Allen Key bolts - how do they make the hexagonal hole in the bolt head?

[NY]

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?

[Tim+]

I've always assumed they start with a round hole and then crimp the head around a hex former. Can't see how else you would do it.

Tim

[F Murtz]

I think you imagine wrongly, :) Probably stamped with a die or forged, there would be other methods but not as easy.

[Syd Rumpo]

Doesn't show the hex head, but it'll just be a different set of dies. Dice? Dunno.

Cheers

[newshound]

I'm pretty sure they will be hot forged.

[Bob Minchin]

There is an article in the latest Model Engineers Workshop magazine on an amateur rotary broaching technique that will make sharp cornered polyagonal holes on either a lathe or a mill. Quite an interesting read but certainly not the way socket cap screws will be made commercially.

Bob

[Andrew Mawson]

They will be hot stamped or rotary broached in quantity, but this is how I did a 'one off' :

Andrew

[Clive George]

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.

[GB]

Cheers. I enjoyed that video. Those automatic machines are impressive.

[Syd Rumpo]

I liked the thread forming best, I was expecting some sort of 'normal' rotating die.

Cheers

[Bob Eager]

I can watch 'How It's Made' (when it's on, usually Quest) for hours.

[News]

Extremely. Not knowing anything about these processes, I wonder why the bolts are cold forged, but the nuts require heat.

[Syd Rumpo]

I would guess that the bolts are cold forged because there are several stages and it would be difficult to clamp them firmly enough while hot and keep them hot through the stages.

The nuts are made in a single motion with no need to hold them other than with the hex forming die. The threads are cut cold. Also, the nuts don't have to be as strong.

Pure speculation.

Cheers

[PeterC]

ISTR, from many, many Fortnights ago, the term "cold-heading", i.e. wham-bang-goodbye mam.

[Steve Walker]

I am pretty sure that rolling threads is both faster and results in a stronger bolt or screw.

[GB]

I'm impressed that it lines up perfectly all the way round, and there's no join. :)