If I understand correctly, you wish to use a generic tacho. similar to those offered on Ebay without the supplied sensor.
I looked into this on a Sieg mill that I used to own. It appears to be far from straightforward.The motor was DC, there's no speed -proportional frequency in the system so just wiring one in was not an option. The solution that I found on-line involved a micro-processor and code and hence over my pay-grade, so I fitted a magnet and hall-effect sensor.
Over to the experts.