*You need any cheap laser pointer in additional to this print. *

Also known as laser knife goniometer or protractor, this tool can be easily used for precisely measure angle between knife (or any other cutting tool) cutting edge bevels. Sharpening quality can also be checked a bit.

Here is some clips of similar tools, where described how it work.

https://www.youtube.com/results?search_query=laser+knife+goniometer