The push button used to hold the collet in place whilst the spanner is used to loosen it engages in a slot in the shaft. Rotate the shaft slowly while pressing firmly on the button. If the slot is not located after a full turn of the shaft, the button must have seized. If so, you will need to dis-assemble the router to free it. There are articles on the net on how to do this for most routers.
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The push button used to hold the collet in place whilst the spanner is used to loosen it engages in a slot in the shaft. Rotate the shaft slowly while pressing firmly on the button. If the slot is not located after a full turn of the shaft, the button must have seized. If so, you will need to dis-assemble the router to free it. There are articles on the net on how to do this for most routers.