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In order to reduce the overall error in a Rockwell hardness measurement, it is
important that the different sources of error are identified and the significant
error sources be reduced if possible. This can only be accomplished by
assessing the separate parameters of the Rockwell test and verifying whether
each is within acceptable limits. The test method standards specify two
categories of methods that can be used to assess many of the aspects of the
test. They are: direct verification and indirect verification of the hardness
machine. Direct verification is a process for verifying that critical components
of the hardness machine are within allowable tolerances by directly measuring
such parameters as the test forces, depth measuring system, and machine
hysteresis. Indirect verification is a process for verifying the measurement
performance of the hardness machine by performing Rockwell hardness tests
using standardized reference blocks and indenters.
Good Practice Recommendations
• Although ASTM and ISO test method standards presently do not require
periodic direct verification of Rockwell machines, it is recommended that
both direct and indirect verifications be performed periodically based on the
usage and condition of the individual machine.
• When a testing machine fails to pass indirect verification of one or more
Rockwell scales, direct verification should be used as a tool to determine the
source of the problem rather than making blind adjustments of a machine
component or electronic offsets to correct errors.
• When a testing machine fails to pass direct verification of one or more of its
components, and cannot be brought within tolerances, it should be repaired
or replaced.
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