High-speed cinematography of supersonic mixing layers

Billet milling equipment including first frames which are laid on a traveling path of a billet, a pair of upper and lower clamping frames which are movable so as to follow a deformation of the billet and which are installed on the first frames, and milling cutter units which normally follow a surface of the billet to be cut in correspondence with the deformation of the billet and which are installed on the clamping frames in a manner to be capable of advancing or retreating relative to the surface to-be-cut. On the basis of a signal from a flaw detector, a milling cutter of the milling cutter unit which is following the deformation of the billet is rotated in pressed contact with the surface to-be-cut of the billet, whereby a flaw is removed by the cutting.