STEP FEED MACHINING METHOD

PURPOSE:To prevent breakage of a drill by providing a slit ring between a drill and a spindle, detecting the load on the drill as the displacement of a deformation ring in the radial direction, stopping and reversing the drill when its value exceeds the specified set value, and re-driving the drill forward after the load is removed. CONSTITUTION:When a torque is applied to a drill 6, the center ring 10b of a slit ring 10 is displaced in the radial direction with a ring connecting point 1 as a fulcrum. The amount of the displacement (equivalent to the torque value of the drill) is detected by a non-contact displacement gauge 7a and its peak value 9a is held by an overload detector 9. When the peak value 9a exceeds the predetermined value, an overload signal 9c is output to an NC control device 16 and a stop and reverse command signal 2a are given to a cutting motor 3. When the cause of the overloading is eliminated, cutting is re-started and continued. By replacing the above process automatically, breakage of the drill 6 can be prevented and reliability of boring can be increased.