This is the last tool I made, it's a depth gage.
It's a quite classical design, but a feature thing I wanted is to be able to lock the measure with a single hand.
That's why I put this large brass knob, so that I can turn it with the thumb, while holding the tool in my right hand, and possibly holding the piece I want to measure in the left hand.
Here I am measuring the location of the tone holes on a flute that I made. I gauge the hole using the right drill bit, then measure with the depth gauge from the bottom up to the drill bit. Just need to add half the size of the bit to get the position of the axis.
Obviously, I need both hands to do that, hence the thumb knob.
To get minimum friction of the rod in the body of the tool, I actually drilled the hole larger and put some boxwood bushes at both sides, so that it slides easily, but with no play.
It's made out of oak, and the rod is a painted steel rod, salvaged from I don't remember what.