Tap tool and tapping method

A tap machining method, comprising:

• aligning an axis of rotation of a spindle of a machine tool on which a tap tool is installed and a central axis of a pilot hole provided in a workpiece, where the pilot hole is machined by the tap tool into a screw hole;
• where the tap tool includes a thread section, formed asymmetrically relative to the axis of rotation, and having a blade section configured to machine a threaded groove of the screw hole, a pad section positioned rearward of the thread section in the direction of rotation, wherein the pad section does not have a blade portion, and is configured to engage with the threaded groove machined by the blade section during machining of the threaded groove so as to support a cutting resistance acting on the thread section by frictionally contacting the surface of the threaded groove, a non-engagement section configured to form a space between itself and the screw hole in a cross-sectional view in which the axis of rotation and the central axis of the pilot hole are aligned, and wherein the space has a size which enables simultaneous release of engagement between the thread section and the threaded groove and engagement between the pad section and the threaded groove when the tap tool shifts in the screw hole in a direction orthogonal to the axis of rotation from a state in which the axis of rotation and the central axis of the pilot hole are aligned;
• performing tap machining on the pilot hole by advancing the tap tool in the direction of the axis of rotation while rotating the tap tool;
• shifting the tap tool in the direction orthogonal to the axis of rotation so as to release engagement between the thread section of the tap tool and the threaded groove;
• and
• withdrawing the tap tool in the direction of the axis of rotation without rotating the tap tool.