M3 Screw Knob

i HAVE REMIXED THIS DESIGN TO CATER FOR M3, M4 AND M5 SCREWS - IT CAN BE FOUND AT https://www.thingiverse.com/thing:5262487.

I have been making a Drawing Robot (https://www.thingiverse.com/thing:2349232), but had problems getting the thumbscrew knob to work for me. The hole for the nut was too small (but this may be a printing problem!). As a result I decided to make my own design using OpenSCAD.
Everything is configurable, and hopefully the variables are sufficiently documented if anyone needs to make changes. I used a Nylok M3 nut to fix mine, requiring a deeper recess, but there are clues in the comments if you choose to use ordinary nuts.
The depth of the knob is calculated - depth of the screw head + depth of the nut + 3mm.
NOTE: This is offered in the hope that it may be useful to someone - I will not be developing it any further.
MINOR UPDATE: Fixed a silly bit of OpenSCAD coding - didn't affect the print, but the bad code annoyed me!

IMPORTANT: DO NOT TRY TO USE TH