DIAGNOSIS Internal corrosion or debris has locked the chuck's jaws. This happens after exposure to moisture or if metal shavings worked their way inside during use.
FIX Break the corrosion free.
- Put a large Allen key or chuck key in the jaws.
- Gently tap the end of the key with a hammer in the direction to *open* the chuck (counter-clockwise). The shock often breaks the bond.
- If it moves slightly, spray penetrating oil (like PB Blaster) into the seams and work it back and forth.
IF THAT DIDN'T WORK If tapping and oil don't free it, the chuck is permanently frozen. You'll need to replace it, which may require cutting it off if the left-hand screw inside is also seized. Use a Dremel to carefully cut a slot in the chuck body and split it.
HEADS UP Do not put the drill in a vise and apply extreme torque to the chuck body. You will twist and damage the spindle, which is a much more expensive repair (requires a new transmission).