NSSCAS Design and Technology (8256) cheat sheet
NSSCAS Design and Technology (8256) · Keep the key ideas within reach.
Design Process
Iterative cycle: identify problem → research → specify → generate ideas → develop → model/prototype → test → evaluate → refine; evaluation feeds back into any earlier stage.
Divergent thinking widens options (brainstorming, SCAMPER, morphological analysis); convergent thinking narrows them (screening matrices, weighted scoring).
Morphological chart: list key functions as columns, possible means as rows, combine one means per column to create new solution concepts.
Weighted decision matrix: score each concept 1–5 per criterion, multiply by criterion weight, sum; highest total is the justified choice, not the automatic one.
Prototype fidelity ladder: sketch → card/foam model → CAD render → functional prototype → pre-production prototype; each step costs more and answers different questions.
Anthropometric data used at the 5th–95th percentile range; design for adjustability rather than an average user when the range is wide.
Design fixation is reduced by deferring judgement, generating quantity before quality, and combining unrelated existing products (analogy).
Evaluation must reference the original specification point by point; unsupported opinions are not evaluation evidence.
Problem Identification and Specification
Problem statement names the user, the context and the deficiency; it must not contain the solution.
Primary research = interviews, questionnaires, observation, user trials; secondary = published data, standards, competitor teardown.
A design specification is a set of measurable requirements: dimension, mass, load, cost, finish, lifetime, safety standard.
Each specification point should be testable - 'lightweight' fails, 'under 1.2 kg' passes.
Constraints are fixed limits (budget, deadline, machinery, regulations); criteria are desirable qualities ranked by importance.
Market analysis covers price band, competing products, target user profile and retail channel.
User needs (what the user says) differ from latent needs (observed difficulties); observation reveals the second.
Specification is a live document: it is revised when testing shows a requirement was wrong or unachievable.
Materials, Components and Their Properties
Mechanical properties: tensile strength, compressive strength, stiffness (Young's modulus), hardness, toughness, ductility, malleability, elasticity.
Density = mass ÷ volume; a lower-density material gives a stiffer structure per unit mass when stiffness-to-weight ratio is compared.
Ferrous metals contain iron (mild steel, cast iron); non-ferrous include aluminium, copper, brass, zinc; stainless steel resists corrosion via chromium oxide layer.
Thermoplastics (acrylic, HDPE, ABS, nylon) soften reversibly on heating; thermosets (epoxy, polyester resin, melamine) cross-link permanently.
Hardwood from broad-leaved trees (oak, ash, beech); softwood from conifers (pine, spruce); manufactured boards (MDF, plywood, chipboard) are stable and sheet-form.
Composites combine fibre and matrix: GRP (glass-reinforced polyester) and CFRP give high strength-to-weight but are hard to recycle.
Material selection uses service conditions: UV exposure, moisture, temperature, abrasion, food contact and cost per part.
Smart and modern materials: shape-memory alloy, photochromic pigment, thermochromic pigment, piezoelectric ceramic, biodegradable PLA.
Manufacturing Processes, Tools and Equipment
Wasting processes: sawing, drilling, turning, milling, filing, sanding - material removed by cutting; accuracy depends on marking out and clamping.
Forming processes: press forming, bending, vacuum forming, injection moulding, extrusion, casting, forging - shape produced by force or flow.
Joining: permanent (welding, brazing, soldering, adhesive, rivet) versus temporary (bolt, screw, knock-down fitting); choose by disassembly need and load type.
Injection moulding needs draft angle (typically 1–2°), uniform wall thickness and no undercuts without side-action tooling.
Vacuum forming requires a former with draft and radiused edges; webbing and thinning occur at sharp corners and deep draws.
Tolerance and allowance: tolerance is permitted variation; allowance is the intentional difference between mating parts (e.g. clearance fit).
Quality control: go/no-go gauges, jigs and templates for repeatability; CNC gives repeatability, not automatic correctness of the program.
Scale of production: one-off, batch, mass; tooling cost amortised over volume determines the economic process choice.
Design Communication, Graphics and Technical Drawing
Third-angle orthographic projection (common in UK/NSSCAS): the view is placed on the side from which it is seen; first-angle places it on the opposite side.
Orthographic set: front, plan (top) and end elevation, aligned and projected; hidden detail shown as dashed lines.
Isometric drawing uses 30° axes with true lengths along axes; oblique uses one face true shape with receding lines at 45°.
Scale drawing ratio 1:2 means drawing is half full size; always state units and scale on the sheet.
Dimensioning rules: dimension lines outside the outline, arrowheads touching extension lines, no repeated or redundant dimensions, no dimension on a hidden line.
Section views show internal detail; hatching at 45° indicates cut material, adjacent parts hatched in opposite directions.
Line conventions: continuous thick = visible outline; continuous thin = dimension/leader; dashed = hidden; chain thin = centre line.
CAD benefits: parametric editing, layers, libraries, rapid export to CAM/STL; risks include file incompatibility and loss of drawing convention discipline.
Design in Context: Sustainability, Health and Safety, Social and Ethical Issues
6 Rs hierarchy: refuse, reduce, reuse, repair, recycle, rot - prevention before recovery before disposal.
Life-cycle assessment (LCA) covers raw material extraction, manufacture, distribution, use phase and end-of-life; energy in the use phase often dominates.
Cradle-to-cradle treats products as nutrient flows; cradle-to-grave ends in landfill.
Design for disassembly: reversible fasteners, material labelling, avoiding bonded dissimilar materials improve end-of-life recovery.
Hazard control hierarchy: eliminate, substitute, engineering control, administrative control, PPE - PPE is last, not first.
Risk assessment = likelihood × severity; record control measures and residual risk.
COSHH-type logic: obtain safety data, assess exposure route (inhalation, skin, ingestion), control dust and solvent vapour at source.
Ethical issues: planned obsolescence, built-in exclusion of users with disabilities, data privacy in smart products, fair sourcing of materials.
Structures, Mechanisms, Control and Systems
Static equilibrium: sum of forces = 0 and sum of moments about any point = 0; use to solve unknown reactions and member forces.
Moment = force × perpendicular distance from pivot; a lever balances when load × load arm = effort × effort arm.
Stress = force ÷ cross-sectional area (N/mm²); strain = extension ÷ original length; Young's modulus = stress ÷ strain in the elastic region.
Beam types: simply supported, cantilever, fixed; bending moment is greatest at the centre of a simply supported beam under a central point load.
Failure modes: tension, compression, shear, bending, torsion, buckling (compression in a slender member), fatigue.
Mechanical advantage = load ÷ effort; velocity ratio = effort distance ÷ load distance; efficiency = MA ÷ VR × 100%.
Gear ratio = teeth driven ÷ teeth driver; a chain-and-sprocket or belt drive transmits torque between parallel shafts.
Control systems: open loop has no feedback; closed loop compares sensor feedback with set point and corrects error (thermostat, servo).
Four-bar linkage: crank-rocker, double-crank and double-rocker behaviour depends on link lengths (Grashof condition).
Practical Project / Design-and-Make Coursework
Coursework evidence chain: problem and context → research → specification → concept sketches → development → manufacturing plan → making log → testing → evaluation against specification.
Manufacturing plan lists sequence of operations, tools, jigs, quality checks and estimated time for each stage.
Making log records date, process, settings, difficulties and corrective action - it is evidence of decision-making, not a diary.
Photograph stages of making with a scale rule visible; retain offcuts, test pieces and rejected parts as evidence of testing.
Testing must use the specified criteria and a measurable method; record raw data in a table before interpreting it.
Modelling before manufacture reduces risk: card or foam mock-up checks ergonomics and proportions before cutting expensive material.
Health and safety evidence: risk assessment for the workshop, correct PPE, machine guard use, supervised operation of hazardous equipment.
Evaluation structure: does it meet each specification point, how well does it work for the user, what would be changed and why.
Design Process (Supplementary: Modelling and Testing)
Physical model tests geometry and ergonomics; mathematical model tests forces, cost or thermal performance; use each for what it can answer.
Virtual modelling (CAD/FEA) allows rapid iteration but results depend on assumed loads and boundary conditions.
Fair test: change one variable, keep others constant, repeat and average to reduce random error.
Failure testing to destruction identifies the weak point and the actual safety factor, not the assumed one.
User trial protocol: defined task, observed errors, timed completion, post-trial interview - triangulate to avoid bias.
Scale model laws: a half-scale beam does not carry half the load; do not scale results linearly without calculation.
Materials and Manufacturing (Supplementary: Finishing and Corrosion)
Surface finishes: paint, varnish, oil, powder coating, anodising (aluminium), electroplating, galvanising (zinc on steel).
Anodising thickens the natural aluminium oxide layer for wear and dye uptake; it is not a paint.
Galvanic corrosion occurs when dissimilar metals are in contact in the presence of an electrolyte; separate them with an insulator.
Wood movement: solid timber expands across the grain with humidity; allow clearance in frames and panels.
Heat treatment of steel: annealing softens, hardening (quench) then tempering restores toughness.
Adhesive selection depends on substrates, gap filling, flexibility and service temperature; surface preparation determines bond strength.
Design Communication (Supplementary: Presentation and Standards)
Working drawing must contain enough information for manufacture without verbal explanation: views, dimensions, tolerances, material, finish.
BS/ISO 8888-style drawing frames and title blocks carry drawing number, scale, projection symbol, material and revision.
Exploded views and assembly drawings show part order and fastening; number parts to a parts list.
Flowcharts and Gantt charts communicate process and time plan; critical path shows the sequence that sets project duration.
Graph choice: bar for categories, line for continuous change, scatter for correlation between two variables.
Annotation on design sketches should state function and material, not just label the shape.