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28 AEROSPACEMANUFACTURING | jANUARy 2010
axes of
ACCURACy
High tolerance machined parts: Northrop Grumman is encouraging the use of VCS to achieve high tolerance machined parts for the F-35 Joint Strike Fighter
Although it’s still true to say that you can’t make a silk purse out of a pig’s ear,
Dr Neil Calder discovers it’s now at least possible to make a good CNC machine
tool significantly more accurate.
N
othing in life is ever quite what it plus three angular errors from rotary and interconnected. The 840D sl
seems, and similarly no machine axes. In larger machines, the volumetric (solution line) VCS corrects the position,
tool axis is ever perfectly straight error caused by an unfavourable movement and orientation vector at
or aligned. Once you add in the effects constellation of the axis positions can the tool centre point, resulting in a
of rotational axes in 5-axis systems, and reach several hundred microns. substantial improvement in volumetric
the long cantilevered axes found in larger The Sinumerik 840D control, accuracies for 5-axis machines with fork
machine tools, then inaccuracies at the which for some years has been one of head kinematics. Absolute accuracies
tool centre point really do start to build the mainstays of the high precision of between ±50 – 75µm have been
up. In the more craft intensive precision machining sector and something of an demonstrated in practice over long
machining industry of previous times, unofficial industry standard for 5-axis machine tool beds of some tens of metres.
the black art of an operator’s knowledge systems, can now provide realtime
of a particular machine’s sweet spots internal compensation for geometric A blueprint for precision
could achieve increases in accuracy and feedback errors. The ‘internal’ bit The work here has significance for
beyond the machine’s nominal. Now is important here as the algorithms the entire F-35 Joint Strike Fighter
there is an electronic sixth sense that for compensating for deviations in programme supply chain as it is being
enables this. drive and encoder positions from their taken as the technical standard for
Siemens has a solution to enable nominal values are achieved within the achieving required tolerances on
optimised machine tool accuracies in controller rather than externally as in machined components. This technology
its Volumetric Compensation System, many other systems which interrupt the has been selected for use by Northrop
or VCS. Siemens cites 21 independent drive system to achieve compensation Grumman to achieve high tolerance
inaccuracies that are summed accuracy. The system, incorporating machined parts and has been the
geometrically at the tool holder: six error innovative Sinamics S120 drives, has outcome of collaboration with Siemens
types for each of the three linear axes been designed to have an architecture stretching back to 2002 with a CNC
(Cartesian positioning and orientation) which is distributed, scalable, open technology symposium in Fort Worth,
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