Numerical control apparatus for machine tool

For use with a machine tool having a spindle head which applies different levels of a preload to a bearing rotatably supporting a main spindle in accordance with a rotation speed of the main spindle, a numerical control apparatus for numerically controlling movement of the spindle head, comprising:

• a numerical control section for numerically controlling the movement of the spindle head;
• a spindle control section for controlling rotation of the main spindle on the basis of a rotation command from the numerical control section;
• a preload change control section for changing the level of the preload to be applied to the bearing in accordance with the rotation speed of the main spindle on the basis of the rotation command from the numerical control section;
• and
• a correcting section for outputting correction data for correction of an axial displacement of the main spindle caused due to a change in the preload level and an axial displacement of the main spindle occurring in accordance with the rotation speed of the main spindle to the numerical control section on the basis of the rotation command from the numerical control section,
• wherein the numerical control section corrects the axial displacement of the main spindle occurring in accordance with the rotation speed of the main spindle and the axial displacement of the main spindle caused due to the change in the preload level on the basis of the correction data.