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How can you tell that they're 3d printed? Because they don't have a part number or the manufacturer's name and logo on them?

I don't have a 3d printer myself so maybe it'd be obvious if I printed some.

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Most 3D printed parts have a telltale texture resulting from the layer-by-layer deposit of material. The same goes for many milled/CNCed parts bearing evidence of tool marks. Once you've seen and held enough, it's relatively easy to identify whether a given part was printed, cast, milled, lathed, etc.

I say most because there are finishing methods which can largely obscure these details and make it less obvious as to which method produced a given part.

They had the layer lines you see on FDM prints.

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