Concept choice and joint failure in a flat-pack laptop stand
A student team must design a flat-pack stand for a 15-inch laptop weighing 2.2 kg. The brief requires: cut from 3 mm birch ply on a 600 mm × 400 mm laser bed; assemble without glue or fasteners; support a 10 kg vertical load; fold flat into an A4 envelope (297 mm × 210 mm); and use no more than one 600 × 400 sheet per stand. Concept A uses six finger-jointed parts and scores highly on strength but poorly on assembly time. Concept B is a one-piece folding stand with two living hinges; it scores highly on aesthetics and assembly time but cracked at one hinge in a 1 m drop test. A prototype of Concept A has joints that are too loose: the laser kerf measured 0.15 mm, but all slots were drawn at 3.00 mm nominal. An 8 kg test load caused the front lip to split along a ply layer. The team must choose a concept and present a justified modification before the next prototype.
1. Choose between the two concepts using weighted criteria, and justify the decision.
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Use a weighted decision matrix with safety/strength at 40%, assembly time 20%, material and cutting cost 15%, flat-pack storage 15%, and aesthetics 10%. Concept A scores 4/5 on strength, 2/5 on assembly, 4/5 on cost, 3/5 on storage and 2/5 on aesthetics: weighted score = 0.40(4) + 0.20(2) + 0.15(4) + 0.15(3) + 0.10(2) = 1.6 + 0.4 + 0.6 + 0.45 + 0.2 = 3.25. Concept B scores 2/5 on strength, 5/5 on assembly, 3/5 on cost, 5/5 on storage and 5/5 on aesthetics: 0.8 + 1.0 + 0.45 + 0.75 + 0.5 = 3.50. On those scores B wins, but the drop-test failure is a safety issue, so strength should be a gate, not just a weighted criterion. A hinge crack in 3 mm ply means B cannot be accepted as drawn. Choose A, but reduce its assembly penalty by redesigning the joints; alternatively accept B only with a redesigned hinge, such as a laminated or interlocking hinge. A weighted matrix informs the choice, but a failure against a mandatory requirement overrides the total.
2. Diagnose the loose joints and the split front lip, and give numeric corrections.
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The loose joint is a kerf-compensation error. The laser removes about 0.15 mm on each cut, so a slot drawn at 3.00 mm finishes about 3.15 mm wide, not 3.00 mm. The slot should be drawn at 3.00 − 0.15 = 2.85 mm, but because plywood varies, cut a test comb with slots at 2.75, 2.85 and 2.95 mm and choose the fit that holds without force. The front lip split along a ply layer because the load was applied across the grain and the lip is only 10 mm wide. Rotate the part so the grain runs along the lip, increase the lip to 15 mm, and add a 3 mm fillet at the internal corner to reduce stress concentration. The 8 kg test was also below the 10 kg requirement, so the failure is more serious than it looks. A cross-grain brace or a folded lip would be a better fix than simply thickening the part.
3. Propose a test plan and the drawing communication needed for the next prototype.
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Test at least three identical stands, not one. Record assembly time from flat to ready, then load to 12 kg (1.2 × the 10 kg requirement) for one hour, measure deflection at the front lip, and repeat assembly 50 times. Drop one stand from 1 m onto a hard floor in both the flat-packed and assembled states. For communication, produce an exploded isometric with part numbers, a cutting layout on 600 × 400 mm showing grain-direction arrows and the 2.85 mm slot note, and a section detail through the finger joint. Add a simple assembly sequence with four steps. The drawing should state the kerf assumption and the ply thickness tolerance so the next prototype is not a one-off. Photograph the failure surfaces and attach them to the test report; that evidence is more convincing than a score in the matrix.