reflective disc and sensors.

make before break rotary switch

The OP said

My plan is to use stationary Hall sensors and a rotating disc with magnets.

Hood

have the magnets set up on a disc so they dont have a gap, using that flexible magnetic strip

Yes a possibility but then you would have to be accurate and with the backlash in the gears it may not be so good, but possibly worth a try to save on a switch if you needed to.
Hood

4 sounds good to me too

there will be a fair bit of work to get them to fire at the same time anyway (a read delay in the macro possibly). An 8 position rotary switch would be the easiest to work with.

Possibly if it would stand up. I have just written a ladder for a friend who has a duplomatic 8 pos turret and it uses a rotary switch with 8 positions so I suppose they can last, then again I dont think its a standard off the shelf switch they use.
I use a servos with indexing drive on mine and makes it a breeze

Hood

Barrafuldi use one in there 8 station toolchanger, and it's been known to fail.

My original turret used normal limit switches and it worked well, it was 6 pos and just had 3 different cams (one for each switch) and it worked well but when I first got the lathe they were a bit hit and miss. I put it down to the switches internal springs being weak, the lathe hadnt been used much in the 20 or more years before I got it, probably only about 50hours so some of the switches had likely been sitting in a compressed sate for years at a time. I replaced the limits and it never missed a beat up until a month ago when I hauled it off to replace with the new turret.
I think if I was going to use switches again I would use optos and slotted discs or possibly if space wasnt a problem some of the honeywell limit switches that I use on the axis for limits.

Hood

Gytarr,

You mention avoiding PLC's but my take on this is that a PLC could really simplify the design. I say that having 12 years in factory automation that almost eveything ran on PLC. My concept on this would be to use a combined mechanical and electrical/electronic solution much like the orginal turret. If you could index by turning the turret with a stepper or other motor to get you into the near vacinity, turn the motor off, then use a simple air cylinder to clamp the tool table secure with air pressure, you could get a secure repeatable position on every tool change. This is a bit oversimplified but you get the concept.

Regardless of my opinion I am fascinated by the concept of a tool turret and would like to keep tabs with you on how you work this out. Gang tooling is killing me!

Robert