After converting from white wheels to a 180 grit CBN a couple years ago, I started wondering about finer grit CBN. I actually inquired here about it and got a mixed bag of answers but some folks had them and liked them. I did end up buying one (8" on a Rikon 1 hp slow speed grinder) and have been using it a while. I've not been compiling objective measurements comparing the edges from 180 and 600, these are just my subjective observations.
It's surprisingly aggressive. I guess all CBN's are when they're new, but just because it's fine doesn't mean it doesn't remove metal. It also generates notably more heat when grinding. I've more than once blued some steel when accidentally pausing. This isn't supposed to affect modern high speed steels, but it does get your attention.
The tool rides very smoothly on the wheel, so it's easy to use a light touch when grinding -- there's essentially no "bounce" from the abrading of the steel. The 180 grit CBN is way, way smoother than the 120 white wheel I used to use, but the 600 is notably smoother than the 180 grit CBN. If you have repeatable setups on your grinder, you can sharpen with removing very little steel. Using a 600 grit does make it a little more important to get a reproducible setup, because it takes longer to grind to the edge if you set up a little bit on the heel.
I like the edge. I hear statements that the edge from a coarser grit lasts longer. I don't really have an opinion on that from my experience, there are so many differences among woods, even woods within the same species, and I haven't tried to do any kind of actual head to head comparison. What I will say is that I've gotten some amazing cuts since getting it (which may or may not be cause/effect).
The experience that prompted me to write this came a few days ago when I was turning a sphere from a piece of mulberry, and I was taking some pictures along the way to show steps in the process, in case I put together a tutorial on the process I use for freehanding spheres (which is an agglomeration of tips I've picked up from watching the tutorials of many others). I was looking at the pictures later and saw the one below of the endgrain cut. I was just using a smart phone but for some reason the focus was just right and you could see really see how clean the cut was by the detail in the end grain. This cut was with a 1/2" Thompson V flute bowl gouge sharpened on the 600 grit CBN. With the sphere in this end grain orientation, I use a high shear angle cut with the bevel rubbing, bringing the tool from near the centerline of the lathe outward (that's actually the downhill cut when the sphere is in this orientation, even though it's opposite a normal spindle cut). Second pic is a zoomed in bit of the first. The lines are just pencil lines marking the second equator of the sphere. There's a tiny bit of crushing and dust in the pores, but I'd say that's a pretty clean end grain cut. How much credit the 600 grit CBN gets I'm not sure, but the edge had to have been pretty darn sharp.
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I'm not at all suggesting a 600 grit CBN is any kind of necessity. But I have become convinced it does give an awfully keen edge. I will suggest that it's hard for me to imagine anyone that's currently using white wheels not being happy with converting to a CBN wheel, and I would recommend the 180 grit as the first one to get. The 600 grit wheel I have is a Woodturner's Wonders. I've no reason to think it's any different than others, but I've no way to know.
Best,
Dave
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Originally Posted by Allen Mattsen
