Spindle Encoder (milling machine or anything with a pulley stack)

I have been pondering how to attach a spindle encoder to my milling machine for rigid tapping. Older vertical mills with pulleys and quills with drawbars pose a problem as there isn't much space or a good place to add an encoder. My preference would be to add a toothed wheel and optical encoders (A/B/Index). But I could not figure out a good way to implement this. I saw a lathe spindle encoder video by Kent VanderVelden, https://www.youtube.com/watch?v=0nCH2G9oAmU and decided this approach could work with my mill.

You will need to adjust the openscad script modifications to fit your pulley profile. See comments and added code starting at line 128 and line 204. Use the original OpenScad script for a pulley to fit your encoder shaft.

I designed an encoder mount which replaces the left spindle housing belt cover.