A quality straight edge is invaluable for determining the flatness of your stones
A Quality Straightedge
Ordinary rulers are not effective to measure the true flatness of sharpening stones. Rulers are just not designed for that specific purpose, so the edge isn't nearly flat enough under the best circumstances. In addition, any flexibility in the ruler will make the issue even worse.
Our Premium Straight Edge is made of solid steel 5mm thick (that’s just under ¼”) and will not bend. One edge is beveled to provide a crisp reference surface, and it is also marked with a full 12” scale, divided to 64ths, making it a useful measuring device as well.
We recommend using the beveled edge for determining the flatness of your stone. That side is manufactured to an accuracy of .001", which is more than accurate to make sure your stones are flat.
A Quick Check for Flatness
It is important to keep your sharpening stones flat to get the best results. But how do you determine the flatness of your stones? One of the best ways is to use a quality straight edge. Hold the straight edge against the stone, then hold them both up to a light. If you see light between them, then the stone is not flat.
Alternatively, you can set the straight edge against the stone and then attempt to slip a piece of paper between them. If the paper slides freely between stone and straightedge, then the stone is not flat.
Q. Is this used to measure the flatness of waterstones or all sharpening stones?
A. While waterstones are more routinely flattened than other stones, this straight edge can be used to determine the flatness of any sharpening stone.