an apparatus is provided which enables the jaws of a chuck 10 to be removed from and inserted into jaw - ways 12 , 13 , 14 using a conventional workpiece handler , rather than a sophisticated and highly accurate dedicated jaw handling mechanism . a conventional workpiece handler cannot position a jaw to close tolerances but this problem is overcome by using a guide shoe 17 which can receive a jaw from the workpiece handler in any one of a range of positions and then accurately guide the jaw into one of the jaw - ways 12 , 13 , or 14 . a mechanism 16 is provided for releasing the jaws from the jaw - ways , and for re - locking the jaws in position .