Spindle speed sensor for a Bridgeport 2J head

And you completely lost me on the "top drive" and "back gear". The bottom pulley is fixed to the spindle, how would its relationship to the spindle change?

You have a variable drive mechanism (the 2 cone pulleys) that in normal drive mode lock to a splined gear hub shaft. When you move the backgear lever, the shaft separates and becomes driven by the backgear. The vari-disk always turns a timing belt that drives a gear that engages the backgear when the lever is moved. Power is only transmitted through the lower splined gear hub. #33

www.machinerypartsdepot.com/store/1478157/page/547181

Edit:
You can see how the backgear moves up/down in the video around the 1 min mark. The gear is part of the splined hub.

Yup. I see that now. About the only place to get rotation in every case is that small gear in the transmission.

I'm supprised many people have not already done this. I'd think the Bridgeport must be one of the more common machines out there? Of course, I grew up and still live 10 miles from Bridgeport so for me, Bridgeport is the mill. Clearly, this design is not electronics friendly.

Doesn't the large bull gear drive the spindle and have the actual speed? There is of course the issue of it going up and down. Could I mount a sensor reading the bull gear teeth? Or would all of that grease affect the reading? I've seen speed sensors that read gear teeth mounted on the inside of truck differentials filled with gear oil, so it seems plausible.

Do you think all of that grease around the bull gear would affect a gear tooth sensor?

Do you think all of that grease around the bull gear would affect a gear tooth sensor?

If you look over the Pico info, I think he uses an epoxy on the sensors that is rated for grease immersion.

Doesn't the large bull gear drive the spindle and have the actual speed? There is of course the issue of it going up and down.

Yes and Yes. You'll have to look over how pico did it. The up down isn't an issue for picking up the teeth, it's the index. Pico mounts the index sensor inboard on the cover plate. So probably only picks up the signal in the up or down position.

Yup. I see that now. About the only place to get rotation in every case is that small gear in the transmission.

No. I know the illustrations are confusing. You really have to have taken the head apart to make sense of it. The big gear (bull gear)
and pressed on splined gear hub and spindle are the only 1:1 final drive components. There is a remote chance you could tie into the quill drive mechanism and make something but I gave up on that. (Spiral gear on the splined hub to quill drive) I assume that does not apply to the CNC'd version since it has a different quill casting.

I would take it apart and see if there is room to drive it off the splined spindle shaft in the quill area. The nice thing with BP's is that
you can service different sections of the head without taking it all apart.

Looking at a video of the 1J head apparently, that one is packed with grease also. The big difference between the 1J & 2J is the bull gear on the 1J is stationary where the 2J, the bull gear raises and lowers. I'm wondering if there is a sweet spot where I can mount the sensor to catch the top or bottom of the tooth in both positions? Or I can make the sensor raise and lower with the bull gear, but now things get a little more complicated.

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The big difference between the 1J & 2J is the bull gear on the 1J is stationary where the 2J, the bull gear raises and lowers. I'm wondering if there is a sweet spot where I can mount the sensor to catch the top or bottom of the tooth in both positions? Or I can make the sensor raise and lower with the bull gear, but now things get a little more complicated.

I stand corrected on the J1 but that does not apply to your machine, focus on J2.
You could probably add 2 index sensors. One at top, one at bottom if you want index for backear and for normal. I would not try to make a sensor that travels. It's hard enough to adjust the up/down travel on the back gear.

Try to dig up some docs on your machine specific quill or take it apart. I'm not ruling out other possibilities. I hate the messy backgear and I would go there as a last resort.