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The hardness measurement performance of a Rockwell machine does not
depend solely on the parameters assessed during a direct verification. Once
the components of the Rockwell hardness machine and indenter are considered
to be operating within acceptable limits, its overall measurement performance
must also be verified. This is accomplished by periodic indirect verification,
coupled with daily verifications. It is important that the verifications of the
hardness machine be made together with the indenter that will be used for
routine testing.
7.2.1 Verification Frequency
The test method standards specify the maximum time allowed between indirect
verifications of Rockwell machines as well as between daily verifications.
These time intervals may not be adequate. More frequent verifications may
be necessary depending on the condition of the machine, the level of machine
usage, and the required measurement accuracy. As discussed in section 6, the
verification results should be monitored and tracked to alert the user to a drift
or erratic behavior in the machine’s performance. These types of problems
may be an indication of an escalating mechanical problem.
7.2.2 Uncertainty in the Certified Hardness Values of Reference Test Blocks
When a high level of measurement accuracy is important, performance
verifications should be made using reference test block standards having as
low an uncertainty as is practical. This applies in the cases of both indirect
and daily verifications. The uncertainty in the certified values of the reference
standards used for machine verifications will contribute to the overall
measurement uncertainty of the hardness machine.
It is also important to consider to what standard the certified value of the
reference test block is traceable. For example, an indirect verification, or daily
verifications made with reference standards traceable to NIST standards, may
not be appropriate when testing materials that must meet the national standards
of another country or a company’s own internal standards. This will continue to
be an issue until international harmonization of the Rockwell scales is achieved.
Even when reference standards having the lowest available uncertainty are
used for machine verifications, it may not provide sufficient measurement
agreement in cases where a very high level of agreement is needed between
two Rockwell machines. Bear in mind that machine performance verifications
are normally considered acceptable when the measurement bias or error
falls within tolerance limits. The combined levels of bias of the two machines
coupled with the uncertainty of the certified values of the two test blocks
may exceed the level of measurement agreement that is required. This
measurement difference can be reduced, to a degree, by “correcting" future
measurements of each hardness machine based on the biases determined
from the respective machine verifications. The measurement difference can
be further reduced by performing verifications of both machines using the same
test blocks, and then, determine corrections based on the total measurement
difference. When making any corrections of hardness measurements, the
discussions in 5.3 should be considered.
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