Why do errors occur when using a digital Rockwell hardness tester?

Taiwan Nakazawa Co., Ltd. answers for you: Possible causes of error include: the error in hardness testing for inclined, spherical, and cylindrical parts is greater than that for flat surfaces.
When the indenter presses into the surface of such parts, the resistance around the indentation point will be smaller than that of a flat surface, and there may even be phenomena of deviation or slipping. As the indentation depth increases, the hardness value decreases. The smaller the radius of curvature and the greater the slope, the more significant the decrease in the hardness value. Diamond indenters are also more easily damaged. For these types of parts, a dedicated workbench should be designed to ensure the workbench and the indenter are concentric.