Photos by Chris Wong
No woodworking tool or accessory has impressed me more than my 80-grit cubic boron nitride (CBN) wheel. CBN is a synthetic abrasive that’s almost as hard as diamond and is best used on hard ferrous metals. A CBN wheel consists of a metal body (usually steel) with CBN grit on the working surfaces.
I had my first experience with a CBN wheel when I was restoring a 1-1/4″ wide chisel. It seemed somebody had been abusing it, as there was a big chunk missing from the corner of the edge. I set the bench grinder’s tool rest at approximately 90° to the wheel and, bevel up, proceeded to grind the edge straight, removing about 1/4″ of steel. Then I flipped the chisel over, adjusted the tool rest and ground the bevel. All this was done in a matter of minutes without needing to pause to cool the tool.
With the “cool wheels” I was used to, I would have had to quench the steel in water every 10 seconds to avoid overheating, and the process would have probably taken three times longer.
Most grinding wheels are made of either silicon carbide or aluminum oxide abrasive bonded together to form the wheel. Both materials are insulators. Through use, the grit particles are released from the wheel to expose sharp, unworn abrasive. This keeps the wheel cutting efficiently but has the downsides of the wheel needing to be dressed frequently and a continually changing diameter.
CBN, on the other hand, is a conductor so it cools the tool as it works. CBN wheels don’t wear so they don’t need to be dressed or trued, and they remain the same diameter, meaning sharpening jigs don’t need to be reset in order to maintain the same geometry. The wheels have an exceptionally long service life and I don’t expect to replace mine in my lifetime.
An added benefit of wheels that stay flat is that you can rely on them to aid you in grinding edges straight.
CBN wheels are available in a variety of grits and a 6″ wheel can easily cost over $100 and an 8″ wheel over $200. I started with a medium-grit wheel, 180. I found it painfully slow for reshaping and modifying tool geometry, something I do quite often, so I subsequently purchased a coarse 80-grit wheel. If you don’t often radically change the geometry of your tools, consider a 180-grit CBN wheel.
Outfitting CBN wheels on a slow-speed grinder will further reduce the chance of overheating, but I have been more than happy with my 80- and 180-grit CBN wheels on my high-speed 6″ bench grinder. Regardless of what grinder you plan to use, look for balanced wheels or be prepared to invest some time repositioning the wheel on the arbor for the best balance and smoothest performance. Properly set up, the grinder will hum.