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Working in a ball-bearing manufacturing plant in 1919, Stanley P. Rockwell
invented the Rockwell hardness test as a tool for obtaining a rapid and
more accurate measure of the hardness of ball races(1) Soon after,Charles H.
Wilson expanded on Rockwell’s invention, and he advanced the
Rockwell hardness test into what is today the most widely used method for
acceptance testing and process control of metals and metal products. Since
its development, the popularity of the Rockwell hardness test has steadily
grown. The Rockwell hardness test continues to be applied as a tool for
assessing the properties of a product while the tolerances on the acceptable
material hardness have become tighter and tighter. The once-thought-of
manufacturing tool has developed into a metrological instrument. To achieve
meaningful measurement results in these circumstances, it is important that
the user make every effort to reduce measurement errors. This is more easily
accomplished when the influences contributing to the error in a Rockwell
hardness test are known, and there is an understanding of what can be done to
reduce these errors. Adhering to “good practice" procedures when performing
Rockwell hardness measurements and calibrations is a crucial step to reducing
measurement errors.
The purpose of this Guide is not to specify the requirements for conducting
a Rockwell hardness test. Test method standards published by national and
international standards writing organizations, such as the American Society for
Testing and Materials (ASTM) and the International Standards Organization
(ISO), provide specific requirements and procedures for Rockwell hardness
testing. The intention of this Guide is to explain the causes of variability in
Rockwell hardness test results and to supplement the information given in test
method standards with good practice recommendations. Although this Guide
is directed more towards the users of Rockwell hardness having the greatest
concern for accuracy in their measurements, much of the information given
is also applicable for users that only require test results to be within wide
tolerance bands, where high accuracy is not as critical. It is recognized that
Rockwell hardness is often used for testing non-metallic materials such as
plastics; however, this Guide is primarily applicable to the testing of metallic
materials.
This Guide also provides recommendations for conducting verifications of
Rockwell hardness machines based on the procedures specified by the test
method standards. Some procedures recommended by this Guide exceed
current requirements of the test methods; however, they can be very useful
in helping to determine and limit sources of measurement error.
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