A milling apparatus has a housing, a drive shaft extending along and rotatable about an axis in the housing and an axially compressible and radially expansible washer stack on the front end of the drive shaft. A tool is held by the on the shaft. A bolt projects radially in a slot through the shaft and is limitedly axially displaceable in the shaft. An actuator element shiftable axially in the passage projects axially forward out of the drive shaft and is fixed in the bolt. A clamping assembly axially compresses and radially expands the stack so as to lock the tool to the drive shaft. A spring assembly is braced axially forward against the housing and axially rearward against the bolt. A piston axially displaceable on the drive shaft in the housing bears axially forward on the bolt.