How Operator Influence Accuracy of Microhardness Tester

No other discipline of hardness testing is influenced as much by the operator as microhardness. Although this is true for both Vickers and Knoop testing, Knoop testing with its fine, seemingly endless tips is the most susceptible.

In general, the ability of an operator to accurately and repeatably resolve the ends of the impressions most often is the cause of error. Getting two operators to agree exactly, when measuring the same impression, is indeed rare. This problem is often masked by users performing daily verifications of their machines.

Here, operators can take their time measuring these impressions on test blocks of a known hardness whose test surfaces are typically in the optimal condition. In production environments, operators are sometimes rushed to perform tests and get parts out the door. All the care that was taken to ensure the proper results on the hardness standard is nowhere to be found.

Additionally, it can be a mundane and tiring task looking through the eyepiece for any period of time. Often, the operator mentally knows the proper dimension of the impression and, when he goes to measure it, he “sees” that value, which is human nature. Proper focus is critical in achieving accurate results. As blurriness increases, so does perceived image size. Consistency of focus helps increase consistency of results. Most automated systems feature some means of focusing automatically, virtually eliminating this concern.

Recording and converting results from microns to Vickers or Knoop hardness numbers is another common source of error. Digital microhardness testers that utilize optical encoders that assist with measuring help to eliminate this error. However, they do not find the impression ends. To help eliminate these problems, cameras can be attached to virtually any microhardness tester, as well as most of the “vintage” models. These cameras feed an image to a computer, which enables the operator to view the image on the computer’s monitor.

Here, Vickers and Knoop impressions can be measured manually by clicking the mouse on the corners of the impression, or in more sophisticated software, the computer will perform a form of gray scaling to automatically determine the tips of the impression and display the hardness value, converted scale (HRC, etc.) and typically the average diagonal.

The enhanced magnification afforded by the camera and the monitor enable the operator to more precisely resolve the tips of the impressions. Also, viewing impressions on the monitor is far more comfortable and relaxing than squinting through an eyepiece, thereby reducing operator fatigue.

Modern modular systems can go from a basic “click on the tips” type of a system all the way through to full automation. This enables users to seek the sophistication level that best matches their needs and, should requirements change, allows their system to grow. Operating in a Windows environment, this type of system collects, files, and stores the data, eliminating errors of transposition or data co-mingling.

Automatic measuring is one of the most popular features of this system. Operator influence over the measurement of impressions is eliminated, as is the sometimes time-consuming process associated with measuring. Once the operator clicks on the “measure” icon, the image is captured and the impression is measured in a blink of the eye. Actual diagonal length, hardness value, and converted scale are displayed on the screen, and saved to a file.