This is to prevent contact with the external thread to protect the threads or protect other things the threads would rub against.
In addition the dome gives a more finished appearance.
316 Stainless Steel Cap Nut (DIN 1587) M12
The cap nut is a kind of hardware fastener. It is also called tree nut, acorn nut.
The cap nut is a type of fasteners. And it gets its name from its shape.
There is a cap shape above a hex head nut. So it is also known as hex head cap nut.
Acorn nut is its common name and it is also called a crown hex nut, blind nut, cap nut or domed cap nut.
It is a nut that has a domed top to cover the threads.
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