Use the chuck key to open the jaws of the chuck as wide as possible. Look down into the chuck and you will see a screw head. Use a screw driver to take this screw out BUT turn the screw CLOCKWISE as if you were tightening it as the bolt will have what is known as a left hand thread. With the screw bolt out put your key back on the chuck and using a wooden or rubber mallet give the key a tap as if to turn the whole chuck ANTI-CLOCKWISE. This should enable you to rotate the chuck off by hand. If it is still stuck give it a sharper tap.
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Use the chuck key to open the jaws of the chuck as wide as possible. Look down into the chuck and you will see a screw head. Use a screw driver to take this screw out BUT turn the screw CLOCKWISE as if you were tightening it as the bolt will have what is known as a left hand thread. With the screw bolt out put your key back on the chuck and using a wooden or rubber mallet give the key a tap as if to turn the whole chuck ANTI-CLOCKWISE. This should enable you to rotate the chuck off by hand. If it is still stuck give it a sharper tap.