CBN Wheel

[phil harold]

The other thread got me wondering
what actually is a CBN wheel
what difference compared to a diamond lapidary wheel?

how long do they actually last?

[Ralph Lindberg]

Well, CBN is not diamonds for one, CBN stands for Cubic Boron Nitride.

The theory is "forever" as the material is not worn away. Dave (D-way) has been using one for almost two years and has not worn it out, this is after hundreds, maybe thousands of tools were shaped and sharpened.

[Reed Gray]

You could probably ask Dave at D Way tools about the specific differences. There was also an article in the AAW magazine a year or two ago. Basically, CBN is a material (Boron is extremely hard, almost as hard as diamonds) that is made specifically for grinding the harder steels. Diamond is better for grinding carbide. The diamond lapidary wheels are generally a finer grit for cutting and polishing stones, and the diamond concrete/masonry blades will work for dressing standard grinding wheels, but probably not good for sharpening your tools.

robo hippy

[Don McManus]

First a disclaimer...I''m an electrical guy, but I work with a lot of mechanical types and machinists a lot...so I'm just remembering what they told me. Anyone is welcome to correct me if I say something incorrect here, but this is how I try to visualize it...

CBN is the second hardest substance known to man, twice as hard as Alumina(sandpaper abrasive), but still much softer than diamond. It also has a thermal advantage that it does not become chemically soluble in steel, where as diamond is made of carbon, which dissolve in iron as carbides at high temperatures(ie hot sparks from grinding). IT also is more significantly more thermally conductive, so it conducts heat away, and grinds cooler.

Another way of thinking about it : While you want the hardness of the cutting/sharpening media to be harder than the material to be abraded, TOO much harder isn't an improvement, because you lose toughness(flexibility). I can recall a machinist explaining to me that 316L stainless was better cut with softer cobalt HSS tools, as opposed to hard carbides, because the carbide is more brittle, which means easier fractures at the edge when cutting a tough, gummy material like 316L.

In another case, we also were told to hone some stainless parts with CBN abrasive, rather than diamond hones. I think the issue, in that case, was that the diamond would embed further into the relative soft "gummy" steel, than break off at the microscopic level, and would not continue to cut, as it was stuck in the part you were trying to hone. The CBN somehow bonds better to the steel wheel, and doesn't break free as easily.

It may be similar to the the A2 vs O1 debate in handplane irons. The theory goes something like this: A2 will last longer on hard abrasive woods, but O1 will supposedly leave a cleaner surface on softer woods, because it doesn't have carbide chunks in the edge (think serrated knife vs. smooth blade). Now if you are cutting an abrasive wood with a high sand content, the A2 wont dull as fast because the carbides are harder, whereas the O1 steel just "deforms" at the edge, or dulls quickly, from our perspective as woodworkers.

Back to the diamond wheels, imagine the diamond to steel interface is sort of like glassy rocks in a clay binder, and the diamonds(rocks) will just break free when rubbed on something soft and gummy(tool steel). I imagine it's sort of like using a broken beer bottle to turn wood with, the edge is sharp and hard, but it just keeps breaking off at the the tip...its too brittle.

Hope this rambling helps,
Don

[phil harold]

Thanks for all the info!
It is clear to me now that a cbn wheel is the need list!

[Bob Rotche]

If one were to invest in a CBN wheel, what grit would be recommended?

[Prashun Patel]

180. It gives a grind capable of a finish cut that allows me to often sand starting at 150-180. It also is coarse enough to remove a lot of steel to regrind bevel angles on my gouges and profiles on my scrapers like the previous Norton 3x 80g I was using.

[Reed Gray]

I do have both, the 80 and 180 grit wheels. They cut very fast, faster than standard wheels. I bounce back and forth between the wheels a lot, but use the 180 most of the time, but use the 80 for my scrapers as it leaves a great burr.

robo hippy