Quality Control in Manufacturing: Tolerances, Checking Devices and Go/No-Go Gauges

Quality control (QC) ensures that products meet the required standard and are safe, functional and consistent. This lesson covers the key quality control concepts in the AQA GCSE Design and Technology specification (8552), Section 3.2.8, including tolerances, checking devices and go/no-go gauges.

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What Is Quality Control?

Quality control is the process of checking and testing products during and after manufacture to ensure they meet the design specification. It differs from quality assurance (QA), which is the overall system of managing quality throughout the entire production process.

Term	Meaning
Quality Control (QC)	Inspection and testing of products at specific stages
Quality Assurance (QA)	The whole system ensuring quality from design to delivery
Tolerance	The acceptable range of variation in a dimension

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Tolerances

A tolerance is the permitted variation in a measurement. No manufacturing process is perfectly precise, so designers specify an acceptable range.

How Tolerances Are Written

A dimension of 50 mm with a tolerance of ±0.5 mm means the part is acceptable if it measures anywhere between 49.5 mm and 50.5 mm.

Notation	Meaning
50 ± 0.5 mm	Acceptable range: 49.5 mm to 50.5 mm
100 ± 1 mm	Acceptable range: 99 mm to 101 mm
25 +0.2 / -0.0 mm	Acceptable range: 25.0 mm to 25.2 mm

Why Tolerances Matter

• Too tight a tolerance increases manufacturing cost because more precise (and expensive) machines and processes are needed.
• Too loose a tolerance may mean parts do not fit together or the product does not function correctly.

AQA Exam Tip: If a question asks you to explain why tolerances are important, link your answer to both fit/function AND cost. The best answers discuss the trade-off between precision and manufacturing expense.

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Checking Devices

Manufacturers use a range of tools and instruments to check that products are within tolerance.

Common Measuring and Checking Tools

Tool	What It Measures	Typical Accuracy
Steel rule	Length	± 0.5 mm
Vernier caliper	Internal/external dimensions, depth	± 0.02 mm
Micrometer	External dimensions (e.g. shaft diameter)	± 0.01 mm
Digital caliper	Internal/external dimensions	± 0.01 mm
Surface plate and DTI	Flatness and concentricity	± 0.01 mm
Protractor / angle gauge	Angles	± 0.5°
Multimeter	Voltage, current, resistance in circuits	Varies by model

In-Process vs Final Inspection

• In-process inspection — checks are made during manufacture (e.g. measuring every 10th component on a lathe). This catches errors early.
• Final inspection — the finished product is tested against the specification before dispatch.

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Go/No-Go Gauges

A go/no-go gauge is a simple, fast quality control tool that checks whether a dimension is within tolerance without needing to read a measurement. This makes it ideal for high-speed production lines.

How It Works

A go/no-go gauge has two ends:

1. Go end — represents the minimum acceptable size. The part should fit through (or onto) this end.
2. No-go end — represents the maximum acceptable size. The part should NOT fit through (or onto) this end.

If the part passes the go end but fails the no-go end, it is within tolerance. If it passes both ends, it is too small. If it fails both, it is too large.

Example: Checking a Shaft Diameter

A shaft must be 20 ± 0.1 mm.

• Go ring gauge = 20.1 mm internal diameter — the shaft should pass through.
• No-go ring gauge = 19.9 mm internal diameter — the shaft should NOT pass through.

Result	Meaning
Passes go, fails no-go	Within tolerance — PASS
Passes go, passes no-go	Too small — FAIL
Fails go, fails no-go	Too large — FAIL

AQA Exam Tip: Go/no-go gauges are a favourite exam topic. Be ready to draw and label a simple diagram showing both ends, and explain the pass/fail criteria clearly.

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graph TD
    A["Quality Assurance QA<br/>whole quality system"] --> B["Quality Control QC<br/>inspection + testing"]
    A --> C[Calibration + procedures]
    A --> D[Operator training]
    A --> E[Process design]
    B --> F{Sample tested}
    F --> G{Within tolerance?}
    G -->|Pass go,<br/>fail no-go| H[ACCEPT]
    G -->|Fail go| I["Too large<br/>adjust machine"]
    G -->|Pass go + no-go| J["Too small<br/>adjust machine"]
    H --> K["SPC chart<br/>plot dimension"]
    K --> L{Drift trend?}
    L -->|Yes| M["Adjust process<br/>before reject"]
    L -->|No| N[Continue run]

Quality Control in Different Material Areas

Material Area	Typical QC Checks
Timbers	Checking dimensions with caliper, visual inspection for defects, testing joint strength
Metals	Go/no-go gauges, micrometer checks, hardness testing, visual weld inspection
Polymers	Checking wall thickness, visual inspection for flash or sink marks, pressure testing
Textiles	Checking seam allowances, colour matching, fabric weight per m², tensile strength
Electronics	Continuity testing, functional testing, visual inspection of solder joints

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Statistical Quality Control

In large-scale production, manufacturers use statistical process control (SPC) — sampling a set number of products at regular intervals and plotting measurements on a control chart. If measurements drift towards the tolerance limits, the machine can be adjusted before defective products are made.

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Summary

Quality control is about checking that products are made correctly. Tolerances define the acceptable range of variation. Go/no-go gauges provide a quick pass/fail check without needing to read a measurement. Understanding these concepts is essential for both the written exam and your NEA project.

AQA Exam Tip: In your NEA, show evidence of quality control by photographing yourself using measuring tools and recording measurements against your specified tolerances. This demonstrates understanding of Section 3.2.8.

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Worked example — go/no-go gauge check on turned steel dowels