How are sizes, tolerances and surface finish specified on a working drawing?
Dimensioning to British Standards with tolerances, fits and surface-finish symbols, choosing datums and dimensioning systems for manufacture.
An SQA Advanced Higher Graphic Communication answer on dimensioning and tolerancing, covering British Standard dimensioning, tolerances, fits, surface-finish symbols, datums and the dimensioning systems used for manufacture.
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What this key area is asking
The SQA wants you to dimension a working drawing to British Standards and to apply tolerances, fits and surface-finish symbols. You should choose sensible datums and dimensioning systems, read and quote limits, and understand how a fit between a hole and a shaft is specified so the parts work together as intended.
Dimensioning to British Standards
Good dimensioning is functional and unambiguous. Choosing a datum (a reference edge, face or centreline) and dimensioning features from it controls how the part is measured and made, and avoids errors building up. Over-dimensioning (giving the same size twice) is an error because it can create conflicting information; under-dimensioning leaves a feature undefined. The marker expects every size needed, given once, from sensible references.
Tolerances
Tolerances exist because no part can be made to an exact size. They are written as upper and lower limits, or as a nominal size with plus and minus deviations. A tight tolerance costs more to achieve, so you specify only as tight a tolerance as the function needs. Reading limits and calculating the tolerance (maximum minus minimum) is a standard exam task, as is recognising bilateral versus unilateral.
Fits and surface finish
The fit is chosen from function. A rotating shaft in a bearing needs a clearance fit; a gear pressed permanently onto a shaft needs an interference fit; a locating dowel that must be tight yet removable suits a transition fit. Surface finish is specified only where it matters, for example a smooth bearing surface, because finer finishes cost more. Matching the fit and finish to the job is the engineering judgement behind the symbols.
Examples in context
A bearing housing carries diametral limits to give the running clearance for the shaft. A gear on a shaft is dimensioned for an interference fit so it transmits torque without slipping. A machined sealing face carries a surface-finish symbol with a roughness value. A bracket is dimensioned from a single datum edge so its hole positions do not accumulate error. In each case the dimensions, tolerances and finish are set by how the part must function.
Try this
Q1. State how the tolerance on a dimension is calculated from its limits. [1 mark]
- Cue. Tolerance = maximum size minus minimum size.
Q2. State what is always true of the hole and shaft sizes in a clearance fit. [1 mark]
- Cue. The hole is always larger than the shaft, so there is always a gap.
Q3. State the difference between a unilateral and a bilateral tolerance. [1 mark]
- Cue. Unilateral allows variation in one direction only; bilateral allows variation both above and below the nominal size.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA AH style4 marksA hole is dimensioned 25.0 plus 0.04 minus 0.00 mm. State the maximum and minimum acceptable sizes, the tolerance, and explain what unilateral tolerance means.Show worked answer →
The maximum size is 25.0 plus 0.04 = 25.04 mm and the minimum is 25.0 minus 0.00 = 25.00 mm.
The tolerance is the difference between them, 25.04 minus 25.00 = 0.04 mm.
Unilateral tolerance means the permitted variation is all in one direction from the stated size (here the hole may be larger but not smaller than 25.00 mm), as opposed to bilateral tolerance, where variation is allowed both above and below.
Markers reward the correct maximum and minimum, the tolerance of 0.04 mm, and the definition of unilateral as variation permitted in one direction only.
SQA AH style3 marksExplain what a clearance fit is and give one example of where a clearance fit would be required.Show worked answer →
A clearance fit is one where the hole is always larger than the shaft across their tolerance ranges, so there is always a gap and the parts can move or be assembled freely.
An example is a shaft that must rotate freely in a bearing or a bolt that must pass easily through a clearance hole.
Markers reward defining a clearance fit as hole always larger than shaft (a guaranteed gap allowing free movement or assembly) and a sensible example such as a rotating shaft in a bearing or a bolt clearance hole.
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