This is a replacement knob and platter for the common J38 morse code telegraph key. The original had an embedded threaded rod to screw into the key arm. Obviosly that can't be 3D printed, so you'll have to find an appropriate screw and thread it up from the bottom of the arm as pictured. It should extend no more than 3/8 of an inch or 1cm above the arm.
The platter slips over the screw, the knob should thread right on. If it's too loose or too tight, that's due to dimensional inaccuracy of your printer. You can scale the part 1-2 % in your slicer before printing to compensate.