CBN (cubic boron nitride) grinding wheels have advantages for grinding woodturning tools. They are the latest must-have tool. Unlike conventional wheels, they don’t require dressing to clean and true them, so make less dust, and they don’t wear down to a smaller diameter, supposedly being very long-lasting. This means that any jigs used with them never need adjusting after initial set-up. They produce an excellent sharp edge on the tools. They are suitable for high speed steel and hardened carbon steel, but apparently unhardened steel may clog them. I also occasionally grind tungsten carbide.
I recently invested in a 180 grit 200 x 40 mm CBN wheel from Optigrind and installed it on my VEM grinder, which though old is a reasonably good quality machine. Some small burrs from the old wheel bushes made them too tight for the new wheel. Holding the steel bushes in my woodturning chuck, I skimmed them very lightly with a graver. They would have gone in as they were, but perhaps never come out again!
I was able to keep the wheel shroud in place. People say that CBN wheels are inherently safe. They say you don’t need to guard them because they cannot burst. But this ignores the possibility of entanglement in the spinning wheel. It could catch long hair or loose clothing. Of course, those are dangerous around the lathe too. An unenclosed wheel seems likely to disperse grinding dust more widely, and because they take a long time to stop, they may come into contact with something while still spinning. So it’s best to keep them enclosed.
The wheel runs fairly true, but not as true as I hoped. There is some vibration. I don’t know if the problem is in the wheel or my grinder. The abrasive layer is very thin, so you can’t true it up with a diamond dressing tool. I found also that the grinding surface has some visible ridges and grooves. Not enough to significantly affect the grinding, but a little disappointing.
CBN wheels are fierce when new
In use, the new wheel is fierce. I understand this is normal, and am expecting it to settle down as the grit begins to wear, but when sharpening a gouge I could see it shrinking before my eyes. It quickly re-ground a heavy scraper. The tools were sharp after this, but I can’t say they were sharper than the old ruby wheel achieved. Some people recommend a coarser 80 grit wheel, but I think that would be too aggressive. I would rather take it slower when reshaping a tool and have a more gentle and controllable grind when sharpening. The wheel creates fewer sparks than the old wheel, which might make it a little harder to judge when the grinding is complete. But as the jigs are accurate, a single pass over the wheel is normally enough.
The CBN wheel is wider than the old one. This makes grinding easier as there is less tendency for careless use to allow a gouge held in a grinding jig to fall off the side. But it does make the wheel heavier, so slower to wind up to speed and slower to stop. In one way, this is helpful as I can sharpen small tools as the grinder is running down. It’s then the equivalent of a slow-speed machine (my grinder runs at high speed).
It’s nice not to have to dress the wheel to keep it clean. I hope that when the wheel is ‘run in’ and becomes less aggressive, it will be easier to use. UPDATE: The wheel has indeed become less aggressive and now I sometimes wish it worked quicker. But for sharpening, it’s fine. I do think a CBN wheel needs to go on a good quality grinder, because it’s hard to eliminate any runout.