Aluminium bracket batch: CNC, die casting or sand casting?
A workshop must supply 500 identical aluminium brackets for solar panel mounts. Each bracket is 120 mm × 60 mm × 6 mm, made from 6082-T6 plate, with two Ø8 H8 holes spaced 40 mm ±0.2 mm and a 15 µm anodised finish. The current route is CNC milling from plate: 11 min cycle at €45/h, material €6.50, and 3% scrap. A die-caster quotes €9,500 for tooling and €2.10 per part, but says the holes would still need machining and porosity could cause rejects. A sand caster quotes €1,200 tooling and €3.80 per part to ±0.5 mm, also requiring hole machining. The customer may repeat the order annually for five years at 500 units, but this is not guaranteed. The workshop has one CNC mill, a drill press, and a budget that cannot absorb a €9,500 tool without a firm order.
1. Calculate the relevant costs for the 500-unit order under each route, making reasonable assumptions where needed. Recommend a process and justify with both cost and capability.
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CNC: 11 min = 0.183 h × €45 = €8.25; material €6.50; total before scrap €14.75. With 3% scrap, cost per good part = 14.75 / 0.97 = €15.21, so 500 good parts cost about €7,600.
Die casting: tooling €9,500 + 500 × €2.10 = €10,550. The holes still need machining; assume 2 min at €45/h plus reaming = €2.00 per part, giving €11,550. A 5% porosity reject rate adds about €550, so roughly €12,100, or €24 per good part.
Sand casting: tooling €1,200 + 500 × €3.80 = €3,100. Hole machining at €2.00 per part adds €1,000, so about €4,100. However, ±0.5 mm as-cast tolerance cannot guarantee the 40 mm ±0.2 mm hole spacing without a post-cast machining fixture.
Recommend CNC for this order: it avoids tooling risk, holds H8 and ±0.2 mm directly, and gives a consistent anodised finish. If a firm five-year order for 2,500 units is signed, die casting becomes attractive: tooling falls to €3.80 per part, so even with €2.00 machining the variable cost is about €7.90 against €15.21 for CNC. The decision should be conditional on a 50-off die-casting trial and porosity inspection.
2. Describe a jig or fixture and a gauging method to ensure the 40 mm ±0.2 mm hole spacing at 500 units.
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Use a steel fixture with two datum edges locating the 120 × 60 mm plate, a toggle clamp, and a drill-bushing plate containing two hardened 8 mm H8 bushes set exactly 40 mm apart. Drill and ream both holes in one setup so the spacing is controlled by the fixture, not by the operator marking out. Locate from the same datum edges on every part.
For gauging, use a go/no-go plug gauge: the go gauge is 39.8 mm and the no-go gauge is 40.2 mm, so the pair checks the spacing. Check hole diameter separately with an 8.00 mm H8 plug gauge. Because 15 µm anodising builds on the surface, mask the holes or specify the pre-coating hole size; otherwise a 7.98 mm hole may close below H8. Inspect the first-off part and then every 25th part, and record the results.
3. Explain why die casting might still fail the specification even if the cost is lower.
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Die casting can be cheaper at volume but is not automatically capable. Porosity can appear at bosses and thick sections, weakening threads and causing rejects that only appear after machining. The 40 mm ±0.2 mm hole spacing may still need a post-cast machining fixture, so the casting does not remove the tight-tolerance operation. Anodising is also a risk: cast Al-Si alloys anodise differently from 6082 plate, and silicon particles can produce a dark, patchy finish that fails a cosmetic specification. A 15 µm coating on a porous casting may look uneven. The correct approach is a 50-off qualification trial with sectioning or X-ray, a machining trial, and an anodising trial before committing to the €9,500 tool.