On 19/04/2016 09:43, NY wrote:
When I was tightening up some bolts that used an Allen key, I started to
wonder: how do they make the hexagonal hole in the top of the bolt that
the key fits into ?

I imagine they drill a pilot hole and then enlarge it with a milling
machine, but the corners of the hexagon are angular, not rounded as
you'd expect a milling machine to make. Or do they finish it off with a
very small diameter milling tool so the corners, although still rounded,
have a very small radius or curvature and so look to be a sharp-cornered
hexagon?

If it's been custom machined, you can often see the metal from where
they drilled a hole then pushed a tool down to make the corners. There
are six little tails all squashed down at the bottom.

For regular bolts, I'd imagine it would be pressed/stamped in some way.