Do you know what's the set screw?

A set screw is a type of fastener designed to pass through one object to connect it to another. For example, when a gear or collar needs to be fastened to a shaft, set screws are the typical solution.

In situations where registering the gear or collar to a specific location on the shaft is desirable, a flat can be machined into the shaft, which the end of the set screw then presses against to lock it into position. In fact there are a variety of different head types one can choose from, depending on the application:

Set screws can technically have heads, as seen in this conduit fitting:

However, they most often do not have heads and consist entirely of threaded shaft. (This is probably why they're called "grub screws" in the UK, as a headless set screw can be said to resemble bug larvae.)

With a little outside-of-the-box thinking, you can probably think of more uses for set screws than merely affixing things. Their recessed nature makes them ideal for anti-tampering applications, particularly when manufactured with a proprietary drive system, like this hex pin incorporated by manufacturer Loss Prevention Fasteners:

On that note, here are some set screw resources: