A very simple replacement knob for broken, stripped or missing lamp knobs. Prints in around 20 minutes (at .15mm layer height, 100% infill), requires about 2g of filament, and costs of about $ 0.10 USD.
I designed this awhile back for the numerous "antique" lamps around our house that my wife and I have inherited over the years from our parents and grandparents, that have broken, stripped or missing knobs. The lamp knob .stl is for a threaded 3mm shaft, but I've used the Autodesk Fusion 360 model to adjust the thread and knob parameters for a number of different lamps.
If this particular lamp knob .stl doesn't work with your lamp, I've included the Autodesk Fusion 360 .f3d model file for the threaded 3mm version to allow customization of the knob for your application if desired.
Designed using Autodesk Fusion 360, sliced using Cura 3.4.1, and printed in PLA at .15mm layer height and 100% infill on an Ultimaker 2+ Extended and an Ultimaker 3 Extended.