in a method for machining grooves , especially fir - tree grooves , which are provided for accommodating blades , on a rotor of a turbine , especially a low - pressure steam turbine , great simplification and high flexibility is achieved by the fact that in the grooves , or fir - tree grooves , material is removed by a program - controlled milling device , wherein the milling device is inserted into the groove , or fir - tree groove , with a milling tool which is guided along the places in the groove , or fir - tree groove , which are to be machined , by a milling tool positioning device , which is freely movable in space .