Replace the knife when the knife does not cut your media correctly anymore.
Use Océ Colorado 1640 prescribed:
Screwdriver (Torx-20) for the cover
Screwdriver (Torx-6) for the knife
Knife
For information on the prescribed maintenance materials, see List of Océ Colorado 1640 maintenance materials.
Mind your head.
Hold a piece of paper under the knife to collect any falling screws. This ways you make sure you do not lose any of the screws inside the machine.
Make sure you position the knife in such a way it does not hit the sensor.