Cut an M8 thread into the hole in the 3d printed mounting plate. Just attach the sensor holder to the existing mounting bolts that fasten the 4th axis to the work bed. Add a modified steel spacer to one of the bolts that fasten the chuck to the face plate. Adjust the position of the sensor to leave a gap of no more then 2mm. Connect the NPN sensor to VCC (brown), GND(blue) and to the signal input (black) with a 10KOhm resistor to VCC.