The big idea: Design for assembly asks how many parts there are and how they go together. First check whether parts can be combined without losing function, safety or repairability.
Why removing a part wins: A part that is not there costs nothing to buy, stock, inspect, fit or get wrong — and a lower parts count is usually a lower cost AND a lower environmental impact at once.
Design for assembly — the second strategy, and with a check that the product can still be taken apart.
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| Change | What it looks like | What it saves |
|---|---|---|
| Combine parts | Mould the button, the seal groove, the hinge and the mounting bosses into the body instead of fitting them | Each part removed takes a purchase order, a stock line, an inspection, a station and a chance of error with it |
| Use snap-fits instead of screws | A moulded clip that engages when the halves are pressed together | No fastener to buy or feed, no driver, and fewer separate fastener and tool operations |
| Make parts symmetrical | A component that works either way round, or one so asymmetric it can only go one way | Either removes the orientation decision, or makes the wrong orientation impossible — both prevent the error |
| Assemble from one direction | Every part dropped in from above, with nothing to be fitted from underneath | No turning the product over, no awkward reaches, and a fixture that can hold it in one position throughout |
| Design out tangling | Avoid open hooks and thin springs that lock together in a bin | A feeder that does not jam, which is what actually stops an automated line |
| Standardise fasteners | One screw size and head type throughout | One tool, one torque setting, and no wrong screw in the wrong hole |
The three questions that remove a part: For every part, ask: does it move relative to the part beside it? Must it be a different material? And must it be separable for assembly or service?
If all three answers are no, it can probably be combined with its neighbour — and that is the whole method.
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| A 14-part design | A 6-part design | |
|---|---|---|
| Stock lines | 14 to order, store, count and pay for | 6 |
| Assembly stations | More stations, or a longer time at each | Fewer stations and a shorter line |
| Chances of error | 14 chances of a part being wrong, missing or backwards | 6 |
| Training | Longer, and a new operator takes longer to become reliable | Shorter |
| End of life | More parts to separate — though usually fewer materials bonded together | Fewer parts, but only better if they are not glued into one lump |
Fewer parts is not automatically better
Check end of life
Six parts bonded into one inseparable lump is worse at end of life than fourteen screwed together.
Check tooling and the other strategies
And combining two parts into one complicated moulding can need a tool so much more expensive that the saving disappears. Design for assembly has to be checked against design for process and design for disassembly before it is adopted.
How this is tested — outlining the advantages of design for assembly and applying it. It comes up two ways:
Paper 1 — multiple choice
- Identify the design-for-assembly change in a described product.
- Choose the change that reduces assembly errors.
Paper 2 — analysing a product
- Explain how a product could be redesigned for assembly.
- Outline the advantages of design for assembly for a named product.
The trap: Reducing the parts count at any cost. Check the result against design for process and design for disassembly before recommending it.
A torch has a body, an end cap, a reflector, a lens, a bezel ring, a switch body, a switch cap, a spring and six screws. Explain how it could be redesigned for assembly.
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