Tool holding spindle assembly particularly for a grinding machine

A tool-holding spindle assembly, particularly for a grinding machine, comprising:

• a power drive, for rotating the spindle,
• at least one electromagnetic radial bearing having excitation coils, an annular armature mounted on the shaft of the spindle and a fixed armature surrounding this annular armature,
• at least one electromagnetic axial bearing having a disc-shaped armature mounted on the shaft of the spindle and a fixed armature,
• at least one radial position detector comprising two pairs of detecting units disposed along two different radial directions for delivering signals representative of the radial position of the spindle,
• at least one axial position detector delivering a signal representative of the axial position of the spindle,
• a control chain for controlling the radial and axial position of the spindle, said control chain being connected to said radial and axial detectors for receiving said signals delivered thereby, and said control chain being connected to excitation coils of the armatures of each magnetic bearing, for controlling the position of the spindle in response to said signals,
• and means for selectively modifying at least one of the signals delivered by said detectors and thereby varying the reference position of the spindle for machining purposes, said signal modifying means being connected between said detectors and said control chain.