Precise Knife Straight Edge is a simple tool to test the flatness of the sharpening stone by a light slit. The Straight Edge has a two-sided bevel in the shape of a knife.
The Straight Edge provides a level of accuracy far greater than that of a conventional metal ruler. The Straight Edge is made of stainless steel of high hardness (at least 61 HRC) and is equipped with a heat-insulating holder.
To test the sharpening stone, engage the straight edge with the surface. Look towards the light source through the straight edge in contact with the stone, and try to see a light slit. If light passes through any part of the straight edge, the sharpening stone surface is not flat.
Both the straight edge and the sharpening stone should be dry during the test.