Design for manufacture and project management · Designing and making principles
Revision notes
Quality Control
Quality control (QC) involves monitoring, checking, measuring and testing materials, components, equipment and products throughout production to detect defects and ensure they conform to specified standards and acceptable tolerances.
QA plans for quality; QC checks whether the required quality has been achieved. QC therefore focuses on checking actual materials, components, processes and outputs.
QC throughout production
Quality control should not only happen at the end. Checks can be planned at several stages:
Incoming raw materials and components: check dimensions, grade/specification, condition, colour or finish where relevant before faulty material enters production.
During manufacture: check critical dimensions, hole positions, joints, surface quality, machine settings and other features at appropriate stages so errors are found before more time and material are added.
Prototype and pre-production checks: measure the prototype against drawings and specification, check fit and assembly, test function and safety, and identify changes before committing to larger-scale manufacture.
Completed products: inspect dimensions, finish, assembly and function before products are accepted or dispatched.
A tolerance is the permitted variation from a specified dimension or value. QC determines whether a component or product lies within that acceptable range.
Go/no-go gauges
A go/no-go gauge quickly checks whether a manufactured feature lies within its permitted limits. The ‘go’ part should fit and the ‘no-go’ part should not. It gives a fast pass/fail result rather than an exact measurement, making it useful when large numbers of repeated components need checking.
Digital and precision measuring
Digital vernier callipers can accurately measure internal, external and depth dimensions. Micrometers are used for very precise measurements such as material thickness or shaft diameter. Laser or probe scanning can also compare manufactured components with required dimensions or digital data.
Non-destructive testing
Non-destructive testing (NDT) checks a material or component for defects without damaging it. AQA examples include X-rays and ultrasound, which can reveal internal faults that may not be visible from the surface.
Choosing the QC check
The check must suit the feature being controlled. A go/no-go gauge is efficient for a repeated critical size; callipers or micrometers provide numerical measurements; visual inspection can identify finish defects; functional testing checks whether the assembled product performs as required.
QA vs QC
QA is preventative: procedures and systems are planned to reduce the chance of faults occurring. QC is detective: materials, components and products are inspected, measured or tested to identify whether the required quality has actually been achieved.
Go/no-go gauges give a fast production check that a feature remains within specified limits.Vacuum forming: a heated thermoplastic sheet is drawn over a former and held while it cools.Injection moulding: granules are melted and driven by a screw through a nozzle into a closed mould.Extrusion forces softened material through a shaped die to produce a continuous cross-section.Blow moulding: air expands a hot polymer parison against the inside of a mould.Rotational moulding coats the inside of a heated rotating mould before the hollow product cools.Compression moulding uses heat and pressure to form a measured charge inside a mould.A pattern forms a cavity in sand before molten metal is poured, cooled and removed.Die casting injects molten metal into a reusable metal die under pressure.Investment casting forms a ceramic mould around a sacrificial pattern for detailed components.MIG welding feeds a consumable wire electrode into a gas-shielded weld pool.TIG welding uses a non-consumable tungsten electrode and separate filler where required.Spot welding clamps sheet between electrodes and creates localised heat through electrical resistance.Steam softens lignin so timber can be bent around a former and held until dry.Thin veneers are glued and clamped around a former to create strong curved components.Flexography uses a relief plate and rollers to transfer ink to the substrate.Offset lithography transfers the image from plate to blanket cylinder and then to the substrate.Screen printing pushes ink through open areas of a mesh stencil onto the substrate.A focused laser follows a programmed path while assist gas helps clear the cut.Milling removes material with a rotating multi-point cutter while the work is accurately positioned.In turning, the workpiece rotates while a cutting tool removes material to create cylindrical forms.FEA divides a model into elements so stresses and deformation can be calculated.CFD predicts how fluids move around or through a product.A go/no-go gauge quickly checks whether a feature lies within its permitted tolerance.Vernier callipers measure external, internal and depth dimensions accurately.A micrometer uses a precision screw to measure small dimensions accurately.A tensile test records how a standard specimen responds as an increasing axial load is applied.Hardness tests compare resistance to permanent indentation under controlled conditions.Anthropometric data provides measured human dimensions for designing appropriate product sizes and clearances.Ergonomics applies knowledge of people to improve comfort, safety, usability and performance.Bauhaus design favoured functional forms, modern materials and design suited to industrial production.Art Deco is recognisable through geometric decoration, symmetry, luxury materials and machine-age influence.Memphis challenged restrained modernism with bold colour, pattern, geometry and playful forms.Arts and Crafts valued skilled making, natural inspiration, material honesty and resistance to poor-quality mass production.The life-cycle model helps explain how sales, competition and redesign priorities change over time.Sustainable design decisions should reduce resource use and keep products and materials useful for longer.
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