Greenhouse Shelf: Drawing Deficiencies and the Wrong Material
A team of three students must build a shelf in the school greenhouse to hold a 6.0 kg watering can at waist height. The client's constraints: the shelf spans 40 cm between two wooden uprights, the budget is $15, it must tolerate near-100% humidity and weekly watering, and it must be finished within two 75-minute workshop periods. Available stock: 12 mm plywood at $9 per 30 × 60 cm panel, 3 mm aluminum sheet at $14 per 30 × 30 cm, and 18 mm particle board at $5 per 30 × 60 cm. The team's technical drawing is a freehand 3-D sketch with no scale, no dimensions, one unexplained dashed line and no title block. Their report states: "We chose 3 mm aluminum because metal is stronger than wood." The teacher requires an orthographic front view and right-side view at 1:5, dimensions in millimetres, and a material justification based on at least two properties. One team member worries the 40 cm shelf will sag in the middle once the can is full.
1. Identify the deficiencies in the team's drawing and state exactly how each one should be corrected.
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A working drawing must let someone else build the part without asking questions. This one fails on five points:
- No scale. Add a title block reading "Scale 1:5" and draw every view to that scale, so a 400 mm span is drawn 80 mm long.
- No dimensions. Add dimension lines with arrowheads and the value in millimetres: overall span 400 mm, depth, thickness, and the position and diameter of the mounting holes. Dimensions go outside the outline, never on it.
- No defined views. Replace the single 3-D sketch with a front view and a right-side view, aligned horizontally on a common baseline, so a feature in one view lines up with the same feature in the other.
- Unexplained line type. A dashed line means a hidden edge; a chain line means an axis of symmetry. Pick the correct convention and keep it consistent, or remove the line.
- No title block. Add project name, student names, date, scale, units and material.
The 1:5 scale also matters for building: measuring the drawing and multiplying by 5 must reproduce the real part.
2. Evaluate the claim "metal is stronger than wood" and recommend a material using at least two properties.
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The claim confuses strength with stiffness. Strength is the stress at which a material breaks; stiffness (Young's modulus) is its resistance to bending. For a shelf, stiffness and thickness control the sag, and stiffness in bending grows with the cube of thickness.
Compare the two candidates for the 400 mm span carrying 6.0 kg at mid-span:
- 12 mm plywood: modulus roughly 8 GPa, but four times the thickness of the aluminum. The cube term (4³ = 64) more than offsets the lower modulus, so it deflects about 0.2 mm.
- 3 mm aluminum: modulus about 70 GPa, yet the thin sheet deflects roughly 1.7 mm - about seven times more.
Both are acceptable in deflection, so the decision moves to other properties:
- Cost and area: $9 buys a 30 × 60 cm plywood panel; $14 buys only 30 × 30 cm of aluminum, and the shelf needs more than one piece.
- Moisture: exterior-grade plywood with a sealed finish resists the humid greenhouse; particle board swells and crumbles; aluminum is fine but corrodes in wet fertilizer.
- Workability: plywood can be cut, drilled and screwed with school tools in the available periods.
Recommendation: 12 mm exterior plywood, sealed, with a stiffening rib.
3. The team is worried about sag. Where should a stiffener go, and how would it appear in the two required views?
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A 6.0 kg load concentrated at mid-span is the worst case, and the weakest point of a flat panel is its lack of depth. Deflection falls with the cube of the depth in the bending direction, so adding depth is far more effective than adding thickness: a 40 mm deep rib along the front edge raises the stiffness many times over for a few grams of material.
Two options:
- a rib glued and screwed under the front edge, running the full 400 mm span;
- a mid-span bracket tying the shelf to the back wall, which shortens the effective span.
In the drawing:
- Front view: the rib appears as a rectangle directly below the shelf edge, 400 mm long, with its height dimensioned (e.g. 40 mm); hidden lines show the screws.
- Right-side view: the rib appears as a small rectangle hanging below the shelf section, showing its depth and thickness, dimensioned in millimetres.
Both views must be drawn at 1:5 and share the same height reference so the rib lines up.