This is a dressing wheel in use. when stones are used for grinding they can become “glazed over” as the surface becomes clogged with dust, slurry and metal filings. In use this feels like the tool you are trying to grind just skates over the surface. The dressing wheel “opens up” the stone, as well as helping keeping it true or round. A newly dressed stone almost grips the tool applied to it and drags it in rather than it skating away. Brian Alcock the old grinder who we learned from told us thi