AQA AQA 2022-style practice: A drawing dimensions a slot width as 30 mm with a tolerance of 0.2 mm. Calculate the upper and lower limits and the tolerance, and explain why a tolerance is given rather than a single size.

A good answer shows the limit arithmetic and explains the purpose of tolerance. Upper limit = 30 + 0.2 = 30.2 mm. Lower limit = 30 - 0.2 = 29.8 mm. The tolerance (total allowed variation) is 30.2 - 29.8 = 0.4 mm. A tolerance is given because no real process makes a part to an exact size; allowing a stated range means any slot between 29.8 mm and 30.2 mm still fits and works, so parts are interchangeable without being identical. A tighter tolerance gives a better fit but costs more to make. Markers reward both limits, the tolerance value, and the point that tolerance allows interchangeable parts within an acceptable range.

EnglandEngineeringSyllabus dot point

How do engineers communicate a design accurately using drawings?

Engineering drawing conventions, third-angle orthographic and isometric projection, dimensioning and tolerancing to the relevant standards.

A focused answer to AQA GCSE Engineering on engineering drawing conventions, third-angle orthographic and isometric projection, dimensioning, line types and tolerancing to BS 8888 standards.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
Drawing conventions
Types of projection
Dimensioning and tolerancing
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What this dot point is asking

AQA wants you to use engineering drawing conventions correctly, recognise and produce third-angle orthographic and isometric views, dimension a drawing properly, and apply tolerances to the relevant British Standard. Drawing questions in the written paper often give you a part and ask you to identify a view, name a line type, read a dimension or work out limits from a tolerance.

Drawing conventions

Conventions exist so that any engineer, anywhere, reads the same drawing the same way. A centre line marks the axis of a hole or shaft; a hidden detail line shows an edge that is behind the visible surface; a leader line with an arrowhead points to a feature being labelled. Following BS 8888 removes ambiguity, which is the whole purpose of a working drawing.

Types of projection

Orthographic is the drawing for manufacture because each view carries the dimensions that the maker needs, with no perspective distortion: a $50 \text{ mm}$ edge measures $50 \text{ mm}$ on the page at full scale. Isometric is for showing appearance to a client because it reads as a recognisable 3D object. The two are complementary: a design package usually has both, an isometric to show the product and orthographic views to make it.

Dimensioning and tolerancing

Dimensions are added with thin extension and dimension lines, arrowheads and a stated size, placed so the reader takes them in from the bottom or right of the sheet, and never duplicated (each size appears once on the clearest view). A tolerance states the allowed variation, for example $25 \pm 0.1 \text{ mm}$ , which gives an upper limit of $25.1 \text{ mm}$ and a lower limit of $24.9 \text{ mm}$ , so the part can be made and inspected against a clear pass or fail range.

Worked example: reading limits and choosing the inspection tool from a drawing

A drawing shows a pin dimensioned $\varnothing 12 \pm 0.05 \text{ mm}$ that must slide into a hole. Work out what the maker and inspector need.

step Find the limits

Upper limit $= 12 + 0.05 = 12.05 \text{ mm}$ . Lower limit $= 12 - 0.05 = 11.95 \text{ mm}$ . Any pin diameter between these passes.

step Find the tolerance

Tolerance $= 12.05 - 11.95 = 0.10 \text{ mm}$ . This is a fairly tight tolerance, so the process and the measuring tool must both be capable of it.

step Choose the measuring tool

A steel rule reads only to about $0.5 \text{ mm}$ , far too coarse. Vernier callipers read to about $0.02 \text{ mm}$ and a micrometer to about $0.01 \text{ mm}$ , so a micrometer (or callipers) is needed to confirm the pin is inside the $0.10 \text{ mm}$ band.

step Check against the drawing

Measure the made pin: if it reads $12.02 \text{ mm}$ it is within limits and accepted; if it reads $12.08 \text{ mm}$ it is above the upper limit and rejected. The drawing, not the maker's judgement, decides pass or fail.

Try this

Q1. Name the three standard views in an orthographic drawing. [3 marks]

Cue. Front elevation, plan, side elevation.

Q2. State the angle used for the non-vertical edges in isometric projection. [1 mark]

Cue. 30 degrees to the horizontal.

Exam-style practice questions

Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

AQA 20184 marksExplain the difference between an orthographic drawing and an isometric drawing, and when each is used.

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A good answer describes each drawing type and links it to its purpose.

An orthographic drawing shows several separate flat views of an object (front elevation, plan and side elevation) drawn to scale and fully dimensioned. In third-angle projection the views are arranged so the plan sits above the front view and the side view is placed on the side it is seen from. It is used for manufacture because every dimension can be shown clearly on the correct view.

An isometric drawing shows the object as a single 3D pictorial view, with vertical edges kept vertical and the other edges drawn at 30 degrees to the horizontal. It is used to show what the product looks like to a client or non-technical reader because it is easy to understand at a glance.

Markers reward the multi-view, dimensioned nature of orthographic for making, and the single 3D pictorial nature of isometric for showing appearance.

AQA 20224 marksA drawing dimensions a slot width as

30 \text{ mm}

with a tolerance of

\pm 0.2 \text{ mm}

. Calculate the upper and lower limits and the tolerance, and explain why a tolerance is given rather than a single size.

Show worked answer →

A good answer shows the limit arithmetic and explains the purpose of tolerance.

Upper limit $= 30 + 0.2 = 30.2 \text{ mm}$ . Lower limit $= 30 - 0.2 = 29.8 \text{ mm}$ . The tolerance (total allowed variation) is $30.2 - 29.8 = 0.4 \text{ mm}$ .

A tolerance is given because no real process makes a part to an exact size; allowing a stated range means any slot between $29.8 \text{ mm}$ and $30.2 \text{ mm}$ still fits and works, so parts are interchangeable without being identical. A tighter tolerance gives a better fit but costs more to make.

Markers reward both limits, the tolerance value, and the point that tolerance allows interchangeable parts within an acceptable range.

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Sources & how we know this

AQA GCSE Engineering (8852) specification — AQA (2017)