I have a modular synthesizer with lots of knurled nuts on 3.5 mm jacks. During use they come loose regularly, so I designed and printed this tool to help me get the nuts tighter than I can get them with my fingers. 3 chunks of 20 mm long steel rod (music wire from Amazon) are used for the center guide and notch grips. 3.0 mm for the center and 0.8 mm for the sides. The dimensions are fairly critical on this print, so you'll need a calibrated printer. Even then, you'll likely need to make some clearance adjustments different from what worked best on my own printer.