Design Presentation and Graphical Conventions
This domain covers the standard language of technical drawing - line types, dimensioning, lettering and sheet layout - together with the presentation drawings used to sell a design idea.
Line conventions (ISO 128 / BS 8888)
Continuous thick (about 0.5–0.7 mm) for visible outlines. Continuous thin for dimension, projection and leader lines and for hatching. Dashed for hidden edges. Long-dash-dot chain for centre lines and cutting planes, thickened at the ends of a cutting plane. Arrowheads about three times as long as they are wide, filled, and touching the line they terminate.
Dimensioning rules
- Dimension to visible outlines only, never to hidden edges.
- Give the fewest dimensions that fully define the part; avoid closed chains unless functionally necessary.
- Prefixes and symbols: Ø diameter, R radius, SØ and SR spheres, □ square, countersink and counterbore symbols, THRU, and thread callouts such as M10 × 1.5.
- Place dimension lines outside the outline where possible, spaced about 8–10 mm, with the smallest dimensions nearest the view.
- For machined parts use baseline (ordinate) dimensioning from one datum.
Worked example
A stepped shaft is Ø30 for 40 mm, then Ø20 for 25 mm. Dimension from the left-hand datum: 40 and 65 overall, with Ø30 and Ø20 written on the elevation. Do not chain 40 and 25, which would allow tolerance to accumulate, and do not dimension either diameter on the plan, where it would duplicate the elevation.
Presentation
Design sheets combine pictorial views with tone, colour, cast shadow and annotation, balanced inside a border with the title block in the bottom right and notes above it.
Traps
Arrowheads stopping short of the outline; dimension lines crossing each other; missing Ø or R symbols; rendering so heavily that the form becomes unclear.