Ok, this is a pretty stupid project. Felice uses the end of a stepped ring mandrel to make her Sterling Silver Fancy Loop Hoop Earrings but was having problems getting the top and bottom features in line. She asked if I could scribe a line across the center of the face. So I did.
I put the end of the mandrel in a vee block, and supported the far end with a Starrett Jack.
I brought the scriber of the digital height gage down onto the top of the mandrel with a .002" thick piece of paper in between, until the scriber grabbed the paper.
I then zeroed the gage, moved down .002, then zeroed again and moved down .499", which is half the diameter of the mandrel end.
I moved the scriber across the face to make a line.
A nice line on center. Hard to see in the pictures, but much more pronounced in real life.
Now Felice can make sure things line up.
I then remembered that I could use the center finding head on my Starrett Combination Square to do the same thing in about 30 seconds. Oh well...