Patent ID: 8579561

Claim:
A spindle device of machine tool comprising: a spindle rotatably supported by a housing; a rod movably supported along an axial direction in the spindle; a collet that clamps and unclamps a tool by movement of the rod; a biasing unit that biases the rod in a tool clamping direction by the collet; a drive that presses the rod against a biasing force of the biasing unit to move the tool in an unclamping direction; a rod-pressing-force detecting unit that detects a pressing force to the rod by the drive; a rod-displacement detecting unit that detects displacement of the rod; a tool-clamping-force detecting unit that detects a tool clamping force by the collet based on the pressing force to the rod detected by the rod-pressing-force detecting unit and the displacement of the rod detected by the rod-displacement detecting unit, the tool clamping force being determined by detecting, by the rod-pressing-force detecting unit, the pressing force at a time when the rod-displacement detecting unit detects that the rod has started to be displaced; and a replacement-timing determining unit that determines a replacement timing of the biasing unit based on the tool clamping force detected by the tool-clamping-force detecting unit and a preset biasing force characteristic value of the biasing unit.