Folding display stand: constraints, criteria and a prototype that wobbles
A local science museum has asked a candidate to develop a portable display stand for its outreach team. The stand must fold to fit inside a hatchback boot measuring 1000 × 600 × 400 mm, carry up to 12 kg of demonstration equipment on a 700 × 400 mm top surface, and be assembled by one adult in under three minutes without tools. Eight units are needed and the materials budget is £45 per unit. The school workshop has a scroll saw, pillar drill, vacuum former, strip heater and hand tools, but no welder, CNC router or laser cutter. The museum wants the stand to look professional and to work on grass as well as on hard floors in schools. A first prototype folds correctly and costs £38 in materials, but the 6 mm plywood top deflects 9 mm under a 6 kg load placed at its centre, the frame rocks 5 mm when standing on uneven grass, and the tool-free catches work loose in transit.
1. Sort the requirements into fixed constraints and design criteria, and explain why 'assembled in under three minutes without tools' pulls against 'carries 12 kg'. State what evidence you would collect before changing the design.
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Constraints (not yours to change): folded size ≤ 1000 × 600 × 400 mm; eight units; ≤ £45 of materials each; only the listed workshop equipment; one-person assembly.
Criteria (targets you design towards, and must make measurable): 12 kg carried with the top deflecting ≤ 3 mm; stable on a 1:20 slope and on grass; assembly ≤ 180 s by one adult with no tools; 'professional' translated into something testable, e.g. no exposed fixings, one consistent finish, no more than two hand actions to erect.
The tension: a tool-free joint must have clearance to slide together, and clearance is play. Play at four joints is what produces the 5 mm rock and the 9 mm bounce. Adding more clips slows assembly (breaking the three-minute criterion) without adding much stiffness, because stiffness comes from the geometry of the connection and from triangulation, not from how hard you clamp it.
Resolution to test: change the joint type rather than the clamp force - a taper or drop-in slot with a self-tightening cam gets tighter as load is applied, giving near-zero play with a one-second action. Add a diagonal brace so the frame acts as triangles rather than rectangles.
Evidence first: deflection at the design load of 12 kg (not 6 kg), measured against a straight edge; assembly timed with three different users; 25 erect/dismantle cycles to see when the catches let go; stability on a 1:20 slope. Record all of it in the design log, because the justification for the redesign is the data, not an opinion.
2. The top is 6 mm ply, 700 × 400 mm, spanning 600 mm between two rails and currently deflecting 9 mm at 6 kg. Using I = bd³/12, evaluate doubling the thickness to 12 mm against keeping 6 mm and adding a 40 mm deep front lip. Recommend one.
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For a flat panel, deflection at constant load varies as 1/d³, so the numbers are quick and decisive.
Baseline, 6 mm: I = 0.70 × 0.006³ / 12 = 1.26 × 10⁻⁸ m⁴. Measured 9 mm at 6 kg, so at the 12 kg design load expect roughly 18 mm - about six times the 3 mm target. The prototype is not marginal, it is badly under-stiff.
Option A, 12 mm ply: I = 0.70 × 0.012³ / 12 = 1.01 × 10⁻⁷ m⁴, eight times larger, so deflection falls to about 18/8 = 2.25 mm. It works. The cost is roughly doubling the panel price (a large slice of the £45), doubling the weight one person must lift, and 6 mm of the 400 mm fold height.
Option B, 6 mm plus a 40 mm lip: local depth rises from 6 to 46 mm, so at the stiffened edge I rises by (46/6)³ ≈ 450 times. In practice the panel behaves as a stiffened box rather than a plate, and a 3–4× reduction (to roughly 4–6 mm at 12 kg) is realistic - still not quite 3 mm unless a central rib is added.
Judgement: use Option B with a lip plus one central rib. It stiffens the frame at the same time, keeps the weight down and leaves budget for better catches. But test it: the glue line and rib spacing decide whether the predicted stiffness is actually delivered, and a bonded joint that creeps changes everything.
3. Critique this test plan: 'Load 12 kg, look at it, and if it does not break, sign it off.' Write a better plan with pass/fail values.
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What is wrong with it: one sample, no measurement, no repeated cycles, no failure criterion beyond catastrophic break, no record, no safety check, and a static 12 kg does not represent carrying, dropping or a child leaning on the front edge. It also proves nothing about the three-minute criterion or stability on grass, which are two of the main specification points.
A defensible plan:
- Static deflection - 0, 6 and 12 kg at the centre and at the front edge; measure with a steel rule against a straight edge to ±0.5 mm. Pass: ≤ 3 mm at 12 kg, and permanent set ≤ 1 mm after unloading.
- Stability - stand on a 1:20 slope and on 40 mm grass; hang 5 kg from the front edge. Pass: no rocking, no tip.
- Assembly - five users, timed, no tools. Pass: ≤ 180 s with at most one failed attempt.
- Durability - 25 erect/dismantle cycles, then repeat test 1. Pass: catches still hold, deflection unchanged.
- Safety - all edges radiused, no trapping points, non-toxic finish, no sharp fastener ends inside the fold volume.
Record results with dates and photographs in the design log. The failures are the valuable part: a test that only confirms success is not evidence of iteration, and the specification should be amended only after the data has been reviewed.