Boring spindle with automatic tool liftoff

A spindle unit for the fine boring of a workpiece, comprising:

• a spindle housing;
• a shaft received in said housing and formed with a tool end and a drive end;
• at least two axially spaced main bearings received in said housing and journaling said shaft therein, one of said main bearings being disposed proximal to said tool end and the other of said main bearings being disposed proximal to said drive end;
• at least one further bearing received in said housing proximal to said tool end and constructed and arranged to take up the journaling of said shaft, said further bearing having an inner running surface having a center offset from the axis of rotation of said main bearings whereby, during the boring of said workpiece, the main bearings are effective to journal the shaft and said further bearing is received with radial play in the housing while during return movement of the shaft, the said one of said main bearings being received with radial play in the housing while said further bearing is effective to journal the shaft about its axis;
• a respective frustoconical support ring for at least one of the bearings proximal to said tool end, the radial play of said one of the bearings proximal to said tool end being produced by axial displacement of said frustoconical bearing support ring;
• a hydraulic piston operatively connected to said bearing support ring for axially displacing same;
• and
• a spring bearing upon said bearing support ring in a direction opposite the direction in which said piston is effective.