Dimensioning, Scales, and Notation
Dimensioning, Scales, and Notation
This domain tests clear, unambiguous size communication: dimension lines, symbols, tolerances and scale calculations.
Core rules
- Dimension lines run parallel to the feature and terminate in arrowheads touching the extension lines. Extension lines start with a small gap from the object and extend slightly beyond the dimension line.
- Numerals sit above a horizontal dimension line and are read from the bottom or right-hand edge. Keep them outside crowded areas.
- Use Ø for diameter, R for radius, SØ for a spherical diameter, □ for a square, and C for a 45° chamfer. A tolerance may be written as 20 ±0.1.
- Do not repeat a size; place each dimension where the feature is clearest.
- Scale is drawing length : real length. At 1:50, divide real size by 50; at 2:1, multiply by 2.
Worked example
A wall is 4.8 m long. At 1:50, drawing length = 4800/50 = 96 mm. A door opening 900 mm wide becomes 900/50 = 18 mm. If a 64 mm line is measured on a 1:50 drawing, real length = 64 x 50 = 3200 mm = 3.2 m. Reasoning: the scale factor applies equally to every length; convert all dimensions to the same unit before dividing or multiplying.
Traps
- Inverting the scale, e.g. multiplying at 1:50.
- Using extension lines that touch the object or cross other dimensions.
- Dimensioning hidden lines instead of visible features.
- Rotating numerals so they read upside down.
- Adding a redundant overall dimension to a chain.