Dimensioning and tolerances: the British Standards rules for dimension, projection and leader lines, dimensioning diameters and radii, and how a tolerance states the allowed variation in a size.

What this key area is asking

The SQA wants you to add sizes to a drawing using the British Standards dimensioning conventions, dimension diameters and radii correctly, and read or state a tolerance as the allowed variation in a size.

The dimensioning conventions

Dimensions are added with three thin line types working together, and a set of British Standards rules controls exactly how they look.

These rules keep the size information separate from the outline, so the drawing stays readable and the figures are never ambiguous.

Dimensioning diameters and radii

Round features have their own conventions because a number alone would not say whether it meant a full diameter or just a radius.

The symbol carries real information: it tells the maker whether the value spans the whole circle or only half of it.

Tolerances

A drawn size is a target, but a workshop can never make a part to an exact figure, so a tolerance states how much variation is allowed.

A tolerance is usually written as a nominal size with a plus and minus value, for example $25 \pm 0.1$. The largest acceptable size is the nominal plus the upper tolerance ($25.1$) and the smallest is the nominal minus the lower tolerance ($24.9$); the difference between them is the total tolerance. Any part made within that range is accepted. Tolerances let parts be made economically while still fitting together, which is why mating parts such as a shaft and a hole each carry their own tolerance.

Why dimensioning and tolerances matter

A drawing without correct dimensions cannot be made, and dimensions added carelessly can be read two ways. The conventions remove that risk, and tolerances make the drawing realistic by stating the range a real workshop can hit. Together they turn an orthographic drawing into a complete, makeable specification, which is why dimensioning rules and tolerance reading are tested directly in the production-graphics context.

How this key area is examined

Questions ask you to add dimensions to a view following the conventions, state a rule (figure placement, the gap, the phi symbol), identify a dimension line, projection line or leader, or work out the limits from a tolerance. Learn the small set of rules precisely and practise calculating the largest and smallest size from a plus-or-minus tolerance. These are accessible marks once the conventions are automatic.

For the official course specification

The SQA publishes the full National 5 Graphic Communication course specification, specimen question paper and coursework task at sqa.org.uk. Always revise from the current specification and SQA past papers, because question style, conventions and terminology are board-specific.