The Chicago pneumatic 3/8-inch drill is much like an electric drill, except that it uses air from an air compressor to power the drill. The chuck on a Chicago pneumatic drill is typically a keyed chuck, but this can be replaced with an after-market keyless chuck. The chuck is held in place by a set screw through the back of the chuck into the drill's drive shaft. Once this screw is taken out, the drill chuck can be loosened and removed.
Remove the air hose from the coupling on the bottom or rear of the drill handle. Grasp the coupling and pull away from the drill, which will disengage the coupling and remove it from the drill.
Insert the chuck key into the drill and twist counterclockwise to loosen the jaws and remove any bit in the chuck. Once the drill bit is removed, twist the chuck counterclockwise until the jaws are open to their widest point.
Peer inside the opened jaws of the chuck and locate the set screw at the back of the chuck. Insert an Allen wrench into the screw, and turn it clockwise to remove the screw.
Place the chuck key back into the chuck. Position the drill flat on a work table, and tap the chuck key with a mallet, turning the chuck counterclockwise to break it loose from the drive shaft. When the chuck breaks loose, remove the chuck key.
Turn the chuck counterclockwise until it is removed from threaded drive shaft.
Before working on repairing your drill, always disconnect the tool from the power source (in this case, the pneumatic air hose).