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Indirect verification, as specified by the test standards, involves assessing two
aspects of the hardness machine: (1) its repeatability, or how well the hardness
machine can repeatedly measure the same value on uniform material; and
(2) its error (or bias), or how well the machine’s measurement agrees with
reference standards. The methods specified by the ASTM and ISO standards
for assessing these parameters involve making hardness measurements
distributed across the surface of reference blocks. Each Rockwell scale is
evaluated in this manner usually by testing three reference blocks per scale;
the hardness levels of the three blocks are chosen to cover the hardness range
of each scale. It is important that the verifications of the hardness machine be
made with the indenter that will be used for routine testing.
The assessment of both the repeatability and error are usually based on
the same set of hardness measurements (typically five per reference block).
The range of the measured values (maximum minus minimum) of each block
provides an indication of the repeatability of the hardness machine when
testing that specific hardness level. The difference between the average of the
measured values and the certified average values of the respective reference
block provides an estimation of the measurement error or bias.
5.2.1 Measurement Repeatability
When the repeatability measurements are based on tests made across the
surface of a test block, the repeatability value will include an error contribution
due to the non-uniformity of the test block. Depending on the degree of the
hardness non-uniformity of the block, this error contribution can be significant.
When performing an indirect verification, it is best to use reference test blocks
having the highest degree of uniformity as possible.
Good Practice Recommendations
• The repeatability of the hardness machine should be assessed periodically
and tracked over time. An increase in the lack of repeatability may indicate
a problem with the Rockwell machine such as worn parts or the need for
cleaning and maintenance.
• A better estimate of repeatability than suggested by the test method
standards may be obtained by making a set of measurements in close
proximity to each other, adhering to indentation spacing restrictions
(see 3.4.7) such that there is no influence from a previous indentation.
A pattern such as illustrated in Figure 8 is recommended. The close
proximity of the measurements will reduce the effect of hardness
nonuniformity in the test block. This procedure must not be used for
assessing the measurement error as described below.
5.2.2 Measurement Bias or Error
The indirect verification of the measurement error or bias is the final
indication of how all the errors in the test machine have combined together
to influence the Rockwell hardness measurement. Even when all the
parameters of a Rockwell testing machine are within specified tolerances,
the final measurement result can be outside the allowable limits for the total
error. This is because the errors associated with the separate Rockwell
hardness test parameters each have acceptability limits that are relatively wide.
If one were to combine all of the maximum allowable errors for the individual
parameters, the combination would far exceed the specified allowable total
error in measurement capability. Therefore, either the errors associated with
the individual parameters must be reduced to as small a level as possible so
that the combination of the individual errors does not exceed the total error
tolerance, or the individual parameters must be adjusted within tolerances to
produce offsetting errors so that when combined, the total error tolerance is
not exceeded. Both of these techniques rely on direct verification of the
adjustments.
Today, the most commonly used technique for handling measurement errors is
to make an adjustment to one or more machine components to reduce the total
measurement error. Unfortunately, since direct verification is rarely performed,
it is not known whether it is the problem component that is being adjusted to
reduce its error, or a within tolerance component that is being adjusted, possibly
out of tolerance, to offset the error. It is generally felt by some hardness
equipment manufacturers that, in the United States, a majority of Rockwell
hardness machines would not pass a full direct verification due to individual
parameters being out of tolerance and the associated error being offset by
adjustments of other parameters. This practice is not recommended, and it can
lead to problems when testing materials at hardness levels other than the test
blocks levels used for the indirect verification or when testing materials other
than the test block material.
Good Practice Recommendation
It is recommended that the as-found condition of the testing machine be
assessed as part of an indirect verification. This is important for documenting
the historical performance of the machine. This procedure should be made
prior to any cleaning, maintenance, adjustments, or repairs. The as-found
condition of the testing machine should be determined with the user’s
indenter(s) that are normally used with the testing machine. One or more
standardized test blocks in the range of normal testing should be tested for
each Rockwell scale that will subsequently undergo indirect verification.
If the as-found condition verifications fall outside specified tolerances, it is
an indication that hardness tests made since the last indirect verification may
be suspect.
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