Why is cubic boron nitride better than diamond for grinding HSS?
- Crystals of boron nitride (CBN) withstand the heat of grinding (diamonds will burn!) and can be used at high speeds on a normal bench grinder
- Diamond grit also reacts chemically with steel during high speed grinding and this blunts the diamonds
- CBN does not react with steel in this way so stays sharp and cool cutting
- Boron nitride is nearly as hard as diamonds and is much harder than aluminium oxide abrasive
- Boron nitride grit is tougher and stronger than diamond crystals so will survive the stress of grinding woodturning gouges
What is Cubic Boron Nitride?
CBN abrasive is made in a similar way to synthetic diamonds. It is composed of nitrogen and boron grown as extremely hard crystals - second in hardness only to diamond. It is so hard and tough a single layer of CBN grit on the surface of a metal disc can grind the hardest steel and last for years. Electroplated CBN grinding wheels are generally machined from steel covered in a layer of CBN grit. The particles of grit are held in place by a layer of electroplated nickel.
Using a CBN wheel
It is used in the same way as a conventional grinding wheel on a standard bench grinder. It is fine to use a grinder running at 3000 rpm. There is no advantage in using a "low speed" grinder. Do not press hard. Very light pressure is needed as the wheel will remove metal quicker than an aluminium oxide wheel. You will notice that very few sparks are produced so you will have to watch the edge very carefully. Stop when you see metal vanishing from the edge as that part of the blade is now sharp. You will not see sparks coming over the top of the edge when it is sharp as you do with a conventional wheel. Do not knock the tool accidentally against the spinning wheel because you might dislodge some of the precious particles of CBN grit. Remember you only have one layer of grit there. Try not to put extreme pressure onto the edge of the wheel for the same reason.
Mounting a wheel on the grinder
The bore of the CBN wheel is 32mm. Plastic or metal bushes are available for different sizes of grinder shaft. For best results the wheel must run perfectly true with no sideways wobble. Ideally the CBN wheel should be on the left hand side so you can easily grind side cutting scrapers without the motor getting in the way.
The shop is now closed.You cannot buy CBN or any grinding wheels on this website. However you can buy CBN wheels from The Toolpost here The Toolpost
Next page - technical information on grinding. How grindstones work