Dimensioning and Line Work
What the exam tests
Dimensioning and line work tests whether a drawing can be manufactured without scaling. You must know dimension lines, extension lines, arrowheads, leaders, aligned dimensioning, symbols, and the BIS rule that all dimensions are in millimetres unless stated otherwise.
Core rules
- In the aligned system, dimension text is placed above the dimension line and oriented so it reads from the bottom or from the right-hand edge; do not use the unidirectional system unless asked.
- Extension lines start about 1 mm from the object and extend 2–3 mm beyond the dimension line. Arrowheads touch the extension lines, not the object.
- Give each feature once. Do not repeat a dimension in two views, and do not dimension hidden edges if a visible view can carry the size.
- Use Ø for diameter, R for radius, SR for spherical radius, □ for square, and t for thickness. Omit the unit symbol mm from each dimension when the title block states millimetres.
Worked example
A rectangular plate is 80 mm × 50 mm and has four Ø10 holes, each 15 mm from the adjacent edges. Dimension the overall 80 and 50 outside the view. On the plan, locate hole centres at 15 and 65 mm horizontally and 15 and 35 mm vertically. Give the hole callout once as 4 × Ø10 with a leader to one hole; the centre spacing dimensions define the pattern.
Traps
Double dimensioning is a classic error. Leaders should point to the feature and have a short horizontal shoulder for the text. Do not let dimension lines cross extension lines unnecessarily, and never place text inside a hatched section. Missing arrowheads or gaps at extension lines cost marks.