This is a very useful fix that can be designed in very little time, once all the correct measurements are taken. It was designed to be press fit on the mating component so that it wouldn't slip off after repeated handling and fiddling. The texturing on the sides isn't strictly necessary, but it does make the knob easier to grip at the cost of increasing print time. Each of these knobs took half an hour to print at .15mm layer height and 20% infill, but making the knob a simple drafted cylinder would have cut down on the time appreciably.
Luckily, there are only a few measurements which characterize the entire knob, so spending a couple minutes with a pair of calipers reveals everything you need to be able to design it.
- Outer diameter of the metal shafts
- Thickness of the slot
- Depth of the slot
- Overall length of the shaft
- Travel of the "push"
Dimensions 1-3 determine the inside of the 3D printed piece, as seen in image 3. Dimension 4 controls how tall to make the entire knob. Dimension 5 is necessary because, as can be seen in image 1, the knobs need to be pushed in, in order to be used. The outside diameter can be changed as desired.
The tolerancing on this part is somewhat finicky, as it largely depends on your 3D printer's calibration and whether the printer is smart enough to realize the filament will spread out on the build plate or if it simply extrudes on the centerlines. For my printer (Prusa i3 MK2) I left the modeled dimension the same as the nominal diameter of the metal piece, and this was exactly what i needed for a press fit.