This is a customizable knob, designed to be part of a featherboard or miter clamping system. Most of the knobs on Thingiverse are designed to be used with a press fit hex bolt, or to slip onto a bolt. I wanted one I could tap into, because drilling and tapping takes a lot less time than prototyping the correct size hex nut hole. A lot of the knobs on Thingiverse also have too many parameters - this one only has 6:
►knob diameter
►knob height
►shaft height (as a ratio of full height)
►number of grippy cutouts
►bolt diameter
►shaft resolution ($fn).

The following are algebraically calculated, so as to not be stupid:
►diameter of the grippy cutouts
►shaft diameter (calculated as the mean of twice the bolt diameter, and half the knob diameter)
►suggested filename (which doesn't render in the final part) - a nice feature when you are prototyping

►Print with 3-5 concentric layers, so the tap has something to grab onto.
►When calculating bolt diameter, use the inner diameter of the bolt, and then multiply by 1.035 (for PLA, or 1.050 for ABS), and add 0.5mm.
►print upside down so you don't need supports

Drill and tap with appropriate tools.

tappable_knob-d5.5x40h30x0.5n12.stl means it has a bolt diameter of 5.5mm, a knob diameter of 40mm, a height of 30mm, with the shaft taking up 0.5 of the total height, and 12 grippy cutouts. d5.5 is suitable for a 1/4" bolt

US Size     Bolt diameter
═══════════════
№1-64              2.1 mm
№2-56              2.4 mm
№3-48              2.7 mm
№4-40              3 mm
№5-40              3.3 mm
№6-32              3.5 mm
№8-32              4.2 mm
№10-24            4.7 mm
№12-24            5.4 mm
1/4"-20             6.1 mm
5/16"-18           7.6 mm
3/8"-16             9.2 mm
7/16"-14           10.6 mm
1/2"-13             12.2 mm
9/16"-12           13.7 mm
5/8"-11             15.3 mm
3/4"-10             18.4 mm
7/8"-9                21.5 mm
1"-8                    24.5 mm