Engineering Drawing: Dimensioning and Tolerancing — CSEC Technical Drawing

Dimensioning gives the sizes and positions of features so a part can be made accurately. Tolerancing states how much a dimension may vary.

Elements of a dimension

• Extension (projection) lines — thin lines projecting from the feature so the dimension can be placed clear of the outline.
• Dimension line — a thin line between the extension lines, ending in arrowheads.
• Dimension figure — the size, placed above or within the dimension line.

Engineering drawings are normally dimensioned in millimetres (mm).

Rules of good dimensioning

• Dimension each feature once; do not repeat.
• Keep dimensions clear of the outline and hatching.
• Place dimensions outside the view where possible.
• Use the symbol Ø before a value for a diameter and R before a value for a radius.

Tolerancing

A tolerance is the permissible variation in a dimension — the difference between the largest and smallest acceptable size.

• Written as 50 ± 0.1 mm means the size may be 49.9 to 50.1 mm.
• Can also be shown as upper and lower limits (e.g. 50.1 / 49.9).

Tolerances allow parts to be made within an acceptable range so they fit and function, while still being economical to manufacture.

Exam tips

• Learn the parts of a dimension: extension lines, dimension line, arrowheads, figure.
• Ø = diameter, R = radius.
• A tolerance is the allowed variation; 50 ± 0.1 means 49.9–50.1 mm.
• Dimension a feature once, kept clear of outlines and hatching, in mm.