Machining device, machining method and cutting tool

A machining device comprising:

• a workpiece holder holding a workpiece;
• a tool post holding a multi-blade tool for cutting the workpiece;
• a feeder feeding the multi-blade tool in a predetermined feed direction relative to the workpiece by relative movement of the workpiece holder and the tool post;
• a vibrator relatively and reciprocally vibrating at least one of the workpiece holder and the tool post in a radial direction of the workpiece;
• and
• a rotator relatively rotating at least one of the workpiece and the multi-blade tool,
• the multi-blade tool having a first cutting blade and a second cutting blade that are arranged side by side along the feed direction, the second cutting blade being located behind the first cutting blade and having length shorter than length of the first cutting blade, and
• the machining device threading the workpiece to form a threaded portion on the workpiece by relatively rotating at least one of the workpiece and the multi-blade tool and relatively moving at least one of the workpiece and the multi-blade tool along the feed direction and performing multiple times of cutting processes in the radial direction of the workpiece along the same cutting path in a predetermined spiral form, wherein
• the machining device includes a controller controlling operations of the feeder, the vibrator and the rotator so that a groove machining to form the threaded portion by a first cutting blade and a second cutting blade of the multi-blade tool with the vibration in the radial direction of the workpiece and a finish machining to form the threaded portion by bringing only the first cutting blade of the multi-blade tool into contact with the grooved portion of the workpiece without the vibration in the radial direction of the workpiece are performed, and
• the controller sets amplitude of a vibration waveform, which is set for the vibrator, to a value at which a cutting edge of the second cutting blade does not come into contact with a screw bottom surface of the workpiece in the finish machining.