I needed a bracket to mount a small fan controller to the side of a shelf in the rack. A simple thing. A flat plate, two screw holes one end, two the other, a slight offset so the board cleared the shelf lip. I spent twenty minutes searching for one to buy and found a dozen that were nearly right, which is the most frustrating category of object there is. Nearly right means you buy it, wait for it, and then spend an afternoon with a file making it actually right.
So I did the thing the printer has been sitting in the corner waiting for me to do. I modelled the exact bracket I needed.
I'm not a CAD person by trade and I won't pretend the model is elegant. I use FreeCAD, which is free, capable, and has a learning curve like a brick wall, but for a part this simple I didn't need to understand the whole thing. A sketch on a plane, a rectangle, four holes positioned by measurement, pad it out to a few millimetres thick, add a bend by sketching the profile from the side instead. The whole design took maybe forty minutes, most of which was me measuring the board twice and the screws three times, because the one rule of printing your own parts is that the model is only ever as good as your measurements.
The measuring is genuinely the hard bit. The board's mounting holes weren't on a tidy round spacing, they were on something like 31.5mm by 18mm, the kind of dimension that exists purely to punish anyone who guesses. Calipers, not a ruler. I learned that one the expensive way on an earlier print that came out a confident two millimetres wrong in every direction.
From there it's a familiar loop. Export an STL, slice it, print it. For a flat bracket the print is trivial, no supports, fifteen minutes, a few grams of PLA. I held the first one up to the board and it was nearly right, which made me laugh, because I'd become the thing I was trying to avoid. The offset was a millimetre shy. So I nudged the sketch, reprinted, and the second one dropped onto the board and the shelf like it had always belonged there.
That's the part shop-bought can't give you. Not the saving, which is real but small, a few grams of plastic against the postage on a part that nearly fits. It's that the thing I made fits exactly, because I made it for exactly this, and when the next slightly-odd mounting problem turns up I'll solve it the same way in under an hour rather than searching listings for the least-wrong compromise.
I bought the printer telling myself it would pay for itself in brackets. It absolutely will not, not at this rate. But every time I print one of these instead of filing down something that nearly fits, it pays for itself in a different currency, and that one I'm happy to keep spending.