Results of a CTMA project, Volumetric Accuracy for Large Machine Tools completed in 2010, play a prominent role in a recent feature article in Aerospace Manufacturing and Design Magazine. The CTMA project was a collaboration of Siemens Energy and Automation, The Boeing Company, Cincinnati Machine (now a part of FIVES Automation), Automated Precision Inc. (API), Warner Robins Air Logistics Center, and the Naval Foundry and Propeller Center. The feature article of the October 2017 issue of Aerospace Manufacturing and Design magazine titled “CNC’s Changing Role in Aerospace Factory Automation” has a direct link to the CTMA project.
Aerospace manufacturers long ago recognized that large parts such as ribs, spars, and beams produced by machining a single monolithic part from a large block of forged aluminum plate reduce cost by reducing overall assembly time and also increase reliability. But producing these big parts requires the use of very large 5 axis CNC machine tools, many with working volumes that exceed 150 cubic feet. Tolerances necessary to ensure fit with mating parts requires precision motion control over the entire working volume, which in turn requires careful alignment and calibration of the motion axes of the machine tool. Traditional methods for measuring error were effective but time-consuming, using lasers to map errors one axis at a time for the X, Y, and X axes and measuring roll, pitch, and yaw error for all rotary axes.
Siemens Energy and Automation, a world-leading provider of CNC machine controls, had developed its Volumetric Compensation Software which ran in real time in the controller and could be used to adapt axis motion in real time but given the potential for catastrophic damage to expensive machines as well as danger to human operators, the capability was available only to select customers. Given the capability of the software to adapt motion control “on the fly,” it was an obvious candidate for the application of error compensation.
The project team produced and demonstrated a system that uses an API Laser Tracker rigidly mounted on the machine tool’s base and Spherically Mounted Retroreflector (SMR) mounted in the spindle to measure the actual position and pose of a few hundred random points over the full working volume of the machine tool. API’s proprietary Volumetric Error Compensation (VEC) software, developed in the project, uses a complex algorithm to decouple error sources and build an extremely accurate volumetric error map.
Besides the measuring system, the API solution also includes a sophisticated proprietary calibration software that uses a polynomial equation-based, kinematic model of the machine to map errors throughout the volume. To compensate the machine in real time, the software had to reside on the machine’s controller, interfaced with the controller’s real-time motion control system. Within the project, the solution was the SINUMERIK 840D Solution Line Control machine controller, which in 2010 was the only CNC control system with real-time motion control compensation capability. The project was successful, winning an R&D 100 award, and API now offers volumetric error compensation as a turnkey service.
Final report available apon request.