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.
