Wanna improve the connection strength between your project's 3D-printed parts, and also have nice clean surfaces? Instead of gluing bits together, or screwing plastic screws directly into your 3D prints, use strong and reusable machine screws and heat-set inserts. Heat set inserts are only a few cents a piece and have a grooved outside, with theads on the inside. Originally they were designed for injection molded parts but they work fabulously for 3D printed plastic as well.
You could use a plain soldering iron to try to heat-press these in - Heat up the iron and use it to press the insert in, wait for it to cool, then use like a hex nut that's embedded inside the plastic. We strongly recommend our heat-set insert tool for soldering irons because it's really hard to insert them straight otherwise and you risk damaging your print!
Check out this video from Noe & Pedro that shows you how to do it
Comes as a 50 pack of M3-threaded inserts that are 4mm long.