A common computer mouse part that breaks is the scroll wheel axle. Initially, I tried printing a replacement out of curiosity but its actually more practical than I thought it is.

The SW17 and stl files for the Microsoft 1850 mouse are attached for reference but I will briefly elaborate on the steps required for such a replacement.

Firstly disassemble the mouse and remove the shaft, remove the axle assembly and cad it out in a program of your choice and a vernier caliper.

Next, print out the part with the shaft aligned horizontally. This is important as the scroll wheel will be stressed in that way. Sand the wheel to ensure it can rotate smoothly in its housing before proceeding to the next step.

One of the biggest issues is interfacing with the encoder as the printer is not fine enough to print a hex shape at that size. To solve this issue I used moldable plastic, in particular, a single esun emorph pellet but hot glue works too. After it is applied shove it into the encoder hole and it should fill in the void and act as a key.

And its done, after throwing out broken mice in the past because of this issue I realized that its actually quite easily fixed.

EDIT(27/5/2021): The cheap pla plastic I used seemed to have broken off at the exact spot where the original broke. I suspect this is due to the differences in height between middle click button and the encoder. Instead of getting it precisely right by filing, I will be testing it with nylon filament(in particular Taulman Nylon230). When tested, it does not seem to add a significant amount of play or flex. This should add a significant amount of toughness. Will update if it breaks.