Sorry that this runs long, and apologies to Rex for hijacking his thread, but we need to think this through.
I do not buy into the notion that a microscopic ragged-edge-off-the-wheel provides a superior cutting experience on a scraper. (Or any tool for that matter.) It simply does not. The "burr" left behind from the grinding process is not usable cutting steel, it is waste metal, the deformed and unsupported damaged material (as seen under magnification) that sits on top the actual tool edge. Think of it as... sparks that didn't become sparks... (Think the edge of torn paper vs. the edge of sheared paper.) That waste metal burr is so fragile and unsupported that one can break away the most fragile parts of it with a fingertip, and the rest will disappear in a fraction of a second once the wood finds it. It serves us no productive purpose. Someone name one other woodworking cutting tool that cuts in that matter. None. Drill bits, saw blades, carving knives, router bits, jointer/planer knives... none. What's the difference between our woodworking process and all the others? For us, the wood moves and we hold the tool more or less stationary, the others it is just the opposite, but a cutter is a cutter is a cutter... It
cuts.
Think of an edge as a line where two plains come together. Ideally that line has no saw tooth pattern to it when viewed under magnification, it is a smooth, essentially 1-dimensional line. An edge has no thickness. The edge on a gouge is the 1-dimensional line where the bevel below the edge and the tool flute above the edge meet, and nothing more. Working through successively finer grinding grits results in a smoother and smoother transition line between those two plains. The grinding burr... is the sawdust on the surface waiting to be swept away, so to say.
(I'd argue that the best quality edges, regardless of the grinding angle, will be found at that line where two ultra-smooth, highly polished surfaces meet each other, creating an even smoother intersection of the two plains. But I've never heard of anyone polishing the bevels and flutes of their turning tools. Someone must. The closest we get is turners who hand-hone their skews with 600 grit or finer diamond plates. Knife makers live and die by finely polished bevels on each side of an edge, grits to 8000 and higher, and edges that make our best sharpened tools look like butter knives. Imagine today's exotic steel turning tools, how much better- sharper- their edges could be if the flutes were finely polished like drill bits, then grind the bevels on the fine CBN wheels.)
What makes one edge tough and long-lasting and another edge fragile and short-lived is the angle at which the two plains come together, how much mass is supporting that edge. A scalpel is incredibly sharp, has no discernible waste metal burr, and is ground in the single-digit degree angle range. But that edge is not strong due to the thinness of the metal behind the edge. We use turning tools with bevel angles between about 30-60 degrees- lots of beef supporting the edge, with scrapers having bevel angles up to about 80 degrees. The wider the angle in degrees, the "less sharp" an edge can be. That 80 degree scraper edge isn't going to cut, it is going to tear.
But, a scraper, when properly prepared, does not scrape, it cuts. It cuts not with a waste metal grinding burr (that we turners over time have adopted as an acceptable cutter), it cuts after we hone off that waste metal and then use a material that is just as hard, or harder, than the scraper to burnish, or roll the edge upward to create a microbevel hook edge. The exact same way a cabinet scraper or card scraper is prepared and presented to a flat board. (This does not happen from a grinding wheel.) The "relocated edge" atop that microbevel, which rubs on the surface of the wood and supports the cutting edge just like the bevel on any other turning tool, is what cuts the wood. Bludgeoning the wood off with a non-microbeveled edge that has a grinding waste burr on it is not cutting. It is tearing fibers through friction, not shearing fibers by cutting them. Not to advertise, but the Veritas "Scraper Burnisher for Woodturners" does a wonderful job of proper final tool preparation before cutting with a scraper.
https://www.leevalley.com/en-us/sho...as-scraper-burnisher-for-turners?item=05K3501 See the instructions for details on the process.
In the end, just like sanding abrasives for wood, it's up to the user to decide how fine of a grinding grit to progress through in an effort to create an "acceptable edge" for the purpose. We don't need woodturning tools to be scalpel sharp, we need them to be acceptably sharp and durable for
cutting wood effectively. That includes scrapers. Maybe we need to rename scrapers, and quit thinking of them and using them as that name would imply. Poor scrapers, all they hope to do is cut, we need to help them get to that goal.
Thank you for your time,
Steve.
