Traditionally, manufacturers have relied on the physical attributes of gages and
coordinate measuring machines (CMMs) to maintain acceptable levels of quality.
For CMMs, the criteria considered most important for accuracy are the stiffness
and stability of component materials and structures. More recently, however,
with widespread use of error mapping to compensate for mechanical variations,
the focus has shifted away from these physical parameters.
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| Mechanically accurate CMMs
are composed of dimensionally stable materials manufactured to precise
specifications. |
Since the early 1980s, the development of CMMs that incorporate “virtual accuracy”
has had some significant ramifications for users. Originally, error mapping
involved software used to aid linear accuracy calibration of machine axes. Using
this method, a CMM’s software scales are adjusted by using scale tensioners.
Because this allows scale accuracy to be equated with a laser interferometer
readout, it improves the CMM’s accuracy without compromising its mechanical
qualities.
Soon after it was introduced, however, CMM manufacturers began using error
mapping to compensate for more extensive mechanical inaccuracies. This allows
builders to reduce costs by designing machines with considerably less precision
than mechanically accurate machines. Error mapping also requires more frequent
intervention by the OEM to maintain the accuracy of “locked” CMM maps that are
inaccessible to third parties. This can create higher levels of uncertainty and
inconvenience for users.
“Updating map parameters requires more time for the calibration process
because it frequently requires verification of other related parameters,” says
Otto Geiseman, president of Wenzel America, Ltd. (Wixom, Michigan).
Additionally, previous calibrations of a mapped CMM may not be as complete as
the user assumes. “Since the mapping process is so laborious, it’s not uncommon
to find residual map files that don’t correlate with the calibration date,” says
Mr. Geiseman. In certain cases, therefore, updating a mapped CMM can consume as
much as twice the amount of time necessary to calibrate a mechanically accurate
CMM.
Wenzel applies a contemporary approach to the traditional quality paradigm.
The company has introduced a line of CMMs that emphasizes high-precision
mechanical components as an alternative to error mapping systems. In place of
computer algorithms, these CMMs incorporate structures composed of stable
materials that are well suited for metrological applications. To achieve high
accuracy, for example, the CMM guideways are precision ground and hand-lapped by
technicians using autocollimators to monitor their work. Combined with
high-precision bearings, this technique produces an extremely accurate machine.
According to Mr. Geiseman, materials commonly used in the manufacture of
components for mapped CMMs may be less than ideal. “Today, most CMM guideways
comprise a combination of materials, with aluminum being the most prevalent,” he
says. “While aluminum does provide some advantages in the manufacturing process,
it’s a soft material that creeps and deforms easily. It’s probably not
sufficiently stable for metrology purposes.” Besides being a less stable
material in terms of weight, aluminum has a coefficient of expansion almost 4
times greater than granite.
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| Monitored by
surface-measuring equipment, hand-lapping operations produce extremely
accurate machine guideways. |
Another important factor is that CMM mapping obscures the mechanical
parameters of a particular measuring machine. “During mapping, the raw
inaccuracy is measured but never revealed to the user,” says Mr. Geiseman.
Because no governing body exists to sanction metrology standards for the design
and accuracy of error maps, users face considerable uncertainty when maps are
replaced.
These factors increase the appeal of a more conventional approach to
measurement that is based on meticulous hardware design instead of software.
Wenzel subscribes to the belief that a highly accurate machine is still the best
means to ensure measurement quality. “Unlike mapped CMMs, mechanically accurate
CMMs have nothing to hide,” says Mr. Geiseman. “Users have an intrinsic right to
know if a CMM has zero compensation or 10, 20, 30 times more inaccuracy than the
OEM’s stated values.”
Article courtesy of MMS Online.
