I have already showed some pictures of this one, but as I have written an article on my site about it and to show my personal touch on this tool, here is my version of Rick Sparber's self centering scribe.
La Belle Note - Centering calipers
My personal touch is the simple 1mm hole for the scribing end. Rick does not give too much details on how the scribe is actually made in his article.
As well, in my article, I describe the theory around this great tool, and describe my process to build it accurately.
In addition, at the end of the article, you will find an alternative from a friend of mine, to make quite the same thing with a much simpler and cheap solution... (he laughed at me when I showed the high tech tool )