I bought a 3D printer DIY kit back in 2012. The idea was to make a custom prosthetic hand for my amputee father. I downloaded a fully articulating human hand model from thingverse, printed all the pieces of the digits, assembled it, intentionally slammed it in the bathroom door, and it broke. So I spent the next year or so experimenting with every kind of polymer that was available on the market at the time, doing stress tests, measuring elongation, chemical resistance, impact testing, etc.. Some were much stronger than others, but all of them suffered the same flaw. Printed parts, due to the way they're printed layer-by-layer, always have a weak axis. There is no molecular bond between layers; just a "got hot and smooshed together" sort of relationship, resulting in a "grain" like wood, and for anything more trivial than chess pieces you have to devote some careful thought to what axis you'll print a part in, so that the resultant grain is oriented in the direction where it's least like to get broken. But nevertheless it will get broken eventually, if your part is expected to handle real-world abuse from all angles. Something like the human hand (what marvel of engineering! - or bizarre coincidence) endures forces in all axes all day, from within and without. You can dip it in oils, solvents, leave it out in the sun, beat on things with it, absent-mindedly smash it, and it's no worse for wear (usually). It self-repairs, and with no way to replicate that, a prosthesis must be nigh indestructible. No way to do what I wanted with it, so I decided I need a CNC mill to make my parts out of solid materials. 8 years later, still never made a hand (turns out dad wasn't that interested), but now I'm neck deep in a metal machining hobby that I never foresaw myself having. Still have the 3D printer, sitting in a dusty box somewhere. Much prefer to tinker with the mill and the lathe. I'll be building a CNC plasma table this year. It's odd how 3D printing got me into machining; I think the more common avenue is 180 degrees out.
