Captive Bolt

thingiverse

Print at 1.5 scale, it works out okay, although breaking free can be quite challenging. A 1/2" captive bolt with knurled faces is what you need to print, and don't forget to use support material - scaling down smaller will negatively affect the thread spacing. Try printing at least 1.5 times the original size because it's pretty tough to get it loose after printing right now. 1.5 worked for me without any issues but I'll update the model after making a few modifications.