The present disclosure includes a shaft and spindle assembly for retaining a part in a part processing assembly. The part is retained on the shaft via a downward force from a part hold-down assembly. The shaft is retained in the spindle assembly that is coupled to a turntable of the part processing assembly. The shaft includes an annular step that abuts against a portion of the spindle assembly to block downward movement of the shaft when the downward force is applied. In this way, the part is retained in a precise location relative to processing nozzles of the part processing assembly even after multiple parts are held down by the part hold-down assembly and processed.