OK, now have a look at the key developed by Arbortech to do that job. They called it the Ian Key after the employee who invented it, and since it is like the more usual Allen key.
Image source: AS170 user manual
The beautiful concept here is: the user applies torque until, because of elastic deformation, the loop just touches itself.
Clearly the size of the loop, and of the gap, will govern the torque so indicated. But those things can be controlled to acceptable precision when forming the loop from steel hex bar stock.
I have to say: this is one of the best designs I know. So simple, so clean, so durable. Elegant! Go IAN!
Postscript: likely materials and processes
Since this is a tool, the steel will have to be hardenable. So not just “mild steel” but something with more carbon content, say 0.8%, and it should be heat treated after bending. There is a lot of marketing hype surrounding steels used in commercial products which can make it hard for the beginner. Steel selection for engineering designers will be the subject of a later lecture. This source suggests SAE 4340 or SAE 4140 steels might be suitable for a wrench and therefore probably also for an Allen (hex) key.
Video of an automated slide folding machine in a production setting, making about one bend per second.Y
nathotron 