Thereby preventing the entry of cutting debris into the gap between the inner end surface of the master bath and the chuck cover. The top plate 310 is installed at the center of the end surface 112 of the chuck main body 110. The top cover 360 moving in accordance with the movement of the master bath 120 when the chuck is opened and closed is provided in the three top cover guide concave portions 330 of the top plate 310. The top cover 360 is in contact with the center side end portions of the celestial surfaces 125 and 126 on both sides in the width direction of the master trough 120 and the center side end portion of the stern 240 of the stair attachment claw 200 therebetween, The gap generated at the side of the center of gravity is always followed by the movement of the masterbatch 120 and is always closed.
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