This is a dynamic knob and crank builder. Basically, measure the widest (round) part of the knob stem, then measure the flat part, figure out the shrinkage factor for your printer, and select styles, etc.

This can generate a crank as well. See the Ryobi Drill Press Crank for an example derived using this. The default settings should make a knob for the RepRapDiscount LCD panel controller.

The RepRapDiscount LCD panel knob depicted was printed in two colors on a single extruder using the M600 Marlin firmware experimental code.

Version 2, updated 24-October-2013: stem dimensions on RepRapDiscount knob were inaccurate; changed sizing for medium and larger sizes to make a more reasonable size if the stem is small.