A machinist's version of a woodworker's tool. No I don't use it to mark metal; I have a better homemade tool for doing that. It was made as a compact tool to mark plastic for a one-off job, sadly non-paying. The body is made in two pieces, screwed together. This makes it easy to construct the square hole for the beam. I've always meant to add a steel wear strip but I ran out of round toits and the new shipment hasn't arrived yet.