Are you using later type Drives? As modern ones run a DC motor in the torque mode, so the tach can be eliminated.
If you really need a tach, there are several simple dedicated IC's that will produce digital to analogue signal ones I have used in the past is the National LM2907 the IC manufacturers have application sheets on their sites.
You will only need to use one of the encoder channels i.e. A or B.
Of course, you will also need the encoder for feedback in a CNC system.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.