A processor for centrifugal processing of semiconductor wafers and similar units. The processor is specially constructed to provide an automatically extendible and retractable rotor head to allow automated loading and unloading of wafers. The processor includes a novel shaft assembly construction which includes an axially extendible shaft controlled using pressurized fluid. The pressurized fluid is supplied to the shaft assembly by a pressure supply which is controllably extendable and retractable for engagement with the shaft assembly when rotation is stopped. The shaft assembly also preferably incorporates an axial locking mechanism which holds the axially movable components of the shaft assembly in fixed axial position during rotation, while allowing release when rotation is stopped so that movable portions of the shaft assembly can be axially extended.