Flatness is an important quality criterion for sharpening stone.
Here we introduce a simple method for checking the flatness of sharpening stone.
Tilt the vernier caliper so that its bottom surface forms a line that contacts the sharpening stone surface. Hold this line up to the light and check gap between the vernier caliper and sharpening stone. If the gap is not straight or of varing widths, that means the surface is not flat. Test several locations on the surface of sharpening stone to ensure the accuracy of the test results.