Office of Technology Transfer – University of Michigan

New Method of Error Compensation for Angular Errors in Machining (Droop Compensation)

Technology #2057

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Zbigniew J. Pasek
Managed By
Richard Greeley
Senior Licensing Specialist, Engineering 734-936-2093
Patent Protection
US Patent Pending


During any machining process, relative motion between the cutting tool and part must occur. In the ideal working condition, the machine tool moves to the position commanded by the machine tool controller and the machining operation commences. However, the machining operation is associated with several sources of error. More often than not, effects of these errors do not completely cancel each other out, and their net effect generates errors in machined features. If sufficient degrees of freedom are available, all the errors can be minimized or eliminated. However, in most machine tools, the available degrees of freedom are usually limited to three. Various techniques exist for reducing the angular errors associated with a machine tool; however, they are time consuming to execute and very laborious. In the case of errors due to gravity, there is no easy method to correct for such errors on hee axis machine tools that have only one actuator per axis.


Researchers at University of Michigan have invented methods to generate more degrees of freedom in a single axis machine tool to compensate for errors by employing multiple linear actuators in place of a single drive. The invention also generates an extra degree of freedom in a single axis machine tool to compensate for errors by employing two ball-screw actuators. Another object of the invention is to use two actuators on the same axis of a machine tool to generate both linear and rotary motion in order to compensate for positional errors of the tool.

Applications and Advantages


  • Machine tools


  • Reduces errors induced by gravity or by geometric inaccuracies in the structure of the machine
  • Saves labor and time