The Ergonomic Assembly Jig
A pupil team is making 30 identical small desk lamps for a school sale. Each lamp has a base, a stem and a shade, joined by M4 screws. In the first batch of 10, assembly takes 14 minutes per lamp, screws are cross-threaded on 4 of 30 joints, and two pupils report wrist ache from holding the stem steady while driving screws. The team has 4 weeks, 2 hours of workshop time per week, and 10 lamps are already sold. Their specification includes 'comfortable to assemble repeatedly' and 'joints must be sound'. They have plywood offcuts, a pillar drill, a vice, and hand tools.
1. Using the numbers, judge whether the team can meet the order, and identify the biggest lever for improvement.
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At 14 min per lamp, 20 remaining lamps need 280 min of assembly, ignoring rework. Available time is 4 weeks × 2 h = 8 h = 480 min, but that must also cover finishing, testing and packing, so assembly is roughly 58% of all remaining time - tight but feasible only if the rate holds and defects do not rise. The biggest lever is not working faster; it is removing the two causes of lost time. Cross-threading on 4 of 30 joints means 13% of joints need rework, and rework plus re-gripping is what inflates cycle time. A jig that holds the stem square to the base and guides the screw axis attacks both the defect rate and the wrist ache at once. Judgement: build the jig before batch 2, then re-time; if the rate falls to about 9 min, the order is comfortably met.
2. Describe the jig you would make from the available stock and explain how it addresses the ergonomic and quality requirements.
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A location jig: a plywood cradle with a machined recess that seats the base, a vertical plywood block with a drilled bore that holds the stem at exactly 90° to the base, and a drill-guide bush or a clearance hole aligned to the screw axis. Clamp the jig in the vice so both hands are free. Quality: the bore fixes the screw axis, so the screw starts square and cross-threading is designed out; the base recess stops the part rotating. Ergonomics: the jig takes the holding force, so the pupil no longer grips and steadies the stem against torque - wrist load drops and the task becomes push-and-turn rather than squeeze-and-hold. Practicalities: plywood is stiff enough for M4 driving torque, the pillar drill can bore the guide holes accurately, and the jig is cheap and repairable. Evaluation plan: run batch 2, record assembly time per lamp and count cross-threaded joints; success is a fall in both, with the jig surviving 30 cycles without wear loosening the guide.