Insert an Allen wrench into the chuck and turn it to tighten up the chuck as soon as you've gotten rid of the screw. When the wrench is firmly in position, set the drill's gearbox to the lowest setting to minimize resistance from the equipments. After that, tap the Allen wrench with a mallet to loosen up the chuck from the drill. You can unscrew the chuck by hand once it's loosened. Screw your brand-new chuck in its location, tighten it with your Allen wrench, then replace the screw in the center.
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