New Compensation Method Improves Machine Tool Accuracy

Siemens' volumetric compensation system battles thermal, geometric errors

Joseph Ogando, Senior Technical Editor -- Design News, October 6, 2008

Thanks to thermal distortion and some slop in even the best mechanical systems, machine tool accuracy isn't always all it's cracked up to be. Over the years, the desire to improve accuracy has spawned a variety of controller-based compensation methods, including lead-error, cross-axis and 3D-spatial compensation. Siemens Energy & Automation has now come up with a much-needed addition to these earlier methods.

At the International Manufacturing Technology Show in Chicago, the company previewed its new Volumetric Compensation System (VCS). Running on SINUMERIK 840 D controllers, VCS targets the geometric errors that can throw off the location and orientation of the tool center point.

"VCS can't turn a bad machine into a good machine, but it can make a good machine better. That sounds corny, but it's true," says Timothy Shafer, Siemens' director for aerospace, one of the first industries to evaluate the new compensation method.

How much better? Shafer reports VCS has in testing typically improved accuracy by 75-80 percent, though in some tests the system has performed even better. In one recent customer trial on a portal milling machine, for example, VCS reduced the volumetric error throughout the machine's work envelope from as much as 0.016 inch to less than 0.001 inch. Shafer says all the tests Siemens and its customers have conducted took place on "fully-compensated machines" – that is, those using a range of earlier compensation methods. "We didn't use any worst-case machines for the testing," he says.

Some of that testing has taken place at Lockheed Martin, which has been evaluating VCS for use as part of the Joint Strike Fighter program. The company didn't immediately respond to requests for an interview, but one of the company's engineers gave a presentation on VCS at the SAE Aerospace Manufacturing and Automated Assembly Conference held last month in Charleston, SC.

How It Works

To create VCS, Siemens borrowed a page from the makers of coordinate-measuring machines, which have a long track record when it comes to controlling positioning accuracy. Shafer says like CMM controls VCS is based on a "21-parameter model" of geometric errors. This model accounts for all 21 linear and rotational errors that affect three-axis and five-axis cartesian machines – including those related to the machine's squareness, linear positioning, yaw, pitch, roll and straightness (see slideshow). Shafer says earlier compensation methods, while effective for the linear errors, didn't account for the rotational errors.

VCS begins with a calibration process that identifies the magnitude of these 21 error sources for a given machine. Once that work has been done and machine is ready to run, the system's control algorithms come into play. As Shafer says, these algorithms work within the controller's interpolation cycle to align the tool tip's programmed and actual position and orientation.

Siemens' VCS isn't the only attempt to come up with a volumetric compensation system. University researchers have been working on the problem for years. To take one example, the Precision Technologies Centre of Industrial Collaboration at the University of Huddersfield has come up with its own volumetric compensation software. 

What sets the Siemens system apart, however, is that it has been tightly integrated into a commercial machine-tool controller that has enough muscle to run the compensation algorithms in real-time. "Everything takes place within the closed-loop control cycle as part of the position calculation, so there's no affect on cycle time," Shafer says, adding that in this regard VCS is unlike CMM controllers which tend to apply their compensation algorithms offline.

The most likely early users of VCS will be in the aerospace industry. At first glance, it would seem that existing compensation methods would do an adequate job of addressing typical dimensional tolerances for large, machined aerospace parts. Shafer puts these tolerances at +/- 0.030 inch, which is already achievable by a half-decent machine tool running other compensation methods. Yet more accurate machines would nonetheless provide benefits, by allowing users tighten control limits, improve yields and avoid stack-up issues for mating parts. "For those reasons, there's a very strong interest in the aerospace industry," Shafer says. What's more, some large components, particularly those used in military aircraft, have dimensional tolerances within 0.005 inch, which makes an even stronger case for VCS.

Non-aerospace applications seem likely too. Shafer says VCS could just as easily be applied to the machining of large parts used in construction equipment, oil-and-gas rigs, wind turbines and more. "It could used anywhere someone has to machine large, monolithic parts accurately," he says.