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