Compare materials and joining methods against a defined brief
A material and its joint must work as a pair. An inexpensive sheet can be unsuitable if the connection cannot be removed, and a removable fastener can still exceed a mass allowance. Use the supplied evidence to make a theoretical selection for a fictional indoor display. No metal fabrication or joining is performed.
Teacher-use draft: Original Grade 10 intended activity. Exact current subject, grade, strand and term alignment remains unverified and needs subject-teacher review. These are original fictional classroom examples, not an official curriculum or examination resource.
The brief and the evidence boundary
A display designer needs two overlapping strips to form a small flat label support. Each strip is 100 mm long and 30 mm wide; overlap is 20 mm, giving an assembled length of 180 mm. There are no holes to design in this task: the relevant options come with appropriately prepared matching holes where their fictional cards require them. The actual hole positions, diameter, strip thickness and load capacity are deliberately not specified, so this is a selection diagram, not a manufacturing drawing.
Brief A requires dry indoor use, separation of the strips for storage without destroying the strips or fastener, recorded suitability for this light display, total mass at most 40 g and total classroom cost at most KSh 55. Choose the lowest-cost fully supported pair. All masses, costs and performance statements below are invented values for these prepared options. They do not describe every aluminium or steel product and do not certify safety.
| Option | Pair mass | Pair cost | Recorded conditions |
|---|---|---|---|
| M1: prepared aluminium pair | 28 g | KSh 40 | Both strips have inspected smooth prepared edges and the specified flat-display stiffness; compatible with J1 and J2 in this invented record. |
| M2: prepared coated steel pair | 46 g | KSh 24 | Same recorded dry-display stiffness and prepared-edge check; compatible with J1 and J2. Suitability outdoors is not recorded. |
| M3: recovered unlabelled pair | Unknown | KSh 0 | Material identity, edge condition and compatibility are unrecorded. |
| Joint | Added mass and cost | Evidence for this task |
|---|---|---|
| J1: prepared bolt-and-nut connection | 4 g; KSh 12 | Can be separated without destroying the specified fastener or strips. The given design record confirms suitability for the stated display. |
| J2: prepared riveted connection | 2 g; KSh 6 | Recorded suitable for the stated display, but separation would destroy the rivet. No installation or removal procedure is supplied. |
| J3: proposed adhesive lap | 1 g; KSh 8 | Suitability, material compatibility and separation behaviour are not established. Do not assume a bond or a curing result from this cost alone. |
Read the three joint diagrams
Prepared original illustration: joining-comparison.png. Publication media URL has not been assigned.
Complete text alternative: A plan shows two 100-by-30 mm strips overlapping by 20 mm to form a 180 mm overall length. Three separate local schematic sections show: J1, a bolt head and nut on opposite sides connected by a shank through two overlapping layers; J2, a rivet with formed heads retaining two layers; J3, a thin adhesive layer between the overlap surfaces. Simplified symbols omit thread detail, hole size and structural specifications.
The overlap is shared by both strip lengths: 100 + 100 − 20 = 180 mm. Adding 20 instead would count the shared area twice. The diagrams show three different methods at one possible overlap; they do not instruct anyone to combine all three. A local section reveals the layered connection, while the plan gives the strip arrangement.
Worked selection for Brief A
M1 with J1 has total mass 28 + 4 = 32 g and cost 40 + 12 = KSh 52. It meets the 40 g and KSh 55 inclusive limits, provides the stipulated display suitability and can be separated as required. Its allowances are 8 g and KSh 3. M2 with J1 costs only KSh 36 but weighs 50 g, exceeding the mass limit by 10 g. Lower price cannot cancel that failure.
M1 with J2 would weigh 30 g and cost KSh 46, but J2 fails the non-destructive separation requirement. “Rivets are cheaper” does not answer the whole brief. J3 has unresolved evidence; its low mass does not establish compatibility, display performance or removability. M3 is similarly incomplete. Unknown is neither a pass nor proof that the option necessarily fails every possible future test.
Completed recommendation: “Choose the supplied M1 pair with J1 for Brief A. Its 32 g mass and KSh 52 cost meet both limits, and the recorded reversible connection suits storage. Keep the dry-use condition. This decision applies to the stated prepared components and fictional evidence; it is not a general approval for a real load-bearing metal product.”
What a qualified supervisor would need before real work
A real proposal would need the actual alloy or grade, thickness, finish, dimensions, service conditions, edge preparation, hole and fastener specifications, compatibility, load case and an appropriate risk assessment. Different metals and environments can create corrosion concerns; this exercise does not test them. Adhesive choice would need its own manufacturer information and suitable handling arrangements. No heating, welding, cutting, drilling, rivet setting, chemical preparation or powered-machine operation is part of this activity.
Paper, a pencil and a ruler are sufficient for comparing the cards and drawing the selected joint. A card overlap may illustrate shared length if the teacher supplies smooth prepared pieces; it cannot reproduce the stiffness, strength, conductivity or durability of metal. Do not transfer an apparent success of a card model to a metal structure.
Independent changed brief
Brief B retains the same dry, light-display purpose and the supplied suitability condition. Separation is no longer required, total mass may be at most 50 g, and cost may be at most KSh 32. Keep all option cards unchanged.
- Which fully supported pair meets Brief B? Show its mass and cost.
- Why does the original M1/J1 recommendation not carry over automatically?
- For two 120 mm strips with 30 mm overlap, what is the assembled length? Does that calculation establish joint strength?
Explained answers and review
- M2/J2 weighs 46 + 2 = 48 g and costs 24 + 6 = KSh 30. Both limits pass and the recorded display suitability applies. Destructive rivet removal no longer violates a requirement. M2/J1 costs KSh 36 and fails budget; M1’s material cost alone exceeds KSh 32. Unknown J3 evidence cannot be treated as a qualifying alternative.
- M1/J1 still has the same properties, but its KSh 52 cost exceeds the new allowance by KSh 20. The change is in the design criteria; a previous recommendation is not universal.
- Length = 120 + 120 − 30 = 210 mm. This determines the arrangement only. Strength requires material, joint and loading evidence absent here.
Review each answer for complete constraints, whole-assembly arithmetic, a justified joint choice and explicit unknowns. Do not award extra credit for suggesting more dangerous processes. Background: TWI’s Mechanical Joining overview describes the family of mechanical joining methods. The precise components, records, dimensions, criteria and conclusions here are original hypothetical teaching data.
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