Engineering Components and Assembly Drawing
This domain tests the link between detail drawings of single parts and the assembly drawing that shows how they fit together, plus the standard representations of common engineering components.
Components you must recognise
Threads (metric, unified, BSP), bolts, nuts, washers, studs, screws, keys, splines, circlips, pins, bushes, ball and roller bearings, gears, springs, couplings, shafts, pulleys, valves and pipe fittings. Draw them using standard simplified conventions rather than attempting photographic realism.
Detail versus assembly drawings
A detail drawing gives one part fully dimensioned with tolerances, surface finish and material. An assembly drawing shows the parts in their working positions, usually in section, with overall dimensions, fits and a parts list. Individual part dimensions are not repeated on the assembly.
Numbering and parts list
Each component is identified by a balloon - a circle on a leader line touching the part - containing the item number. Numbers run clockwise or in a logical order and match a parts list in the corner of the sheet, giving item number, description, material, quantity and drawing reference. Hidden detail is normally omitted from assembly drawings; adjacent parts in section are hatched in opposite directions.
Worked example
A shaft, a ball bearing and a key: draw the shaft, step the bearing onto its seat at the correct shoulder, and place the key in its keyway. Balloon the shaft as 1, bearing as 2, key as 3. Parts list: 1 Shaft, mild steel, 1 off; 2 Ball bearing 6205, 1 off; 3 Key 8 × 7 × 40, key steel, 1 off. Overall dimensions and the shaft-to-bearing fit (e.g. k6) are given; the key's own length is left to its detail drawing.
Traps
Dimensioning every part on the assembly view; balloons whose leader lines do not touch the part; parts list quantities missing; hatching all components in the same direction so joints disappear; drawing threads as a series of V's instead of thin and thick lines with the crest convention.