design for assembly
Design for Assembly
A part drawing carries the machining cost and hides the assembly cost. The assembly cost is labour, and labour is where a design that looks cheap on paper becomes expensive on the line: five parts, four screws and a bracket that must be held in place while a nut is started.
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- Rule one
- One part that does two jobs beats two parts
- Rule two
- Parts should self-locate before fastening
- Rule three
- Every fastener is a hidden cost line
- Typical saving
- 30–50% of assembly time from part-count reduction
Assembly cost is the part of a design that nobody draws and everybody pays for. The fixes are structural: fewer parts, better location, fewer fasteners.
Specifications
| First principle | Merge parts that do not move relative to each other |
|---|---|
| Second principle | Location before fastening — spigots, shoulders, chamfers |
| Third principle | One fastener type, as few as possible |
| Tolerance strategy | One deliberate clearance or slot instead of a tight stack |
| Typical result | 30–50% less assembly time from part-count reduction alone |
Frequently asked questions
What is design for assembly?
Designing a product so it can be assembled quickly and correctly: fewer parts, self-locating features, fewer and standardised fasteners, and tolerances that do not fight the assembly sequence.
How do I reduce part count?
Merge parts that do not move relative to each other. Two bolted parts often become one machined part with a step, which is cheaper in material, faster to assemble and stiffer at the joint.
Why are fasteners so expensive in assembly?
Each one adds a part number, a bin, a pick, an insertion, a torque operation and an inspection. Twelve screws can cost more in labour than the machining of the parts they hold.
What is the cheapest DFA feature?
A chamfer. A 0.5 mm chamfer on a bore and a matching lead on the shaft turns a fiddly insertion into a one-second push, and it costs nothing at the machine.
Should I send the assembly drawing with a part inquiry?
Yes. A machinist who can see the stack-up will flag the tolerance that will not assemble, which is usually cheaper to fix on the drawing than on the line.
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