Abstract: A spindle assembly configured to be assembled at the internal taper socket of a tool holder, the assembly including: a rotating shaft configured for rotation within the tool holder; a bearing assembly, comprising a front end bearing assembly and rear end bearing assembly, configured to radially and axially support the rotating shaft within the tool holder and to enable high-speed rotation of the shaft within the tool holder; a turbine operatively connected with the rotating shaft, the turbine configured to rotate the rotating shaft; a plurality of passages configured to allow high pressure coolant fluid flowing from the tool holder to drive the turbine, wherein at least one bearing of the bearing assembly is disposed in a part of the spindle assembly which is configured to be situated in the space defined by the taper socket of the tool holder and a front clamping nut.