Here's my last thought for the day: quick and dirty. Buy adhesive metal measuring tapes from Lee Valley 70539-adhesive-bench-tapes. if you buy the inch/metric scale tape and cut the tape in half lengthwise and then use the mm scale and wrap it around the base of the compound, total error error is about 1.1 degrees over 360 degrees or less because you already have part of the circle covered by your existing scale.
Good morning.! You talked about favouring a graduated ring, to slip over your tool post, rather than disk between the post and compound. I thought that the metal tape applied directly to the tool post would do as well as a ring. I said compound which should have been wrap around the base of your tool post. Tape's annealed to make that easy. I went metric because you quoted a 115 mm D. Turns out when you do the math that the metric graduations work better though. $20 solution.
They look expensive! What is the approximate price?These aren't AliEx priced but they may also give more flexibility in mounting and certainly would have better tech support:
AksIM™ Off-Axis Rotary Absolute Magnetic Encoder Module
The sparkies (said with respect....) gave me one of this family to build into a high end slip ring assembly I worked on in a past life. I'll refer you to the data sheet for the important details but I can somehow imagine feeding the output into a TouchDRO......
Lst try, then I'll assume that I'm missing something. 1 mm = roughly 1 degree if your tool post is 115mm D. 115mmx Pi(D) = circumference of 361.1, an error of .003/division. Once installed, you can reference from your existing graduations. Write major divisions on the tape, if you want. No math, just counting graduations.
They look expensive! What is the approximate price?
I've been thinking about this since the first post...it's still not like a ring marked in degrees with bold numbers at every 15 degrees, even bolder at 30, and still bolder yet every 90. In my opinion, that is how all lathe compounds should be marked.
Good fortune in your journey Pilgrim.
If your mill and lathe aren't in daily use, it could be spread over several days, to mitigate the tedium.
With all the "tedium" involved in the electronics part of this thread ...I can see a job opportunity for some healthy person making etched plates for everyone of us that has a crude "Taiwanese's" scale on their import lathe....after all. there are guys that can etch/mark scope turrets' that can reliably & repeatably hit small targets at atrocious ranges . an adapto plate for a lathe should be child's play.