text: This invention relates generally to machine tools having rotatable, tool-carrying spindles, and more specifically, to a servo-controlled drive system for angularly orientating the tool-carrying spindle of a machine tool.
  Machine tools, of the type having a rotatably driven tool-carrying spindle, usually include a spindle gear train for transmitting rotational energy from a spindle drive motor to the spindle. In the past, such spindle gear trains have comprised at least two gears each keyed on a separate one of the spindle shaft and the input shaft, respectively, the input shaft being driven by the spindle drive motor. A cluster gear is journaled into the machine tool spindlehead parallel to the pair of gears and is axially slidable so that each of the cluster gear members can be moved into and out of engagement with each of the gears on the input and on the spindle shaft to vary the ratio of motor shaft rotation to spindle shaft rotation. Accurate angular positioning of the spindle shaft is accomplished by a feed back position transducer typically taking the form of a resolver whose shaft is coupled to the spindle drive shaft through one or more pinion gears. Electrical signals from the resolver, indicative of spindle shaft position, are employed by the machine tool control system to servo-control the spindle drive motor.
  Prior art spindle drive systems, such as described above, are subject to the following disadvantages. Firstly, the feedback positioning loop, that is, the resolver and the gear train coupling the resolver to the spindle shaft, invariably exhibits excessive backlash, due to play between the cluster gear shaft and the motor shaft, which often results in unsatisfactory servo-control of the spindle drive motor. Secondly, to assure meshing of the cluster gear and each of the gears keyed on the spindle shaft and on the input shaft, it is usually necessary to "creep", that is to say, rotate the spindle gear train members very slowly during shifting, to prevent locking or jamming. This is often very time consuming. Thirdly, when the cluster gear of such prior art spindle drive systems is axially shifted to change spindle speed, synchronism between the drive motor shaft and the spindle shaft is not always maintained.
  In contrast, the present invention concerns an improved servo-controlled spindle drive system which is not subject to the aforementioned disadvantages.
  It is an object of the present invention to provide an improved servo-controlled spindle drive system in which there is zero backlash between the feedback transducer and the spindle drive system motor.
  It is another object of the present invention to provide an improved servo-controlled spindle drive system having a gear train which may be shifted without the necessity of creeping the spindle.
  It is yet another object of the present invention to provide an improved servo-controlled spindle drive system in which constant synchronism between the spindle motor shaft and the spindle is maintained.