There's a particular satisfaction to a machine that fixes itself, even when you're the one doing the actual fixing. My Ender clone has been running near enough daily through this lockdown, and last week the extruder arm let go. Not catastrophically. Just a hairline crack across the lever where the spring tensions it, the sort of thing you don't notice until your prints start under-extruding and you spend forty minutes blaming the nozzle.
The lever is an injection-moulded plastic part. Cheap to replace, except every supplier was quoting two weeks because the whole country is at home ordering bits for projects they'll abandon by June. So I did the obvious thing and printed a new one.
That's the bit that still tickles me. The broken part on a 3D printer is, more often than not, a printable part. I pulled a known-good model off Thingiverse, sliced it in PETG rather than PLA because the original failure was almost certainly heat creep softening the plastic right next to the hotend, and ran it overnight. PETG is a faff to dial in: it strings, it doesn't like being printed too fast, and it'll happily weld itself to a smooth bed if you let the first layer go down too hot. But it survives sitting next to a 250°C block in a way PLA simply won't, and that was the entire point.
Whilst the printer was occupied I had a proper look round the carriage and found the fan shroud had been quietly vibrating loose for weeks. One of the mounting ears had snapped clean off, so the shroud was held on by a single screw and a great deal of optimism. The part-cooling airflow it's supposed to direct at the print was instead blowing somewhere off to the left, which explains a few overhangs I'd written off as "just how this filament behaves".
So I printed that too. A sturdier shroud with thicker ears and a bit more meat around the screw holes, again in PETG. Whilst I was at it I added a small relief so the fan wire stops getting pinched, which it had been doing since roughly the day I built the thing.
The repaired printer is better than it was new. The lever tensions properly, the cooling actually points at the print, and the wire isn't slowly being sliced in half. None of these were design flaws exactly, more the accumulated wear of a cheap machine doing real work. The nice part is that a cheap machine doing real work is also a machine that can manufacture its own spares, given a roll of filament and an evening.
I keep a small folder now: extruder lever, fan shroud, belt tensioners, the little feet, the filament guide. Anything plastic that could plausibly fail, pre-sliced and ready to go. It feels slightly absurd to stockpile printed parts for the printer, but the alternative is a fortnight of downtime over a part that costs about thirty pence in filament and four hours of machine time I'd be asleep through anyway. The machine maintains itself. I just press go and wander off.