When I first saw Eagle’s Marking Center Finder, I thought, “Cool, that works on the same principle as Rockler’s mortise-centering router baseplate.” Looking at the PVC-made jig, I figured it’d be 5 to 10 bucks tops, but then I saw $25 price tag and figured I’d tell everyone they should spend 15 minutes in the shop and make one with a with a piece of scrap wood and a section of dowel instead.
It’s simple geometry that if you build it right, drilling three evenly spaced holes on a line, the resulting jig should be pretty accurate in finding the center of a board. And if you build your own you won’t be limited to the width of a 2×4 like Eagle’s model.
Why would you want to build one in the first place? It’s handy for marking the center of board edge when you’re laying out mortises, holes for dowels, centering biscuits, and doing plenty of other operations. If you find you’d actually want to purchase Eagle America’s marking center finder, they generously throw in a pencil for your $25.