Dimensioning a Working Drawing

Dimensioning a Working Drawing

A dimension states a size the part must be made to. The conventions exist so that a dimension can never be misread.

• Extension lines project from the feature, with a small gap where they leave the outline.
• Dimension lines run between them, ending in narrow filled arrowheads.
• The figure sits above the dimension line, readable from the bottom or the right of the sheet — never upside down.
• Dimensions go outside the view where possible, and the smallest chain sits closest to the object so lines do not cross.
• Millimetres are assumed on SANS drawings, so units are not repeated on every figure.
• Diameters take Ø, radii take R.

Worked example. A hole 20 mm from the left edge and 30 mm from the bottom must be dimensioned from those two edges — the datums. Dimensioning it from a neighbouring hole instead means the tolerance of that hole is inherited, and errors accumulate down a chain of features.

Core checkpoint: Every dimension needed to make the part appears exactly once. Repeating one invites contradiction when a drawing is revised.