How are tolerances and fits used to guarantee parts work together, and how is quality controlled?
Tolerance and its role in manufacture (nominal size, upper and lower limits, tolerance band, bilateral and unilateral tolerance), types of fit (clearance, interference, transition), how tolerance affects cost and interchangeability, and the role of quality control and quality assurance including go and no-go gauges in checking parts.
A focused answer to the Edexcel 9DT0 content on tolerance and fits, covering nominal size and limits, the tolerance band, clearance, interference and transition fits, the cost and interchangeability of tolerance, and quality control with go and no-go gauges.
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What this dot point is asking
Edexcel wants you to understand tolerance (nominal size, upper and lower limits, the tolerance band, bilateral and unilateral tolerance), the types of fit (clearance, interference, transition), how tolerance affects cost and interchangeability, and the role of quality control and assurance including go and no-go gauges.
The answer
Tolerance, limits and the tolerance band
Bilateral and unilateral tolerance
Types of fit
When two parts mate (a shaft in a hole), the fit describes how they go together:
- Clearance fit: the hole is always larger than the shaft, so there is always a gap and the parts slide or rotate (a wheel on an axle).
- Interference fit: the shaft is always larger than the hole, so the parts are forced together and grip (a bearing pressed onto a shaft, a dowel in a hole).
- Transition fit: the limits overlap, so the fit may be slightly loose or slightly tight, used for accurate location where the part is occasionally dismantled.
Tolerance, cost and interchangeability
A tighter (smaller) tolerance band gives parts that are more precise and interchangeable (any part fits any assembly), which is essential for mass production and spares. But it costs much more: more accurate machines, careful setup, slower production and more inspection and rejected parts. So designers specify the widest tolerance the function allows, tight only where it matters.
Quality control, quality assurance and gauges
Examples in context
A wheel on an axle uses a clearance fit so it spins freely, while a bearing pressed onto a motor shaft uses an interference fit so it grips without slipping. Mass-produced parts are toleranced so any unit is interchangeable, letting a spare fit any product. On the line, go and no-go gauges let operators check thousands of parts quickly, passing those within the limits and rejecting the rest, while the wider QA system (documented procedures, controlled processes) prevents faults arising. Calculating limits, the tolerance band and the resulting fit, and explaining the cost of tight tolerance, are the core skills Edexcel tests here.
Try this
Q1. A part is mm. State its upper limit, lower limit and tolerance band. [2 marks]
- Cue. Upper mm, lower mm, tolerance band mm.
Q2. Explain the difference between a clearance fit and an interference fit. [2 marks]
- Cue. A clearance fit always leaves a gap (hole larger than shaft, parts move); an interference fit is always tight (shaft larger than hole, parts grip).
Q3. State what a go and no-go gauge checks. [1 mark]
- Cue. Whether a dimension lies within its limits (the go end fits, the no-go end does not), without measuring the exact size.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20204 marksA shaft is specified as 25.0 mm with a tolerance of plus or minus 0.05 mm. State the upper and lower limits and the tolerance band, and explain what tolerance means.Show worked answer →
Award marks for the limits, the band and the meaning.
Upper limit mm. Lower limit mm.
Tolerance band (total tolerance) mm.
Tolerance is the permitted variation in a dimension: any shaft made between and mm is acceptable, because in real manufacture no part can be made exactly to size, so a range is allowed within which the part still functions.
Markers reward the two limits, the mm band, and a correct definition (the allowable variation between the limits).
Edexcel 20226 marksExplain the difference between a clearance fit and an interference fit, and discuss how tightening a tolerance affects manufacturing cost. Use a calculation to support your answer.Show worked answer →
Extended-response item marked on levels (correct fits, the cost relationship and a supporting calculation).
A clearance fit always leaves a gap: the hole is always larger than the shaft, so the parts slide or rotate (for example a wheel on an axle). An interference fit is always tight: the shaft is always larger than the hole, so the parts are forced together and grip (for example a dowel pressed into a hole, or a bearing onto a shaft). A transition fit may be either.
Cost: a tighter (smaller) tolerance band needs more accurate machines, more careful setup, slower production and more inspection and rejects, so unit cost rises sharply as the band shrinks; a wider tolerance is cheaper but less precise.
Calculation: a hole of to mm with a shaft of to mm always leaves a gap (smallest hole minus largest shaft mm), confirming a clearance fit.
A strong answer defines both fits, links tighter tolerance to higher cost with reasons, and supports the fit with a max-min calculation.
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Sources & how we know this
- Pearson Edexcel A-Level Design and Technology: Product Design (9DT0) specification — Pearson Edexcel (2017)