LGCSE Design & Technology (0189) cheat sheet

Iterative cycle stages: brief → research → specification → ideation → development → prototyping → testing → evaluation → refinement; evaluation feeds back into any earlier stage, not only the end.

Product design specification (PDS) criteria must be measurable: state a value plus a test method (e.g. 'withstand 50 N without permanent deformation, tested by ...'), otherwise the criterion cannot be evaluated.

Primary research = data collected first-hand (interviews, questionnaires, user trials, anthropometric measurement); secondary research = existing sources (standards, patents, market reports, competitor teardown).

Anthropometric design rule: design to the 5th percentile for reach limits and the 95th percentile for clearance limits; use the 50th percentile only for non-critical comfort dimensions.

Inclusive design targets the widest practical user range; a design that excludes users with reduced grip strength, vision or dexterity fails accessibility evaluation.

Design fixation is reduced by generating a fixed quota of concepts (e.g. 20) before judging any of them; judging during ideation suppresses divergent output.

Evaluation against the PDS, not personal preference, determines whether a prototype passes; record evidence (measurements, test data, user feedback) for each criterion.

Iteration is justified when test evidence shows a criterion is unmet; iteration without new evidence adds cost and time but not design quality.

Stress σ = F/A (N/mm² or MPa); strain ε = extension/original length (no unit); Young's modulus E = σ/ε in the linear-elastic region only.

Hardwood = broad-leaved trees (oak, ash, beech, mahogany); softwood = conifers (pine, spruce, cedar). Hardness of the wood is not implied by the hardwood/softwood label.

Thermoplastics soften reversibly on heating (acrylic, HDPE, ABS, PET); thermosetting plastics char and cannot be remelted (epoxy, polyester resin, urea formaldehyde).

Ferrous metals contain iron (mild steel, cast iron, stainless steel); non-ferrous metals do not (aluminium, copper, brass, zinc). Stainless steel resists corrosion via a chromium oxide surface layer.

Alloying changes properties without melting-point collapse: brass = copper + zinc (harder, more corrosion-resistant than pure copper); bronze = copper + tin; solder = tin + lead or tin + silver.

Material selection decision rule: first eliminate by service requirements (strength, temperature, corrosion, food contact), then by manufacturing process compatibility, then by cost and availability.

Composite = reinforcement (e.g. glass or carbon fibre) in a matrix (e.g. polyester resin); the fibre carries tensile load, the matrix transfers load and protects the fibre.

Ductile materials (mild steel, copper, aluminium) deform before fracture and suit bending and drawing; brittle materials (cast iron, ceramics, acrylic) fracture with little warning and suit compressive loading.

Cutting speed rule: harder material and smaller cutter diameter require lower spindle speed; feed rate rises with cutter diameter and tooth count.

Drilling: centre-punch the mark, use a pilot hole for holes above roughly 6 mm in metal, clamp the workpiece, and withdraw the bit to clear swarf.

Tapping a thread requires a tapping-size hole: tap drill diameter ≈ major diameter − thread pitch (e.g. M6 × 1.0 → 5.0 mm).

Bending allowance must be allowed for: the neutral axis does not change length, so flat-pattern length = sum of straight lengths + bend allowance for each bend.

Batch size decides process: one-off → hand tools and general-purpose machines; medium batch → jigs, fixtures and CNC; mass production → dedicated tooling, dies and moulding.

Vacuum forming suits thin thermoplastic sheet over a male or female mould; it cannot form undercuts or sharp internal corners without a draft angle.

Injection moulding requires draft angle on all vertical faces and uniform wall thickness to avoid sink marks and warping; internal undercuts need side-action tooling and raise cost.

Quality control: go/no-go gauges give fast pass/fail on a single dimension; coordinate measuring machines give measured values for tolerance verification and first-off inspection.

Velocity ratio of a gear train = product of driven tooth numbers ÷ product of driving tooth numbers; VR > 1 reduces speed and increases torque.

Mechanical advantage MA = load ÷ effort; velocity ratio VR = distance moved by effort ÷ distance moved by load; efficiency = MA/VR × 100%.

Lever classes: Class 1 fulcrum between effort and load; Class 2 load between fulcrum and effort (always MA > 1); Class 3 effort between fulcrum and load (always MA < 1, used for speed/range).

Four-bar linkage: the shortest link can rotate fully only if the shortest + longest link lengths ≤ sum of the other two (Grashof condition); otherwise the linkage only rocks.

Belt drives slip under overload and absorb shock; chain and gear drives do not slip but need lubrication and alignment.

Static equilibrium: sum of forces = 0 and sum of moments about any point = 0; moment = force × perpendicular distance from pivot (N·m).

Triangulation gives structural rigidity; a square frame without a diagonal racking under load, so add a diagonal brace or gusset.

Beam behaviour: a beam under bending has compression on one face and tension on the other; the neutral axis carries no direct bending stress. Increasing depth raises stiffness far more than increasing width.

Ohm's law V = I × R; power P = V × I = I²R = V²/R. Series: R_total = R1 + R2 + ...; parallel: 1/R_total = 1/R1 + 1/R2 + ...

Potential divider: V_out = V_in × R2/(R1 + R2) with output taken across R2. A light-dependent resistor in the top position makes V_out rise as light falls.

LEDs need a series resistor: R = (V_supply − V_LED)/I_LED. Typical values: red LED ≈ 2 V, forward current ≈ 10–20 mA.

Open-loop control has no feedback (e.g. a timed motor run); closed-loop control compares sensor feedback with a set point and corrects the output.

Sensor-to-actuator chain: input transducer (LDR, thermistor, switch, microphone) → signal processing (comparator, op-amp, microcontroller) → output device (LED, buzzer, relay, motor).

A relay or transistor driver is required when a microcontroller output (typically a few mA at 5 V) must switch a load drawing more current than the output can supply.

A transistor as a switch: base current above the threshold saturates the transistor so collector current flows through the load; a flyback diode protects against back-EMF from inductive loads.

Pulse-width modulation controls average power to a motor or LED by varying duty cycle; average voltage ≈ duty cycle × supply voltage.

CAD workflow: parametric sketch → constrain dimensions and geometry → extrude/revolve → assemble with mates → produce orthographic drawing with dimensions and tolerances.

Parametric modelling rule: change a driving dimension and all dependent features update; unconstrained sketches drift and are the main cause of failed updates.

STL for additive manufacture tessellates the surface into triangles; smaller chord tolerance gives a finer mesh and larger file, and the model must be manifold (watertight) to slice correctly.

3D printing (fused deposition modelling) suits one-offs and complex internal geometry; it has lower strength across layer boundaries and slower cycle time than injection moulding for high volume.

CNC requires a toolpath and post-processed G-code; feed rate, spindle speed and depth of cut are set per material, and work zero must be set before running.

Laser cutting produces 2D profiles from vector files; kerf width (typically 0.1–0.3 mm) must be compensated in the drawing for accurate fits.

Rapid prototyping is used for form/fit checks and client sign-off before committing to tooling; it does not replace functional testing where load or fatigue matters.

Emerging technologies to cite with a trade-off: generative design (lightweight optimised geometry vs. difficult manufacture), digital twins (predictive maintenance vs. data infrastructure cost), IoT sensing (real-time data vs. security and privacy risk).

The 6 Rs in priority order: Rethink, Refuse, Reduce, Reuse, Repair, Recycle; recycling is the last resort because it still consumes energy and loses material quality.

Design for disassembly: use reversible fasteners (screws, snap fits) instead of adhesives or welding where components must be separated for repair or recycling.

Material selection for sustainability: prefer a single material or a compatible pair per assembly; mixed-material bonded assemblies cannot be economically separated.

Embodied energy is the total energy used to extract, process and manufacture a material; aluminium has high embodied energy per kilogram but is economically recyclable, unlike thermosets.

Life cycle assessment (LCA) covers raw material extraction, manufacture, distribution, use and end-of-life; comparing products on one stage alone (e.g. manufacture only) gives a misleading conclusion.

Extended producer responsibility and WEEE-type regulations place take-back and recycling obligations on the producer, so end-of-life must be designed in at the concept stage.

Planned obsolescence (technical, functional or aesthetic) shortens product life; designing for upgradability or repair extends it and reduces lifetime environmental impact.

Energy in use often dominates the life cycle for powered products; a lower-power motor or standby cut-off can outweigh a higher-impact casing material.

Third-angle orthographic projection (common in the UK): the view is placed on the side of the object from which it is seen; first-angle places it on the opposite side.

Standard view layout in third angle: plan above the front elevation, and the right-hand side view to the right of the front elevation.

Line conventions: continuous thick = visible outlines; short dashes = hidden detail; chain dotted = centre lines; thin continuous = dimension and projection lines.

Dimension rules: dimension lines outside the outline, arrowheads touching the extension lines, never dimension a feature twice, and never scale a drawing by measurement.

Scale notation: 1:1 full size, 1:2 half size, 2:1 twice full size. State the scale on every drawing.

Isometric drawing uses 30° axes from the horizontal with all lines true length; oblique uses one face true shape with receding lines at 45°; both are pictorial, not working drawings.

Section views: hatch cut surfaces at 45°, omit hidden detail in the sectioned view, and use a cutting-plane line with arrows showing the viewing direction.

Exploded views show assembly order along a common axis and are used for assembly instructions and parts lists, not for manufacturing dimensions.

Risk assessment hierarchy: eliminate the hazard, substitute a safer process, use engineering controls (guards, extraction), then administrative controls (signs, training), then PPE as the last line of defence.

Risk = likelihood × severity; a low-likelihood but severe hazard (e.g. chuck key left in a lathe) still requires control.

Machine guarding: never remove or bypass an interlock guard; where an interlock is defeated, the machine must be isolated and locked off before adjustment.

Isolation procedure (lock-out/tag-out) before changing a blade, bit or belt: switch off, isolate, lock and tag, then verify the machine cannot start.

COSHH-type duty: obtain and read the safety data sheet before using a substance, use the minimum quantity, and use local exhaust ventilation for dusts, solvent vapours and solder fume.

PPE selection by hazard: impact/chemical splash goggles for eye protection, face shield for grinding and lathe work, hearing protection above roughly 85 dB(A), and dust masks rated for fine wood dust.

Electrical safety in the workshop: RCD protection on portable tools, no damaged leads, no trailing cables across walkways, and no water near electrical equipment.

Hot-work and fire rules: clear the area of combustible material, keep a fire extinguisher of the correct class (CO₂ for electrical, foam/water for solids), and know the emergency stop and first-aid locations.