I built/assembled a small drill press recently and used a part from a Junk Drill because I couldn't figure out what a particular component I needed was called.
What I was looking for was a way to step up in chuck size so I could accommodate a bigger bit on a rod that was a smaller size (see: 3.17mm motor shaft driving 1/4"+ bits).
I now know the function of what I was looking for, it's a chamber of gears driven by a central D shaft gear the motor shaft can be inserted into. It's the internal component used to drive keyless chucks available in most retail drills.
Does anyone know what this would be called so I could look for it properly next time? I resorted to stripping an existing finished product for a part just because I couldn't figure it out, I was originally wanting to mount a keyed chuck