DIY spindle encoder problem

Physical width of Teeth and slots

This is the most critical factor. The teeth and slots must be large enough to be
properly recognized by your photointerrupter. My photointerrupter has a slit width of 1mm.
you see in detail what happens when an obstacle moves through the slit
of the photointerrupter. This behaviour has influence to the minimum width of your stripes
and spaces. This in turn means that you have to increase the diameter of your
index disc when you want to have more pulses per turn. And the maximum diameter depends
on how much space you have. As a thumb rule the minimum witdh of your teeth and
also of your slot should be two times the slit width of the photointerrupter.
Much smaller teeth and slots will make you problems in obtaining a reliable signal.
When you run into this problem (you need more pulses per turn but cannot increase the
diameter of your index disc) then choose another photointerrupter with smaller slit width.

Right, that is good advice: make the disk as large as possible to
"swamp out" the effects of detector/source edge detection errors

My DIY encoder. forum.linuxcnc.org/26-turning/33983-maxi...encoder-speed#105521 . Slots were 1.6mm wide at about 32mm radius, with 60 slots in the disc. 3000rpm no prob with parallel port. The index pulse was a piece of white tape on the disc, that tripped the standard Sherline tach encoder.

I am working away from home, so have a big backlog (this message is being sent from an RPi plugged in to the hotel TV)
Equal slot and tooth widths is something to aim for. You can have any number of slots, so choose the slot number to suit your tool.

Thank you for your commitment and help