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Choosing datums should start with how the part functions, not with whichever face looks convenient in CAD. Good datum selection in GD&T connects the drawing to the way the component is located and inspected.
Start With the Assembly
Ask which surface actually locates the part during use. That functional surface is often a strong candidate for the primary datum.
Primary, Secondary and Tertiary
The datum reference frame establishes orientation and location progressively. The order matters because each reference removes degrees of freedom.
Think Like the Inspector
If a datum scheme is impossible to reproduce on the inspection equipment, it is worth reviewing. The drawing should describe a realistic measurement condition.
Think Like Manufacturing
Datums should also be practical to establish during manufacturing. A beautiful theoretical datum that cannot be located consistently on the shop floor can create problems.
Functional Datums Win
When in doubt, go back to function: how is the part mounted, what must align, and what surfaces control that relationship?
Keep learning: Explore more practical engineering articles in the GetCADFile Design Guide.