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.
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