PURPOSE: To control vertical oscillation of a curved cutting edge during the sitting by inserting a curved cutting edge into a support shaft and fixing it with a hydraulic pressure sealed thereinto so as to expand the diameter thereof.

CONSTITUTION: A curved cutting edge 22 grasped on both sides thereof with a spacer 21 is inserted into a support shaft 11 before a pressure oil is ingroduced into a hydraulic chamber 14 and the pressure oil is introduced thereinto to release air out of the chamber 14. With an air vent valve 29 closed, the pressure oil is fed into the hydraulic chamber 14 with a hydraulic pump 20. When the hydraulic chamber 14 is pressurized, the circumference of the fitted section 13 of the support shaft 11 is made so expansible by about 70μ as to reduce a clearance between the inner diameter of a hole 23 for mounting the curved cutting edge 22 and the outer diameter of the support shaft 11, a condition under which the cutting edge 22 is ground. This controls vertical oscillation of the curved cutting edge 22 during the slitting.