A rock-boring machine of the type comprising rotatable headplate having a plurality of cutting units mounted on the front face of the headplate. Each cutting unit includes a cutter journalled for rotation on a shaft. The ends of the shaft are hexagonal in cross-section and are supported in complementally-slotted saddles to prevent rotation of the shaft in the saddles. The shaft is locked in the saddles by means of a single locking member at each end of the shaft. Each locking member is secured to a saddle and is adapted to bear against one of the surfaces of the hexagonal ends of the shaft.