CBN and PCD, the Newer Alternatives
By Dave Monyhan
There has been quite a rush to CBN (Cubic Boron Nitride) in cutting iron in the last ten years. Some machines are just wet grinders converted to use a CBN bit. Others have been designed to be a CBN cutting machine. Set up is critical when using CBN. The aggressive cutting action it delivers is directly related to how true the set up is and how conscientious the operator is.
CBN, an ultra-hard cutting material consisting of polycrystalline cubic boron nitride with a metallic or ceramic binder is primarily used to machine hardened ferrous material. It’s available either as a tip brazed to a carbide insert carrier or as a solid insert.
CBN has its limitations. It likes to cut large amounts of material with out interruption. Guess what? The average cylinder head or block has many holes that interrupt the cutting action of the tool bit. With that in mind it’s even more critical to have correct speeds and feeds on your surfacer to adjust for machining cylinder heads and blocks. CBN must be driven by rigid machine tools with secure holding fixtures. As I stated earlier, some CBN machines are simply converted wet grinders, and others are designed as CBN machine. But not all CBN tips are the same.
The purity of CBN content separates price from quality. I recommend the titanium coated CBN tips, due to the witness mark left on the tip after you have surfaced a workpiece. Plus the titanium coating delivers longer life. These tips can be rotated about every 2-3 degrees so you can get the maximum life out of the tip. Plus if you’re using a solid CBN tip, after one side is worn out you simply flip it over and use it again. It’s essential that when you rotate the tip, that the holder is clean and the holding finger is sound. There’s nothing worse than having your machine throw a $250 tool bit somewhere in the shop, because chances are you are not going to find it. CBN can also be sharpened. If the tip isn’t fractured, it can be lapped back to spec, extending the life of the tip.
PCD (Polycrystalline Diamond) is an ultra-hard tool material (substrate) consisting of a synthetic polycrystalline diamond tip brazed to a carbide insert carrier. Primarily used to machine non-ferrous materials at high speeds, PCD has been developed specifically to machine aluminum. CBN only smears the aluminum when removing metal without producing the necessary finish today’s aluminum heads require. To extend the life of your tooling, use adequate lubrication.