There's a small loop of satisfaction that's unique to 3D printing: the machine breaks, and you fix it with a part the machine itself made. A fan duct cracks, you print a better fan duct. A belt tensioner is flimsy, you print a sturdier one. The printer becomes the means of its own repair, which is either elegant or slightly absurd depending on your mood, and after a year of doing it I've landed on "both".
This sounds like a perpetual-motion machine and it absolutely is not, which is the first thing worth saying plainly.
the bootstrapping problem
You can only print the part that fixes the printer if the printer is working well enough to print it. Obvious, and exactly the trap you fall into. The classic version: the part that's failing is the one holding your extruder square, so prints are already coming out warped, so the replacement part you print to fix it comes out warped too, and now you've made the problem permanent and circular.
I learned this the hard way with a cracked X-carriage. By the time I noticed the crack, every print had a slight skew. The replacement carriage I printed inherited that skew. I fitted it, things got marginally worse, and I spent an evening confused before I realised I'd been printing the fix on an already-broken machine.
The rule I follow now: if a structural part fails, the kind that affects geometry or rigidity, beg or buy the replacement, or print it on someone else's known-good machine. Print your own replacements only once the machine is trustworthy again. Don't photocopy a photocopy.
For everything non-structural, print away. Fan shrouds, cable guides, knobs, spool holders, the little drawer of organisational tat that quietly makes the machine nicer to live with. None of it affects print geometry, so a slightly imperfect part is fine.
the parts that earned their place
A few prints have genuinely paid for themselves in aggravation saved.
- A geared extruder knob. Manual filament loading went from a fingertip-pinching chore to a satisfying twist. Trivial part, used every single day.
- A filament guide and runout-sensor mount. Long prints used to fail at 3am when the spool ran dry mid-job. The sensor pauses the print instead. The mount that holds it was, fittingly, printed.
- A proper fan duct. The stock part dumped air everywhere except where I needed it. A redesigned duct focused the cooling on the nozzle tip and noticeably tidied up my overhangs and bridging.
- Belt tensioners on both axes. The original tensioning was "loosen a screw, pull, hope". The printed tensioners let me dial belt tension with a thumbscrew, which improved dimensional accuracy more than I expected.
the material question
Here's a sharp edge people skip. The printer gets hot near the hotend and, depending on your enclosure, fairly warm everywhere. PLA, the easy default, softens at temperatures a printer can plausibly reach. A PLA bracket near the heater block will slowly sag and creep, and your carefully-printed fix deforms its way back into a fault over weeks.
For anything mechanical or anywhere near heat, I print in PETG. It's nearly as easy to print as PLA, it's tougher, and it holds its shape at the temperatures the machine actually sees. ABS is better still for heat but a pain to print without a heated chamber, so PETG is my sensible middle ground for the printer's own parts.
the honest accounting
The printer maintains itself only up to a point, and pretending otherwise leads to bad evenings. The motors, the board, the heater, the bearings, the smooth rods, none of that comes off the print bed. What you can make is the connective tissue: the brackets, ducts, mounts and guides that hold the bought parts in the right relationship to each other.
That turns out to be most of what wears out and most of what you want to improve. The expensive bits are reliable. The cheap fiddly bits are the ones that crack, and those are exactly the ones the machine can replace for the cost of a few grams of filament and an hour of its own time. A machine that mends its own small ailments and leaves the serious surgery to me. I can work with that.