ISO 129-1 dimensioning: arrowheads, extension lines and symbols

How dimensions are presented on technical drawings: arrowheads, extension lines, the dimension text and the symbols that go in front of the value.

Arrowheads

ISO 129-1:2004, 5.3.2, Figure 11 and Annex A, Figure A.1 (h = lettering height of the dimensions). One style of arrowhead only on a single drawing; where space is too small, an oblique stroke or a dot may be substituted (ISO 129:1985, 4.3.3).

Extension lines

• Overshoot beyond the dimension line: about 8 × the line width — ISO 129-1:2004, 5.4
• Permitted gap from the feature: about 8 × the line width — ISO 129-1:2004, 5.4
• Extension lines are continuous narrow lines per ISO 128-20 (now ISO 128-2) (ISO 129-1:2004, 5.4).
• Drawn perpendicular to the dimensioned length; may be oblique but parallel to each other (ISO 129-1:2004, 5.4).
• Intersecting projected contours extend about 8 line widths beyond their intersection (ISO 129-1:2004, 5.4).

Dimension text

• The text of all dimensions, graphical symbols and annotations shall be indicated above the dimension line and read from the bottom. When the text is presented vertically, it shall read from the right. Orientation is determined at the centre of the dimension, symbol or annotation. — ISO 129-1:2018, 4.1.1
• Dimensional values in decimal notation shall use a comma as the decimal marker. — ISO 129-1:2018, 4.1.1
• Characters per the ISO 3098 series; only one character height for dimension and tolerance presentation on a drawing; a space separates the elements of the dimension indicator. — ISO 129-1:2018, 4.1.7
• Number of repetitions, a multiplication sign and a space before the dimension, e.g. '2× 18'. — ISO 129-1:2018, 4.1.2

Symbols in front of the value

ISO 129-1:2018, 5.2 and Table 1 (property indicators). The indicator precedes the value without a space. Unicode characters are MechPilot's mapping, by Unicode character name.

Tolerances

• When upper and lower limits are on two lines, the decimal markers of both are aligned; without a decimal marker, the digits are aligned as if it were shown.
• The dimension value and the lower deviation are at the same distance from the dimension line (so the upper deviation sits above the lower).
• Deviations always carry a + or - sign, except a zero value, which has no sign.
• Trailing zeros may or may not be presented.
• For a tolerance class per ISO 286-1 (e.g. 25 H7), the deviation values need not be shown unless needed.

ISO 129-1:2018, 4.1.7, Examples 1 and 2 and Figure 1

Notes from the data file

• Left out because no clause giving a number was seen (docs/to-verify.md): spacing between parallel dimension lines, distance of the first dimension line from the outline, gap between the text and the dimension line, and the chamfer notation (ISO 129-1:2018, Annex B, informative).
• Origin of running dimensions: a small open circle (origin symbol, ISO 129-1:2018, 3.1.4); ISO 129:1985, 4.3.1 c) gave about 3 mm diameter. Its 2018 size is in Annex A (not seen).
• ISO 129:1985 also allowed 'method 2' (values read from the bottom only, dimension line interrupted); ISO 129-1:2018 keeps only the rule in textPlacement.