After it lifts a splinter. Beyond that, with high rig count osage, where you need a magnifying glass to see the rings, I have learned that a solid ring isn't needed, close enough works. Just make certain you dont feel any gouge9 or sharp dips, all violations need to be feathered out for as long a distance as you can. If its flat stave with no knots, you can just flatten the back with a belt sander and get to work.