In the previous discussion of the repeatability of a hardness machine in 5.2.1, it was imagined that a small number of Rockwell measurements were made on a perfectly uniform sample of material. Now, consider that the small set of Rockwell hardness measurements were reproduced at some periodic interval, for example once a day over an extended period of time always using the same test block and indenter. As before, each set of measurements would again vary within a fairly consistent range. In addition to this within-set variation, it also would be found that the average for each day’s measurements would vary from day-to-day, as illustrated in Figure 10. This day-to-day variation is known as the level of reproducibility of the hardness machine. The variation is principally due to time dependent sources such as a change of operator or environmental factors.
The user should monitor the performance of the hardness machine over an extended period of time to assess the acceptable level of reproducibility. Subsequent monitoring can provide an indication that the Rockwell machine may be in need of maintenance or is being operated incorrectly. It is
recommended that control charts, or other comparable methods, be used to monitor the performance of the hardness machine between verifications. Control charts provide a method for detecting lack of statistical control. Control chart data should be interpreted by the user based on past experience. The need for corrective action does not depend solely on data falling outside the control limits but also on the prior data leading to this occurrence. As a general rule, however, once the hardness machine is determined to be in control, a single occurrence of data falling outside the control limits should alert the user to a possible problem.