A machine tool assembly having a rotatable element for driving a tool to operate on a workpiece and structure for driving the rotatable element in rotation. The driving structure has a first power source and first structure cooperating between the first power source and the rotatable tool driving element for selectively a) allowing the first power source to be operated at one speed to drive the rotatable tool driving element at a first operating speed with the machine tool assembly in a first state and b) allowing the power source to be operated at the one speed without rotating the rotatable tool driving element at the first operating speed with the machine tool assembly in a second state. Structure is provided for changing the machine tool assembly from one of its first and second states to the other of its first and second states.
