole circles, dimensioning to a hidden edge, and forgetting the general tolerance note.