I made a center finder thing to mark the center of a chucked piece.  I often have a bowl or hollow form that has a tenon for my chuck, but no center mark.   If I want to reverse mount it, like in a jam chuck or vacuum chuck, it’s really helpful to know where the center is.

This tool fits right down into the spindle threads, and I can tap the knob to mark the center.

I took a scrap piece of wood and carefully turned a cylinder to just match the threads.  I then drilled a hole to match some scrap rod I had laying around.  I sharpened the rod to a point and epoxied a knob onto the end.