Dimensioning principles

Every feature needs its size and location defined once. Missing dimensions force guessing or scaling; duplicated dimensions can contradict each other. Overall dimensions are useful but, if they duplicate a chain, one is shown as reference (in brackets) or omitted.

Dimension what matters to function and assembly. Choose datum features — surfaces or holes the part locates on — and dimension from them. Two holes that must line up with a mating part are dimensioned to each other or from a common datum, not via unrelated edges.

Chain dimensioning (end-to-end) is compact but tolerances accumulate: five dimensions at ±0.1 can stack to ±0.5 at the end. Baseline (parallel) dimensioning from one datum keeps each feature's tolerance independent. Use baseline where positions matter.

• Place dimensions outside the outline where possible, on the view that shows the feature most clearly.

• Don't dimension to hidden lines.

• Keep extension lines from crossing dimension lines where you can; smaller dimensions inside, larger outside.

• Diameters with ⌀, radii with R; hole callouts can give size, depth and count (e.g. 4 × ⌀8 THRU).

• State units once in the title block rather than on every dimension.