Daleman,
This is a very common repair to replace the motor shaft encoder pulley on top of the spindle motor.
Do you have a steel or an aluminum pulley?
Do you TSC (through spindle coolant)?
95% of the time in the field I have to cut the pulley off the shaft with a bare hacksaw blade. BE VERY CAREFUL NOT TO DAMAGE THE MOTOR SHAFT. If you ding the shaft it's ok, but make sure you clean in up good so it doesn't cause any runout on that pulley. Any pulley runout will cause spindle load fluctuation and RPM fluctuation. You'll know you did a good job if after it's back together the load doesn't move at 12,000RPM.
Don't forget to float that motor back in at 1000RPM before you tighten the bolts.
If you have TSC the rotary union runout NEEDS to be <0.0001" at the top of the union. If it's out, check the motor shaft flatness on the top surface, should not exceed 0.0002". If it does, get out the sandpaper and make it flat.
Thank you,
Phil