Sunday, November 14, 2010

How To Design A Metrology Test Artifact

Everyone who works in the manufacturing world is accountable to the
overarching watchdog of traceability.  It comes in different forms: ANSI,
ISO, NIST, and others, but the concept is the same: the manufacturing
process should be controlled, through the use of metrology, to
within published guidelines.  It is common to talk about a 10:1
rule in the 3D measurement world, whereby if a manufactured component has
a blueprint tolerance of 0.010″, the measurement system used to verify the
component should be accurate to within 0.001″ at some standard
deviation (usually 2 or 3 sigma for normally distributed data sets). 

By the same logic, when we confirm that the measurement system is accurate to
0.001″, we should use a test artifact that is certified and known to be within
0.0001″, for example.  Therein lies the design.  A test artifact that
is stable to within 0.0001″ over applicable size ranges, various temperature
ranges, with inherent physical properties capable of exposing small variances in
the measurement system takes some serious thought and planning. 

We all have an arsenal of readily-available tools to accomplish this: surface
plates, tooling balls, temperature-controlled rooms, and in some cases,
reference measurement systems such as CMMs and interferometers, but the key is
to understand the limitations of the materials, techniques, thermals, and even
the warm-up periods for the metrology equipment under test.  Things like
adhesives, body temperatures of the operators, residual stresses, gravity, and
human handling characteristics can impact the validity of a metrology
study.  We as metrologists know that extra time spent in setup and planning
can make or break our results, so why not spend the extra time when testing our
own equipment? 

If we think of the metrology artifact as “measuring” our metrology system,
then we’re thinking the right way, and we can pass this traceability through to
the important stuff-the products we’re measuring!

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