Lettering, dimensioning and line work
Lettering, dimensioning and line work
This domain tests the drawing's grammar: legible single-stroke lettering, complete dimensions and correct line conventions. Marks are lost for inconsistent letter heights, missing arrows or dimensions that do not match the view.
Core rules
- Use upright, single-stroke Gothic capitals. Standard heights are 2.5, 3.5, 5, 7, 10, 14 and 20 mm; keep the height-to-width ratio and stroke thickness consistent. Draw light guide lines for the top and bottom of each line of lettering, and space letters evenly.
- A dimension has extension lines, a dimension line, arrowheads and a numeral. Leave a 1–2 mm gap between the object and the extension line, and extend the line 2–3 mm past the dimension line. Arrowheads should touch the extension lines; the numeral sits above the dimension line and is read from the bottom or the right.
- Give the smallest dimensions nearest the view. Do not repeat a dimension on another view, and do not dimension hidden lines. Use Ø for diameter, R for radius, □ for square and SR for spherical radius. Dimensions are in millimetres unless another unit is stated.
- Visible edges: continuous thick. Hidden edges: short dashes. Centre lines: long-short chain. Dimension, extension and hatching lines: continuous thin. Break lines: wavy or zigzag thin.
Worked example
To dimension a 40 mm wide plate, draw extension lines from the two corners with a 1.5 mm gap. Draw the dimension line 8 mm below the plate, parallel to the edge, and add arrowheads pointing inward. Write 40 above the centre of the dimension line. Reason: the dimension line is parallel to the measured edge, and the numeral is not crossed by any line.
Traps
Do not place numerals upside down or across another line, and do not use a thick line for dimensioning. A common error is dimensioning the same feature on both views; another is leaving arrowheads floating short of the extension lines.