A calibration device is used for calibrating a position of a cutting tool relative to a machine rotation axis, such as for setting the position of the small boring tool for a CNC machine. The device includes a body with a machine contact surface, a tool contact surface, a machine rotation axis locating surface and a worm access opening. An anti-backlash worm gear assembly is mounted in the body and includes a worm in contact with the wheel. A bubble level is attached to rotate with the wheel with respect to the body responsive to rotation of the worm. At least one anti-backlash wedge is biased into contact with the worm.