Gage repeatability & reproducibility of hardness testers

During the past 15 years, Statistical Process Control has become an integral part of manufacturing. Heat treaters are now asked to supply capability statistics on process characteristics such as: temperature, carbon potential, quenching oil speeds and agitation severity. Along with process characteristics they are also asked to demonstrate process capability on product characteristics such as: tensile strength, decarburization, case depths and hardness. As heat treaters become more involved in control charting, one of the questions that is most often asked is, ''How accurate is my hardness tester and how much variation does the actual hardness test add to my process.'' This study hopes to evaluate the precision and accuracy of hardness testing (Brinnell, Rockwell, Knoop) and how it affects process capability information. Addressed will be the issue of test block differences, operator technique, test machine resolution and how to set up control charts to test whether the hardness test is in control.