Abstract: A machine tool comprises a support mounted for rotation about a machining axis, a first motor operable to rotate the support about the machining axis, a tool holder mounted on the support for rotation about a tool positioning axis which is parallel to and off-set from the machining axis, a second motor operable to rotate the tool holder about the tool positioning axis, and control means. For a constant diameter machining circle, the support and the tool holder are rotated about their respective axes at equal angular velocities so that they rotate as a unit. To adjust the diameter of the machining circle, one of the motors operates for a period at a different angular velocity to the other motor, thereby altering the position of the tool holder relative to the support about the tool positioning axis.